Simulations of momentum feedback by black hole winds (2009)
Nayakshin, Sergei, Power, Chris
The observed super-massive black hole (SMBH) mass -- galaxy velocity dispersion ($M_{\rm bh} - \sigma$) correlation may be established when winds/outflows from the SMBH drive gas out of the potential...
Competitive feedback in galaxy formation (2009)
Nayakshin, Sergei, Wilkinson, Mark I., King, Andrew
It is now well established that many galaxies have nuclear star clusters (NCs) whose total masses correlate with the velocity dispersion (sigma) of the galaxy spheroid in a very similar way to the...
Dynamic Monte Carlo radiation transfer in SPH. Radiation pressure force implementation (2009)
Nayakshin, Sergei, Cha, Seung-Hoon, Hobbs, Alexander
We present a new framework for radiation hydrodynamics simulations. Gas dynamics is modelled by the Smoothed Particle Hydrodynamics (SPH) method, whereas radiation transfer is simulated via a...
Star formation in the GC: observational constraints and theory (2008)
Abstract. We review some observational aspects and theoretical models for the formation of the young massive stars in the central parsec of our Galaxy. It is concluded that an in-situ star formation...
Hobbs, Alexander, Nayakshin, Sergei
Young massive stars in the central parsec of our Galaxy are best explained by star formation within at least one, and possibly two, massive self-gravitating gaseous discs. With help of numerical...
Nayakshin, Sergei, Hobbs, Alexander
Young massive stars in the central parsec of our Galaxy are best explained by star formation within at least one, and possibly two, massive self-gravitating gaseous discs. With help of numerical...
Forced accretion in stochastically fed AGN and quasars (2007)
Nayakshin, Sergei, King, Andrew
Steady state accretion discs larger than ~ 0.01-0.1 pc are known to be gravitationally unstable for the accretion rates needed to explain super-massive black hole (SMBH) activity. We propose that...
Variable accretion and emission from the stellar winds in the Galactic centre (2007)
Cuadra, Jorge, Nayakshin, Sergei, Martins, Fabrice
We present numerical simulations of stellar wind dynamics in the central parsec of the Galactic centre, studying in particular the accretion of gas on to Sgr A*, the super-massive black hole. Unlike...
Can the soft excess in AGN originate from disc reflection? (2007)
Done, Chris, Nayakshin, Sergei
The soft excess seen in many AGN is most probably due to partially ionized material moving at relativistic speeds close to the black hole. There are currently two potential geometries for this...
Simulations of star formation in a gaseous disc around sgr A* - a failed AGN (2007)
Nayakshin, Sergei, Cuadra, Jorge, Springel, Volker
We numerically model fragmentation of a gravitationally unstable gaseous disc under conditions that may be appropriate for the formation of the young massive stars observed in the central parsec of...
Stellar processes near AGN (2007)
Precise mechanisms by which Active Galactic Nuclei (AGN) receive their gaseous fuel is still a mystery. Here I draw attention to the extra ordinary star formation event that took place in the central...
News from the year 2006 Galactic Center workshop (2007)
Morris, Mark, Nayakshin, Sergei
We summarize some of the new results from contributions made to the Galactic Centre Workshop that took place in Bad Honnef, Germany, on April 18-22, 2006.
Constraining the number of compact remnants near Sgr A* (2006)
Deegan, Patrick, Nayakshin, Sergei
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...
X-rays from cusps of compact remnants near galactic centres (2006)
Nayakshin, Sergei, Sunyaev, Rashid
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...
Anisotropic X-ray emission in AGN accretion discs (2006)
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...
The M-sigma relation for nucleated galaxies (2006)
McLaughlin, Dean E., King, Andrew R., Nayakshin, Sergei
The definitive version of this paper is available at http://www.journals.uchicago.edu/ApJ/journal/issues/ApJL/v650n1/20872/20872.html
The M-sigma Relation for Nucleated Galaxies (2006)
McLaughlin, Dean E., King, Andrew R., Nayakshin, Sergei
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...
Clumpy winds and the obscuration of Active Galactic Nuclei (2006)
Nayakshin, Sergei, Cuadra, Jorge
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...
