arXiv subjects

Sergei Nayakshin

Publications and source records attributed to Sergei Nayakshin.

At least 109 records · Page 6Linked to original sources

arXiv2006

Constraining the number of compact remnants near Sgr A*

Due to dynamical friction stellar mass black holes and neutron stars are expected to form high density cusps in the inner parsec of our Galaxy. These compact remnants, expected to number around 20000, may be accreting cold dense gas present there, and give rise to potentially observable X-ray emission. Here we build a simple but detailed time-dependent model of such emission. We find that at least several X-ray sources of this nature should be detectable with Chandra at any one time. Turning this issue around, we also ask a question of what current observational constraints might be telling us about the total number of compact remnants. A cusp with ~ 40 thousand black holes over-predicts the number of discrete sources and the total X-ray luminosity of the inner parsec, and is hence ruled out. Future observations of the distribution and orbits of the cold ionised gas in the inner parsec of Sgr A* will put tighter constraints on the cusp of compact remnants.

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arXiv2006

X-rays from cusps of compact remnants near galactic centres

Compact remnants -- stellar mass black holes and neutron stars formed in the inner few parsec of galactic centres are predicted to sink into the central parsec due to dynamical friction on low mass stars, forming a high concentration cusp (Morris 1993). Same physical region may also contain very high density molecular clouds and accretion discs that are needed to fuel SMBH activity. Here we estimate gas capture rates onto the cusp of stellar remnants, and the resulting X-ray luminosity, as a function of the accretion disc mass. At low disc masses, most compact objects are too dim to be observable, whereas in the high disc case most of them are accreting at their Eddington rates. We find that for low accretion disc masses, compact remnant cusps may be more luminous than the central SMBHs. This ``diffuse'' emission may be of importance for local moderately bright AGN, especially Low Luminosity AGN. We also briefly discuss how this expected emission can be used to put constraints on the black hole cusp near our Galactic Centre.

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arXiv2006

Anisotropic X-ray emission in AGN accretion discs

Straight-forward models of X-ray reflection in the inner region of accretion discs predict that primary X-ray flux and the flux reflected off the surface of the disc should vary together, albeit a short light travel time delay. Most of the observations, however, show that the X-ray flux can vary while the reflected features remain constant. Here we propose a simple explanation to this. In all likelihood, the emission of a moderately optically thick magnetic flare atop an accretion disc is anisotropic. A constant energy release rate in a flare will appear to produce a variable X-ray flux as the flare rotates with the accretion disc anchoring the magnetic tube. The reflector, on the other hand, receives a constant X-ray flux from the flare. Since the reflected emission is azimuthally symmetric, the observer will see a roughly constant reflected flux (neglecting relativistic effects). The model does not produce quasi-periodic oscillations (QPO) if magnetic flux tubes are sheared out faster than they complete one orbit.

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arXiv2006

The M-sigma Relation for Nucleated Galaxies

Momentum feedback from super-Eddington accretion offers a simple explanation for the observed M-sigma and M-M_{spheroid} relations between supermassive black holes (SMBH) and the spheroids of their host galaxies. Recently Ferrarese et al. and Wehner & Harris observed analogous relations between the masses of central star clusters and their hosts. We show that stellar winds and supernovae from such nuclear clusters (NC) give similar feedback explanations for this case also, and we discuss the connection to the Faber-Jackson relation for the spheroids themselves.

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arXiv2006

Clumpy winds and the obscuration of Active Galactic Nuclei

The role of star-formation driven outflows in the obscuration of the central source in the Active Galactic Nuclei (AGN) is discussed. The outflow from a sub-parsec scale accretion disc is numerically modelled for parameters appropriate to the Galactic Centre. The resulting obscuration pattern is very patchy, with some lines of sight becoming optically thick to Thomson scattering. A fixed observer would see column depth changing by factors of many over time scales of order months to hundreds of years, depending on the physical size of the outflow region. Such winds may be relevant for obscuration of some AGN and especially "changing look AGN". However, averaged over the sky as seen from the central source, these winds are always optically thin unless wind outflow rates are super-Eddington. A simple scaling argument shows that this is true not only for stellar-driven winds but for any AGN winds. We therefore conclude that AGN winds are unable to account for the vast majority of optically thick obscured AGN (a significant fraction of all AGN). We suggest that the most likely source of optically thick obscuration in AGN is a warped parsec scale accretion disc.

