To see the other types of publications on this topic, follow the link: Active Galactic Nuclei, quasars, emission lines, Seyfert.
Journal articles on the topic 'Active Galactic Nuclei, quasars, emission lines, Seyfert'
Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles
Consult the top 50 journal articles for your research on the topic 'Active Galactic Nuclei, quasars, emission lines, Seyfert.'
Next to every source in the list of references, there is an 'Add to bibliography' button. Press on it, and we will generate automatically the bibliographic reference to the chosen work in the citation style you need: APA, MLA, Harvard, Chicago, Vancouver, etc.
You can also download the full text of the academic publication as pdf and read online its abstract whenever available in the metadata.
Browse journal articles on a wide variety of disciplines and organise your bibliography correctly.
1
Zhou, Yipeng, Lei Hao, Peng Jiang, Xiang Pan, Luming Sun, Chichuan Jin, Xuejie Dai, et al. "Discovery of a Radio-loud Narrow-line Seyfert 1 Galaxy at z = 0.9 Reddened with a Strong 2175 Å Bump." Astrophysical Journal 941, no. 2 (December 1, 2022): 111. http://dx.doi.org/10.3847/1538-4357/aca085.
Full textAbstract:
Abstract We present a broadband spectrophotometric analysis of an unusual quasar, SDSS J160558.86+474300.1, at z = 0.9, found in a systematic search for reddened radio-loud narrow-line Seyfert 1 galaxies (NLS1s). J1605+4743 qualifies for the conventional definition of an NLS1 in its permitted emission line widths and Hβ/[O iii] flux ratio. It also shows strong Fe ii emissions. J1605+4743 has a steep radio spectrum. With a radio loudness of ∼150 (70) before (after) reddening correction, it is among the most radio-loud NLS1s discovered. In addition, the blueshifted emission lines are also found to be weak, suggesting that it can be an exotic “wind-dominated” Seyfert similar to weak-line quasars, especially the ones with strong reddening. As an extreme hybrid central engine, its environment and host galaxy become rather intriguing. We discover in it a proximate 2175 Å absorber with a bump strength similar to the average value of 2175 Å absorbers in the Milky Way, which we suspect can be intrinsic to the quasar. In addition, the quasar seems to have enhanced instead of suppressed star formation at a rate of ≳94 M ⊙ yr−1. The combination of these unusual properties, i.e., the unique extinction, strong Fe ii and [O ii] lines, weak [O iii], Mg ii, and Balmer lines, and large radio loudness, might hold important clues to accretion physics, the formation/destruction of dust grains in the environment of active galactic nuclei, and possibly also the coevolution of supermassive black holes and their host galaxies in general.
2
Marziani, Paola, Marco Berton, Swayamtrupta Panda, and Edi Bon. "Optical Singly-Ionized Iron Emission in Radio-Quiet and Relativistically Jetted Active Galactic Nuclei." Universe 7, no. 12 (December 8, 2021): 484. http://dx.doi.org/10.3390/universe7120484.
Full textAbstract:
The issue of the difference between optical and UV properties of radio-quiet and radio-loud (relativistically “jetted”) active galactic nuclei (AGN) is a long standing one, related to the fundamental question of why a minority of powerful AGN possess strong radio emission due to relativistic ejections. This paper examines a particular aspect: the singly-ionized iron emission in the spectral range 4400–5600 Å, where the prominent HI Hβ and [Oiii]λλ4959,5007 lines are also observed. We present a detailed comparison of the relative intensity of Feii multiplets in the spectral types of the quasar main sequence where most jetted sources are found, and afterwards discuss radio-loud narrow-line Seyfert 1 (NLSy1) nuclei with γ-ray detection and with prominent Feii emission. An Feii template based on I Zw 1 provides an accurate representation of the optical Feii emission for RQ and, with some caveats, also for RL sources. CLOUDY photoionization simulations indicate that the observed spectral energy distribution can account for the modest Feii emission observed in composite radio-loud spectra. However, spectral energy differences alone cannot account for the stronger Feii emission observed in radio-quiet sources, for similar physical parameters. As for RL NLSy1s, they do not seem to behave like other RL sources, likely because of their different physical properties, that could be ultimately associated with a higher Eddington ratio.
3
Liu, Chenxu, Karl Gebhardt, Erin Mentuch Cooper, Dustin Davis, Donald P. Schneider, Robin Ciardullo, Daniel J. Farrow, et al. "The Active Galactic Nuclei in the Hobby–Eberly Telescope Dark Energy Experiment Survey (HETDEX). I. Sample Selection." Astrophysical Journal Supplement Series 261, no. 2 (July 21, 2022): 24. http://dx.doi.org/10.3847/1538-4365/ac6ba6.
Full textAbstract:
Abstract We present the first active galactic nuclei (AGN) catalog of the Hobby–Eberly Telescope Dark Energy Experiment Survey (HETDEX) observed between 2017 January and 2020 June. HETDEX is an ongoing spectroscopic survey (3500–5500 Å) with no target preselection based on magnitudes, colors or morphologies, enabling us to select AGN based solely on their spectral features. Both luminous quasars and low-luminosity Seyferts are found in our catalog. AGN candidates are selected with at least two significant AGN emission lines, such as the Lyα and C iv λ1549 line pair, or with a single broad emission line with FWHM > 1000 km s−1. Each source is further confirmed by visual inspections. This catalog contains 5322 AGN, covering an effective sky coverage of 30.61 deg2. A total of 3733 of these AGN have secure redshifts, and we provide redshift estimates for the remaining 1589 single broad-line AGN with no crossmatched spectral redshifts from the Sloan Digital Sky Survey Data Release 14 of QSOs. The redshift range of the AGN catalog is 0.25 < z < 4.32, with a median of z = 2.1. The bolometric luminosity range is 109–1014 L ☉ with a median of 1012 L ☉. The median r-band magnitude of our AGN catalog is 21.6 mag, with 34% having r > 22.5, and 2.6% reaching the detection limit at r ∼ 26 mag of the deepest imaging surveys we searched. We also provide a composite spectrum of the AGN sample covering 700–4400 Å.
4
Lyu, Jianwei, and George H. Rieke. "Polar Dust Emission in Quasar IR SEDs and Its Correlation with Narrow-line Regions." Astrophysical Journal Letters 940, no. 2 (November 25, 2022): L31. http://dx.doi.org/10.3847/2041-8213/ac9e5c.
Full textAbstract:
Abstract Polar dust has been found to play an important role in the mid-infrared emission of nearby Seyfert nuclei. If and how often polar dust exists among the quasar population is unknown due to the lack of spatially resolved observations. In this Letter, we report correlations between the prominence of active galactic nucleus (AGN) forbidden line emission (commonly associated with the narrow-line region) and the dust mid-IR energy output among the archetypal Palomar-Green quasar sample and other bright type-1 AGNs drawn from the Sloan Digital Sky Survey, Spitzer, and Wide-field Infrared Survey Explorer (WISE) archives. The AGN mid-IR color differences traced by WISE W2 (∼4.6 μm)−W3 (∼12 μm) and W2 (∼4.6 μm)−W4 (∼22 μm), and near-IR to mid-IR spectral energy distributions (SEDs) constrained with the Two Micron All Sky Survey, WISE, and Spitzer data have clear trends with the relative strength of the forbidden line regions traced by the optical [O iii] and mid-IR [O iv] emission lines. These observations indicate that, where the lines are strong, a large fraction of the AGN emission at λ ≳ 5 μm comes from dust in the forbidden line regions. We find that the widely quoted universal AGN template is a result of averaging quasar SEDs with different levels of polar dust emission above the torus output and that the typical intrinsic IR SED of compact torus dust emission alone falls with increasing wavelength past 5 μm (in νF ν ). In addition, the association of polar dust with the forbidden lines suggests an alternative to the receding torus hypothesis for the decrease in infrared output with increasing AGN luminosity.
5
Maitra, Chandreyee, Frank Haberl, Valentin D. Ivanov, Maria-Rosa L. Cioni, and Jacco Th van Loon. "Identification of AGN in the XMM-Newton X-ray survey of the SMC." Astronomy & Astrophysics 622 (January 24, 2019): A29. http://dx.doi.org/10.1051/0004-6361/201833663.
Full textAbstract:
Context. Finding active galactic nuclei (AGN) behind the Magellanic Clouds (MCs) is difficult because of the high stellar density in these fields. Although the first AGN behind the Small Magellanic Cloud (SMC) were reported in the 1980s, it is only recently that the number of AGN known behind the SMC has increased by several orders of magnitude. Aims. The mid-infrared colour selection technique has proven to be an efficient means of identifying AGN, especially obscured sources. The X-ray regime is complementary in this regard and we use XMM-Newton observations to support the identification of AGN behind the SMC. Methods. We present a catalogue of AGN behind the SMC by correlating an updated X-ray point-source catalogue from our XMM-Newton survey of the SMC with previously identified AGN from the literature as well as a list of candidates obtained from the ALLWISE mid-infrared colour-selection criterion. We studied the properties of the sample with respect to their redshifts, luminosities, and X-ray spectral characteristics. We also identified the near-infrared counterpart of the sources from the VISTA observations. Results. The redshift and luminosity distributions of the sample (where known) indicate that we detect sources ranging from nearby Seyfert galaxies to distant and obscured quasars. The X-ray hardness ratios are compatible with those typically expected for AGN, and the VISTA colours and variability are also consistent with AGN. A positive correlation was observed between the integrated X-ray flux (0.2–12 keV) and the ALLWISE and VISTA magnitudes. We further present a sample of new candidate AGN and candidates for obscured AGN. Together these make an interesting subset for further follow-up studies. An initial spectroscopic follow-up of 6 out of the 81 new candidates showed that all six sources are active galaxies, although two have narrow emission lines.
