Articoli di riviste: "Accretion phenomena"

1

Walker, Mark. "Accretion-driven nonthermal phenomena". Astrophysical Journal 330 (luglio 1988): L47. http://dx.doi.org/10.1086/185202.

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2

Bailey, Jeremy. "Accretion Phenomena in Cataclysmic Variables". Publications of the Astronomical Society of Australia 13, n. 1 (gennaio 1996): 75–80. http://dx.doi.org/10.1017/s1323358000020579.

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AbstractThe processes of accretion onto the white dwarf in a cataclysmic variable are reviewed. These systems provide the most easily observable examples of accretion disks, and enable the structure of the accretion region to be studied in detail. The properties of column accretion onto magnetic cataclysmic variables are also described.

3

KUNCIC, ZDENKA, e GEOFFREY V. BICKNELL. "TOWARDS A NEW STANDARD THEORY FOR ASTROPHYSICAL DISK ACCRETION". Modern Physics Letters A 22, n. 23 (30 luglio 2007): 1685–700. http://dx.doi.org/10.1142/s0217732307024243.

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We briefly review recent developments in black hole accretion disk theory, placing new emphasis on the vital role played by magnetohydrodynamic (MHD) stresses in transporting angular momentum. The apparent universality of accretion-related outflow phenomena is a strong indicator that vertical transport of angular momentum by large-scale MHD torques is important and may even dominate radial transport by small-scale MHD turbulence. This leads to an enhanced overall rate of angular momentum transport and allows accretion of matter to proceed at an interesting rate. Furthermore, we argue that when vertical transport is important, the radial structure of the accretion disk is modified and this affects the disk emission spectrum. We present a simple model demonstrating that energetic, magnetically-driven outflows give rise to a disk spectrum that is dimmer and redder than a standard accretion disk accreting at the same rate. We briefly discuss the implications of this key result for accreting black holes in different astrophysical systems.

4

Lee, Umin, e Tod E. Strohmayer. "Thermonuclear Excitation of R-modes in Neutron Stars". International Astronomical Union Colloquium 155 (1995): 445–46. http://dx.doi.org/10.1017/s0252921100037921.

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AbstractNeutron stars display phenomena which result from the burning of accreted nuclear fuel in their surface layers. In addition to producing X-ray bursts and other temporal phenomema, such burning can excite oscillation modes of the neutron star. Here we investigate the stability of R-modes in uniformly rotating neutron stars using nonadiabatic pulsation calculations of steady burning envelopes on accreting neutron stars. We find that the fundamental R-modes are unstable for a large range of accretion rates, and thus might manifest themselves observationally in the power spectra of accreting sources above about 1 kHz.

5

Ferreira, J. "Accretion-ejection phenomena from young stars". EAS Publications Series 9 (2003): 33. http://dx.doi.org/10.1051/eas:2003083.

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6

Kim, Hongsu, e Uicheol Jang. "Effect of Radiation Pressure Formed at the Inner Region of the Accretion Disk on the Accretion Flow in the Outer Region". Journal of Astronomy and Space Sciences 40, n. 4 (dicembre 2023): 247–58. http://dx.doi.org/10.5140/jass.2023.40.4.247.

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Studying the accretion phenomena provides a window into understanding most heavenly bodies, from the birth of stars to active galactic nuclei (AGN). We would adopt the effect of the radiation pressure, which reduces accretion rates ( M ˙ ), on the accretion phenomena. The Shakura-Sunyaev α-disk model of disk accretion is a good candidate theory of advection dominated accretion flow (ADAF). Reduction in the angular velocity leads to the suppression the disk luminosity and surface temperature, essentially indicating the transition of the standard accretion disk model from convection dominated accretion flow (CDAF) to ADAF.

7

Yokosawa, M. "Dynamical Evolution of Accretion Flow onto a Black Hole". Symposium - International Astronomical Union 188 (1998): 455–56. http://dx.doi.org/10.1017/s0074180900116006.

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Active galactic nuclei(AGN) produce many type of active phenomena, powerful X-ray emission, UV hump, narrow beam ejection, gamma-ray emission. Energy of these phenomena is thought to be brought out binding energy between a black hole and surrounding matter. What condition around a black hole produces many type of active phenomena? We investigated dynamical evolution of accretion flow onto a black hole by using a general-relativistic, hydrodynamic code which contains a viscosity based on the alpha-model. We find three types of flow's pattern, depending on thickness of accretion disk. In a case of the thin disk with a thickness less than the radius of the event horizon at the vicinity of a marginally stable orbit, the accreting flow through a surface of the marginally stable orbit becomes thinner due to additional cooling caused by a general-relativistic Roche-lobe overflow and horizontal advection of heat. An accretion disk with a middle thickness, 2rh≤h≤ 3rh, divides into two flows: the upper region of the accreting flow expands into the atmosphere of the black hole, and the inner region of the flow becomes thinner, smoothly accreting onto the black hole. The expansion of the flow generates a dynamically violent structure around the event horizon. The kinetic energy of the violent motion becomes equivalent to the thermal energy of the accreting disk. The shock heating due to violent motion produces a thermally driven wind which flows through the atmosphere above the accretion disk. A very thick disk, 4rh≤h,forms a narrow beam whose energy is largely supplied from hot region generated by shock wave. The accretion flowing through the thick disk,h≥ 2rh, cannot only form a single, laminar flow falling into the black hole, but also produces turbulent-like structure above the event horizon. The middle disk may possibly emit the X-ray radiation observed in active galactic nuclei. The thin disk may produce UV hump of Seyfert galaxy. Thick disk may produce a jet observed in radio galaxy. The thickness of the disk is determined by accretion rate, such ashκ κes/cṁf(r) κ 10rhṁf(r), at the inner region of the disk where the radiation pressure dominates over the gas pressure. Here, Ṁ is the accretion rate and ṁ is the normarized one by the critical-mass flux of the Eddington limit. κesandcare the opacity by electron scattering and the velocity of light.f(r) is a function with a value of unity far from the hole.

8

Tanaka, Y. "Outburst Phenomena in X-Ray Binaries". Symposium - International Astronomical Union 151 (1992): 215–24. http://dx.doi.org/10.1017/s0074180900122211.

