Journal articles on the topic 'Active Galactic Nuclei – Blazars – Gamma-ray blazars'
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1
TAVECCHIO, F. "GAMMA-RAY EMISSION FROM AGNS (SPECIAL FOCUS ON BL LAC OBJECTS)." International Journal of Modern Physics D 19, no. 06 (June 2010): 841–48. http://dx.doi.org/10.1142/s0218271810017081.
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Blazars, radio-loud active galactic nuclei with the relativistic jet closely aligned with the line of sight, dominate the extragalactic sky observed at gamma-ray energies, above 100 MeV. We discuss some of the emission properties of these sources, focusing in particular on the "blazar sequence" and the interpretative models of the high-energy emission of BL Lac objects.
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Uwitonze, Emmanuel, Pheneas Nkundabakura, and Tom Mutabazi. "Determination of K4000 of potential blazar candidates among EGRET unidentified gamma-ray sources." Proceedings of the International Astronomical Union 15, S356 (October 2019): 355–57. http://dx.doi.org/10.1017/s1743921320003336.
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AbstractBlazars are radio-loud Active Galactic Nuclei (AGN) with relativistic jets oriented towards the observer’s line-of-sight. Based on their optical spectra, blazars may be classified as flat-spectrum radio quasars (FSRQs) or BL Lacs. FSRQs are more luminous blazars with both narrow and broad emission and absorption lines, while BL Lacs are less luminous and featureless. Recent studies show that blazars dominate (˜93%) the already-identified EGRET sources (142), suggesting that among the unidentified sources (129) there could still be faint blazars. Due to the presence of a strong non-thermal component inside their jets, blazars are found to display a weaker depression at ˜4000 Å (K 4000 ≤ 0.4). In this study, we aimed at determining the K 4000 break for a selected sample among the potential blazar candidates from unidentified EGRET sources to confirm their blazar nature. We used two blazar candidates, 3EG J1800-0146 and 3EG J1709-0817 associated with radio counterparts, J1802-0207 and J1713-0817, respectively. Their optical counterparts were obtained through spectroscopic observations using Robert Stobie spectrograph (RSS) at the Southern African Large Telescope (SALT) in South Africa. The observed Ca II H & K lines depression at ˜4000 Å in spectra of these sources show a shallow depression, K 4000 = 0.35 ± 0.02 and 0.24 ± 0.01, respectively, suggesting that these sources are blazar candidates. Moreover, the redshifts z = 0.165 and 0.26 measured in their spectra confirm the extragalactic nature of these sources.
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Takahashi, T., H. Kubo, and G. Madejski. "ASCA Observations of Blazars." Highlights of Astronomy 11, no. 2 (1998): 812–15. http://dx.doi.org/10.1017/s1539299600018852.
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During the last years it has become evident that blazar class of AGN emit a lot of energy in the gamma-ray regime. It is generally thought that the non-thermal emission from blazars, observed from radio to GeV/TeV 7-rays, is radiation of very energetic particles via both synchrotron and Compton processes. This underlines the importance of high-energy emission for models of the blazar class of active galactic nuclei. Most of the gamma-bright AGN are variable, and flares with time scales as short as a day have been observed. This suggests that the site of gamma-ray emission is very compact and situated in a fluid which moves relativistically at a small angle towards the observer. It is generally assumed that the emission originates from the jet of these objects.
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Langejahn, M., M. Kadler, J. Wilms, E. Litzinger, M. Kreter, N. Gehrels, W. H. Baumgartner, C. B. Markwardt, and J. Tueller. "Hard X-ray properties of radio-selected blazars." Astronomy & Astrophysics 637 (May 2020): A55. http://dx.doi.org/10.1051/0004-6361/202037469.
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Context. Hard X-ray properties of beamed active galactic nuclei have been published in the 105-month Swift/BAT catalog, but there have not been any studies carried out so far on a well-defined, radio-selected sample of low-peaked blazars in the hard X-ray band. Aims. Using the statistically complete MOJAVE-1 sample, we aim to determine the hard X-ray properties of radio-selected blazars, including the enigmatic group of gamma-ray-faint blazars. Additionally, we aim to determine the contribution of radio-selected low-peaked blazars to the diffuse cosmic X-ray background (CXB). Methods. We determined photon indices, fluxes, and luminosities in the range of 20 keV–100 keV of the X-ray spectra of blazars and other extragalactic jets from the MOJAVE-1 sample, derived from the 105-month Swift/BAT survey. We calculated log N–log S distributions and determined the luminosity functions. Results. The majority of the MOJAVE-1 blazars are found to be hard X-ray emitters albeit many at low count rates. The log N–log S distribution for the hard X-ray emission of radio-selected blazars is clearly non-Euclidean, in contrast to the radio flux density distribution. Approximately 0.2% of the CXB in the 20 keV–100 keV band can be resolved into MOJAVE-1 blazars. Conclusions. The peculiar log N–log S distribution disparity might be attributed to different evolutionary paths in the X-ray and radio bands, as tested by luminosity-function modeling. X-ray variability can be ruled out as the dominant contributor. Low-peaked blazars constitute an intrinsically different source population in terms of CXB contribution compared to similar studies of X-ray-selected blazars. The hard X-ray flux and spectral index can serve as a good proxy for the gamma-ray detection probability of individual sources. Future observations combining deep X-ray survey, for example, with eROSITA, and targeted gamma-ray observations with CTA can benefit strongly from the tight connection between these high-energy bands for the different blazar sub-classes.
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Paggi, Alessandro, R. D'Abrusco, F. Massaro, M. Landoni, D. Milisavljevic, N. Masetti, F. Ricci, et al. "Multi-wavelength selection and identification of gamma-ray blazar candidates." Proceedings of the International Astronomical Union 10, S313 (September 2014): 58–63. http://dx.doi.org/10.1017/s1743921315001878.
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AbstractA significant fraction (~ 30%) of the gamma-ray sources detected by the Fermi Gamma-ray Space Telescope is still of unknown origin, being not yet associated with counterparts at lower energies. Many unidentified gamma-ray sources (UGSs) could be blazars, the largest identified population of extragalactic gamma-ray sources and the rarest class of active galactic nuclei. In particular, it has been found that blazars occupy a defined region in WISE three dimensional color space, well separated from that occupied by other sources in which thermal emission prevails. For farther sources with weaker IR emission, additional informations can be obtained combining WISE data with X-ray or radio emission. Alternatively, the low-frequency radio emission can be used for identifying potential gamma-ray candidate blazars. However, optical spectroscopic observations represent the tell-tale tool to confirm the exact nature of these sources. To this end, an extensive observational campaign has been performed with several optical telescopes, aimed at pinpointing the exact nature of gamma-ray candidate blazars selected with the different selection methods mentioned above. The results of this campaign lead to the discovery of 60 new gamma-ray blazars, thus confirming the effectiveness of these selection criteria.
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Bhatta, Gopal. "Blazar Jets as Possible Sources of Ultra-High Energy Photons: A Short Review." Universe 8, no. 10 (October 1, 2022): 513. http://dx.doi.org/10.3390/universe8100513.
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In this paper, I present a qualitative discussion on the prospect of production of ultra-high photons in blazars. The sources are a subclass of active galactic nuclei which host supermassive black holes and fire relativistic jets into the intergalactic medium. The kpc-scale jets are believed to be dominated by Poynting flux and constitute one of the most efficient cosmic particle accelerators, that potentially are capable of accelerating the particles up to EeV energies. Recent IceCube detection of astrophysical neutrino emissions, in coincidence with the enhanced gamma-ray from Tev blazar TXS 0506 + 056, further supports hadronic models of blazar emissions in which particle acceleration processes, such as relativistic shocks, magnetic re-connection, and relativistic turbulence, could energize hadrons, e.g., protons, up to energies equivalent to billions of Lorentz factors. The ensuing photo-pionic processes may then result in gamma-rays accompanied by neutrino flux. Furthermore, the fact that blazars are the dominant source of observed TeV emission encourages search for signatures of acceleration scenarios that would lead to the creation of ultra-high-energy photons.