6
Jha, Vivek Kumar, Hum Chand, Vineet Ojha, Amitesh Omar, and Shantanu Rastogi. "A comparative study of the physical properties for a representative sample of Narrow and Broad-line Seyfert galaxies." Monthly Notices of the Royal Astronomical Society 510, no. 3 (December 21, 2021): 4379–93. http://dx.doi.org/10.1093/mnras/stab3700.
Full textAbstract:
ABSTRACT We present a comparative study of the physical properties of a homogeneous sample of 144 Narrow line Seyfert 1 (NLSy1) and 117 Broad-line Seyfert 1 (BLSy1) galaxies. These two samples are in a similar luminosity and redshift range and have optical spectra available in the 16th data release of Sloan Digital Sky Survey (SDSS-DR16) and X-ray spectra in either XMM-NEWTON or ROSAT. Direct correlation analysis and a principal component analysis (PCA) have been performed using ten observational and physical parameters obtained by fitting the optical spectra and the soft X-ray photon indices as another parameter. We confirm that the established correlations for the general quasar population hold for both types of galaxies in this sample despite significant differences in the physical properties. We characterize the sample also using the line shape parameters, namely the asymmetry and kurtosis indices. We find that the fraction of NLSy1 galaxies showing outflow signatures, characterized by blue asymmetries, is higher by a factor of about 3 compared to the corresponding fraction in BLSy1 galaxies. The presence of high iron content in the broad-line region of NLSy1 galaxies in conjunction with higher Eddington ratios can be the possible reason behind this phenomenon. We also explore the possibility of using asymmetry in the emission lines as a tracer of outflows in the inner regions of Active Galactic Nuclei. The PCA results point to the NLSy1 and BLSy1 galaxies occupying different parameter spaces, which challenges the notion that NLSy1 galaxies are a subclass of BLSy1 galaxies.
7
Filippenko, Alexei V. "Low-Luminosity Active Galactic Nuclei." Symposium - International Astronomical Union 134 (1989): 495–512. http://dx.doi.org/10.1017/s0074180900141804.
Full textAbstract:
I review the basic properties of low-luminosity active galactic nuclei (LLAGNs) — objects in which activity similar to, but intrinsically milder than, that in QSOs and luminous Seyferts is believed to be present. Until recently, most LLAGNs were first recognized as such and studied at optical wavelengths, but evidence for activity and valuable information concerning its nature are now also being obtained in many other spectral ranges. Low-ionization nuclear emission-line regions (LINERs) are the most common LLAGNs; in a large fraction of them the emission lines come from clouds of gas probably photoionized by a nonstellar continuum, rather than heated by shocks. It is also clear, however, that there could be substantial heterogeneity among LINERs, particularly those with extended emission; some may be produced by cooling flows, supernova-driven winds, and galaxy interactions or mergers. LLAGNs can easily be hidden from sight in a variety of ways, including obscuration by galactic or circumnuclear disks and dilution by bursts of star formation. Direct imaging, spectroscopy, and spectropolarimetry can be used to isolate the different components, especially with high spatial and/or spectral resolution. A surprising result is that Seyfert nuclei are sometimes found in dwarf and very late-type galaxies. The nearby Sd III-IV galaxy NGC 4395 contains the intrinsically faintest known Seyfert 1 nucleus, with a broad Ha luminosity only ∼ 0.1 that of M81. The idea that the apparent “activity” in some galaxies is a direct consequence of vigorous star formation has some strong supporting evidence, and should seriously be considered. As an example, I focus on the peculiar supernova 1987F, whose optical spectrum bears a striking resemblance to that of typical Seyfert 1 nuclei.
8
Makishima, K., N. Iyomoto, Y. Ishisaki, and Y. Terashima. "X-Ray Observations of Low-Luminosity Active Galactic Nuclei." International Astronomical Union Colloquium 159 (1997): 52–53. http://dx.doi.org/10.1017/s0252921100039610.
Full textAbstract:
AbstractThrough X-ray observations with ASCA, low-luminosity active galactic nuclei have been found in at least seven near-by spiral galaxies. Some of them exhibit very intense, and possibly broad, Fe-K emission lines. Their time variability is relatively insignificant, in contrast to lowluminosity Seyfert galaxies.
9
Foschini, Luigi, Matthew L. Lister, Heinz Andernach, Stefano Ciroi, Paola Marziani, Sonia Antón, Marco Berton, et al. "A New Sample of Gamma-Ray Emitting Jetted Active Galactic Nuclei." Universe 8, no. 11 (November 4, 2022): 587. http://dx.doi.org/10.3390/universe8110587.
Full textAbstract:
We considered the fourth catalog of gamma-ray point sources produced by the Fermi Large Area Telescope (LAT) and selected only jetted active galactic nuclei (AGN) or sources with no specific classification, but with a low-frequency counterpart. Our final list is composed of 2980 gamma-ray point sources. We then searched for optical spectra in all the available literature and publicly available databases, to measure redshifts and to confirm or change the original LAT classification. Our final list of gamma-ray emitting jetted AGN is composed of BL Lac Objects (40%), flat-spectrum radio quasars (23%), misaligned AGN (2.8%), narrow-line Seyfert 1, Seyfert, and low-ionization nuclear emission-line region galaxies (1.9%). We also found a significant number of objects changing from one type to another, and vice versa (changing-look AGN, 1.1%). About 30% of gamma-ray sources still have an ambiguous classification or lack one altogether.
10
Moorwood, A. F. M., and E. Oliva. "Infrared [FeII] and H2 Line Emission in Active Galactic Nuclei." Symposium - International Astronomical Union 134 (1989): 365–71. http://dx.doi.org/10.1017/s007418090014135x.
Full textAbstract:
Infrared spectra around [FeII](1.644μm), H2 (v = 1-0 S(1) at 2.121μm) and Brγ (H 7-4 at 2.167μm) have been obtained of 35 emission line nuclei whose optical spectra range from pure HII regionlike to Seyfert 1. All three emission lines have been detected in a large fraction of those nuclei exhibiting starburst activity but not in “pure” Seyferts. The highest detection frequency, however, was found for composite nuclei showing evidence for both starburst and Seyfert activity. Several arguments suggest that both the [FeII] and H2 are shock excited and could be largely attributed to supernova remnants. The fact that the [FeII]/Brγ and H2/Brγ ratios tend to be somewhat larger in composite than in pure starburst nuclei, however, plus significant spatial displacements between the [FeII] and H2 emitting regions found recently during follow-up observations of selected galaxies suggest that large scale phenomena, e.g. SN winds or mass outflows from AGN, may also be important.
11
Sales, D. A., M. G. Pastoriza, and R. Riffel. "A 5.5–35 μm Spectral Analysis of Active Galactic Nuclei." Proceedings of the International Astronomical Union 5, S267 (August 2009): 137. http://dx.doi.org/10.1017/s1743921310005983.
Full textAbstract:
The spectroscopic properties of a sample of nine Seyfert 1, six NLS1, 26 Seyfert 2, and three starburst galaxies have been studied at mid-infrared wavelengths in order to determine if the frequency of detection of the brightest emission lines and the continuum shape are correlated with the degree of activity. The raw data were obtained from the Spitzer Infrared Spectrograph (IRS) archive and reduced with the pipeline (version 17.2). The spectra of the galaxies were grouped by type of activity. We conclude through this study that in general the continuum shape of the active galactic nuclei (AGN) is flat between 5 μm and 12 μm, and strong variations are found at longer wavelengths in this sample.
12
Genzel, R., D. Lutz, E. Sturm, L. Tacconi, N. Thatte, and A. Sternberg. "Infrared to Millimeter Spectroscopy and Imaging of Active Galactic Nuclei." International Astronomical Union Colloquium 159 (1997): 312–24. http://dx.doi.org/10.1017/s0252921100040355.
Full textAbstract:
AbstractObservations in the 1 µm to 1 cm wavelength band give important information on the physical processes occurring in and immediately around active galactic nuclei. Concentrating on recent results on ionic, atomic, and molecular emission lines, we discuss as examples the first ISO results on the nature of ultra-luminous infrared galaxies, and near-infrared and millimeter measurements of the central 100 parsecs of the Seyfert 2 galaxy NGC 1068.
13
Persson, S. E. "High-density gas in bipolar outflow sources and active galactic nuclei." Canadian Journal of Physics 64, no. 4 (April 1, 1986): 421–25. http://dx.doi.org/10.1139/p86-075.
Full textAbstract:
The highest density regions near the base of the outflow in young stellar objects with bipolar molecular outflows are discussed. Bright emission lines of Ca II and O I characterize the spectra and lead to estimates of n ~ 1010 cm−3, temperatures of a few thousand Kelvins, and mass motions of several hundred kilometres per second. Typically the rate of momentum transfer within this region fails by more than an order of magnitude to provide the force necessary to drive the molecular outflow on the parsec scale. The overall near-infrared spectra of these young stars are similar to those of certain active galactic nuclei having strong Fe II emission.A comparison is made between the Ca II and O I lines in the infrared core source of the bipolar H II region S106 and in the Seyfert galaxy Mrk 42. The spectra are shown to be scaled versions of each other, and we conclude that the physical conditions are broadly similar in the high-density emission-line regions of these two objects. The Ca II lines appear self-reversed in both objects, but possible Ca II absorption in the underlying galaxy in Mrk 42 compromises the detailed comparison of line shapes. Nevertheless, it is likely that the geometries and velocity fields are also analogous. Derived parameters of the emission-line regions in S106 and the Seyfert galaxies I Zw 1 and Mrk 42 are compared.