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Three distinctly different types of outburst phenomenon in X-ray binaries are discussed: (1) the so-called type 1 burst which is a thermonuclear flash of matter accreted on the surface of neutron stars, (2) the so-called type 2 burst which is a burst-like X-ray emission caused by a spasmodic accretion, and (3) the nova-like outburst which is triggered by a sudden commencement of accretion.

9

Iijima, T. "Rapid Mass Accretion in the Symbiotic Star AG Dra". International Astronomical Union Colloquium 93 (1987): 759–62. http://dx.doi.org/10.1017/s0252921100105640.

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AbstractThe mass accretion process onto the hot component of AG Dra and its explosive phenomena are discussed. The hot component seems to be a massive white dwarf (M > 1 M⊙). The mass accretion rate is estimated to be about 10−7M⊙/year. Many properties of the explosive phenomena agree with those of mild hydrogen flashes expected from this rapid mass accretion.

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de Gouveia Dal Pino, Elisabete M., e Alex C. Raga. "JD7 -Astrophysical Outflows and Associated Accretion Phenomena". Proceedings of the International Astronomical Union 5, H15 (novembre 2009): 235–36. http://dx.doi.org/10.1017/s1743921310009002.

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Highly collimated supersonic jets and outflows are very frequent in several astrophysical environments. They are seen in young stellar objects (YSOs), proto-planetary nebulae, compact objects (like galactic black holes or microquasars, and X-ray binary stars), active galactic nuclei, and are also possibly associated to gamma-ray bursts (GRBs) and to ultra-high energy cosmic rays sources (UHECRs). Despite their different physical scales, all these outflow classes have strong morphological similarities, but questions such as - what physics do they share? - or - can we find a universal mechanism of acceleration and collimation that operates in all classes? - remain matters of debate. The most accepted mechanism for their origin relies on a rotating accretion disk threaded by perpendicular large-scale magnetic fields and, though most of the systems producing jets contain an accretion disk around the central source, the real role that rotation and magnetic fields play in these processes is still not fully understood, nor are the highly non-linear physical processes connected to these jet-disk systems in the large parameter space involved.

11

Baan, Willem A., Hiroshi Imai e Gabor Orosz. "Fallback in bipolar planetary nebulae?" Research in Astronomy and Astrophysics 21, n. 11 (1 dicembre 2021): 275. http://dx.doi.org/10.1088/1674-4527/21/11/275.

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Abstract The subclass of bipolar Planetary Nebulae (PNe) exhibits well-defined low-power outflows and some shows shock-related equatorial spiderweb structures and hourglass structures surrounding these outflows. These structures are distinctly different from the phenomena associated with spherical and elliptical PNe and suggest a non-standard way to simultaneously energise both kinds of structures. This paper presents evidence from the published literature on bipolar PN Hb 12 and other sources in support of an alternative scenario for energising these structures by means of accretion from material shells deposited during earlier post-AGB and pre-PNe evolutionary stages. In addition to energising the bipolar outflow, a sub-Eddington accretion scenario could hydrodynamically explain the spiderweb and outer hourglass structures as oblique shockwaves for guiding the accreting material into the equatorial region of the source. Estimates of the accretion rate resulting from fallback-related spherical accretion could indeed help to drive a low-power outflow and contribute to the total luminosity of these sources.

12

Schneider, P. Christian, H. Moritz Günther e Kevin France. "The UV Perspective of Low-Mass Star Formation". Galaxies 8, n. 1 (21 marzo 2020): 27. http://dx.doi.org/10.3390/galaxies8010027.

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The formation of low-mass ( M ★ ≲ 2 M ⊙ ) stars in molecular clouds involves accretion disks and jets, which are of broad astrophysical interest. Accreting stars represent the closest examples of these phenomena. Star and planet formation are also intimately connected, setting the starting point for planetary systems like our own. The ultraviolet (UV) spectral range is particularly suited for studying star formation, because virtually all relevant processes radiate at temperatures associated with UV emission processes or have strong observational signatures in the UV range. In this review, we describe how UV observations provide unique diagnostics for the accretion process, the physical properties of the protoplanetary disk, and jets and outflows.

13

Simon, Jacob B., Kris Beckwith e Philip J. Armitage. "Emergent mesoscale phenomena in magnetized accretion disc turbulence". Monthly Notices of the Royal Astronomical Society 422, n. 3 (11 aprile 2012): 2685–700. http://dx.doi.org/10.1111/j.1365-2966.2012.20835.x.

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14

Annibali, F., M. Tosi, A. Aloisi, M. Bellazzini, A. Buzzoni, M. Cignoni, L. Ciotti et al. "Accretion phenomena in nearby star-forming dwarf galaxies". Proceedings of the International Astronomical Union 11, S321 (marzo 2016): 211–13. http://dx.doi.org/10.1017/s1743921316009017.

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AbstractWe present two pilot studies for the search and characterization of accretion events in star-forming dwarf galaxies. Our strategy consists of two complementary approaches: i) the direct search for stellar substructures around dwarf galaxies through deep wide-field imaging, and ii) the characterization of the chemical properties in these systems up to large galacto-centric distances. We show our results for two star-forming dwarf galaxies, the starburst irregular NGC 4449, and the extremely metal-poor dwarf DDO 68.

15

King, A. R. "Accretion Disc Instabilities". Symposium - International Astronomical Union 151 (1992): 195–203. http://dx.doi.org/10.1017/s0074180900122193.

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I give a brief discussion of accretion disc instabilities, concentrating mainly on tidal instabilities caused by the presence of a binary companion. The superhumps observed in superoutbursts of SU UMa dwarf novae probably result from the excitation of a resonance in the accretion disc near the 3:1 commensurability with the binary orbit. This resonance can only appear for mass ratios q = M2/M1 < qerit ≃ 0.25 – 0.33: for larger mass ratios the available resonances are considerably weaker. Application of this picture to other types of binary suggests that the condition q < qerit may be necessary but not sufficient. Further, some cataclysmic systems show phenomena which could be tidal in origin even though the condition evidently fails.

16

Leonov, I. I., e N. N. Sokolikhina. "Severe hard rime accretion in the Far North of Russia: observations and modeling". IOP Conference Series: Earth and Environmental Science 1023, n. 1 (1 maggio 2022): 012012. http://dx.doi.org/10.1088/1755-1315/1023/1/012012.