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arXiv2005

Massive stars in sub-parsec rings around galactic centers

We consider the structure of self-gravitating marginally stable accretion disks in galactic centers in which a small fraction of the disk mass has been converted into proto-stars. We find that proto-stars accrete gaseous disk matter at prodigious rates. Mainly due to the stellar accretion luminosity, the disk heats up and geometrically thickens, shutting off further disk fragmentation. The existing proto-stars however continue to gain mass by gas accretion. As a results, the initial mass function for disk-born stars at distances R ~ 0.03-3 parsec from the super-massive black hole should be top-heavy. The effect is most pronounced at around R ~ 0.1 parsec. We suggest that this result explains observations of rings of young massive stars in our Galaxy and in M31, and predict that more of such rings will be discovered.

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arXiv2005

Weighing the young stellar discs around Sgr A*

It is believed that young massive stars orbiting Sgr A* in two stellar discs on scales of 0.1-0.2 parsecs were formed either farther out in the Galaxy and then quickly migrated inward, or in situ in a massive self-gravitating disc. Comparing N-body evolution of stellar orbits with observational constraints, we set upper limits on the masses of the two stellar systems. These masses turn out to be few times lower than the expected total stellar mass estimated from the observed young high-mass stellar population and the standard galactic IMF. If these stars were formed in situ, in a massive self-gravitating disc, our results suggest that the formation of low-mass stars was suppressed by a factor of at least a few, requiring a top-heavy initial mass function (IMF) for stars formed near sgr A*.

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arXiv2005

The "missing" YSOs in the central parsec of the Galaxy: evidence for star formation in a massive accretion disk and a top-heavy IMF

Few dozens of young high mass stars orbit Sgr A* at distances as short as 0.1 parsec, where star formation should be quenched by the strong tidal shear from Sgr A*. The puzzling young stellar population is believed to come into existence in one of the two ways: (i) "normal" star formation at several tens of parsec in a very massive star cluster that then spiraled in, or (ii) star formation in situ in a massive self-gravitating disk. We propose to constrain these two scenarios via the expected X-ray emission from young low mass stars that should have formed alongside the massive stars. To this end we compare the recent Chandra observations of X-ray emission from young stars in the Orion Nebula, and the Chandra observations of Sgr A* field. We show that the cluster inspiral model is ruled out irrespectively of the initial mass function (IMF) of the young stars. In addition, for the in situ model, we find that no more than few thousand low-mass stars could have formed alongside the massive stars. This is more than a factor of ten fewer than expected if these stars were formed with the standard IMF as elsewhere in the Galaxy. The young stars in the GC are thus the first solid observational evidence for star formation in AGN disks and also require the IMF of these stars to be top-heavy. We briefly consider implication of these results for AGN in general.

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arXiv2004

Warped accretion disks and the unification of Active Galactic Nuclei

Orientation of parsec-scale accretion disks in AGN is likely to be nearly random for different black hole feeding episodes. Since AGN accretion disks are unstable to self-gravity on parsec scales, star formation in these disks will create young stellar disks, similar to those recently discovered in our Galactic Center. The disks blend into the quasi-spherical star cluster enveloping the AGN on time scales much longer than a likely AGN lifetime. Therefore, the gravitational potential within the radius of the black hole influence is at best axi-symmetric rather than spherically symmetric. Here we show that as a result, a newly formed accretion disk will be warped. For the simplest case of a potential resulting from a thin stellar ring, we calculate the disk precession rates, and the time dependent shape. We find that, for a realistic parameter range, the disk becomes strongly warped in few hundred orbital times. We suggest that this, and possibly other mechanisms of accretion disk warping, have a direct relevance to the problem of AGN obscuration, masing warped accretion disks, narrow Fe K-alpha lines, etc.