14
Netzer, Hagai. "High-Energy Lines in Active Galactic Nuclei." International Astronomical Union Colloquium 159 (1997): 20–33. http://dx.doi.org/10.1017/s0252921100039567.
Full textAbstract:
AbstractThe recent observations of a highly ionized, X-ray absorbing gas in active galactic nuclei (AGN) suggest a new nuclear component, the so-called ‘warm absorber’. This gas is likely to be at a temperature of ~ 1–2 × 105 K and is most easily detected in the 0.5–10keV range, where several oxygen absorption edges are often observed.This review describes the properties of warm absorbers and the relation to other nuclear components, such as the broad-line emitting gas. The stability of the gas is a key issue and analysis shows that it is likely to be thermally stable, at the above mentioned temperature. When successful models are compared to the data, they can be used to infer the column density, composition and level of ionization of the X-ray absorbing gas. They also show that, on top of the strong continuum absorption, the gas must emit X-ray lines that are at the limit of detection by present day X-ray instruments.New calculations of X-ray emission lines emitted by ionized X-ray absorbers are shown and discussed. Various line equivalent widths are defined and examples are shown over a large range of column density and ionization parameter. The equivalent width of the strongest 0.5-5 keV lines is only a few tens of eV, but in cases of obscured X-ray source, like in Seyfert 2s, the lines are measured against the scattered and diffuse radiation with much larger equivalent widths. The X-ray absorbing and emitting gas is responsible also for a fraction of the observed flux of some UV emission lines. It is also the cause of the detection of several UV absorption lines. The calculations predict that some of those absorption lines, in particular 0 VI λ1035, are very sensitive warm-absorber tracers. Thus, analysis of the combined X-ray and UV properties is the best way to identify the location and properties of this gas.Understanding the origin and properties of warm X-ray absorbers is a major challenge of AGN research. Several new ideas are briefly discussed, trying to relate the location, mass, and motion of this gas to what is known about other observed nuclear components.
15
Doddamani, Vijayakumar H., and P. Vedavathi. "Ultraviolet Line-Continuum properties of Seyfert 1 galaxies and quasars." Proceedings of the International Astronomical Union 9, S304 (October 2013): 233–35. http://dx.doi.org/10.1017/s1743921314003883.
Full textAbstract:
AbstractActive galaxies are most luminous objects in the universe whose spectra are characterized by both permitted and forbidden emission line features. The spectra of Seyfert 1 galaxies and quasars contain strong and broad emission lines of wide ranging ionization potentials. The velocity widths of the lines range from a minimum of ≈ 500 km/s for narrow lines to a maximum of 20,000 km/sec for broad lines. The UV spectra of the active galaxies contain strong and broad emission lines such as Lyα, NV, SiIV, OIV], CIV, CIII] and MgII lines. The widths of the broad lines are attributed to the differential doppler shifts of the emission lines due to the bulk motions of individual clumpy gas clouds in the BLR region. We have anlysed UV spectra of Seyfert 1 galaxies and quasars from IUE satellite archival database to understand the nature of dependence of the emission line properties with the underlying UV continuum. We have undertaken line luminosity correlation studies for Lyα and CIV lines with their underlying UV continuum luminosity at 1125Å, 1425Å & 1625Å. The IUE archival spectra have been reduced for galactic reddening using the E(B-V) and NHI values published continuum luminosity has been observed at 1125Å, 1425Å & 1625Å. The Lyα line line has exhibited strongest linear correlation wavelengths while CIV line has shown at 1425Å and 2625Å wavelengths. These results are empirically consistent with the predictions of the general multi-component photo-ionization models suggesting that the central strong UV continuum has been reprocessed by the clumpy gas clouds of the broad emission region (BLR). A detailed account of the data reduction, UV flux measurement and the significance of line-luminosity correlations are discussed in this paper.
16
GHISELLINI, GABRIELE. "JETTED ACTIVE GALACTIC NUCLEI." International Journal of Modern Physics: Conference Series 08 (January 2012): 1–12. http://dx.doi.org/10.1142/s2010194512004345.
Full textAbstract:
Most of the electromagnetic output of blazars (BL Lac objects and Flat Spectrum Radio Quasars) comes out in the γ–ray band, making the Large Area Telescope [0.1–100 GeV] onboard the Fermi satellite and the Cherenkov telescopes crucial for gather crucial data and thus to understand their physics. These data are complemented by the observations of the Swift satellite in the X–ray and optical–UV bands, and by ground based optical and radio telescopes. This rich coverage of the spectrum allows a robust modelling, from which important trends start to emerge. In powerful sources we see the contribution of the accretion disk that, once modeled, give us the black hole mass and the accretion rate. Even when not directly visible, the disk luminosity can be derived through the broad emission lines. Therefore we start to know the jet power, the disk luminosity, and the black hole mass, 3 crucial ingredients if we want to draw a general scenario. At the start, jets are believed to be magnetically dominated. And yet, on the scale where they emit most of their luminosity, their power is already in the form of kinetic energy of particles. Relativistic jets are formed for a very broad range of the disk luminosity, from close to Eddington down to at least 10-4 Eddington. Their power correlates with the accretion rate, and can be even more powerful than the accretion disk luminosity.
17
Fricke, K. J., and W. Kollatschny. "Relationships of the Active Nucleus, Galaxy, and Environment." Symposium - International Astronomical Union 134 (1989): 425–44. http://dx.doi.org/10.1017/s0074180900141580.
Full textAbstract:
35 years ago Baade and Minkowski (1954) suggested that a galaxy collision - diagnosed from the peculiar appearance of the parent object and its strong emission lines - is responsible for the strong radio-source CygA. This was the first time that gravitational interactions between galaxies were suggested to trigger nuclear activity. Over the following decades after the detection of the quasars and the gradual realization that quasars, comparable to the Seyfert phenomenon, are events at the nuclei of seemingly isolated galaxies, the collision hypothesis was abandoned. Efforts concentrated on the understanding of the activity as internal processes in the host galaxies, possibly aided by infall of gas from the intergalactic medium (cf. Rees, 1978; Gunn, 1979).
18
García-Benito, Rubén, Enrique Pérez-Montero, Oli L. Dors, José M. Vlchez, Monica V. Cardaci, and Guillermo F. Hägele. "A new tool to derive chemical abundances in type-2 active galactic nuclei." Proceedings of the International Astronomical Union 15, S356 (October 2019): 61–65. http://dx.doi.org/10.1017/s1743921320002586.
Full textAbstract:
AbstarctWe present a new tool for the analysis of the optical emission lines of the gas in the Narrow Line Region (NLR) around Active Galactic Nuclei (AGNs). This new tool can be used in large samples of objects in a consistent way using different sets of optical emission-lines taking into the account possible variations from the O/H - N/O relation. The code compares certain observed emission-line ratios with the predictions from a large grid of photoionization models calculated under the most usual conditions in the NLR of AGNs to calculate the total oxygen abundance, nitrogen-to-oxygen ratio and ionization parameter. We applied our method to a sample of Seyfert 2 galaxies with optical emission-line fluxes from the literature. Our results confirm the high metallicity of the objects of the sample and provide consistent values with the direct method. The usage of models to calculate precise ICFs is mandatory when only optical emission lines are available to derive chemical abundances using the direct method in NLRs of AGN.
19
Binette, L., M. Villar Martín, G. Magris C., M. Martínez-Paredes, A. Alarie, A. Rodríguez Ardila, and I. Villicaña-Pedraza. "TEMPERATURE DISCREPANCY WITH PHOTOIONIZATION MODELS OF THE NARROW-LINE REGION." Revista Mexicana de Astronomía y Astrofísica 58, no. 1 (April 1, 2022): 133–53. http://dx.doi.org/10.22201/ia.01851101p.2022.58.01.11.
Full textAbstract:
Using published work on the narrow-line region of Active Galactic Nuclei, a comparison is carried out among the [OIII] λ4363Å/5007Å ratios (ROIII) observed in quasars, Seyfert 2's and the spatially resolved ENLR plasma. Using the weak [ArIV] λ4711Å/λ4740Å doublet observed by Koski (1978) among Seyfert 2's, we find evidence of a Narrow-Line Region (NLR) populated by low density emission clouds (≲ 104 cm-3). After considering calculations of the [Ar] and [OIII] ratios that assume a powerlaw distribution of plasma densities, no evidence of collisional deexcitation is found. The plasma temperature that is inferred is 13 500 °K, which is problematic to reproduce with standard photoionization calculations. The simplest interpretation for the near coincidence of the ROIII ratios among the ENLR and Seyfert 2 measurements (ROIII ≃ 0.017) is that the low density regime applies to both plasmas.
20
Binette, L., M. Villar Martín, G. Magris C., M. Martínez-Paredes, A. Alarie, A. Rodríguez Ardila, and I. Villicaña-Pedraza. "TEMPERATURE DISCREPANCY WITH PHOTOIONIZATION MODELS OF THE NARROW-LINE REGION." Revista Mexicana de Astronomía y Astrofísica 58, no. 1 (April 1, 2022): 133–53. http://dx.doi.org/10.22201/ia.01851101p.2022.58.01.11.