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Paggi, A., M. Bonato, C. M. Raiteri, M. Villata, G. De Zotti, and M. I. Carnerero. "A new multiwavelength census of blazars." Astronomy & Astrophysics 641 (September 2020): A62. http://dx.doi.org/10.1051/0004-6361/202038430.
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Context. Blazars are the rarest and most powerful active galactic nuclei. They play a crucial and expanding role in the multifrequency and multimessenger astrophysics of today. Dominating the high-energy extragalactic sky, they have been recently associated with high-energy astrophysical neutrinos and they may be among the accelerators of the ultra-high energy cosmic rays. Current blazar catalogs, however, are incomplete and they are depleted, in particular, at low Galactic latitudes. Aims. Our study is aimed at augmenting the current blazar census, starting from a sample of ALMA calibrators that provides more homogeneous sky coverage, especially at low Galactic latitudes, to build a catalog of blazar candidates that can provide candidate counterparts to unassociated γ-ray sources and sources of high-energy neutrino emission or ultra-high energy cosmic rays. Methods. Based on the ALMA Calibrator Catalog, we compiled a selection of 1580 ALMA Blazar Candidates (ABCs) for which we collected multiwavelength information, including Gaia photometric, parallax, and proper motion data, as well as SDSS and LAMOST photometric and spectral data, WISE photometric data, X-ray (Swift-XRT, Chandra-ACIS and XMM-Newton-EPIC) count-rates and spectra, and Fermi-LAT fluxes and spectral slopes. We also compared our ALMA Blazar Candidates with existing blazar catalogs, such as 4FGL, 3HSP, WIBRaLS2, and the KDEBLLACS. Results. The ABC catalog fills in the gaps with regard to low Galactic latitude sources in current blazar catalogs. By cross-matching this catalog with Gaia DR2, SDSS DR12, LAMOST DR5, AllWISE, and 4FGL catalogs, we obtained 805, 295, 31, 1311, and 259 matches, respectively. ALMA Blazar Candidates are significantly dimmer than known blazars in the Gaia g band, while the difference in the Gaia b − r color between the two populations is less pronounced. Also, ABC sources appear bluer in SDSS than known blazars, although with low statistical significance. Most ABCs classified as QSO and BL Lac fall into the SDSS color regions of low redshift quasars, with some QSOs entering the regions of higher redshift quasars. We collected 110 optical spectra in SDSS DR12 and LAMOST DR5, which mostly classify the corresponding sources as QSO (98), while 12 sources turned out to be galactic objects. Regarding the WISE colors, we found that ABC sources are significantly bluer than known blazars in the w2 − w3 and w3 − w4 colors. In X-ray, we detected 173 ABCs and we were able to extract X-ray spectra for 92 of them. Our sources are, on average, similar in the X-ray band to known blazars in terms of count rates and spectral slopes, implying that our sample covers the same region of the blazar parameter space in this band. A comparison of γ-ray properties shows that the ABCs are, on average, dimmer and that their γ-ray spectra are softer than known blazars, indicating a significant contribution on the part of FSRQ sources. Making use of WISE colors, we classified 715 ABCs as candidate γ-ray blazar of different classes. Conclusions. We built a new catalog of 1580 candidate blazars with a rich multiwavelength data-set, filling in the gaps for low Galactic latitude sources in current blazar catalogs. This will be particularly important for the identification of the source population of high-energy neutrinos or ultra-high energy cosmic rays, or to verify the Gaia optical reference frame. In addition, ALMA Blazar Candidates can be investigated both through optical spectroscopic observation campaigns or through repeated photometric observations for variability studies. In this context, the data collected by the upcoming LSST surveys will provide a key tool for investigating the possible blazar nature of these sources.
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Xiao, Hubing, Junhui Fan, Zhihao Ouyang, Liangjun Hu, Guohai Chen, Liping Fu, and Shaohua Zhang. "An Extensive Study of Blazar Broad Emission Line: Changing-look Blazars and the Baldwin Effect." Astrophysical Journal 936, no. 2 (September 1, 2022): 146. http://dx.doi.org/10.3847/1538-4357/ac887f.
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Abstract It is known that the blazar jet emissions are dominated by nonthermal radiation, while the accretion disk jets are normally dominated by thermal emission. In this work, our aim is to study the connection between the two types of emission by investigating the correlation between the blazar emission-line intensity property, which embodies the nature of an accretion disk, and the γ-ray flux property, which is the representative of jet emission. We compiled a sample of 656 blazars with available emission-line equivalent widths (EWs), the GeV γ-ray flux, and the spectral energy distribution (SED) information from the literature. In this work, we found 55 previous blazar candidates of uncertain types (BCUs) that are now identified as flat-spectrum radio quasars (FSRQs), and found 52 “changing-look” blazars based on their EWs, 45 of which are newly confirmed. These changing-look blazars have a larger accretion ratio ( M ̇ / M ̇ Edd ) than BL Lacertae (BL Lac) objects. In addition, we suggest that the lower synchrotron peak blazars (LSPs) could be the source of changing-look blazars because 90.7% of the changing-look blazars in this work are confirmed as LSPs. An anticorrelation between EW and continuum intensity, the so-called global “Baldwin effect” (BEff), has been confirmed. We suggest that the steeper global BEff observed for the blazar than for radio-quiet active galactic nuclei (RQ-AGNs) is caused by the inverse Compton scattering of broad-emission-line photons. This interpretation is further supported by the positive correlation between the emission-line EW and intrinsic inverse Compton luminosity.
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Lott, B., E. Cavazzuti, S. Ciprini, S. Cutini, and D. Gasparrini. "Gamma-ray blazars and active galactic nuclei seen by the Fermi-LAT." Proceedings of the International Astronomical Union 10, S313 (September 2014): 12–16. http://dx.doi.org/10.1017/s1743921315001787.
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AbstractThe third catalog of active galactic nuclei (AGNs) detected by the Fermi-LAT (3LAC) is presented. It is based on the third Fermi-LAT catalog (3FGL) of sources detected with a test statistic (TS) greater than 25 using the first 4 years of data. The 3LAC includes 1591 AGNs located at high Galactic latitudes, |b| > 10 (with 28 duplicate associations, thus corresponding to 1563 gamma-ray sources among 2192 sources in the 3FGL catalog), a 71% increase over the second catalog based on 2 years of data. A very large majority of these AGNs (98%) are blazars. About half of the newly detected blazars are of unknown type, i.e., they lack spectroscopic information of sufficient quality to determine the strength of their emission lines. The general properties of the 3LAC sample confirm previous findings from earlier catalogs, but some new subclasses (e.g., intermediate- and high-synchrotron-peaked FSRQs) have now been significantly detected.
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Sikora, Marek. "High-Energy Radiation From Active Galactic Nuclei." International Astronomical Union Colloquium 142 (1994): 923–28. http://dx.doi.org/10.1017/s0252921100078325.
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AbstractTwo recent findings concerning high-energy radiation properties of active galactic nuclei—discovery of breaks in hard X-ray spectra of Seyfert galaxies, and discovery of huge fluxes of hard gamma rays from blazars—seem to press us to change our standard views about radiation production in these objects. I review briefly the existing radiation models, confront them with the newest observations, and discuss newly emerging theoretical pictures which attempt to account for the discoveries.Subject headings: galaxies: active — galaxies: nuclei — radiation mechanisms: nonthermal
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Fraga, Bernardo M. O., Ulisses Barres de Almeida, Clécio R. Bom, Carlos H. Brandt, Paolo Giommi, Patrick Schubert, and Márcio P. de Albuquerque. "Deep learning Blazar classification based on multifrequency spectral energy distribution data." Monthly Notices of the Royal Astronomical Society 505, no. 1 (May 17, 2021): 1268–79. http://dx.doi.org/10.1093/mnras/stab1349.