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Abstract Severe weather means any hazardous meteorological phenomena which can cause significant damage, serious social disruption, or loss of human life. Such weather phenomena include thunderstorms, hurricanes, tornados, ice storms, and many others. The main purpose of this research is a study of formation mechanisms of rime accretion and methods for their prediction. This is necessary to forecast these phenomena and minimize their economic and other losses. This work is dedicated to a detailed study of an extraordinary case of formation of rime accretion in the territory of the Yamalo-Nenets Autonomous Okrug, which occurred in early January 2016. Numerical experiments are used for an analysis of various methods for predicting areas of possible ice accretion formation. For the numerical experiments, the atmospheric model WRF-ARW has been chosen. For a theoretical assessment of icing intensity, the Makkonen formula is used. The results of this research are as follows: the main mechanisms of ice accretion formation have been identified for the territory of the Far North of Russia. Numerical experiments with the WRF-ARW model have been successful and reproduced the main signs of rime accretion. The results show that the model has successfully reproduced the icing start time and the icing intensity distribution throughout the entire numerical experiment. The quality of the model's assessment of the mass of rime accretion is difficult to estimate from data of only two meteorological stations; however, the values obtained are close to the observed ones.

17

Court, J. M. C., S. Scaringi, C. Littlefield, N. Castro Segura, K. S. Long, T. Maccarone, D. Altamirano et al. "EX draconis: using eclipses to separate outside-in and inside-out outbursts". Monthly Notices of the Royal Astronomical Society 494, n. 4 (24 aprile 2020): 4656–64. http://dx.doi.org/10.1093/mnras/staa1042.

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ABSTRACT We present a study of the eclipses in the accreting white dwarf EX draconis (EX Dra) during TESS Cycles 14 and 15. During both of the two outbursts present in this data set, the eclipses undergo a hysteretic loop in eclipse-depth/out-of-eclipse-flux space. In each case, the direction in which the loops are executed strongly suggests an outburst that is triggered near the inner edge of the accretion disc and propagates outwards. This in turn suggests that the outbursts in EX Dra are ‘inside out’ outbursts; events predicted by previous hydrodynamic studies of dwarf nova accretion discs and confirmed spectroscopically in a number of other accreting white dwarf systems. We therefore propose that the direction of the loop executed in eclipse-depth/out-of-eclipse flux space be used as a test to phenomenologically distinguish between ‘inside out’ and ‘outside in’ outbursts in other eclipsing dwarf novae; a reliable and purely photometric test to differentiate between these phenomena.

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Stuchlík, Zdeněk, e Jan Schee. "Observational phenomena in the field of Kerr Superspinars". Proceedings of the International Astronomical Union 8, S290 (agosto 2012): 313–14. http://dx.doi.org/10.1017/s1743921312020170.

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AbstractWe test observational consequences of primordial Kerr superspinars indicated by string theories. We demonstrate that Kerr superspinars can serve as ultra-high energy accelerators and explore specific optical phenomena related to accretion discs orbiting them.

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Hawley, J. F. "Numerical Simulations of MHD Accretion Disks". Proceedings of the International Astronomical Union 5, H15 (novembre 2009): 237–38. http://dx.doi.org/10.1017/s1743921310009014.

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AbstractNumerical simulations play an increasingly important role in investigating accretion disks and associated phenomena such as jets. This paper provides a few examples of recent results that have been obtained with simulations, both local or global.

20

Buckley, D. A. H., S. B. Potter, E. Kotze, M. Kotze e H. Breytenbach. "New Observations of Accretion Phenomena in Magnetic Cataclysmic Variables". EPJ Web of Conferences 64 (2014): 07005. http://dx.doi.org/10.1051/epjconf/20136407005.

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21

Migliori, Giulia, R. Margutti, B. D. Metzger, R. Chornock, C. Vignali, D. Brethauer, D. L. Coppejans et al. "Roaring to Softly Whispering: X-Ray Emission after ∼3.7 yr at the Location of the Transient AT2018cow and Implications for Accretion-powered Scenarios*". Astrophysical Journal Letters 963, n. 1 (26 febbraio 2024): L24. http://dx.doi.org/10.3847/2041-8213/ad2764.

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Abstract We present the first deep X-ray observations of luminous fast blue optical transient (LFBOT) AT 2018cow at ∼3.7 yr since discovery, together with the reanalysis of the observation at δ t ∼ 220 days. X-ray emission is significantly detected at a location consistent with AT 2018cow. The very soft X-ray spectrum and sustained luminosity are distinct from the spectral and temporal behavior of the LFBOT in the first ∼100 days and would possibly signal the emergence of a new emission component, although a robust association with AT 2018cow can only be claimed at δ t ∼ 220 days, while at δ t ∼ 1350 days contamination of the host galaxy cannot be excluded. We interpret these findings in the context of the late-time panchromatic emission from AT 2018cow, which includes the detection of persistent, slowly fading UV emission with ν L ν ≈ 1039 erg s−1. Similar to previous works (and in analogy with arguments for ultraluminous X-ray sources), these late-time observations are consistent with thin disks around intermediate-mass black holes (with M • ≈ 103–104 M ☉) accreting at sub-Eddington rates. However, differently from previous studies, we find that smaller-mass black holes with M • ≈ 10–100 M ☉ accreting at ≳the Eddington rate cannot be ruled out and provide a natural explanation for the inferred compact size (R out ≈ 40 R ☉) of the accretion disk years after the optical flare. Most importantly, irrespective of the accretor mass, our study lends support to the hypothesis that LFBOTs are accretion-powered phenomena and that, specifically, LFBOTs constitute electromagnetic manifestations of super-Eddington accreting systems that evolve to ≲Eddington over a ≈100-day timescale.

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Mohanty, Subhanjoy. "Star-disk interaction in brown dwarfs: implications for substellar formation". Proceedings of the International Astronomical Union 3, S243 (maggio 2007): 345–56. http://dx.doi.org/10.1017/s1743921307009702.

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AbstractI review the current state of knowledge regarding disk accretion in young brown dwarfs (BDs), and the interaction of the disk with the central object. In particular, I discuss (1) observations of accretion/outflow phenomena in BDs; (2) techniques for measuring accretion rates (Ṁacc); (3) the dependence of Ṁacc on the central mass from stars to brown dwarfs; (4) the temporal evolution of Ṁacc; and (5) observations of variability in the accretion line profiles. I then examine the implications of these issues for the formation mechanism of BDs, and discuss new observations that can further constrain substellar origins.