Full textAbstract:
Using published work on the narrow-line region of Active Galactic Nuclei, a comparison is carried out among the [OIII] λ4363Å/5007Å ratios (ROIII) observed in quasars, Seyfert 2's and the spatially resolved ENLR plasma. Using the weak [ArIV] λ4711Å/λ4740Å doublet observed by Koski (1978) among Seyfert 2's, we find evidence of a Narrow-Line Region (NLR) populated by low density emission clouds (≲ 104 cm-3). After considering calculations of the [Ar] and [OIII] ratios that assume a powerlaw distribution of plasma densities, no evidence of collisional deexcitation is found. The plasma temperature that is inferred is 13 500 °K, which is problematic to reproduce with standard photoionization calculations. The simplest interpretation for the near coincidence of the ROIII ratios among the ENLR and Seyfert 2 measurements (ROIII ≃ 0.017) is that the low density regime applies to both plasmas.
21
Kheirandish, Ali, Kohta Murase, and Shigeo S. Kimura. "High-energy Neutrinos from Magnetized Coronae of Active Galactic Nuclei and Prospects for Identification of Seyfert Galaxies and Quasars in Neutrino Telescopes." Astrophysical Journal 922, no. 1 (November 1, 2021): 45. http://dx.doi.org/10.3847/1538-4357/ac1c77.
Full textAbstract:
Abstract Particles may be accelerated in magnetized coronae via magnetic reconnections and/or plasma turbulence, leading to high-energy neutrinos and soft γ-rays. We evaluate the detectability of neutrinos from nearby bright Seyfert galaxies identified by X-ray measurements. In the disk-corona model, we find that NGC 1068 is the most promising Seyfert galaxy in the Northern sky, where IceCube is the most sensitive, and show prospects for the identification of aggregated neutrino signals from Seyfert galaxies bright in X-rays. Moreover, we demonstrate that nearby Seyfert galaxies are promising targets for the next generation of neutrino telescopes such as KM3NeT and IceCube-Gen2. For KM3NeT, Cen A can be the most promising source in the Southern sky if a significant fraction of the observed X-rays come from the corona, and it could be identified in few years of KM3NeT operation. Our results reinforce the idea that hidden cores of supermassive black holes are the dominant sources of the high-energy neutrino emission and underlines the necessity of better sensitivity to medium-energy ranges in future neutrino detectors for identifying the origin of high-energy cosmic neutrinos.
22
Agostino, Christopher J., Samir Salim, Sara L. Ellison, Robert W. Bickley, and S. M. Faber. "A New Physical Picture for Active Galactic Nuclei Lacking Optical Emission Lines." Astrophysical Journal 943, no. 2 (February 1, 2023): 174. http://dx.doi.org/10.3847/1538-4357/acac99.
Full textAbstract:
Abstract In this work, we use ∼500 low-redshift (z ∼ 0.1) X-ray active galactic nuclei (AGNs) observed by XMM-Newton and the Sloan Digital Sky Survey (SDSS) to investigate the prevalence and nature of AGNs that apparently lack optical emission lines (“optically dull AGNs”). Although one quarter of spectra appear absorption-line dominated in visual assessment, line extraction with robust continuum subtraction from the MPA/JHU catalog reveals usable [O iii] measurements in 98% of the sample, allowing us to study [O iii]-underluminous AGNs together with more typical AGNs in the context of the L [O III]–L X relation. We find that “optically dull AGNs” do not constitute a distinct population of AGNs. Instead, they are the [O iii]-underluminous tail of a single, unimodal L [O III]–L X relation that has substantial scatter (0.6 dex). We find the degree to which an AGN is underluminous in [O iii] correlates with the specific star formation rate or D 4000 index of the host, which are both linked to the molecular gas fraction. Thus the emerging physical picture for the large scatter seems to involve the gas content of the narrow-line region. We find no significant role for previously proposed scenarios for the presence of optically dull AGNs, such as host dilution or dust obscuration. Despite occasionally weak lines in SDSS spectra, >80% of X-ray AGNs are identified as such with the Baldwin–Phillips–Terlevich diagram. More than 90% are classified as AGNs based only on [N ii]/Hα, providing more complete AGN samples when [O iii] or Hβ are weak. X-ray AGNs with LINER spectra obey essentially the same L [O III]–L X relation as Seyfert 2s, suggesting their line emission is produced by AGN activity.
23
Bochkarev, Nikolay G. "1–3-Day Variations of the Broad Emission Lines of Seyfert Nuclei." Symposium - International Astronomical Union 121 (1987): 219–22. http://dx.doi.org/10.1017/s0074180900155172.
Full textAbstract:
One of the principal problems of investigation of AGNs is to understand the nature of the central objects and the physical processes which yield their gigantic luminosities. This requires us to consider the innermost parts of AGNs (3–30 gravitational radii, i.e. ≲ 1016 cm: angular size less than 1 milliarcsecond).1–3d variations of the relative shape of broad emission line profiles of active galactic nuclei (AGN) are perhaps the most informative probe of the innermost parts of AGNs (r = 1015–16 cm from the centre). Optimal objects for these observations are Sy 1.5 with strong variations of the non-thermal continuum (e.g. NGC 4151). The expected amplitude of the variations ranges from a few percent to 10–20%. The optimal spectral resolution is 0.5–5 Å. It is very desirable to complement spectral observations with photometric data. The results of the first set of such observations with the 6-meter telescope are reported.
24
Filippenko, Alexei V., and Wallace L. W. Sargent. "A Search for Low-Level Seyfert Activity in the 500 Brightest Northern Galaxies." Symposium - International Astronomical Union 121 (1987): 451–60. http://dx.doi.org/10.1017/s0074180900155470.
Full textAbstract:
A sensitive search for intrinsically faint Seyfert nuclei in the 500 brightest northern galaxies is being conducted with the Hale 5.08 m telescope. The primary aim is to extend the luminosity function of active galactic nuclei (AGNs) to lower luminosities. Thus far, we have detected Hα emission having full-width near zero-intensity (FWZI) ≳ 4000 km s−1 in a significant fraction (≳10%) of nearby galaxies. Moreover, the narrow emission lines in a given nucleus often have markedly different widths, as shown in detail for M81. Those associated with high critical densities are generally the broadest, indicating that a very wide range of densities is present in the narrow-line region. The relative intensities of the lines can readily be explained with models in which clouds of gas having ne ≈ 102 to 107 cm−3 are photoionized by nonstellar radiation such as that produced by QSOs and luminous Seyfert 1 galaxies.
25
den Brok, Jakob S., Michael J. Koss, Benny Trakhtenbrot, Daniel Stern, Sebastiano Cantalupo, Isabella Lamperti, Federica Ricci, et al. "BASS. XXVIII. Near-infrared Data Release 2: High-ionization and Broad Lines in Active Galactic Nuclei*." Astrophysical Journal Supplement Series 261, no. 1 (July 1, 2022): 7. http://dx.doi.org/10.3847/1538-4365/ac5b66.
Full textAbstract:
Abstract We present the BAT AGN Spectroscopic Survey (BASS) Near-infrared Data Release 2 (DR2), a study of 168 nearby ( z ¯ = 0.04 , z < 0.6) active galactic nuclei (AGN) from the all-sky Swift Burst Array Telescope X-ray survey observed with the Very Large Telescope (VLT)/X-shooter in the near-infrared (NIR; 0.8–2.4 μm). We find that 49/109 (45%) Seyfert 2 and 35/58 (60%) Seyfert 1 galaxies observed with VLT/X-shooter show at least one NIR high-ionization coronal line (CL; ionization potential χ > 100 eV). Comparing the emission of the [Si vi] λ1.9640 CL with the X-ray emission for the DR2 AGN, we find a significantly tighter correlation, with a lower scatter (0.37 dex) than that for the optical [O iii] λ5007 line (0.71 dex). We do not find any correlation between CL emission and the X-ray photon index Γ. We find a clear trend of line blueshifts with increasing ionization potential in several CLs, such as [Si vi] λ1.9640, [Si x] λ1.4300, [S viii] λ0.9915, and [S ix] λ1.2520, indicating the radial structure of the CL region. Finally, we find a strong underestimation bias in black hole mass measurements of Sy 1.9 using broad Hα due to the presence of significant dust obscuration. In contrast, the broad Paα and Paβ emission lines are in agreement with the M–σ relation. Based on the combined DR1 and DR2 X-shooter sample, the NIR BASS sample now comprises 266 AGN with rest-frame NIR spectroscopic observations, the largest set assembled to date.
26
Valencia-S., Mónica, G. Busch, S. Smajić, N. Fazeli, S. Behroozian, J. Zuther, S. Fischer, and A. Eckart. "AGN with faint broad line regions: Some ‘True’-Seyfert 2s might be Narrow Line Seyfert 1s." Proceedings of the International Astronomical Union 9, S304 (October 2013): 274–77. http://dx.doi.org/10.1017/s1743921314004050.
Full textAbstract:
AbstractThe intriguing existence of ‘True’-Seyfert 2s has opened a debate on the validity of the unified model of active galactic nuclei (AGN). These objects, also called ‘Naked’-AGN, seem to lack a broad line region. In some cases, their X-ray emission is unabsorbed, typical of Seyfert 1 galaxies, indicating a clear view toward the nucleus, but no broad lines are seen in the optical. In ‘True’-Seyfert 2s with higher column densities, observations of polarized light have failed to reveal the hidden broad emission. We performed high-resolution near-infrared integral-field observations of two ‘True’-Seyfert 2 candidates: IRAS 01072+4954 and NGC 7496. We found that the AGN in these sources might power very faint and narrow broad emission lines (FWHMbroad < 1500km s−1). Here, we discuss the properties of the broad components estimated from published X-ray and/or optical measurements and present their near-infrared candidate identification. Both galaxies host intermediate-mass black holes, with masses ~ 105−106 M⊙. Our results favor the unified model in the cases of high accretion rates, and stress the necessity of a multiwavelength approach to unveil the nature of ‘Naked’-AGN.