Galactic centre stellar winds and Sgr A* accretion (2006)
Cuadra, Jorge, Nayakshin, Sergei, Springel, Volker, Di Matteo, Tiziana
Weighing the young stellar discs around Sgr A* (2006)
Nayakshin, Sergei, Dehnen, Walter, Cuadra, Jorge, Genzel, Reinhard
Weighing the young stellar discs around Sgr A* (2006)
Nayakshin, Sergei, Dehnen, Walter, Cuadra, Jorge, Genzel, Reinhard
Massive stars in sub-parsec rings around galactic centers (2005)
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...
Weighing the young stellar discs around Sgr A* (2005)
Nayakshin, Sergei, Dehnen, Walter, Cuadra, Jorge, Genzel, Reinhard
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...
Nayakshin, Sergei, Sunyaev, Rashid
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...
Accretion of cool stellar winds on to Sgr A*: Another puzzle of the galactic centre? (2005)
Cuadra, Jorge, Nayakshin, Sergei, Springel, Volker, Di Matteo, Tiziana
Warped accretion disks and the unification of Active Galactic Nuclei (2004)
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,...
Using close stars as probes of hot accretion flow in Sgr A* (2004)
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...
Self-gravitating accretion disk in Sgr A* few million years ago: was Sgr A* a failed quasar? (2004)
Nayakshin, Sergei, Cuadra, Jorge
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...
Close stars and accretion in Low Luminosity Active Galactic Nuclei (2004)
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...
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...
Cuadra, Jorge, Nayakshin, Sergei, Sunyaev, Rashid
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...
X-ray flares from Sgr A*: star-disk interactions? (2003)
Nayakshin, Sergei, Cuadra, Jorge, Sunyaev, Rashid
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....
The frozen (inactive) disk in Sgr A: freezing the accretion of the hot gas too? (2003)
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...
Close stars and an inactive accretion disk in Sgr A*: Eclipses and flares (2003)
Nayakshin, Sergei, Sunyaev, Rashid
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...
On the accretion geometry of Cyg X-1 in the low/hard state (2002)
Barrio, F. Eugenio, Done, Chris, Nayakshin, Sergei
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...
Nayakshin, Sergei, Kazanas, Demos
We perform a first study of time-dependent X-ray reflection in photo-ionized accretion disks. We assume a step-functional change in the X-ray flux and use a simplified prescription to describe the...
An XMM-Newton observation of the luminous Seyfert 1 galaxy Markarian 509 (2001)
Pounds, Ken, Reeves, James, O'Brien, Paul, Page, Kim, Turner, Martin, Nayakshin, Sergei
We present the spectral analysis of an early XMM-Newton observation of the luminous Seyfert 1 galaxy Markarian 509. We find the hard (2-10 keV) continuum slope, including reflection, to be somewhat...
Narrow Moving Fe K-alpha lines from magnetic flares in AGN (2001)
Nayakshin, Sergei, Kazanas, Demosthenes
We point out that luminous magnetic flares, thought to occur in standard AGN accretion disks, cannot be located much higher than few pressure scale heights above the disk. Using this fact, we...
Modeling the X-ray - UV Correlations in NGC 3516 (2000)
Kazanas, Demos, Nayakshin, Sergei
We test the "reprocessing paradigm" of the optical -- UV AGN variability by detailed modeling of the correlated X-ray -- optical (3590 and 5510 Angstrem) variability of the recent multiwavelength...
On the "failure" of the standard model for Soft X-ray Transients in quiescence (2000)
Nayakshin, Sergei, Svensson, Roland
It is currently believed that the ``standard'' accretion disk theory under-predicts the observed X-ray luminosity from Soft X-ray Transients (SXT) in quiescence by as much as 4 to 6 orders of...
Accretion disk models and their X-ray reflection signatures. I. Local spectra (2000)
Nayakshin, Sergei, Kallman, Timothy R.
X-ray illumination of accretion disks is an invaluable diagnostic of the structure of these disks because of the associated iron K$\alpha$ emission. Here we point out that the resulting reflected...