23

Czerny, B. "Emission of Accretion Disks". Symposium - International Astronomical Union 159 (1994): 261–70. http://dx.doi.org/10.1017/s0074180900175138.

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Accretion disks surrounding massive black hole is an attractive scenario of nuclear activity. A number of arguments support it although there is no unquestionable proof of the existence of accretion disks in active galactic nuclei. Meaningful comparison of the disk model prediction with the data can only be made if emission of accretion disks is calculated taking into account the existence of optically thin parts responsible for the emission of x-ray radiation. Nonlocal reprocessing phenomena have to be also included. Since we have no real understanding of the viscous processes operating in accretion disks some ad hoc parameterization of these processes has to be used and its applicability should be checked by broad band comparison of predictions for continuum emission and spectral features with available data.

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Liska, M. T. P., N. Kaaz, G. Musoke, A. Tchekhovskoy e O. Porth. "Radiation Transport Two-temperature GRMHD Simulations of Warped Accretion Disks". Astrophysical Journal Letters 944, n. 2 (1 febbraio 2023): L48. http://dx.doi.org/10.3847/2041-8213/acb6f4.

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Abstract In many black hole (BH) systems, the accretion disk is expected to be misaligned with respect to the BH spin axis. If the scale height of the disk is much smaller than the misalignment angle, the spin of the BH can tear the disk into multiple, independently precessing “sub-disks.” This is most likely to happen during outbursts in black hole X-Ray binaries (BHXRBs) and in active galactic nuclei (AGNs) accreting above a few percent of the Eddington limit, because the disk becomes razor-thin. Disk tearing has the potential to explain variability phenomena including quasi-periodic oscillations in BHXRBs and changing-look phenomena in AGNs. Here, we present the first radiative two-temperature general relativistic magnetohydrodynamic (GRMHD) simulation of a strongly tilted (65°) accretion disk around an M BH = 10 M ⊙ BH, which tears and precesses. This leads to luminosity swings between a few percent and 50% of the Eddington limit on sub-viscous timescales. Surprisingly, even where the disk is radiation-pressure-dominated, the accretion disk is thermally stable over t ≳ 14,000 r g /c. This suggests warps play an important role in stabilizing the disk against thermal collapse. The disk forms two nozzle shocks perpendicular to the line of nodes where the scale height of the disk decreases tenfold and the electron temperature reaches T e ∼ 108–109 K. In addition, optically thin gas crossing the tear between the inner and outer disk gets heated to T e ∼ 108 K. This suggests that warped disks may emit a Comptonized spectrum that deviates substantially from idealized models.

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Bandyopadhyay, Tanwi, e Ujjal Debnath. "Accretions of Tsallis, Rényi and Sharma–Mittal dark energies onto higher-dimensional Schwarzschild black hole and Morris–Thorne wormhole". Modern Physics Letters A 36, n. 12 (24 marzo 2021): 2150081. http://dx.doi.org/10.1142/s0217732321500814.

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In this work, we study the dark energy accretion phenomena onto [Formula: see text]-dimensional Schwarzschild black hole and [Formula: see text]-dimensional Morris–Thorne wormhole. We obtain the [Formula: see text]-dimensional Schwarzschild black hole mass and [Formula: see text]-dimensional Morris–Thorne wormhole mass and their rate of change of masses due to accretion. For the dark energy component, we consider Tsallis, modified Rényi and “modified” Sharma–Mittal holographic dark energy (HDE) and new agegraphic dark energy (NADE). We also find the black hole mass and the wormhole mass in terms of redshift when cold dark matter and the specified forms of dark energies accrete onto them. In most cases, the black hole mass increases, and wormhole mass decreases for HDE and NADE accretions. The only exception is the Sharma–Mittal NADE, where the black hole mass decreases and wormhole mass increases during the evolution of the Universe. However, the slope of increasing/decreasing mass significantly depends on the dimension in almost all cases.

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Wang, De-Hua, e Cheng-Min Zhang. "The parallel tracks of kHz QPOs in low-mass X-ray binary 4U 1728−34: explorer of the bimodal accretion modes". Monthly Notices of the Royal Astronomical Society 497, n. 3 (11 agosto 2020): 2893–98. http://dx.doi.org/10.1093/mnras/staa2161.

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ABSTRACT Parallel tracks in the relation between the lower kilohertz quasi-periodic oscillation (kHz QPO) frequency ν1 and the X-ray intensity I have been observed in the neutron star low-mass X-ray binary (NS-LMXB) 4U 1728−34, and we try to ascribe these phenomena to the bimodal accretion modes, i.e. the disc accretion and radial accretion. For the source with one constant radial accretion rate $\dot{M}_{\rm r}$, as the increase of the disc accretion rate $\dot{M}_{\rm d}$, the NS magnetosphere-disc radius can shrink from ∼18 to ∼16 km, while ν1 increases from ∼670 to ∼870 Hz. Then, due to the increase of the kHz QPO frequency and total X-ray intensity Itot, the source will trace out an oblique track in ν1–I relation. While, if the source enters another constant radial accretion rate due to the variation of the accretion environment, the increase of the disc accretion rate can arise another parallel track in ν1–I relation. Finally, the reliability of the theoretical assumptions, the variability of the accretion radius and accretion rate, as well as the dependence of the two accretion modes, are also investigated and discussed.

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Tajima, Y., e J. Fukue. "Electron-Positron Pair Winds from Central Luminous Accretion Disk with Radiation Drag". Symposium - International Astronomical Union 188 (1998): 402–3. http://dx.doi.org/10.1017/s0074180900115736.

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The accretion disks are now supposed to be the main driving source of the active astrophysical phenomena. Even the electron-positron pair plasma will be created at the surface of the sufficiently luminous disk. While the effect of radiation drag which causes in the intense radiation fields around the accretion disk is examined recently. Then, we numerically consider the radiative accelerated pair-winds, which blow off from central luminous accretion disk surrounding a black hole, taking into account radiation drag of the order of v/c.