27
Cheng, F. H., J. H. You, and M. Yan. "Cerenkov redshift and the redshift difference among broad emission lines of quasars and active galactic nuclei." Physics Letters A 138, no. 4-5 (June 1989): 165–67. http://dx.doi.org/10.1016/0375-9601(89)90020-0.
Full text
28
Matsuoka, Kenta, Tohru Nagao, Alessandro Marconi, Roberto Maiolino, and Yoshiaki Taniguchi. "The AGN-Starburst Connection traced by the Nitrogen Abundance." Proceedings of the International Astronomical Union 8, S290 (August 2012): 273–74. http://dx.doi.org/10.1017/s1743921312019977.
Full textAbstract:
AbstractThe connection between the active galactic nuclei (AGNs) and star formation activity is one of the most important issues in understanding the coevolution of supermassive black holes (SMBHs) and galaxies. In our recent study, by using SDSS quasar spectra we found that the emission-line flux rations involving a nitrogen line correlate with the Eddington ratio. This correlation suggests that the mass accretion in quasars is associated with a post-starburst phase, when AGB stars enrich the interstellar medium with the nitrogen. Moreover, we focused on nitrogen-loud quasars, that have prominent emission lines of the nitrogen, to investigate whether this argument is correct. In this symposium, we present our results regarding the relation between the star formation and feeding to SMBHs traced by the nitrogen abundance.
29
Kim, Minjin, Luis C. Ho, Carol J. Lonsdale, Mark Lacy, Andrew W. Blain, and Amy E. Kimball. "Evidence of AGN-driven Outflows in Young Radio Quasars Selected from the Wide-field Infrared Survey Explorer." Proceedings of the International Astronomical Union 9, S304 (October 2013): 347–48. http://dx.doi.org/10.1017/s1743921314004268.
Full textAbstract:
AbstractWe present near-infrared spectra of young radio quasars selected by cross-correlating the Wide-field Infrared Survey Explorer (WISE) all-sky survey catalog with the radio catalog [Faint Images of the Radio Sky at Twenty cm (FIRST) and NRAO VLA Sky Survey (NVSS)]. The objects have typical redshifts of z ≈ 2 and [O III] luminosities of 107 erg s−1 comparable to those of luminous quasars. The observed flux ratios of narrow emission lines indicate that these objects appear to be powered by active galactic nuclei. The [O III] line is broad, with full width at half maximum ~1300 to 2100 km s−1, significantly larger than that of ordinary quasars. These large line widths might be explained by jet-induced outflows.
30
Fernández-Ontiveros, Juan A., and Teo Muñoz-Darias. "X-ray binary accretion states in active galactic nuclei? Sensing the accretion disc of supermassive black holes with mid-infrared nebular lines." Monthly Notices of the Royal Astronomical Society 504, no. 4 (April 22, 2021): 5726–40. http://dx.doi.org/10.1093/mnras/stab1108.
Full textAbstract:
ABSTRACT Accretion states, which are universally observed in stellar mass black holes in X-ray binaries, might be expected in active galactic nuclei (AGN). This is the case at low luminosities, when the jet–corona coupling dominates the energy output in both populations. Previous attempts to extend this framework to a wider AGN population have been extremely challenging due to heavy hydrogen absorption of the accretion disc continuum and starlight contamination from the host galaxies. We present the luminosity–excitation diagram (LED), based on the [O iv]25.9 μm and [Ne ii]12.8 μm mid-infrared nebular line fluxes. This tool enables to probe the accretion disc contribution to the ionizing continuum. When applied to a sample of 167 nearby AGN, the LED recovers the characteristic q-shaped morphology outlined by individual X-ray binaries during a typical accretion episode, allowing us to tentatively identify the main accretion states. The soft state would include broad-line Seyferts and about half of the Seyfert 2 population, showing highly excited gas and radio-quiet cores consistent with disc-dominated nuclei, in agreement with previous studies. The hard state mostly includes low-luminosity AGN ($\lesssim 10^{-3}\, \rm {L_{Edd}}$) characterized by low-excitation radio-loud nuclei and a negligible disc contribution. The remaining half of Seyfert 2 nuclei and the bright LINERs show low excitation at high accretion luminosities and could be identified with the bright hard and intermediate states. Their hosts show ongoing star formation in the central kiloparsecs. We discuss the above scenario, its potential links with the galaxy evolution picture, and the possible presence of accretion state transitions in AGN, as suggested by the growing population of changing-look quasars.
31
Saikia, Payaswini, Elmar Körding, Deanne L. Coppejans, Heino Falcke, David Williams, Ranieri D. Baldi, Ian Mchardy, and Rob Beswick. "15-GHz radio emission from nearby low-luminosity active galactic nuclei." Astronomy & Astrophysics 616 (August 2018): A152. http://dx.doi.org/10.1051/0004-6361/201833233.
Full textAbstract:
We present a sub-arcsec resolution radio imaging survey of a sample of 76 low-luminosity active galactic nuclei (LLAGN) that were previously not detected with the Very Large Array at 15 GHz. Compact, parsec-scale radio emission has been detected above a flux density of 40 μ Jy in 60% (45 of 76) of the LLAGN sample. We detect 20 out of 31 (64%) low-ionization nuclear emission-line region (LINER) nuclei, ten out of 14 (71%) low-luminosity Seyfert galaxies, and 15 out of 31 (48%) transition objects. We use this sample to explore correlations between different emission lines and the radio luminosity. We also populate the X-ray and the optical fundamental plane of black hole activity and further refine its parameters. We obtain a fundamental plane relation of log LR = 0.48 (±0.04) log LX + 0.79 (±0.03) log M and an optical fundamental plane relation of log LR = 0.63 (±0.05) log L[O III] + 0.67 (±0.03) log M after including all the LLAGN detected at high resolution at 15 GHz, and the best-studied hard-state X-ray binaries (luminosities are given in erg s−1 while the masses are in units of solar mass). Finally, we find conclusive evidence that the nuclear 15 GHz radio luminosity function (RLF) of all the detected Palomar Sample LLAGN has a turnover at the low-luminosity end, and is best-fitted with a broken power law. The break in the power law occurs at a critical mass accretion rate of 1.2 × 10−3 M⊙ yr−1, which translates to an Eddington ratio of ṁEdd ~ 5.1 × 10−5, assuming a black hole mass of 109 M⊙. The local group stands closer to the extrapolation of the higher-luminosity sources, and the classical Seyferts agree with the nuclear RLF of the LLAGN in the local universe.
32
Hutsemékers, D., B. Agís González, F. Marin, D. Sluse, C. Ramos Almeida, and J. A. Acosta Pulido. "Polarization of changing-look quasars." Astronomy & Astrophysics 625 (May 2019): A54. http://dx.doi.org/10.1051/0004-6361/201834633.
Full textAbstract:
If the disappearance of the broad emission lines observed in changing-look quasars originates from the obscuration of the quasar core by dusty clouds moving in the torus, high linear optical polarization would be expected in those objects. We then measured the rest-frame UV-blue linear polarization of a sample of 13 changing-look quasars, 7 of them being in a type 1.9-2 state. For all quasars but one the polarization degree is lower than 1%. This suggests that the disappearance of the broad emission lines cannot be attributed to dust obscuration, and supports the scenario in which changes of look are caused by a change in the rate of accretion onto the supermassive black hole. Such low polarization degrees also indicate that these quasars are seen under inclinations close to the system axis. One type 1.9-2 quasar in our sample shows a high polarization degree of 6.8%. While this polarization could be ascribed to obscuration by a moving dusty cloud, we argue that this is unlikely given the very long time needed for a cloud from the torus to eclipse the broad emission line region of that object. We propose that the high polarization is due to the echo of a past bright phase seen in polar-scattered light. This interpretation raises the possibility that broad emission lines observed in the polarized light of some type 2 active galactic nuclei can be echoes of past type 1 phases and not evidence of hidden broad emission line regions.
33
Singh, R., G. van de Ven, and K. Jahnke. "The nature of LINER galaxies: Ubiquitous hot old stars and rare accreting black holes." Proceedings of the International Astronomical Union 9, S304 (October 2013): 280–81. http://dx.doi.org/10.1017/s1743921314004074.
Full textAbstract:
AbstractGalaxies, which often contain ionised gas, sometimes also exhibit a so-called low-ionisation nuclear emission line region (LINER). For 30 years, this was attributed to a central mass-accreting supermassive black hole (more commonly known as active galactic nucleus or AGN) of low luminosity, making LINER galaxies the largest AGN sub-population, which dominate in numbers over higher AGN-luminosity Seyfert galaxies and quasars. This, however, poses a serious problem. While the inferred energy balance is plausible, many LINERs clearly do not contain any other independent signatures of an AGN. Using integral field spectroscopic data from the CALIFA survey, we compare the observed radial surface brightness profiles with what is expected from illumination by an AGN. For 48 galaxies with LINER-like emission we show, that the radial emission-line surface brightness profiles are inconsistent with ionisation by a central point-source and hence cannot be due to an AGN alone. The most probable explanation for the excess LINER-like emission is ionisation by evolved stars during the short but very hot and energetic phase known as post-AGB. This leads us to an entirely new interpretation. Post-AGB stars are ubiquitous and their ionising effect should be potentially observable in every galaxy with gas present and with stars older than ~1 Gyr, unless a stronger radiation field from young hot stars or an AGN outshines them. This means, that galaxies with LINER-like emission are not a class defined by a property but rather by the absence of a property. It also explains why LINER emission is observed mostly in massive galaxies with old stars and little star formation.