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ABSTRACT Blazars are among the most studied sources in high-energy astrophysics as they form the largest fraction of extragalactic gamma-ray sources and are considered prime candidates for being the counterparts of high-energy astrophysical neutrinos. Their reliable identification amid the many faint radio sources is a crucial step for multimessenger counterpart associations. As the astronomical community prepares for the coming of a number of new facilities able to survey the non-thermal sky at unprecedented depths, from radio to gamma-rays, machine-learning techniques for fast and reliable source identification are ever more relevant. The purpose of this work was to develop a deep learning architecture to identify Blazar within a population of active galactic nucleus (AGN) based solely on non-contemporaneous spectral energy distribution information, collected from publicly available multifrequency catalogues. This study uses an unprecedented amount of data, with spectral energy distributions (SEDs) for ≈14 000 sources collected with the Open Universe VOU-Blazars tool. It uses a convolutional long short-term memory neural network purposefully built for the problem of SED classification, which we describe in detail and validate. The network was able to distinguish Blazars from other types of active galactic nuclei (AGNs) to a satisfying degree (achieving a receiver operating characteristic area under curve of 0.98), even when trained on a reduced subset of the whole sample. This initial study does not attempt to classify Blazars among their different sub-classes, or quantify the likelihood of any multifrequency or multimessenger association, but is presented as a step towards these more practically oriented applications.
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GHISELLINI, GABRIELE. "EXTRAGALACTIC GAMMA-RAYS: GAMMA RAY BURSTS AND BLAZARS." International Journal of Modern Physics A 20, no. 29 (November 20, 2005): 6991–7000. http://dx.doi.org/10.1142/s0217751x05030673.
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The extragalactic gamma-ray sky is dominated by two classes of sources: Gamma-Ray Bursts (GRBs) and radio loud active galactic nuclei whose jets are pointing at us (blazars). We believe that the radiation we receive from them originates from the transformation of bulk relativistic energy into random energy. Although the mechanisms to produce, collimate and accelerate the jets in these sources are uncertain, it is fruitful to compare the characteristics of both classes of sources in search of enlightening similarities. I will review some general characteristics of radio loud AGNs and GRBs and I will discuss the possibility that both classes of sources can work in the same way. Finally, I will discuss some recent exciting prospects to use blazars to put constraints on the cosmic IR-Optical-UV backgrounds, and to use GRBs as standard candles to measure the Universe.
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d’Antonio, D., M. Giroletti, G. Giovannini, and A. Maini. "Radio spectral properties of cores and extended regions in blazars in the MHz regime." Monthly Notices of the Royal Astronomical Society 490, no. 4 (November 15, 2019): 5798–806. http://dx.doi.org/10.1093/mnras/stz2967.
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ABSTRACT Low-frequency radio surveys allow in-depth studies and new analyses of classes of sources that were previously known and characterized only in other bands. In recent years, low radio frequency observations of blazars have become available as a result of new surveys, such as the GaLactic and Extragalactic All-sky Murchison Widefield Array (MWA) survey (GLEAM). We search for gamma-ray blazars in a low-frequency (ν < 240 MHz) survey, to characterize the spectral properties of the spatial components. We cross-correlate GLEAM with the fourth catalogue of active galactic nuclei (4LAC) detected by the Fermi satellite. This improves on previous works by using a low-frequency catalogue that is wider and deeper, with a better spectral coverage and the latest and most sensitive gamma-ray source list. Compared with a previous study based on the commissioning survey, the detection rate increased from 35 to 70 per cent. We include data from the Australia Telescope 20-GHz (AT20G) survey in order to extract high-frequency high-angular resolution information about the radio cores of blazars. We find low radio frequency counterparts for 1274 out of 1827 blazars in the range of 72–231 MHz. Blazars have flat spectra at the ∼100-MHz regime, with a mean spectral index α = −0.44 ± 0.01 (assuming Sν ∝ να). Low synchrotron peaked objects have a flatter spectrum than high synchrotron peaked objects. Low frequency radio and gamma-ray emissions show a significant but scattered correlation. The ratio between lobe and core radio emission in gamma-ray blazars is smaller than previously estimated.
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Wang, Xiang-Yu, Xiao-Jun Bi, Zhen Cao, Piero Vallania, Han-Rong Wu, Da-Hai Yan, and Qiang Yuan. "Chapter 3 Extra-galactic gamma-ray sources *." Chinese Physics C 46, no. 3 (March 1, 2022): 030003. http://dx.doi.org/10.1088/1674-1137/ac3fa9.
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Abstract Extra-galactic gamma-ray sources, such as gamma-ray bursts, active galactic nuclei, starburst galaxies, are interesting and important targets for LHAASO observations. In this chapter, the prospects of detecting these sources with LHAASO and their physical implications are studied. The upgrade plan for the Water Cherenkov Detector Array (WCDA), which aims to enhance the detectability of relatively lower energy photons, is also presented. In addition, a study on constraining the extragalactic background light with LHAASO observation of blazars is presented.
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WAGNER, ROBERT. "PHYSICS INSIGHTS FROM RECENT MAGIC AGN OBSERVATIONS." International Journal of Modern Physics D 18, no. 10 (October 2009): 1489–92. http://dx.doi.org/10.1142/s0218271809015667.
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The total set of the 14 active galactic nuclei detected by MAGIC so far includes well-studied bright blazars like Mkn 501, the giant radio galaxy M 87, but also the distant flat-spectrum radio quasar 3C 279, and an intriguing gamma-ray source in the 3C 66A/B region, whose energy spectrum is not compatible with the expectations from 3C 66A. Besides scheduled observations, so far MAGIC succeeded in discovering TeV gamma rays from three blazars following triggers from high optical states. I report selected highlights from recent MAGIC observations of extragalactic TeV gamma-ray sources, emphasizing and discussing the new physics insights the MAGIC observations were able to contribute.
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Kurtanidze, Omar M., Maria G. Nikolashvili, Givi N. Kimeridze, Lorand A. Sigua, Bidzina Z. Kapanadze, and Roman Z. Ivanidze. "Long-term photometry of blazars at Abastumani Observatory." Proceedings of the International Astronomical Union 2, S238 (August 2006): 397–98. http://dx.doi.org/10.1017/s1743921307005637.
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AbstractWe give a brief summary of the ongoing Abastumani Active Galactic Nuclei Monitoring Program started in the May 1997. More than 110000 frames are obtained during more than 1300 nights of observations for about 50 target objects, among them gamma-ray, X-ray and optical blazars. All observations were done in the BVRI bands using ST-6 CCD based photometer attached to the Newtonian focus of 70-cm meniscus telescope.
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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.
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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.
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Donea, Alina-C., and Raymond J. Protheroe. "Radiation Fields in Blazars — a Possible Extension of the Small Scale Symbiosis (Disk/Jet) into a Large Scale (Dust/Dust) Symbiosis." Publications of the Astronomical Society of Australia 19, no. 1 (2002): 39–42. http://dx.doi.org/10.1071/as01098.
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AbstractIn blazar models both protons and electrons may be efficiently accelerated in jets and produce γ-rays. Here we discuss the interactions of these γ-rays with different radiation fields. The external radiation fields within a few parsecs from the black hole involved in such interactions could be the direct radiation from the accretion disk coupled with the jet, the infrared radiation from a dusty torus, and the emission line radiation from the broad line region surrounding the accretion disk. The optical thickness for absorption of γ-ray photons in the external radiation fields is analysed for blazars and quasars.Based on the unification theory of active galactic nuclei we briefly review the evidence for the existence of small scale dust tori in blazars/FR I. We propose that the existing jet–accretion disk symbiosis extrapolates to a large scale symbiosis between other important dusty constituents of the blazar/FR I family.
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Rosales de León, Alberto, Anthony M. Brown, and Paula M. Chadwick. "Photohadronic modelling of the 2010 gamma-ray flare from Mrk 421." Monthly Notices of the Royal Astronomical Society 501, no. 2 (December 12, 2020): 2198–208. http://dx.doi.org/10.1093/mnras/staa3839.