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Okuda, T., V. Teresi e D. Molteni. "QPOs expected in rotating accretion flows around a supermassive black hole". Proceedings of the International Astronomical Union 2, S238 (agosto 2006): 423–24. http://dx.doi.org/10.1017/s1743921307005765.

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AbstractIt is well known that rotating inviscid accretion flows with adequate injection parameters around black holes could form shock waves close to the black holes, after the flow passes through the outer sonic point and can be virtually stopped by the centrifugal force. We numerically examine such shock waves in 2D accretion flows with 10−5 to 106 Eddington critical accretion rates around a supermassive black hole with 106M⊙. As the results, the luminosities show QPO phenomena with modulations of a factor 2–3 and with quasi-periods of a few to several hours.

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MUKHOPADHYAY, BANIBRATA. "HYDRODYNAMICAL STUDY OF ADVECTIVE ACCRETION FLOW AROUND NEUTRON STARS". International Journal of Modern Physics D 11, n. 08 (settembre 2002): 1305–19. http://dx.doi.org/10.1142/s0218271802002244.

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Here we study the accretion process around neutron stars, especially for the cases where shock does form in the accretion disk. In case of accretion flows around a black hole, close to the horizon the matter is supersonic. On the other hand for the case of neutron stars and white dwarfs, matter must be subsonic close to the inner boundary. So the nature of the inflowing matter around neutron stars and white dwarfs are strictly different from that around black holes in the inner region of the disk. Here we discuss a few phenomena and the corresponding solutions of hydrodynamic equations of matter in an accretion disk around slowly rotating neutron stars without magnetic field.

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Durjasz, Michał. "Catching unusual phenomena with extensive maser monitoring". Proceedings of the International Astronomical Union 18, S380 (dicembre 2022): 210–12. http://dx.doi.org/10.1017/s1743921323002260.

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AbstractHigh brightness and low interstellar extinction allow the 6.7 GHz methanol (CH3OH) masers to carry the information about what happens in the vicinity of the High-Mass Young Stellar Objects (HMYSOs). Monitoring this transition provides an only one opportunity to catch rare, unusual phenomena. In this paper, I describe three of them: quasi-periodic flares of the red-shifted emission in Cep A HW2, accretion burst in S255-NIRS3 and reappearance of the methanol maser flare in G24.329+0.144.

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Burdonov, K., W. Yao, A. Sladkov, R. Bonito, S. N. Chen, A. Ciardi, A. Korzhimanov et al. "Laboratory modelling of equatorial ‘tongue’ accretion channels in young stellar objects caused by the Rayleigh-Taylor instability". Astronomy & Astrophysics 657 (gennaio 2022): A112. http://dx.doi.org/10.1051/0004-6361/202140997.

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Context. The equatorial accretion scenario, caused by the development of the Rayleigh-Taylor (RT) instability at the disk edge, was suggested by accurate three-dimensional magnetohydrodynamic (MHD) modelling, but no observational or experimental confirmation of such phenomena has been evidenced yet. Aims. We studied the propagation of a laterally extended laser-generated plasma stream across a magnetic field and investigated if this kind of structure can be scaled to the case of equatorial ‘tongue’ accretion channels in young stellar objects (YSOs); if so, this would support the possibility of equatorial accretion in young accreting stars. Methods. We conducted a scaled laboratory experiment at the PEARL laser facility. The experiment consists in an optical laser pulse that is focused onto the surface of a Teflon target. The irradiation of the target leads to the expansion of a hot plasma stream into the vacuum, perpendicularly to an externally applied magnetic field. We used a Mach-Zehnder interferometer to diagnose the plasma stream propagation along two axes, to obtain the three-dimensional distribution of the plasma stream. Results. The laboratory experiment shows the propagation of a laterally extended laser-generated plasma stream across a magnetic field. We demonstrate that: (i) such a stream is subject to the development of the RT instability, and (ii) the stream, decomposed into tongues, is able to efficiently propagate perpendicular to the magnetic field. Based on numerical simulations, we show that the origin of the development of the instability in the laboratory is similar to that observed in MHD models of equatorial tongue accretion in YSOs. Conclusions. As we verify that the laboratory plasma scales favourably to accretion inflows of YSOs, our laboratory results support the argument in favour of the possibility of the RT-instability-caused equatorial tongue accretion scenario in the astrophysical case.

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Jovanovic, P. "Investigation of some galactic and extragalactic gravitational phenomena". Serbian Astronomical Journal, n. 185 (2012): 1–16. http://dx.doi.org/10.2298/saj1285001j.

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Here we present a short overview of the most important results of our investigations of the following galactic and extragalactic gravitational phenomena: supermassive black holes in centers of galaxies and quasars, supermassive black hole binaries, gravitational lenses and dark matter. For the purpose of these investigations, we developed a model of a relativistic accretion disk around a supermassive black hole, based on the ray-tracing method in the Kerr metric, a model of a bright spot in an accretion disk and three different models of gravitational microlenses. All these models enabled us to study physics, spacetime geometry and effects of strong gravity in the vicinity of supermassive black holes, variability of some active galaxies and quasars, different effects in the lensed quasars with multiple images, as well as the dark matter fraction in the Universe. We also found an observational evidence for the first spectroscopically resolved sub-parsec orbit of a supermassive black hole binary system in the core of active galaxy NGC 4151. Besides, we studied applications of one potential alternative to dark matter in the form of a modified theory of gravity on Galactic scales, to explain the recently observed orbital precession of some S-stars, which are orbiting around a massive black hole at the Galactic center.

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Hanami, Hitoshi. "Nonlinear Oscillation of the Magnetosphere around Neutron Stars". International Astronomical Union Colloquium 108 (1988): 242–43. http://dx.doi.org/10.1017/s0252921100093969.

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SummaryWe investigate the unsteady motion of mass reservoir formed by the accretion onto the magnetosphere around rotating neutron stars. The unsteady motion of the reservoir induces secondary accretion to neutron star by R-T instability. The nonperiodic or quasiperiodic phenomena of X-ray bursters seems to be related to this property of mass reservoir on the magnetosphere. We classify the typical dynamical state of the reservoir into three types with the parameters which are accretion rate acc and angular velocity of neutron star Ωs. They are nonsequential oscillation, sequential periodic (quasi-periodic) oscillation, and chaotic oscillation states.