34
Pérez-Montero, E., O. L. Dors, J. M. Vílchez, R. García-Benito, M. V. Cardaci, and G. F. Hägele. "A bayesian-like approach to derive chemical abundances in type-2 active galactic nuclei based on photoionization models." Monthly Notices of the Royal Astronomical Society 489, no. 2 (August 23, 2019): 2652–68. http://dx.doi.org/10.1093/mnras/stz2278.
Full textAbstract:
Abstract We present a new methodology for the analysis of the emission lines of the interstellar medium in the narrow-line regions around type-2 active galactic nuclei. Our aim is to provide a recipe that can be used for large samples of objects in a consistent way using different sets of optical emission lines that takes into the account possible variations from the O/H–N/O relation to use [N ii] lines. Our approach consists of a bayesian-like comparison between certain observed emission-line ratios sensitive to total oxygen abundance, nitrogen-to-oxygen ratio, and ionization parameter with the predictions from a large grid of photoionization models calculated under the most usual conditions in this environment. We applied our method to a sample of Seyfert 2 galaxies with optical emission-line fluxes and determinations of their chemical properties from detailed models in the literature. Our results agree within the errors with other results and confirm the high metallicity of the objects of the sample, with N/O values consistent with a large secondary production of N, but with a large dispersion. The obtained ionization parameters for this sample are much larger than those for star-forming object at the same metallicity.
35
Qi, Yan-Qing, Tong Liu, Zhen-Yi Cai, and Mouyuan Sun. "Nucleosynthesis Contribution of Neutrino-dominated Accretion Flows to the Chemical Evolution of Active Galactic Nuclei." Astrophysical Journal 934, no. 1 (July 1, 2022): 1. http://dx.doi.org/10.3847/1538-4357/ac7a43.
Full textAbstract:
Abstract Recent observations of quasars show high line-flux ratios in their broad emission lines and the ratios appear to be independent of redshift up to z ≳ 6, which indicates that the broad-line regions of these early quasars are surprisingly metal-rich. Here, we revisit the chemical evolution of high-redshift quasars by adding a new ingredient, i.e., the neutrino-dominated accretion flows (NDAFs) with outflows, on top of the conventional core-collapse supernovae (CCSNe). In the presence of the chemical contribution from NDAFs with outflows, the total metal mass (i.e., the summation of the conventional CCSN and NDAFs with outflows) per CCSN depends weakly upon the mass of the progenitor star if the mass is in the range of ∼25–55 M ⊙. We model the chemical evolution by adopting a improved open-box model with three typical initial mass functions (IMFs). We find that, with the additional chemical contribution from NDAFs with outflows, the quasar metallicity can be enriched more rapidly in the very early universe (z ∼ 10) and reaches a higher saturation than the no-NDAF case at z ∼ 8, after which they evolve slowly with redshift. The quasar metallicity can reach ∼20 Z ⊙ (Z ⊙ denotes the metallicity of the Sun, ∼20% of which is produced by NDAF outflows) at z ∼ 8 for the “top-heavy” IMF model in Toyouchi et al., which readily explains the quasar observations on the supersolar metal abundance and redshift-independent evolution.
36
Du, Pu, Shuo Zhai, and Jian-Min Wang. "Rarefied Broad-line Regions in Active Galactic Nuclei: Anomalous Responses in Reverberation Mapping and Implications for Weak Emission-line Quasars." Astrophysical Journal 942, no. 2 (January 1, 2023): 112. http://dx.doi.org/10.3847/1538-4357/aca52a.
Full textAbstract:
Abstract Reverberation mapping (RM) is a widely used method for probing the physics of broad-line regions (BLRs) in active galactic nuclei (AGNs). There is increasing preliminary evidence that the RM behaviors of broad emission lines are influenced by BLR densities; however, the influences have not been investigated systematically from a theoretical perspective. In this paper, we adopt a locally optimally emitting cloud model and use CLOUDY to obtain the one-dimensional transfer functions of the prominent UV and optical emission lines for different BLR densities. We find that the influences of BLR densities to RM behaviors mainly have three aspects. First, rarefied BLRs (with low gas densities) may show anomalous responses in RM observations. Their emission-line light curves inversely respond to the variations in continuum light curves, which may have been observed in some UV RM campaigns. Second, the different BLR densities in AGNs may result in correlations between the time lags and equivalent widths of emission lines, and may contribute to the scatters of the radius–luminosity relationships. Third, the variations in BLR densities may explain the changes in time lags in individual objects for different years. Some weak emission-line quasars are probably extreme cases of rarefied BLRs. We predict that their RM observations may show anomalous responses.
37
Berton, M., V. Braito, S. Mathur, L. Foschini, E. Piconcelli, S. Chen, and R. W. Pogge. "Broadband X-ray observations of four gamma-ray narrow-line Seyfert 1 galaxies." Astronomy & Astrophysics 632 (December 2019): A120. http://dx.doi.org/10.1051/0004-6361/201935929.
Full textAbstract:
Narrow-line Seyfert 1 galaxies (NLS1s) is one of the few classes of active galactic nuclei harboring powerful relativistic jets and detected in γ-rays. NLS1s are well-known X-ray sources. While in non-jetted sources the origin of this X-ray emission may be a hot corona surrounding the accretion disk, in jetted objects, especially beamed ones, the contribution of corona and relativistic jet is difficult to disentangle without a proper sampling of the hard X-ray emission. For this reason, we observed with NuSTAR the first four NLS1s detected at high energy γ-rays. These data, along with XMM-Newton and Swift/XRT observations, confirmed that X-rays originate both in the jet and in the accretion disk corona. Time variability in hard X-rays furthermore suggests that, as observed in flat-spectrum radio quasars, the dissipation region during flares could change its position from source to source, and it can be located both inside and outside the broad-line region. We find that jetted NLS1s, and other blazars as well, seem not to follow the classical fundamental plane of black hole (BH) activity, which therefore should be used as a BH mass estimator in blazars with extreme care only. Our results strengthen the idea according to which γ-NLS1s are smaller and younger version of flat-spectrum radio quasars, in which both a Seyfert and a blazar component co-exist.
38
Ogawa, Shoji, Yoshihiro Ueda, Keiichi Wada, and Misaki Mizumoto. "Warm Absorbers in the Radiation-driven Fountain Model of Low-mass Active Galactic Nuclei." Astrophysical Journal 925, no. 1 (January 1, 2022): 55. http://dx.doi.org/10.3847/1538-4357/ac3cb9.
Full textAbstract:
Abstract To investigate the origins of the warm absorbers in active galactic nuclei (AGNs), we study the ionization-state structure of the radiation-driven fountain model in a low-mass AGN and calculate the predicted X-ray spectra utilizing the spectral synthesis code Cloudy. The spectra show many absorption and emission line features originating in the outflowing ionized gas. The O viii 0.654 keV lines are produced mainly in the polar region much closer to the supermassive black hole than the optical narrow-line regions. The absorption measure distribution of the ionization parameter (ξ) at a low inclination spreads over 4 orders of magnitude in ξ, indicating the multiphase ionization structure of the outflow, as actually observed in many type 1 AGNs. We compare our simulated spectra with the high energy resolution spectrum of the narrow-line Seyfert 1 galaxy NGC 4051. The model reproduces slowly outflowing (a few hundred kilometers per second) warm absorbers. However, the faster components with a few thousand kilometers per second observed in NGC 4051 are not reproduced. The simulation also underproduces the intensity and width of the O viii 0.654 keV line. These results suggest that the ionized gas launched from subparsec or smaller regions inside the torus, which is not included in the current fountain model, must be an important ingredient of the warm absorbers with a few thousand kilometers per second. The model also consistently explains the Chandra/HETG spectrum of the Seyfert 2 galaxy Circinus.
39
Zhen-Guo, Ma. "Iron K Emission Lines in Seyfert(-Like) Active Galactic Nuclei: Revelation of a Rapidly Spinning Central Black Hole." Chinese Physics Letters 19, no. 10 (October 2002): 1537–39. http://dx.doi.org/10.1088/0256-307x/19/10/342.
Full text
40
Marziani, Paola, Ascension del Olmo, Jaime Perea, Mauro D’Onofrio, and Swayamtrupta Panda. "Broad UV Emission Lines in Type-1 Active Galactic Nuclei: A Note on Spectral Diagnostics and the Excitation Mechanism." Atoms 8, no. 4 (December 18, 2020): 94. http://dx.doi.org/10.3390/atoms8040094.
Full textAbstract:
This paper reviews several basic emission properties of the UV emission lines observed in the spectra of quasars and type-1 active galactic nuclei, mainly as a function of the ionization parameter, metallicity, and density of the emitting gas. The analysis exploits a general-purpose 4D array of the photoionization simulations computed using the code CLOUDY, covering ionization parameter in the range 10−4.5–10+1.0, hydrogen density nH∼107–1014 cm−3, metallicity Z between 0.01 and 100 Z⊙, and column density in the range 1021–1023 cm−2. The focus is on the most prominent UV emission lines observed in quasar spectra, namely Nvλ1240, Siivλ1397, Oiv]λ1402, Civλ1549, Heiiλ1640, Aliiiλ1860, Siiii]λ1892, and Ciii]λ1909, and on the physical conditions under which electron-ion impact excitation is predicted to be the dominant line producer. Photoionization simulations help constrain the physical interpretation and the domain of applicability of spectral diagnostics derived from measurements of emission line ratios, reputed to be important for estimating the ionization degree, density, and metallicity of the broad line emitting gas, as well as the relative intensity ratios of the doublet or multiplet components relevant for empirical spectral modeling.