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arXiv2004

Using close stars as probes of hot accretion flow in Sgr A*

The extremely hot and tenuous accretion flow in the immediate vicinity of Sgr A* is believed to be invisible (too dim) in the X-ray band, except for short X-ray flares. Here we point out that during pericenter passages, close brightest stars irradiate the inner region of the accretion flow, providing a plenty of optical/UV photons. These seed photons are Compton up-scattered by the hot electrons of the accretion flow to higher frequencies, some into the X-ray band, potentially making the innermost accretion flow much brighter in X-rays than usual. We propose to use coordinated near infra-red and X-ray observations of close star passages to put constraints onto Sgr A* accretion theories. The absence of a noticeable change in the steady emission of Sgr A* as observed by Chandra in the year 2002, when the star named S2 passed through a pericenter of its orbit, already rules out the hotter of the ``standard'' Advection-Dominated Accretion Flows. The less dense accretion flows, in particular the model of Yuan et al. (2003), passes the test and is constrained to accretion rates no larger than $\sim 10^{-7}$ Solar masses per year.

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arXiv2004

Self-gravitating accretion disk in Sgr A* few million years ago: was Sgr A* a failed quasar?

Sgr A* is extra-ordinarily dim in all wavelengths requiring a very low accretion rate at the present time. However, at a radial distance of a fraction of a parsec from Sgr A*, two rings populated by young massive stars suggest a recent burst of star formation in a rather hostile environment. Here we explore two ways of creating such young stellar rings with a gaseous accretion disk: by self-gravity in a massive disk, and by capturing "old" low mass stars and growing them via gas accretion in a disk. The minimum disk mass is above 10^4 Msun for the first mechanism and is few tens times larger for the second one. The observed relatively small velocity dispersion of the stars rules out disks more massive than around 10^5 Msun: heavier stellar or gas disks would warp each other by orbital precession in an axisymmetric potential too strongly. The capture of "old" stars by a disk is thus unlikely as the origin of the young stellar disks. The absence of a massive nuclear gas disk in Sgr A* now implies that the disk was either accreted by the SMBH, which would then imply almost a quasar-like luminosity for Sgr A*, or was consumed in the star formation episode. The latter possibility appears to be more likely on theoretical grounds. We also consider whether accretion disk plane changes, expected to occur due to fluctuations in the angular momentum of gas infalling into the central parsec of a galaxy, would dislodge the embedded stars from the disk midplane. We find that the stars leave the disk midplane only if the disk orientation changes on time scales much shorter than the disk viscous time.

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arXiv2004

Close stars and accretion in Low Luminosity Active Galactic Nuclei

Quasar accretion disks are believed to form stars by self-gravity. Low Luminosity Active Galactic Nuclei (LLAGN) are much dimmer galactic centers, and are often believed to be quasars that ran out of gaseous fuel. LLAGN accretion disks should thus co-exist with thousands to millions of stars or proto-stars left from the previous stronger accretion activity. In principle, these stars may produce several important effects: (i) contribute to the optical/UV spectra of some LLAGN; (ii) reprocessing of the stellar radiation in the dusty disks could dominate the LLAGN infra-red spectra; (iii) deplete the (accretion) gas disk much faster than it can accrete onto the supper-massive black hole (SMBH); (iv) stars, individually or in groups, may slow down and modulate the accretion flow significantly due to their inertia. In this way they may produce the LLAGN cut-off disks; (v) alternatively, frequent enough stellar collisions and resulting stellar disruptions could keep the inner disk empty. Here we explore these ideas. We find that, despite ``low'' luminosities of LLAGN, unrealistically high stellar densities are required to make a sizable radiative contribution to the (HST) optical/UV spectra of these galactic nuclei. Stellar contribution to the infrared spectrum is more likely. Further, if LLAGN are in a quasi steady-state for as long as 10^7 years or more, too high stellar densities would again be required to significantly affect the dynamics of accretion flow. However, if LLAGN are ``short''-lived phenomena, e.g. t < 10^5 years, quiescent states of quiescence-outburst cycles, then embedded stars may be much more important through the mass effects (iii) -- (v). With observations of LLAGN becoming progressively better, it will be more and more difficult to neglect the presence of close stars in and around nuclear accretion disks.