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Sniegowska, M., B. Czerny, E. Bon e N. Bon. "Possible mechanism for multiple changing-look phenomena in active galactic nuclei". Astronomy & Astrophysics 641 (settembre 2020): A167. http://dx.doi.org/10.1051/0004-6361/202038575.

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Context. The changing-look phenomenon observed in a growing number of active galaxies challenges our understanding of the accretion process close to a black hole. Aims. We propose a simple explanation for the sources where multiple semi-periodic outbursts are observed, and the sources are operating close to the Eddington limit. Methods. The outburst are caused by the radiation pressure instability operating in the narrow ring between the standard gas-dominated outer disk and the hot optically thin inner advection-dominated accretion flow. The corresponding limit cycle is responsible for periodic outbursts, and the timescales are much shorter than the standard viscous timescale due to the narrowness of the unstable radial zone. Results. Our toy model gives quantitative predictions and works well for multiple outbursts like those observed in NGC 1566, NGC 4151, NGC 5548, and GSN 069, although the shapes of the outbursts are not yet well modeled, and further development of the model is necessary.

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DONMEZ, ORHAN. "QUASI-PERIODIC OSCILLATIONS AND FREQUENCIES IN AN ACCRETION DISK AND COMPARISON WITH THE NUMERICAL RESULTS FROM NON-ROTATING BLACK HOLE COMPUTED BY THE GRH CODE". Modern Physics Letters A 22, n. 02 (20 gennaio 2007): 141–57. http://dx.doi.org/10.1142/s021773230702138x.

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The shocked wave created on the accretion disk after different physical phenomena (accretion flows with pressure gradients, star-disk interaction etc.) may be responsible observed Quasi Periodic Oscillations (QPOs) in X-ray binaries. We present the set of characteristics frequencies associated with accretion disk around the rotating and non-rotating black holes for one particle case. These persistent frequencies are results of the rotating pattern in an accretion disk. We compare the frequency's from two different numerical results for fluid flow around the non-rotating black hole with one particle case. The numerical results are taken from Refs. 1 and 2 using fully general relativistic hydrodynamical code with non-selfgravitating disk. While the first numerical result has a relativistic tori around the black hole, the second one includes one-armed spiral shock wave produced from star-disk interaction. Some physical modes presented in the QPOs can be excited in numerical simulation of relativistic tori and spiral waves on the accretion disk. The results of these different dynamical structures on the accretion disk responsible for QPOs are discussed in detail.

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Chamandy, Luke, Adam Frank, Eric G. Blackman, Jonathan Carroll-Nellenback, Baowei Liu, Yisheng Tu, Jason Nordhaus, Zhuo Chen e Bo Peng. "Accretion in common envelope evolution". Proceedings of the International Astronomical Union 14, S343 (agosto 2018): 235–38. http://dx.doi.org/10.1017/s174392131800666x.

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AbstractCommon envelope evolution (CEE) occurs in some binary systems involving asymptotic giant branch (AGB) or red giant branch (RGB) stars, and understanding this process is crucial for understanding the origins of various transient phenomena. CEE has been shown to be highly asymmetrical and global 3D simulations are needed to help understand the dynamics. We perform and analyze hydrodynamic CEE simulations with the adaptive mesh refinement (AMR) code AstroBEAR, and focus on the role of accretion onto the companion star. We bracket the range of accretion rates by comparing a model that removes mass and pressure using a subgrid accretion prescription with one that does not. Provided a pressure-release valve, such as a bipolar jet, is available, super-Eddington accretion could be common. Finally, we summarize new results pertaining to the energy budget, and discuss the overall implications relating to the feasibility of unbinding the envelope in CEE simulations.

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MATSUMIYA, Hisato, Takashi NISHIHARA, Mikio SHIMIZU, Teruo ASO e Teruhiro YUKINO. "Galloping Phenomena of Overhead Transmission Lines Under Snow and Ice Accretion". Wind Engineers, JAWE 37, n. 1 (2012): 27–33. http://dx.doi.org/10.5359/jawe.37.27.

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Meier, D. L., P. Godon, S. Edgington, D. G. Payne e K. R. Lind. "11.18. The magnetic switch and the FR I/FR II break". Symposium - International Astronomical Union 184 (1998): 491–92. http://dx.doi.org/10.1017/s0074180900085727.

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Elbakyan, Vardan G., Sergei Nayakshin, Eduard I. Vorobyov, Alessio Caratti o Garatti e Jochen Eislöffel. "Accretion bursts in high-mass protostars: A new test bed for models of episodic accretion". Astronomy & Astrophysics 651 (luglio 2021): L3. http://dx.doi.org/10.1051/0004-6361/202140871.

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Aims. It is well known that low-mass young stellar objects (LMYSOs) gain a significant portion of their final mass through episodes of very rapid accretion, with mass accretion rates up to Ṁ∗ ∼ 10−4 M⊙ yr−1. Recent observations of high-mass young stellar objects (HMYSOs) with masses M∗ ≳ 10 M⊙ uncovered outbursts with accretion rates exceeding Ṁ∗ ∼ 10−3 M⊙ yr−1. Here, we examine which scenarios proposed in the literature so far to explain accretion bursts of LMYSOs can also apply to the episodic accretion in HMYSOs. Methods. We utilise 1D time-dependent models of protoplanetary discs around HMYSOs to study burst properties. Results. We find that discs around HMYSOs are much hotter than those around their low-mass cousins. As a result, a much more extended region of the disc is prone to the thermal hydrogen ionisation and magnetorotational activation instabilities. The former, in particular, is found to be ubiquitous in a very wide range of accretion rates and disc viscosity parameters. The outbursts triggered by these instabilities, however, always have too low of an Ṁ∗ and are one to several orders of magnitude too long compared to those observed from HMYSOs to date. On the other hand, bursts generated by tidal disruptions of gaseous giant planets formed by the gravitational instability of the protoplanetary discs yield properties commensurate with observations, provided that the clumps are in the post-collapse configuration with planet radius Rp ≳ 10 Jupiter radii. Furthermore, if observed bursts are caused by disc ionisation instabilities, then they should be periodic phenomena with the duration of the quiescent phase comparable to that of the bursts. This may yield potentially observable burst periodicity signatures in the jets, the outer disc, or the surrounding diffuse material of massive HMYSOs. Bursts produced by disruptions of planets or more massive objects are not expected to be periodic phenomena, although multiple bursts per protostar are possible. Conclusions. Observations and modelling of episodic accretion bursts across a wide range of young stellar object (YSO) masses is a new promising avenue to break the degeneracy between models of episodic accretion in YSOs.