41
Xie, Yanxia, and Luis C. Ho. "The Ionization and Destruction of Polycyclic Aromatic Hydrocarbons in Powerful Quasars." Astrophysical Journal 925, no. 2 (February 1, 2022): 218. http://dx.doi.org/10.3847/1538-4357/ac32e2.
Full textAbstract:
Abstract We reanalyze the mid-infrared (5–40 μm) Spitzer spectra of 86 low-redshift (z < 0.5) Palomar–Green quasars to investigate the nature of polycyclic aromatic hydrocarbon (PAH) emission and its utility as a star formation rate (SFR) indicator for the host galaxies of luminous active galactic nuclei (AGNs). We decompose the spectra with our recently developed template-fitting technique to measure PAH fluxes and upper limits, which we interpret using mock spectra that simulate the effects of AGN dilution. While luminous quasars can severely dilute and affect the detectability of emission lines, PAHs are intrinsically weak in some sources that are otherwise gas-rich and vigorously forming stars, conclusively demonstrating that powerful AGNs destroy PAH molecules. Comparing PAH-based SFRs with independent SFRs derived from the mid-infrared fine-structure neon lines and the total infrared luminosity reveals that PAHs can trace star formation activity in quasars with bolometric luminosities ≲1046 erg s−1, but increasingly underestimate the SFR for more powerful quasars, typically by ∼0.5 dex. Relative to star-forming galaxies and low-luminosity AGNs, quasars have a comparable PAH 11.3 μm/7.7 μm ratio but characteristically lower ratios of 6.2 μm/7.7 μm, 8.6 μm/7.7 μm, and 11.3 μm/17.0 μm. We suggest that these trends indicate that powerful AGNs preferentially destroy small grains and enhance the PAH ionization fraction.
42
Guo, Hengxiao, Aaron J. Barth, Kirk T. Korista, Michael R. Goad, Edward M. Cackett, Misty C. Bentz, William N. Brandt, et al. "The Paschen Jump as a Diagnostic of the Diffuse Nebular Continuum Emission in Active Galactic Nuclei*." Astrophysical Journal 927, no. 1 (March 1, 2022): 60. http://dx.doi.org/10.3847/1538-4357/ac4bc6.
Full textAbstract:
Abstract Photoionization modeling of active galactic nuclei (AGN) predicts that diffuse continuum (DC) emission from the broad-line region makes a substantial contribution to the total continuum emission from ultraviolet through near-infrared wavelengths. Evidence for this DC component is present in the strong Balmer jump feature in AGN spectra, and possibly from reverberation measurements that find longer lags than expected from disk emission alone. However, the Balmer jump region contains numerous blended emission features, making it difficult to isolate the DC emission strength. In contrast, the Paschen jump region near 8200 Å is relatively uncontaminated by other strong emission features. Here, we examine whether the Paschen jump can aid in constraining the DC contribution, using Hubble Space Telescope Space Telescope Imaging Spectrograph spectra of six nearby Seyfert 1 nuclei. The spectra appear smooth across the Paschen edge, and we find no evidence of a Paschen spectral break or jump in total flux. We fit multicomponent spectral models over the range 6800–9700 Å and find that the spectra can still be compatible with a significant DC contribution if the DC Paschen jump is offset by an opposite spectral break resulting from blended high-order Paschen emission lines. The fits imply DC contributions ranging from ∼10% to 50% at 8000 Å, but the fitting results are highly dependent on assumptions made about other model components. These degeneracies can potentially be alleviated by carrying out fits over a broader wavelength range, provided that models can accurately represent the disk continuum shape, Fe ii emission, high-order Balmer line emission, and other components.
43
Park, Daeseong, Aaron J. Barth, Luis C. Ho, and Ari Laor. "A New Iron Emission Template for Active Galactic Nuclei. I. Optical Template for the Hβ Region*." Astrophysical Journal Supplement Series 258, no. 2 (February 1, 2022): 38. http://dx.doi.org/10.3847/1538-4365/ac3f3e.
Full textAbstract:
Abstract We present a new empirical template for iron emission in active galactic nuclei (AGNs) covering the 4000–5600 Å range. The new template is based on a spectrum of the narrow-line Seyfert 1 galaxy Mrk 493 obtained with the Hubble Space Telescope. In comparison with the canonical iron template object I Zw 1, Mrk 493 has narrower broad-line widths, lower reddening, and a less extreme Eddington ratio, making it a superior choice for template construction. We carried out a multicomponent spectral decomposition to produce a template incorporating all the permitted and forbidden lines of Fe ii identified in the Mrk 493 spectrum over this wavelength range, as well as lines from Ti ii, Ni ii, and Cr ii. We tested the template by fitting it to AGN spectra spanning a broad range of iron emission properties, and we present a detailed comparison with fits using other widely used monolithic and multicomponent iron emission templates. The new template generally provides the best fit (lowest χ 2) compared to other widely used monolithic empirical templates. In addition, the new template yields more accurate spectral measurements including a significantly better match of the derived Balmer line profiles (Hβ, Hγ, Hδ), in contrast with results obtained using the other templates. Our comparison tests show that the choice of iron template can introduce a systematic bias in measurements of the Hβ line width, which consequently impacts single-epoch black hole mass estimates by ∼0.1 dex on average and possibly up to ∼0.3–0.5 dex individually.
44
Dors, O. L., M. Contini, R. A. Riffel, E. Pérez-Montero, A. C. Krabbe, M. V. Cardaci, and G. F. Hägele. "Chemical abundances of Seyfert 2 AGNs – IV. Composite models calculated by photoionization + shocks." Monthly Notices of the Royal Astronomical Society 501, no. 1 (November 30, 2020): 1370–83. http://dx.doi.org/10.1093/mnras/staa3707.
Full textAbstract:
ABSTRACT We build detailed composite models of photoionization and shock ionization based on the suma code to reproduce emission lines emitted from the Narrow Line Regions (NLR) of Seyfert 2 nuclei. The aim of this work is to investigate diagram active galactic nucleus (AGN) positions according to shock parameters, shock effects on the gas temperature and ionization structures and derive a semi-empirical abundance calibration based on emission-line ratios little sensitive to the shock presence. The models were used to reproduce optical (3000 < λ(Å) < 7000) emission line intensities of 244 local ($z \: \lesssim \: 0.4$) Seyfert 2s, whose observational data were selected from Sloan Digital Sky Survey DR7. Our models suggest that shocks in Seyfert 2 nuclei have velocities in the range of 50–300 $\rm km \: s^{-1}$ and imply a narrower metallicity range ($0.6 \: \lesssim \: (Z/Z_{\odot }) \: \lesssim \: 1.6$) than those derived using pure photoionization models. Our results indicate that shock velocity in AGNs cannot be estimated using standard optical line ratio diagrams, based on integrated spectra. Our models predict a different temperature structure and $\rm O^{+}$/O and $\rm O^{2+}$/O fractional abundances throughout the NLR clouds than those derived from pure photoionization models, mainly in shock-dominated objects. This suggests that, in order to minimize the shock effects, the combination of emission-lines emitted by ions with similar intermediate ionization potential could be good metallicity indicators. Finally, we derive two calibrations between the N/O abundance ratio and the N2O2 = log([N ii]λ6584/[O ii]λ3727) and N2 = log([N ii]λ6584/H α) indexes which agree with that derived from pure photoionization models.
45
Temple, Matthew J., Gary J. Ferland, Amy L. Rankine, Paul C. Hewett, N. R. Badnell, Connor P. Ballance, Giulio Del Zanna, and Roger P. Dufresne. "Fe iii emission in quasars: evidence for a dense turbulent medium." Monthly Notices of the Royal Astronomical Society 496, no. 3 (June 17, 2020): 2565–76. http://dx.doi.org/10.1093/mnras/staa1717.
Full textAbstract:
ABSTRACT Recent improvements to atomic energy-level data allow, for the first time, accurate predictions to be made for the Fe iii line emission strengths in the spectra of luminous, $L_\text{bol}\simeq 10^{46}\!-\!10^{48}\mbox{${\rm \, erg}{\rm \, s}^{-1}\, $}$, active galactic nuclei. The Fe iii emitting gas must be primarily photoionized, consistent with observations of line reverberation. We use cloudy models exploring a wide range of parameter space, together with ≃26 000 rest-frame ultraviolet spectra from the Sloan Digital Sky Survey, to constrain the physical conditions of the line emitting gas. The observed Fe iii emission is best accounted for by dense (nH ≃ 1014 cm−3) gas which is microturbulent, leading to smaller line optical depths and fluorescent excitation. Such high density gas appears to be present in the central regions of the majority of luminous quasars. Using our favoured model, we present theoretical predictions for the relative strengths of the Fe iii UV34 λλ1895, 1914, 1926 multiplet. This multiplet is blended with the Si iii] λ1892 and C iii] λ1909 emission lines and an accurate subtraction of UV34 is essential when using these lines to infer information about the physics of the broad line region in quasars.