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ABSTRACT Blazars are a subclass of active galactic nuclei (AGNs) that have a relativistic jet with a small viewing angle towards the observer. Recent results based on hadronic scenarios have motivated an ongoing discussion of how a blazar can produce high energy neutrinos during a flaring state and which scenario can successfully describe the observed gamma-ray behaviour. Markarian 421 is one of the closest and brightest objects in the extragalactic gamma-ray sky and showed flaring activity over a 14-days period in 2010 March. In this work, we describe the performed analysis of Fermi-LAT data from the source focused on the MeV range (100 MeV–1 GeV), and study the possibility of a contribution coming from the pγ interactions between protons and MeV SSC target photons to fit the very high energy (VHE) gamma-ray emission. The fit results were compared with two leptonic models (one-zone and two-zone) using the Akaike Information Criteria (AIC) test, which evaluates goodness-of-fit alongside the simplicity of the model. In all cases, the photohadronic model was favoured as a better fit description in comparison to the one-zone leptonic model, and with respect to the two-zone model in the majority of cases. Our results show the potential of a photohadronic contribution to a lepto-hadronic origin of gamma-ray flux of blazars. Future gamma-ray observations above tens of TeV and below 100 MeV in energy will be crucial to test and discriminate between models.
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Raiteri, Claudia M., and Massimo Villata. "Polarimetric Properties of Blazars Caught by the WEBT." Galaxies 9, no. 2 (June 11, 2021): 42. http://dx.doi.org/10.3390/galaxies9020042.
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Active galactic nuclei come in many varieties. A minority of them are radio-loud, and exhibit two opposite prominent plasma jets extending from the proximity of the supermassive black hole up to megaparsec distances. When one of the relativistic jets is oriented closely to the line of sight, its emission is Doppler beamed and these objects show extreme variability properties at all wavelengths. These are called “blazars”. The unpredictable blazar variability, occurring on a continuous range of time-scales, from minutes to years, is most effectively investigated in a multi-wavelength context. Ground-based and space observations together contribute to give us a comprehensive picture of the blazar emission properties from the radio to the γ-ray band. Moreover, in recent years, a lot of effort has been devoted to the observation and analysis of the blazar polarimetric radio and optical behaviour, showing strong variability of both the polarisation degree and angle. The Whole Earth Blazar Telescope (WEBT) Collaboration, involving many tens of astronomers all around the globe, has been monitoring several blazars since 1997. The results of the corresponding data analysis have contributed to the understanding of the blazar phenomenon, particularly stressing the viability of a geometrical interpretation of the blazar variability. We review here the most significant polarimetric results achieved in the WEBT studies.
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Weaver, Zachary R., Svetlana G. Jorstad, Alan P. Marscher, Daria A. Morozova, Ivan S. Troitsky, Iván Agudo, José L. Gómez, Anne Lähteenmäki, Joni Tammi, and Merja Tornikoski. "Kinematics of Parsec-scale Jets of Gamma-Ray Blazars at 43 GHz during 10 yr of the VLBA-BU-BLAZAR Program." Astrophysical Journal Supplement Series 260, no. 1 (May 1, 2022): 12. http://dx.doi.org/10.3847/1538-4365/ac589c.
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Abstract We analyze the parsec-scale jet kinematics from 2007 June to 2018 December of a sample of γ-ray bright blazars monitored roughly monthly with the Very Long Baseline Array (VLBA) at 43 GHz under the VLBA-BU-BLAZAR program. We implement a novel piecewise linear fitting method to derive the kinematics of 521 distinct emission knots from a total of 3705 total intensity images in 22 quasars, 13 BL Lacertae objects, and 3 radio galaxies. Apparent speeds of these components range from 0.01c to 78c, and 18.6% of knots (other than the “core”) are quasi-stationary. One-fifth of moving knots exhibit nonballistic motion, with acceleration along the jet within 5 pc of the core (projected) and deceleration farther out. These accelerations occur mainly at locations coincident with quasi-stationary features. We calculate the physical parameters of 273 knots with statistically significant motion, including their Doppler factors, Lorentz factors, and viewing angles. We determine the typical values of these parameters for each jet and the average for each subclass of active galactic nuclei. We investigate the variability of the position angle of each jet over the 10 yr of monitoring. The fluctuations in position of the quasi-stationary components in radio galaxies tend to be parallel to the jet, while no directional preference is seen in the components of quasars and BL Lacertae objects. We find a connection between γ-ray states of blazars and their parsec-scale jet properties, with blazars with brighter 43 GHz cores typically reaching higher γ-ray maxima during flares.
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Lamberts, Astrid, Ewald Puchwein, Christoph Pfrommer, Philip Chang, Mohamad Shalaby, Avery Broderick, Paul Tiede, and Gwen Rudie. "Constraining blazar heating with the 2 ≲ z ≲ 3 Lyman-α forest." Monthly Notices of the Royal Astronomical Society 512, no. 2 (March 4, 2022): 3045–59. http://dx.doi.org/10.1093/mnras/stac553.
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ABSTRACT The intergalactic medium (IGM) acts like a calorimeter recording energy injection by cosmic structure formation, shocks and photoheating from stars and active galactic nuclei. It was recently proposed that spatially inhomogeneous TeV-blazars could significantly heat up the underdense IGM, resulting in patches of both cold and warm IGM around z ≃ 2−3. The goal of this study is to compare predictions of different blazar heating models with recent observations of the IGM. We perform a set of cosmological simulations and carefully compute mock observables of the Lyman-α (Lyα) forest. We perform a detailed assessment of different systematic uncertainties which typically impact this type of observables and find that they are smaller than the differences between our models. We find that our inhomogeneous blazar heating model is in good agreement with the Lyα line properties and the rescaled flux probability distribution function at high redshift (2.5 < z < 3) but that our blazar heating models are challenged by lower redshift data (2 < z < 2.5). Our results could be explained by HeII reionization although state-of-the-art models fall short on providing enough heating to the low-density IGM, thus motivating further radiative transfer studies of inhomogeneous HeII reionization. If blazars are indeed hosted by group-mass haloes of $2\times 10^{13}\, \rm {M}_\odot$, a later onset of blazar heating in comparison with previous models would be favoured, which could bring our findings here in agreement with the evidence of blazar heating from local gamma-ray observations.
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Kifune, Tadashi. "Very High Energy γ-rays from AGN." Publications of the Astronomical Society of Australia 19, no. 1 (2002): 1–4. http://dx.doi.org/10.1071/as01081.
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AbstractEvidence of TeV γ-ray emission has been found for only a handful of active galactic nuclei, with detailed investigations limited to the blazars Mrk 421 and Mrk 501. TeV γ-ray astronomy, as the highest energy band, provides important information that is hard to obtain from longer wavelength electromagnetic radiation. The current status of TeV γ-ray studies of active galactic nuclei is summarized and our understanding of the high energy phenomena taking place in active galactic nuclei is outlined, with the prospects for future TeV γ-ray observations also considered.
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Deng, Xue-Jiao, Rui Xue, Ze-Rui Wang, Shao-Qiang Xi, Hu-Bing Xiao, Lei-Ming Du, and Zhao-Hua Xie. "The physical properties of γ-ray-quiet flat-spectrum radio quasars: why are they undetected by Fermi-LAT?" Monthly Notices of the Royal Astronomical Society 506, no. 4 (August 12, 2021): 5764–73. http://dx.doi.org/10.1093/mnras/stab2095.
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ABSTRACT During a decade of operation of the Fermi-Large Area Telescope (LAT), thousands of blazars have been detected in the γ-ray band. However, there are still numbers of blazars that have not been detected in the γ-ray band. In this work, we focus on investigating why some flat-spectrum radio quasars (FSRQs) are undetected by Fermi-LAT. By cross-matching the Candidate Gamma-ray Blazars Survey catalogue with the Fourth Catalog of Active Galactic Nuclei Detected by the Fermi-LAT, we select 11 γ-ray-undetected (γ-ray-quiet) FSRQs as our sample and collect their quasi-simultaneous multiwavelength data. In the framework of the conventional one-zone leptonic model, we investigate their underlying physical properties and study the possibility that they are undetected in the γ-ray by modelling their quasi-simultaneous spectral energy distributions. In contrast to a smaller bulk Lorentz factor suggested by previous works, our results suggest that the dissipation region located relatively far away from the central super-massive black hole is more likely to be the reason why some γ-ray-quiet FSRQs are undetected by Fermi-LAT.