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Al Farrizi, Abdurrahman, Ankiq Taofiqurohman e Subiyanto Subiyanto. "Shoreline Change Analysis of Pontang Cape of Serang Regency of Banten Province". Omni-Akuatika 16, n. 2 (31 dicembre 2020): 90. http://dx.doi.org/10.20884/1.oa.2020.16.2.800.

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Coastal areas, being vulnerable to environmental problems, have one of the most frequent problems which are the change in the shorelines. Shoreline changes, namely abrasions, can cause problems such as land degradations or loss of land in a coastal zone. This problem occurs in many areas, one of which is Pontang Cape. This study aims to determine the distance and rate of shoreline changes that occured in the Cape and its surroundings, as well as explaining the analysis points based on similar studies that had been conducted. This research used ArcMap software and Digital Shoreline Analysis System (DSAS) toolset to determine the distance and rate of shoreline changes for 19 years (1999-2018). Based on the results, there were two shoreline segments where different phenomena of shoreline change took place, namely Banten Bay (accretion) and Pontang Cape-Lontar (abrasion). The most likely causes of changes in the shorelines are sediment runoffs from rivers that lead to bay and sediment transports that affect Banten Bay accretions, while sea sand mining and conversions of mangrove swamps into fishery ponds are factors affecting abrasions in Pontang Cape.Keywords: Abrasion, Accretion, Pontang Cape, Banten Bay, DSAS

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Stuchlik, Zdenek, e Jan Schee. "ON OBSERVATIONAL PHENOMENA RELATED TO KERR SUPERSPINARS". Acta Polytechnica 54, n. 3 (27 giugno 2014): 254–58. http://dx.doi.org/10.14311/ap.2014.54.0254.

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We investigate possible signatures of a Kerr naked singularity (superspinar) in various observational phenomena. It has been shown that Kerr naked singularities (superspinars) have to be efficiently converted to a black hole due to accretion from Keplerian discs. In the final stages of the conversion process the near-extreme Kerr naked singularities (superspinars) provide a variety of extraordinary physical phenomena. Such superspinning Kerr geometries can serve as an efficient accelerator for extremely high-energy collisions, enabling a direct and clear demonstration of the outcomes of the collision processes. We shall discuss the efficiency and the visibility of the ultra-highenergy collisions in the deepest parts of the gravitational well of superspinning near-extreme Kerr geometries for the whole variety of particles freely falling from infinity. We demonstrate that ultrahigh-energy processes can be obtained with no fine tuning of the motion constants and the products of<br />the collision can escape to infinity with efficiency substantially higher than in the case of near-extreme black holes. Such phenomena influence the radiative processes taking place in the accretion disc, and together with the particular generated geometry they influence the observed radiation field. Here we<br />assume the “geometrical” influence of a Kerr naked singularity on the spectral line profiles of radiation emitted by monochromatically and isotropically radiating point sources forming a Keplerian ring or disc around such a compact object. We have found that the profiled spectral line of the radiating<br />Keplerian ring can be split into two parts because there is no event horizon in the naked singularity spacetimes. The profiled lines generated by Keplerian discs are qualitatively different for a Kerr naked singularity and black hole spacetimes broadened near the inner edge of a Keplerian disc.

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Orlando, Salvatore, Fabio Reale, Giovanni Peres e Andrea Mignone. "Mass Accretion Processes in Young Stellar Objects: Role of Intense Flaring Activity". Acta Polytechnica CTU Proceedings 1, n. 1 (4 dicembre 2014): 108–12. http://dx.doi.org/10.14311/app.2014.01.0108.

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According to the magnetospheric accretion scenario, young low-mass stars are surrounded by circumstellar disks which they interact with through accretion of mass. The accretion builds up the star to its nal mass and is also believed to power the mass out ows, which may in turn have a signicant role in removing the excess angular momentum from the star-disk system. Although the process of mass accretion is a critical aspect of star formation, some of its mechanisms are still to be fully understood. On the other hand, strong aring activity is a common feature of young stellar objects (YSOs). In the Sun, such events give rise to perturbations of the interplanetary medium. Similar but more energetic phenomena occur in YSOs and may in uence the circumstellar environment. In fact, a recent study has shown that an intense flaring activity close to the disk may strongly perturb the stability of circumstellar disks, thus inducing mass accretion episodes (Orlando et al. 2011). Here we review the main results obtained in the eld and the future perspectives.

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Uchiyama, Mizuho, Kohei Ichikawa, Koichiro Sugiyama, Yoshihiro Tanabe e Yoshinori Yonekura. "Mid-infrared and Maser Flux Variability Correlation in Massive Young Stellar Object G036.70+00.09". Astrophysical Journal 936, n. 1 (29 agosto 2022): 31. http://dx.doi.org/10.3847/1538-4357/ac866e.

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Abstract We present the discovery of the simultaneous flux variation of a massive young stellar object (MYSO) G036.70+00.09 (G036.70) both in the maser emission and mid-infrared (MIR; λ = 3–5 μm) bands. Using the ALLWISE and NEOWISE archival databases that cover a long time span of approximately 10 yr with a cadence of 6 months, we confirm that G036.70 indicates a stochastic year-long MIR variability with no signs of a WISE band color change of W1 (3.4 μm) −W2 (4.6 μm). Cross-matching the MIR data set with the high-cadence 6.7 GHz class II methanol maser flux using the Hitachi 32 m radio telescope that discovered its periodicity in the methanol maser of 53.0–53.2 days, we also determine the flux correlations between the two bands at two different timescales, year-long and day-long, both of which have never been reported in MYSOs, except when they are in the accretion burst phase. The results of our study support the scenario that a class II methanol maser is pumped up by infrared emission from accreting disks of MYSOs. We also discuss the possible origins of MIR and maser variability. To explain the two observed phenomena, a stochastic year-long MIR variability with no signs of significant color change and maser-MIR variability correlation or a change in mass accretion rate and line-of-sight extinction because of the nonaxisymmetric dust density distribution in a rotating accretion disk are possible origins. Observations through spectroscopic monitoring of accretion-related emission lines are essential for determining the origin of the observed variability in G036.70.