46
Nakata, Ryo, Kiyoshi Hayashida, Hirofumi Noda, Tomokage Yoneyama, Hironori Matsumoto, and Masatoshi Imanishi. "Spatially resolved X-ray spectroscopy of the archetype type 2 active galactic nucleus NGC 1068 with Chandra." Publications of the Astronomical Society of Japan 73, no. 2 (February 17, 2021): 338–49. http://dx.doi.org/10.1093/pasj/psab001.
Full textAbstract:
Abstract We investigate spatial distributions of iron Kα (Fe-Kα) lines in the cental 100 pc of the Seyfert 2 galaxy NGC 1068 observed with Chandra. The spatial distributions of Fe-Kα lines, neutral and highly ionized, around the center of the galactic nucleus are not isotropic, as consistently confirmed in both image and spectral analyses. The hydrogen number density of the gas clouds responsible for the neutral Fe-Kα line emission is estimated to be 102–103 cm−3 for the sampled regions near the galactic core. The photo-ionization model, where iron is assumed to be ionized by X-rays from the galactic nucleus, yields ionization parameters lower than 19 for these clouds. The range of this ionization parameter is two or three orders of magnitude lower than the theoretically expected value to produce the observed helium-like Fe-Kα line intensities. Therefore, the photo-ionization model is excluded from the explanation of the amount of highly ionized iron that is responsible for the observed Fe-Kα lines. Also, we find anti-correlation in the spatial distributions between the molecular cloud in the area observed with ALMA and that of the Fe-Kα lines, including that from neutral iron. We suggest that X-ray iron-line and radio molecular cloud observations are complementary to probe the distribution of matters in the central regions around the cores of active galactic nuclei.
47
Joh, Kazuma, Tohru Nagao, Keiichi Wada, Koki Terao, and Takuji Yamashita. "Do gas clouds in narrow-line regions of Seyfert galaxies come from their nuclei?" Publications of the Astronomical Society of Japan 73, no. 4 (June 30, 2021): 1152–65. http://dx.doi.org/10.1093/pasj/psab065.
Full textAbstract:
Abstract The narrow-line region (NLR) consists of gas clouds ionized by the strong radiation from the active galactic nucleus (AGN), distributed in the spatial scale of AGN host galaxies. The strong emission lines from the NLR are useful to diagnose physical and chemical properties of the interstellar medium in AGN host galaxies. However, the origin of the NLR is unclear; the gas clouds in NLRs may originally be in the host and photoionized by the AGN radiation, or they may be transferred from the nucleus with AGN-driven outflows. In order to study the origin of the NLR, we systematically investigate the gas density and velocity dispersion of NLR gas clouds using a large spectroscopic data set taken from the Sloan Digital Sky Survey. The [S ii] emission-line flux ratio and [O iii] velocity width of 9571 type-2 Seyfert galaxies and 110041 star-forming galaxies suggest that the gas density and velocity dispersion of NLR clouds in Seyfert galaxies (ne ≈ 194 cm−3 and $\sigma _{\rm [O\,{\small III}]} \approx 147\:$km s−1) are systematically larger than those of clouds in H ii regions of star-forming galaxies (ne ≈ 29 cm−3 and $\sigma _{\rm [O\,{\small III}]} \approx 58\:$km s−1). Interestingly, the electron density and velocity dispersion of NLR gas clouds are larger for Seyfert galaxies with a higher [O iii]/Hβ flux ratio, i.e., with a more active AGN. We also investigate the spatially-resolved kinematics of ionized gas clouds using the Mapping Nearby Galaxies at the Apache Point Observatory (MaNGA) survey data for 90 Seyfert galaxies and 801 star-forming galaxies. We find that the velocity dispersion of NLR gas clouds in Seyfert galaxies is larger than that in star-forming galaxies at a fixed stellar mass, at both central and off-central regions. These results suggest that gas clouds in NLRs come from the nucleus, probably through AGN outflows.
48
Greenwell, Claire, Poshak Gandhi, Daniel Stern, Peter Boorman, Yoshiki Toba, George Lansbury, Vincenzo Mainieri, and Christopher Desira. "A candidate optically quiescent quasar lacking narrow emission lines." Monthly Notices of the Royal Astronomical Society: Letters 503, no. 1 (February 26, 2021): L80—L84. http://dx.doi.org/10.1093/mnrasl/slab019.
Full textAbstract:
ABSTRACT Many active galactic nuclei (AGN) surveys rely on optical emission line signatures for robust source classification. There are, however, examples of luminous AGN candidates lacking such signatures, including those from the narrow-line region, which are expected to be less susceptible to classical nuclear (torus) obscuration. Here, we seek to formalize this subpopulation of AGN with a prototypical candidate, SDSS J075139.06+402810.9. This shows infrared (IR) colours typical for AGN, an optical spectrum dominated by an early type galaxy continuum, an [O iii] λ5007Å limiting flux about two dex below Type 2 quasars at similar IR power, and a k-corrected 12 $\mu$m quasar-like luminosity of ∼ 1045 erg s−1. These characteristics are not consistent with jet and host galaxy dilution. A potential scenario to explain this AGN quiescence in the optical is a sky-covering ‘cocoon’ of obscuring material, such that the AGN ionizing radiation is unable to escape and excite gas on kpc scales. Alternatively, we may be witnessing the short phase between recent triggering of obscured AGN activity and the subsequent narrow-line excitation. This prototype could define the base properties of an emerging candidate AGN subtype – an intriguing transitional phase in AGN and galaxy evolution.
49
Gupta, N., G. Shukla, R. Srianand, J. K. Krogager, P. Noterdaeme, A. J. Baker, F. Combes, et al. "MALS SALT-NOT Survey of MIR-selected Powerful Radio-bright AGN at 0 < z < 3.5." Astrophysical Journal 929, no. 1 (April 1, 2022): 108. http://dx.doi.org/10.3847/1538-4357/ac4220.
Full textAbstract:
Abstract We present results of an optical spectroscopic survey using SALT and the Nordic Optical Telescope to build a Wide-field Infrared Survey Explorer mid-infrared color-based, dust-unbiased sample of powerful radio-bright (>200 mJy at 1.4 GHz) active galactic nuclei (AGN) for the MeerKAT Absorption Line Survey (MALS). Our sample has 250 AGN (median z = 1.8) showing emission lines, 26 with no emission lines, and 27 without optical counterparts. Overall, our sample is fainter (Δi = 0.6 mag) and redder (Δ(g−i) = 0.2 mag) than radio-selected quasars, and representative of fainter quasar population detected in optical surveys. About 20% of the sources are narrow-line AGN (NLAGN)–65% of these, at z < 0.5 are galaxies without strong nuclear emission, and 10% at z > 1.9, have emission line ratios similar to radio galaxies. The farthest NLAGN in our sample is M1513-2524 (z em = 3.132), and the largest radio source (size ∼330 kpc) is M0909-3133 (z em = 0.884). We discuss in detail 110 AGN at 1.9 < z < 3.5. Despite representing the radio loudest quasars (median R = 3685), their Eddington ratios are similar to the Sloan Digital Sky Survey quasars having lower R. We detect four C iv broad-absorption line (BAL) QSOs, all among AGN with least R, and highest black hole masses and Eddington ratios. The BAL detection rate ( 4 − 2 + 3 %) is consistent with that seen in extremely powerful (L 1.4GHz > 1025 W Hz−1) quasars. Using optical light curves, radio polarization, and γ-ray detections, we identify seven high-probability BL Lacertae objects. We also summarize the full MALS footprint to search for H i 21 cm and OH 18 cm lines at z < 2.
50
Green, Paul J., Lina Pulgarin-Duque, Scott F. Anderson, Chelsea L. MacLeod, Michael Eracleous, John J. Ruan, Jessie Runnoe, et al. "The Time Domain Spectroscopic Survey: Changing-look Quasar Candidates from Multi-epoch Spectroscopy in SDSS-IV." Astrophysical Journal 933, no. 2 (July 1, 2022): 180. http://dx.doi.org/10.3847/1538-4357/ac743f.
Full textAbstract:
Abstract Active galactic nuclei (AGN) can vary significantly in their rest-frame optical/UV continuum emission, and with strong associated changes in broad line emission, on much shorter timescales than predicted by standard models of accretion disks around supermassive black holes. Most such changing-look or changing-state AGN—and at higher luminosities, changing-look quasars (CLQs)—have been found via spectroscopic follow-up of known quasars showing strong photometric variability. The Time Domain Spectroscopic Survey of the Sloan Digital Sky Survey IV (SDSS-IV) includes repeat spectroscopy of large numbers of previously known quasars, many selected irrespective of photometric variability, and with spectral epochs separated by months to decades. Our visual examination of these repeat spectra for strong broad line variability yielded 61 newly discovered CLQ candidates. We quantitatively compare spectral epochs to measure changes in continuum and Hβ broad line emission, finding 19 CLQs, of which 15 are newly recognized. The parent sample includes only broad line quasars, so our study tends to find objects that have dimmed, i.e., turn-off CLQs. However, we nevertheless find four turn-on CLQs that meet our criteria, albeit with broad lines in both dim and bright states. We study the response of Hβ and Mg ii emission lines to continuum changes. The Eddington ratios of CLQs are low, and/or their Hβ broad line width is large relative to the overall quasar population. Repeat quasar spectroscopy in the upcoming SDSS-V black hole Mapper program will reveal significant numbers of CLQs, enhancing our understanding of the frequency and duty cycle of such strong variability, and the physics and dynamics of the phenomenon.