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arXiv2004

Feeding the black hole with condensing accretion flows: radiatively efficient and radiatively inefficient cases

We study the accretion flow of a hot gas captured by the black hole gravity in the presence of a thin cold accretion disk. Such geometrical arrangement is expected in Active Galactic Nuclei (AGN) and in galactic X-ray binary systems because both hot and cold gases are present in the black hole vicinity. Previous astrophysical literature concentrated on the evaporation of the cold disk in the classical heat conduction limit. Here we consider the inverse process, i.e. condensation of the hot gas onto the cold disk. We find two distinct condensation regimes. (i) In the classical thermal conduction limit, the radiative cooling in the hot gas itself force condensation above a certain critical accretion rate. Most of the flow energy in this case is re-emitted as X-ray radiation. (ii) Below a certain minimum accretion rate, the hot electrons are collisionless and the classical heat flux description becomes invalid. We use the ``non-local'' heat flux approach borrowed from the terrestrial laser heated plasma experiments. Due to their very large mean free path, the hot particles penetrate deep into the cold disk where the radiative losses are significant enough to enable condensation. In this case the hot flow energy is inconspicuously re-radiated by the transition layer in many UV and especially optical recombination lines (e.g., Ly$α$, $Hα$, H$β$) as well as via the optically thick disk emission. We derive an approximate analytical solution for the dynamics of the hot condensing flow. If the cold disk is inactive, i.e. accumulating mass for a future accretion outburst, then the two-phase flows appear radiatively inefficient. These condensing solutions may be relevant to \sgra, low luminosity AGN, and transient binary accreting systems in quiescence.

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arXiv2003

Bright stars and an inactive disk in Sgr A* and other dormant galaxy centers. I. The optically thick disk

Cold inactive disks are believed to exist in Low Luminosity AGN (LLAGN). They may also exist in the nuclei of inactive galaxies and in the center of our own Galaxy. These disks would then be embedded in the observed dense nuclear stellar clusters. Making the simplest assumption of an optically thick disk, we explore several ways to detect the disk presence through its interaction with the cluster. First of these is the eclipses of close bright stars by the disk. The second is the increase in the infrared flux of the disk due to illumination of its surface by such stars during close passages. Finally the surface brightness of the star cluster should show an anisotropy that depends on the inclination angle of the disk. We apply the first two of the methods to Sgr A*, the super massive black hole in our Galactic Center. Using the orbital parameters of the close star S2, we strongly rule out a disk optically thick in the near infrared unless it has a relatively large inner hole. For disks with no inner holes, we estimate that the data permit a disk with infrared optical depth no larger than about 0.01. Such a disk could also be responsible for the detected 3.8 microns excess in the spectrum of S2. The constraints on the disk that we obtain here can be reconciled with the disk parameters needed to explain the observed X-ray flares if dust particles in the disk have sizes greater than ~ 30 microns. The destruction of small dust particles by strong UV heating and shocks from star passages through the disk, and grain growth during "quiescent" times, are mentioned as possible mechanisms of creating the unusual grain size distribution. (...)

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arXiv2003

X-ray flares from Sgr A*: star-disk interactions?