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LENAIN, J. P., C. BOISSON, and H. SOL. "SSC SCENARIO FOR TEV EMISSION FROM NON-BLAZAR AGNs." International Journal of Modern Physics D 17, no. 09 (September 2008): 1577–84. http://dx.doi.org/10.1142/s0218271808013170.
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M 87 is the first extragalactic source detected in the TeV γ-ray domain that is not a blazar, its large scale jet not being aligned to the line of sight. We present here a multi-blob synchrotron self-Compton model accounting explicitly for large viewing angles and moderate Lorentz factors as inferred from magnetohydrodynamic simulations of jet formation, motivated by the detection of M 87 at very high energies (VHE; E > 100 GeV ). Predictions are presented for the very high-energy emission of active galactic nuclei with extended optical or X-ray jet, which could be misaligned blazars but still show some moderate beaming. We include predictions for 3C 273, Cen A and PKS 0521–36.
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Dzhatdoev, Timur, Vladimir Galkin, and Egor Podlesnyi. "Nonthermal Radiation of the Extreme TeV Blazar 1ES 0229+200 from Electromagnetic Cascades on Infrared Photon Field." Universe 7, no. 12 (December 14, 2021): 494. http://dx.doi.org/10.3390/universe7120494.
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Extreme TeV blazars (ETBs) are active galactic nuclei with jets presumably pointing towards the observer having their intrinsic (compensated for the effect of γ-ray absorption on extragalactic background light photons) spectral energy distributions (SEDs) peaked at an energy in excess of 1 TeV. These sources typically reveal relatively weak and slow variability as well as higher frequency of the low-energy SED peak compared to other classes of blazars. It proved to be exceedingly hard to incorporate all these peculiar properties of ETBs into the framework of conventional γ-ray emission models. ETB physics have recently attracted great attention in the astrophysical community, underlying the importance of the development of self-consistent ETB emission model(s). We propose a new scenario for the formation of X-ray and γ-ray spectra of ETBs assuming that electromagnetic cascades develop in the infrared photon field surrounding the central blazar engine. This scenario does not invoke compact fast-moving sources of radiation (so-called “blobs”), in agreement with the apparent absence of fast and strong variability of ETBs. For the case of the extreme TeV blazar 1ES 0229+200 we propose a specific emission model in the framework of the considered scenario. We demonstrate that this model allows to obtain a good fit to the measured SED of 1ES 0229+200.
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Rajagopal, M., L. Marcotulli, K. Labrie, S. Marchesi, and M. Ajello. "Identifying the 3FHL Catalog. VI. Results of the 2019 Gemini Optical Spectroscopy." Astronomical Journal 165, no. 2 (January 9, 2023): 42. http://dx.doi.org/10.3847/1538-3881/aca1be.
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Abstract Active galactic nuclei with their relativistic jets pointed toward the observer are a class of luminous gamma-ray sources commonly known as blazars. The study of this source class is essential to unveil the physical processes powering these extreme jets, to understand their cosmic evolution, as well as to indirectly probe the extragalactic background light. To do so, however, one needs to correctly classify and measure a redshift for a large sample of these sources. The Third Fermi–LAT Catalog of High-Energy Sources (3FHL) contains 1212 blazars detected at energies greater than 10 GeV. However, ∼25% of these sources are unclassified and ∼56% lack redshift information. To increase the optical completeness of blazars in the 3FHL catalog, we devised an optical spectroscopic follow-up campaign using 4 m and 8 m telescopes. In this paper, we present the results of the last part of this campaign, where we observed 5 blazars using the 8 m Gemini-S telescope in Chile. We report all the 5 sources to be classified as BL Lacs, a redshift lower limit for 2 sources, and featureless spectra for the remaining 3 sources. We also performed a one-zone leptonic fit to the two sources with the redshift lower limits.
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Boula, Stella, Maria Petropoulou, and Apostolos Mastichiadis. "On the Connection of Radio and γ-Ray Emission in Blazars." Galaxies 7, no. 1 (December 25, 2018): 3. http://dx.doi.org/10.3390/galaxies7010003.
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Blazars are a sub-category of radio-loud active galactic nuclei with relativistic jets pointing towards to the observer. They are well-known for their non-thermal variable emission, which practically extends over the whole electromagnetic spectrum. Despite the plethora of multi-wavelength observations, the issue about the origin of the γ -ray and radio emission in blazar jets remains unsettled. Here, we construct a parametric leptonic model for studying the connection between the γ -ray and radio emission in both steady-state and flaring states of blazars. Assuming that relativistic electrons are injected continuously at a fixed distance from the black hole, we numerically study the evolution of their population as it propagates to larger distances while losing energy due to expansion and radiative cooling. In this framework, γ -ray photons are naturally produced at small distances (e.g., 10 − 3 pc) when the electrons are still very energetic, whereas the radio emission is produced at larger distances (e.g., 1 pc), after the electrons have cooled and the emitting region has become optically thin to synchrotron self-absorption due to expansion. We present preliminary results of our numerical investigation for the steady-state jet emission and the predicted time lags between γ -rays and radio during flares.
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Giovannini, Gabriele, and Teddy Cheung. "JD6 - The Connection between Radio Properties and High Energy Emission in AGNs." Proceedings of the International Astronomical Union 10, H16 (August 2012): 187–98. http://dx.doi.org/10.1017/s1743921314005407.
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AbstractWhile observations in the radio band are providing essential information on the innermost structures of relativistic jets in active galactic nuclei (AGN), the recent detection byFermiof gamma-ray emission from many hundreds of blazars shows that the maximum jet power is emitted at high energies. Multi-wavelength monitoring observations further allow variability studies of the AGN spectral energy distributions over 13 orders of magnitude in frequency. The Joint Discussion offered the possibility for a comprehensive discussion of advances in the observational domain and stimulated theoretical discussion about our current understanding of jet physics.
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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.
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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.
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Romoli, Carlo, Nachiketa Chakraborty, Daniela Dorner, Andrew Taylor, and Michael Blank. "Flux Distribution of Gamma-Ray Emission in Blazars: The Example of Mrk 501." Galaxies 6, no. 4 (December 6, 2018): 135. http://dx.doi.org/10.3390/galaxies6040135.
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Flux distribution is an important tool to understand the variability processes in active galactic nuclei. We now have available a great deal of observational evidences pointing towards the presence of log-normal components in the high energy light curves, and different models have been proposed to explain these data. Here, we collect some of the recent developments on this topic using the well-known blazar Mrk 501 as example of complex and interesting aspects coming from its flux distribution in different energy ranges and at different timescales. The observational data we refer to are those collected in a complementary manner by Fermi-LAT over multiple years, and by the First G-APD Cherenkov Telescope (FACT) telescope and the H.E.S.S. array in correspondence of the bright flare of June 2014.
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Glawion, Dorit. "Radio Galaxies at TeV Energies: Preface." Galaxies 8, no. 1 (February 22, 2020): 18. http://dx.doi.org/10.3390/galaxies8010018.
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The majority of the known extragalactic sky from TeV gamma-ray energies consists of blazars having plasma jets pointing in the direction of the line-of-sight, which results in a large Doppler boosting of their emission. Up to now, only six galaxies with a larger viewing angle have been detected in the TeV range. These objects also show fascinating properties, such as fast variability or spectral features and are called “radio galaxies”. The TeV radio galaxies provide a unique laboratory for studying key aspects of active galactic nuclei. This Special Issue of Galaxies targets these exciting objects.
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Orienti, Monica. "Extragalactic relativistic jets." Proceedings of the International Astronomical Union 14, S342 (May 2018): 61–68. http://dx.doi.org/10.1017/s1743921318008153.