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Beuermann, K., e U. Woelk. "Radiation Hydrodynamics of Accreting Magnetic White Dwarfs". International Astronomical Union Colloquium 158 (1996): 199–202. http://dx.doi.org/10.1017/s0252921100038616.

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AbstractWe solved the stationary one-dimensional two-fluid radiation hydrodynamic equations including cyclotron radiation for a wide range of mass flow rates. Here, we discuss the implications for accretion phenomena on the white dwarfs in AM Her binaries.

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Gouveia Dal Pino, E. M. de. "Relativistic jets and accretion phenomena associated with galactic and extragalactic black holes". Brazilian Journal of Physics 35, n. 4b (dicembre 2005): 1163–66. http://dx.doi.org/10.1590/s0103-97332005000700043.

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Matsuura, Taiki, Masaya Suzuki, Makoto Yamamoto, Shinichiro Shishido, Takeshi Murooka e Hiroshi Miyagawa. "Numerical simulation of ice accretion phenomena on rotor blade of axial blower". Journal of Thermal Science 21, n. 4 (27 luglio 2012): 322–26. http://dx.doi.org/10.1007/s11630-012-0550-5.

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Linares, Manuel, Barbara De Marco, Rudy Wijnands e Michiel van der Klis. "X-ray variability of transitional millisecond pulsars: a faint, stable, and fluctuating disc". Monthly Notices of the Royal Astronomical Society 512, n. 4 (31 marzo 2022): 5269–77. http://dx.doi.org/10.1093/mnras/stac720.

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ABSTRACT Transitional millisecond pulsars (tMSPs) have emerged in the last decade as a unique class of neutron stars at the crossroads between accretion- and rotation-powered phenomena. In their (sub-luminous) accretion disc state, with X-ray luminosities of order 1033–1034 erg s−1, they switch rapidly between two distinct X-ray modes: the disc-high (DH) and disc-low (DL) states. We present a systematic XMM–Newton and Chandra analysis of the aperiodic X-ray variability of all three currently known tMSPs, with a main focus on their disc state and separating DH and DL modes. We report the discovery of flat-topped broad-band noise in the DH state of two of them, with break frequencies of 2.8 mHz (PSR J1023 + 0038) and 0.86 mHz (M28-I). We argue that the lowest frequency variability is similar to that seen in disc-accreting X-ray binaries in the hard state, at typical luminosities at least two orders of magnitude higher than tMSPs. We find strong variability in the DH state around 1 Hz, not typical of hard state X-ray binaries, with fractional rms amplitudes close to 30 per cent. We discuss our results and use them to constrain the properties of the accretion disc, assuming that the X-ray variability is produced by fluctuations in mass accretion rate, and that the break frequency corresponds to the viscous time-scale at the inner edge of the disc. In this context, we find that the newly found break frequencies are broadly consistent with a disc truncated close to the light cylinder with $\dot{M}\simeq 10^{13}-5\times 10^{14}$ g s−1 and a viscosity parameter α ≳ 0.2.

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Thé, P. S. "The Properties of Accretion Disks of Herbig AE/BE Stars and Their Influence on the Surface and the Inside". International Astronomical Union Colloquium 137 (1993): 440–44. http://dx.doi.org/10.1017/s0252921100018194.

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AbstractIn this paper we review the observations which lead to the believe that intermediate mass pre-main sequence Herbig Ae/Be stars are surrounded by accretion disks. A discussion is given of their properties and their contributions to surface and inside phenomena of the central stars.

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Malik, Arshid Hussain, e Randhi Uma Devi. "Analysis of shoreline changes in the severe storm surge region of upper Coromandel coast, India, using Remote sensing (RS) and Geographic information system (GIS)". Journal of Applied and Natural Science 15, n. 4 (20 dicembre 2023): 1453–66. http://dx.doi.org/10.31018/jans.v15i4.5074.

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Coastlines have been subject to ongoing vulnerability from natural disasters, resulting in erosion and accretion occurring at varying intervals. The impacts of human activities and alterations in natural processes, including those induced by climate change, exacerbate the existing challenges faced along coastal regions. The present study aimed to analyze the changing shoreline along the Bapatla coastal tract in the upper coromandel coast of Andhra Pradesh due to erosion and accretion using the DSAS toolbar in ArcGIS software. Multiple statistical methods are available in DSAS; the present study confined attention to the end point rate (EPR), linear regression rate (LRR), and net shoreline movement (NSM) for this investigation. The Nizampatnam Mandal had the greatest erosion rate (-16.8 m/yr) and accretion rate (10.5 m/yr), according to EPR, as well as the highest erosion rate (-16.7 m/yr) and accretion rate (10.4 m/yr) according to LRR. As per NSM, between 1990 and 2020, the highest erosion and accretion was seen again in Nizampatnam. The shoreline at Nizampatnam either retreated by as much as -285.2 meters or advanced by 211.1 meters. The research indicated that the Bapatla district's shoreline is susceptible to erosion and accretion. The present investigation revealed that the coastal region of Bapatla district exhibits susceptibility to both erosion and accretion phenomena. The findings of the present study hold significant implications for professionals in the fields of coastal science, engineering, and administration. The findings are of utmost importance in the development of sustainable approaches for coastal management.

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Abarzhi, S. I., e K. R. Sreenivasan. "Turbulent mixing and beyond". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 368, n. 1916 (13 aprile 2010): 1539–46. http://dx.doi.org/10.1098/rsta.2010.0021.

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Turbulence is a supermixer. Turbulent mixing has immense consequences for physical phenomena spanning astrophysical to atomistic scales under both high- and low-energy-density conditions. It influences thermonuclear fusion in inertial and magnetic confinement systems; governs dynamics of supernovae, accretion disks and explosions; dominates stellar convection, planetary interiors and mantle-lithosphere tectonics; affects premixed and non-premixed combustion; controls standard turbulent flows (wall-bounded and free—subsonic, supersonic as well as hypersonic); as well as atmospheric and oceanic phenomena (which themselves have important effects on climate). In most of these circumstances, the mixing phenomena are driven by non-equilibrium dynamics. While each article in this collection dwells on a specific problem, the purpose here is to seek a few unified themes amongst diverse phenomena.

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