Sgr A*, the putative black hole in our Galactic Center (GC), is extraordinary dim in all frequencies. Apparently the black hole is unable to accrete at the Bondi accretion rate for some reason. Another mystery of Sgr A* is the recently discovered large magnitude and short duration X-ray flares (Baganoff et al. 2001). Here we point out that X-ray flares should be expected from star passages through an inactive (i.e. formerly accreting) disk. There are thousands of stars in Sgr A* stellar cluster, and each star will pass through the disk twice per orbit. A shock hot enough to emit X-rays is formed around the star when the latter is inside the disk. We develop a simple yet somewhat detailed model for the X-ray emission from the shock. The duration of the flares, their X-ray spectra, frequency of events, and weakness of emission in the radio and near infra-red appear to be consistent with the available observations of X-ray flares. We therefore suggest that at least a fraction of the observed flares is due to the star-disk passages. Such star-disk flares are also of interest in the nuclei of nearby inactive galaxies, especially in connection with perspective Constellation-X and XEUS X-ray missions.

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arXiv2003

The frozen (inactive) disk in Sgr A: freezing the accretion of the hot gas too?

The black hole in our Galactic Center is extremely underluminous for the amount of hot gas available for accretion. Theoretical understanding of this fact rests on a likely but not entirely certain assumption that the electrons in the accreting gas are much cooler than the protons. In this case the hot gas as a whole is too hot to accrete, and is too tenous to radiate away its gravitational energy. Here we propose a drastically different picture of the accretion process in Sgr A* not based on the unchecked two-temperature assumption. Namely, we argue that there should exist a very cold inactive disk -- a remnant of a past stronger accretion activity in Sgr A*. Such a disk would be a very efficient cooling surface for the hot flow. We show that under certain conditions the cooling due to thermal conduction cannot be balanced by the viscous heating in the hot flow. Along with the heat, the hot flow loses its viscosity and thus ability to accrete. It settles (condences) onto the cold disk slighlty inside of the circularization radius. If the latter is very large, then the liberated energy, and the luminosity emitted, is orders of magnitude less than naively expected. We build a simple analytical model for this flow and calculate the expected spectra that appear to be in a very reasonble agreement with observations. Strong additional support for the presence of the inactive disk comes from the recent observations of X-ray flares in Sgr A*. The properties of these flares are very similar to those produced by stars passing through a cold disk.

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arXiv2003

Close stars and an inactive accretion disk in Sgr A*: Eclipses and flares

A cold neutral and extremely dim accretion disk may be present as a remnant of a past vigorous activity around the black hole in our Galactic Center (GC). Here we discuss ways to detect such a disk through its interaction with numerous stars present in the central ~0.1 parsec of the Galaxy. The first major effect expected is X-ray and near infrared (NIR) flares arising when stars pass through the disk. The second is eclipses of the stars by the disk. We point out conditions under which the properties of the expected X-ray flares are similar to those recently discovered by Chandra. Since orbits of bright stars are now being precisely measured, the combination of the expected flares and eclipses offers an invaluable tool for constraining the disk density, size, plane and even direction of rotation. The winds of the O-type stars are optically thick to free-free absorption in radio frequencies. If present near Sgr A* core, such powerful stellar winds can modulate and even occult the radio source.

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arXiv2002

On the accretion geometry of Cyg X-1 in the low/hard state

We fit the broad-band RXTE PCA and HEXTE spectrum from 3--200 keV with reflection models which calculate the vertical ionization structure of an X-ray illuminated disc. We consider two geometries corresponding to a truncated disc/inner hot flow and magnetic flares above an untruncated disc. Both models are able to fit the PCA 3--20 keV data, but with very different spectral components. In the magnetic flare models the 3--20 keV PCA spectrum contains a large amount of highly ionized reflection while in the truncated disc models the amount of reflection is rather small. The Compton downscattering rollover in reflected emission means that the magnetic flare models predict a break in the spectrum at the high energies covered by the HEXTE bandpass which is not seen. By contrast the weakly illuminated truncated disc models can easily fit the 3--200 keV spectra.

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