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AbstractRelativistic jets are one of the most powerful manifestations of the release of energy produced around supermassive black holes at the centre of active galactic nuclei (AGN). Their emission is observed across the entire electromagnetic spectrum, from the radio band to gamma rays. Despite decades of efforts, many aspects of the physics of relativistic jets remain elusive. In particular, the location and the mechanisms responsible for the high-energy emission and the connection of the variability at different wavelengths are among the greatest challenges in the study of AGN. Recent high resolution radio observations of flaring objects locate the high energy emitting region downstream the jet at parsec scale distance from the central engine. Furthermore, monitoring campaigns of the most active blazars indicate that not all the high energy flares have the same characteristics in the various energy bands, even from the same source, making the interpretation of the mechanism responsible for the high-energy emission not trivial. Here I will discuss gamma-ray properties of blazars obtained by Fermi Large Area Telescope observations and the connection between radio and high-energy emission in relativistic jets, and I will focus on the importance of high angular resolution observations.
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de Menezes, Raniere, Raffaele D’Abrusco, Francesco Massaro, and Sara Buson. "The Isotropic γ-ray Emission above 100 GeV: Where Do Very High-energy γ-rays Come From?" Astrophysical Journal 933, no. 2 (July 1, 2022): 213. http://dx.doi.org/10.3847/1538-4357/ac771d.
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Abstract Astrophysical sources of very high energy (VHE; >100 GeV) γ-rays are rare, since GeV and TeV photons can be only emitted in extreme circumstances involving interactions of relativistic particles with local radiation and magnetic fields. In the context of the Fermi Large Area Telescope (LAT), only a few sources are known to be VHE emitters, where the largest fraction belongs to the rarest class of active galactic nuclei: the blazars. In this work, we explore Fermi-LAT data for energies >100 GeV and Galactic latitudes b > ∣50°∣ in order to probe the origin of the extragalactic isotropic γ-ray emission. Since the production of such VHE photons requires very specific astrophysical conditions, we would expect that the majority of the VHE photons from the isotropic γ-ray emission originate from blazars or other extreme objects like star-forming galaxies, γ-ray bursts, and radio galaxies, and that the detection of a single VHE photon at the adopted Galactic latitudes would be enough to unambiguously trace the presence of such a counterpart. Our results suggest that blazars are, by far, the dominant class of sources above 100 GeV, although they account for only 22.8 − 4.1 + 4.5 % of the extragalactic VHE photons. The remaining 77 − 4.5 + 4.1 % of the VHE photons still have an unknown origin.
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del Palacio, S., V. Bosch-Ramon, and G. E. Romero. "Gamma rays from jets interacting with BLR clouds in blazars." Astronomy & Astrophysics 623 (March 2019): A101. http://dx.doi.org/10.1051/0004-6361/201834231.
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Context. The innermost parts of powerful jets in active galactic nuclei are surrounded by dense, high-velocity clouds from the broad-line region, which may penetrate into the jet and lead to the formation of a strong shock. Such jet-cloud interactions are expected to have measurable effects on the γ-ray emission from blazars. Aims. We characterise the dynamics of a typical cloud-jet interaction scenario, and the evolution of its radiative output in the 0.1–30 GeV energy range, to assess to what extent these interactions can contribute to the γ-ray emission in blazars. Methods. We use semi-analytical descriptions of the jet-cloud dynamics, taking into account the expansion of the cloud inside the jet and its acceleration. Assuming that electrons are accelerated in the interaction and making use of the hydrodynamical information, we then compute the high-energy radiation from the cloud, including the absorption of γ-rays in the ambient photon field through pair creation. Results. Jet-cloud interactions can lead to significant γ-ray fluxes in blazars with a broad-line region (BLR), in particular when the cloud expansion and acceleration inside the jet are taken into account. This is caused by 1) the increased shocked area in the jet, which leads to an increase in the energy budget for the non-thermal emission; 2) a more efficient inverse Compton cooling with the boosted photon field of the BLR; and 3) an increased observer luminosity due to Doppler boosting effects. Conclusions. For typical broad-line region parameters, either (i) jet-cloud interactions contribute significantly to the persistent γ-ray emission from blazars or (ii) the BLR is far from spherical or the fraction of energy deposited in non-thermal electrons is small.
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Kachelrieß, M., S. Ostapchenko, and R. Tomàs. "TeV Gamma Rays from Ultrahigh Energy Cosmic Ray Interactions in the Cores of Active Galactic Nuclei: Lessons from Centaurus A." Publications of the Astronomical Society of Australia 27, no. 4 (2010): 482–89. http://dx.doi.org/10.1071/as09072.
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AbstractTeV gamma rays have been observed from blazars as well as from radio galaxies like M 87 and Centaurus A. In leptonic models, gamma rays above the pair production threshold can escape from the ultrarelativistic jet, because large Lorentz factors reduce the background photon densities compared to those required for isotropic emission. Here we discuss an alternative scenario, where very high energy photons are generated as secondaries from ultrahigh energy cosmic rays interactions in the cores of active galactic nuclei. We show that TeV gamma-rays can escape from the core despite large infrared and ultraviolet backgrounds. For the special case of Centaurus A, we study whether the various existing observations from the far infrared to the ultrahigh energy range can be reconciled within this picture.
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Fichtel, Carl E. "High-Energy Gamma-Ray Observations of Active Galaxies." International Astronomical Union Colloquium 142 (1994): 917–22. http://dx.doi.org/10.1017/s0252921100078313.
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AbstractDuring the period from 1992 May to early 1992 November, the Energetic Gamma-Ray Experiment Telescope (EGRET) on board the Compton Gamma Ray Observatory obtained high-energy gamma-ray data for most of the sky. A total of 18 active galaxies have been seen with high certainty, and it is expected that more will be found in the data when a more thorough analysis is complete. All of those that have been seen are radio-loud quasars or BL Lacertae objects; most have already been identified as blazars. No Seyfert galaxies have been found thus far. If the spectra are represented as a power law in energy, spectral slopes ranging from approximately −1.7 to −2.4 are found. A wide range of z-values exists in the observed sample, eight having values in excess of 1.0. Time variations have been seen, with the timescale for a significant change being as short as days in at least one case. These results imply the existence of very large numbers of relativistic particles, probably close to the central object. Although a large extrapolation is required, their existence also suggests that these active galactic nuclei may be the source of the extragalactic cosmic rays.Subject headings: acceleration of particles — galaxies: active — gamma rays: observations — quasars: general
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DE ALMEIDA, ULISSES BARRES. "RESULTS FROM MAGIC OBSERVATIONS OF EXTRAGALACTIC RELATIVISTIC SOURCES." International Journal of Modern Physics: Conference Series 08 (January 2012): 178–83. http://dx.doi.org/10.1142/s2010194512004576.
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The Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) experiment is an array of two 17-meter telescopes located in the Canary Island of La Palma that observes the very-high energy (VHE) gamma-ray sky in stereoscopic mode since 2009. MAGIC is distinguished by its low-energy threshold of approximately 50 GeV, which grants the system a unique potential in the study of distant extragalactic sources whose gamma-ray emission is significantly attenuated due to absorption by the extragalactic background light (EBL). The observation of non-thermal gamma rays in the GeV-TeV range from extragalactic sources is a characteristic signature of their relativistic nature and therefore fundamentally important for our understanding of the physics of these objects. Since the beginning of its stereo operation, MAGIC has observed a large number of active galactic nuclei (AGN) of different classes, including several blazars and distant quasars. In this paper we will review some of the most important results of these observations.
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Mirabel, I. Félix. "Microquasars: disk–jet coupling in stellar-mass black holes." Proceedings of the International Astronomical Union 2, S238 (August 2006): 19–22. http://dx.doi.org/10.1017/s1743921307004619.
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AbstractMicroquasars provide new insights into: 1) the physics of relativistic jets from black holes, 2) the connection between accretion and ejection, and 3) the physical mechanisms in the formation of stellar-mass black holes. Furthermore, the studies of microquasars in our Galaxy can provide in the future new insights on: 1) a large fraction of the ultraluminous X-ray sources in nearby galaxies, 2) gamma-ray bursts (GRBs) of long duration in distant galaxies, and 3) the physics in the jets of blazars. If jets in GRBs, microquasars and Active Galactic Nuclei (AGN) are due to a unique universal magnetohydrodynamic mechanism, synergy of the research on these three different classes of cosmic objects will lead to further progress in black hole physics and astrophysics.
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Middei, Riccardo, Ioannis Liodakis, Matteo Perri, Simonetta Puccetti, Elisabetta Cavazzuti, Laura Di Gesu, Steven R. Ehlert, et al. "X-Ray Polarization Observations of BL Lacertae." Astrophysical Journal Letters 942, no. 1 (December 28, 2022): L10. http://dx.doi.org/10.3847/2041-8213/aca281.
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Abstract Blazars are a class of jet-dominated active galactic nuclei with a typical double-humped spectral energy distribution. It is of common consensus that the synchrotron emission is responsible for the low frequency peak, while the origin of the high frequency hump is still debated. The analysis of X-rays and their polarization can provide a valuable tool to understand the physical mechanisms responsible for the origin of high-energy emission of blazars. We report the first observations of BL Lacertae (BL Lac) performed with the Imaging X-ray Polarimetry Explorer, from which an upper limit to the polarization degree Π X < 12.6% was found in the 2–8 keV band. We contemporaneously measured the polarization in radio, infrared, and optical wavelengths. Our multiwavelength polarization analysis disfavors a significant contribution of proton-synchrotron radiation to the X-ray emission at these epochs. Instead, it supports a leptonic origin for the X-ray emission in BL Lac.
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Hovatta, T., E. Lindfors, S. Kiehlmann, W. Max-Moerbeck, M. Hodges, I. Liodakis, A. Lähteemäki, et al. "Association of IceCube neutrinos with radio sources observed at Owens Valley and Metsähovi Radio Observatories." Astronomy & Astrophysics 650 (June 2021): A83. http://dx.doi.org/10.1051/0004-6361/202039481.
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Context. Identifying the most likely sources for high-energy neutrino emission has been one of the main topics in high-energy astrophysics ever since the first observation of high-energy neutrinos by the IceCube Neutrino Observatory. Active galactic nuclei with relativistic jets, also known as blazars, have been considered to be one of the main candidates because of their ability to accelerate particles to high energies. Aims. We study the connection between radio emission and IceCube neutrino events using data from the Owens Valley Radio Observatory (OVRO) and Metsähovi Radio Observatory blazar monitoring programs. Methods. We identify sources in our radio monitoring sample that are positionally consistent with IceCube high-energy neutrino events. We estimate their mean flux density and variability amplitudes around the neutrino arrival time, and compare these with values from random samples to establish the significance of our results. Results. We find radio source associations within our samples with 15 high-energy neutrino events detected by IceCube. Nearly half of the associated sources are not detected in the γ-ray energies, but their radio variability properties and Doppler boosting factors are similar to the γ-ray detected objects in our sample, meaning that they could still be potential neutrino emitters. We find that the number of strongly flaring objects in our statistically complete OVRO samples is unlikely to be a random coincidence (at 2σ level). Conclusions. Based on our results, we conclude that although it is clear that not all neutrino events are associated with strong radio flaring blazars, observations of large-amplitude radio flares in a blazar at the same time as a neutrino event are unlikely to be a random coincidence.
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Frey, Sándor, Krisztina É. Gabányi, and Tao An. "The Quasar CTD 135 Is Not a Compact Symmetric Object." Symmetry 14, no. 2 (February 4, 2022): 321. http://dx.doi.org/10.3390/sym14020321.
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The radio-loud quasar CTD 135 (2234+282, J2236+2828) has been proposed as a candidate compact symmetric object (CSO), based on its symmetric radio structure revealed by multi-frequency very long baseline interferometry (VLBI) imaging observations on milliarcsec angular scales. CSOs are known as young jetted active galactic nuclei (AGN) whose relativistic plasma jets are misaligned with respect to the line of sight. The peculiarity of CTD 135 as a CSO candidate was its detection in γ-rays, while the vast majority of known γ-ray emitting AGN are blazars with jets pointing close to our viewing direction. Since only a handful of CSOs are known as γ-ray sources, the unambiguous identification of a single candidate is important for studying this rare class of objects. By collecting and interpreting observational data from the recent literature, we revisit the classification of CTD 135. We present evidence that the object, based on its flat-spectrum radio core with high brightness temperature, variability at multiple wavebands, and infrared colours should be classified as a blazar rather than a CSO.
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Korsmeier, Michael, Elena Pinetti, Michela Negro, Marco Regis, and Nicolao Fornengo. "Flat-spectrum Radio Quasars and BL Lacs Dominate the Anisotropy of the Unresolved Gamma-Ray Background." Astrophysical Journal 933, no. 2 (July 1, 2022): 221. http://dx.doi.org/10.3847/1538-4357/ac6c85.
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Abstract We analyze the angular power spectrum (APS) of the unresolved gamma-ray background (UGRB) emission and combine it with the measured properties of the resolved gamma-ray sources of the Fermi-LAT 4FGL catalog. Our goals are to dissect the composition of the gamma-ray sky and to establish the relevance of different classes of source populations of active galactic nuclei in determining the observed size of the UGRB anisotropy, especially at low energies. We find that, under physical assumptions for the spectral energy distribution, i.e., by using the 4FGL catalog data as a prior, two populations are required to fit the APS data, namely flat-spectrum radio quasars at low energies and BL Lacs at higher energies. The inferred luminosity functions agree well with the extrapolation of the flat-spectrum radio quasar and BL Lac ones obtained from the 4FLG catalog. We use these luminosity functions to calculate the UGRB intensity from blazars, finding a contribution of 20% at 1 GeV and 30% above 10 GeV. Finally, bounds on an additional gamma-ray emission due to annihilating dark matter are also derived.
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D’Ammando, Filippo. "Relativistic Jets in Gamma-Ray-Emitting Narrow-Line Seyfert 1 Galaxies." Galaxies 7, no. 4 (November 7, 2019): 87. http://dx.doi.org/10.3390/galaxies7040087.
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Before the launch of the Fermi Gamma-ray Space Telescope satellite only two classes of active galactic nuclei (AGN) were known to generate relativistic jets and thus to emit up to the γ -ray energy range: blazars and radio galaxies, both hosted in giant elliptical galaxies. The discovery by the Large Area Telescope (LAT) on-board the Fermi satellite of variable γ -ray emission from a few radio-loud narrow-line Seyfert 1 galaxies (NLSy1) revealed the presence of an emerging third class of AGN with powerful relativistic jets. Considering that NLSy1 are usually hosted in late-type galaxies with relatively small black hole masses, this finding opened new challenging questions about the nature of these objects, the disc/jet connection, the emission mechanisms at high energies, and the formation of relativistic jets. In this review, I will discuss the broad-band properties of the γ -ray-emitting NLSy1 included in the Fourth Fermi LAT source catalog, highlighting major findings and open questions regarding jet physics, black hole mass estimation, host galaxy and accretion process of these sources in the Fermi era.
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Sol, Hélène, Andreas Zech, Catherine Boisson, Henric Krawczynski, Lisa Fallon, Elisabete de Gouveia Dal Pino, Jim Hinton, Susumu Inoue, Andrii Neronov, and Richard White. "Prospect on intergalactic magnetic field measurements with gamma-ray instruments." Proceedings of the International Astronomical Union 8, S294 (August 2012): 459–70. http://dx.doi.org/10.1017/s1743921313002925.
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AbstractObserving high-energy gamma-rays from Active Galactic Nuclei (AGN) offers a unique potential to probe extremely tiny values of the intergalactic magnetic field (IGMF), a long standing question of astrophysics, astroparticle physics and cosmology. Very high energy (VHE) photons from blazars propagating along the line of sight interact with the extragalactic background light (EBL) and produce e+e− pairs. Through inverse-Compton interaction, mainly on the cosmic microwave background (CMB), these pairs generate secondary GeV-TeV components accompanying the primary VHE signal. Such secondary components would be detected in the gamma-ray range as delayed “pair echos” for very weak IGMF (B < 10−16G), while they should result in a spatially extended gamma-ray emission around the source for higher IGMF values (B > 10−16G). Coordinated observations with space (i.e. Fermi) and ground-based gamma-ray instruments, such as the present Cherenkov experiments H.E.S.S., MAGIC and VERITAS, the future Cherenkov Telescope Array (CTA) Observatory, and the wide-field detectors such as HAWC and LHAASO, should allow to analyze and finally detect such echos, extended emission or pair halos, and to further characterize the IGMF.
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Krichbaum, T. P., W. Alef, and A. Witzel. "The Sub-Parsec Scale Jets of AGN." Symposium - International Astronomical Union 175 (1996): 11–13. http://dx.doi.org/10.1017/s007418090007981x.
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With an angular resolutions of 0.05–0.2 mas, millimeter-VLBI1 observations (at 22, 43, and 86 GHz) allow to investigate the very central –sub-parsec scale – regions of active galactic nuclei (AGN), which are self-absorbed at lower frequencies. Here we briefly present preliminary results from recent observations of Cygnus A at 22 & 43 GHz, which reveal evidence for subluminal motion in jet and counter-jet, and 86 GHz VLBI observations of two extreme γ – blazars, suggesting a tight correlation between their γ – ray activity and the generation of jets.
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Xiao, Hubing, Zhihao Ouyang, Lixia Zhang, Liping Fu, Shaohua Zhang, Xiangtao Zeng, and Junhui Fan. "The Relativistic Jet and Central Engine of Fermi Blazars." Astrophysical Journal 925, no. 1 (January 1, 2022): 40. http://dx.doi.org/10.3847/1538-4357/ac36da.
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Abstract The origin of jets is one of the most important issues concerning active galactic nuclei, yet it has remained obscure. In this work, we made use of information from emission lines, spectral energy distributions, and Fermi–LAT γ-ray emission to construct a blazar sample that contains 667 sources. We note that jet power originations are different for BL Lacertae objects (BL Lacs) and flat-spectrum radio quasars (FSRQs). The correlation between jet power P jet and the normalized disk luminosity L Disk/L Edd shows a slope of −1.77 for BL Lacs and a slope of 1.16 for FSRQs. The results seem to suggest that BL Lac jets are powered by extracting black hole (BH) rotation energy, while FSRQ jets are mostly powered by accretion disks. Meanwhile, we find the accretion ratio M ̇ / M ̇ Edd increases with the normalized γ-ray luminosity. Based on this, we propose a dividing line, log ( L BLR / L Edd ) = 0.25 log ( L γ / L Edd ) − 2.23 , to separate FSRQs and BL Lacs in the diagram of L BLR/L Edd against L γ /L Edd using a machine-learning method; the method gives an accuracy of 84.5%. In addition, we propose an empirical formula, M BH / M ☉ ≃ L γ 0.65 / 21.46 , to estimate BH mass based on a strong correlation between γ-ray luminosity and BH mass. Strong γ-ray emission is typical in blazars, and the emission is always boosted by a Doppler-beaming effect. In this work, we generate a new method to estimate a lower limit of Doppler factor δ and give δ BL Lac = 7.94 and δ FSRQ = 11.55.
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Müller, C., M. Kadler, R. Ojha, R. Schulz, J. Trüstedt, P. G. Edwards, E. Ros, et al. "TANAMI: Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry." Astronomy & Astrophysics 610 (February 2018): A1. http://dx.doi.org/10.1051/0004-6361/201731455.
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Context. TANAMI is a multiwavelength program monitoring active galactic nuclei (AGN) south of − 30° declination including high-resolution very long baseline interferometry (VLBI) imaging, radio, optical/UV, X-ray, and γ-ray studies. We have previously published first-epoch8.4 GHz VLBI images of the parsec-scale structure of the initial sample. In this paper, we present images of 39 additional sources. The full sample comprises most of the radio- and γ-ray brightest AGN in the southern quarter of the sky, overlapping with the region from which high-energy (> 100 TeV) neutrino events have been found. Aims. We characterize the parsec-scale radio properties of the jets and compare them with the quasi-simultaneous Fermi/LAT γ-ray data. Furthermore, we study the jet properties of sources which are in positional coincidence with high-energy neutrino events compared to the full sample. We test the positional agreement of high-energy neutrino events with various AGN samples. Methods. TANAMI VLBI observations at 8.4 GHz are made with southern hemisphere radio telescopes located in Australia, Antarctica, Chile, New Zealand, and South Africa. Results. Our observations yield the first images of many jets below − 30° declination at milliarcsecond resolution. We find that γ-ray loud TANAMI sources tend to be more compact on parsec-scales and have higher core brightness temperatures than γ-ray faint jets, indicating higher Doppler factors. No significant structural difference is found between sources in positional coincidence with high-energy neutrino events and other TANAMI jets. The 22 γ-ray brightest AGN in the TANAMI sky show only a weak positional agreement with high-energy neutrinos demonstrating that the > 100 TeV IceCube signal is not simply dominated by a small number of the γ-ray brightest blazars. Instead, a larger number of sources have to contribute to the signal with each individual source having only a small Poisson probability for producing an event in multi-year integrations of current neutrino detectors.
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Bhattacharya, Debbijoy, Krishna Mohana A, Subir Bhattacharyya, Nilay Bhatt, and C. S. Stalin. "Multiwavelength study of different flaring and low-activity states of blazar 4C+21.35." Monthly Notices of the Royal Astronomical Society 500, no. 1 (September 26, 2020): 1127–38. http://dx.doi.org/10.1093/mnras/staa2958.
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ABSTRACT Blazars, a class of active galactic nuclei, emit over the entire accessible electromagnetic spectrum and modelling of their broad-band spectral energy distribution (SED) is the key to constrain the underlying emission mechanisms. Here we report the results on the one-zone leptonic emission modelling carried out on the blazar 4C+21.35 using multiwavelength data spanning over the period 2008–2018. Broad-band SED modelling using γ-ray data from Fermi-Large Area Telescope, X-ray data from Swift-XRT and AstroSat, and UV–optical data from Swift-UVOT, AstroSat, and Catalina Real-Time Transient Survey was carried out at seven different epochs, including three γ-ray flaring episodes and four quiescent periods (three long-term averaged ones and one during AstroSat observing period). Our SED modelling suggests that two compact emission regions originating at a different time outside the broad-line region and moving away from the core with variation primarily in the jet electron spectra can explain the emission from the high-, moderate-, and low-activity periods. The emissions from high- and first low-activity states are likely to have originated in the first region. The moderate- and second low-activity states are likely due to the second emission region with fresh particle acceleration/injection at a later time.
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Liodakis, Ioannis, Alan P. Marscher, Iván Agudo, Andrei V. Berdyugin, Maria I. Bernardos, Giacomo Bonnoli, George A. Borman, et al. "Polarized blazar X-rays imply particle acceleration in shocks." Nature 611, no. 7937 (November 23, 2022): 677–81. http://dx.doi.org/10.1038/s41586-022-05338-0.
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AbstractMost of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to around 1 TeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnetic field, which can be probed by observations of the polarization of light from the jets. Measurements of the radio to optical polarization—the only range available until now—probe extended regions of the jet containing particles that left the acceleration site days to years earlier1–3, and hence do not directly explore the acceleration mechanism, as could X-ray measurements. Here we report the detection of X-ray polarization from the blazar Markarian 501 (Mrk 501). We measure an X-ray linear polarization degree ΠX of around 10%, which is a factor of around 2 higher than the value at optical wavelengths, with a polarization angle parallel to the radio jet. This points to a shock front as the source of particle acceleration and also implies that the plasma becomes increasingly turbulent with distance from the shock.