Ionization physics evidence for magnetic flare origin of X-rays in AGN (2000)
We present full disk X-ray reflection spectra for two currently popular accretion flow geometries for AGN -- the lamppost model frequently used to discuss the iron line reverberation in AGN, and the...
On the X-ray heated skin of Accretion Disks (1999)
We present a simple analytical formula for the Thomson depth of the X-ray heated skin of accretion disks valid at any radius and for a broad range of spectral indices of the incident X-rays,...
Thermal Instability and Photoionized X-ray Reflection in Accretion Disks (1999)
Nayakshin, Sergei, Kazanas, Demosthenes, Kallman, Timothy R.
We study the X-ray illumination of an accretion disk. We relax the simplifying assumption of constant gas density used in most previous studies; instead we determine the density from hydrostatic...
Time-Dependent Disk Models for the Microquasar GRS1915+105 (1999)
Nayakshin, Sergei, Rappaport, Saul, Melia, Fulvio
We investigate different accretion disk models and viscosity prescriptions in order to provide a basic explanation for the exotic temporal behavior in GRS 1915+105. Based on the fact that the overall...
Pressure Ionization Instability: Connection between Seyferts and GBHCs (1998)
Spectrum of Seyfert 1 Galaxies (S1G hereafter) is very similar to that of several Galactic Black Hole Candidates (GBHCs) in their hard state, suggestive that both classes of objects have similar...
Corona Energy Budget in AGN and GBHCs (1998)
Recent progress in observations and understanding of spectra of Seyfert Galaxies shows that X-rays are most likely produced by magnetic flares on the surface of the disk, similar to Solar X-ray...
The Physics of Accretion Disks with Magnetic Flares (1998)
Rapid progress in multi-wavelength observations of Seyfert Galaxies in recent years is providing evidence that X-ray emission in these objects may be produced by magnetic flares occurring above a...
X-rays From Magnetic Flares In Cygnus X-1: The Role Of A Transition Layer (1998)
Nayakshin, Sergei, Dove, James B.
The spectrum of Seyfert 1 Galaxies is very similar to that of several Galactic Black Hole Candidates (GBHCs) in their hard state, suggestive that both classes of objects have similar physical...
Magnetic Flares and State Transitions in Galactic Black Hole and Neutron Star Systems (1997)
Nayakshin, Sergei, Melia, Fulvio
We here examine the conditions of the two-phase disk model under which magnetic flares arise above the cold accretion disk due to magnetic buoyancy and produce X-rays via Comptonization of the disk's...
X-rays From Magnetic Flares In Cygnus X-1: A Unified Model With Seyfert Galaxies (1997)
Nayakshin, Sergei, Melia, Fulvio
Some recent work has shown that the spectrum of Seyfert 1 Galaxies is very similar to that of several Galactic Black Hole Candidates (GBHCs) in their hard state. However, the smallness of the...
Magnetic Flares and the Observed Optical Depth in Seyfert Galaxies (1997)
Nayakshin, Sergei, Melia, Fulvio
We here consider the pressure equilibrium during an intense magnetic flare above the surface of a cold accretion disk. Under the assumption that the heating source for the plasma trapped within the...
Physical Constraints on, and a Model for, the Active Regions in Seyfert Galaxies (1997)
Nayakshin, Sergei, Melia, Fulvio
We discuss several physical constraints on the nature of the Active Regions (AR) in Seyfert 1 Galaxies, and show that a plausible model consistent with these constraints is one in which the ARs are...
Self-Consistent Fokker-Planck Treatment Of Particle Distributions in Astrophysical Plasmas (1997)
Nayakshin, Sergei, Melia, Fulvio
High-energy, multi-component plasmas in which pair creation and annihilation, lepton-lepton scattering, lepton-proton scattering, and Comptonization all contribute to establishing the particle and...
Expanding Relativistic Shells and Gamma-Ray Burst Temporal Structure (1996)
Fenimore, E. E., Madras, Claudine, Nayakshin, Sergei
Many models of cosmological gamma-ray bursts involve the sudden release of $\sim 10^{51}$ erg which produce shells which expand at relativistic speeds (Lorentz $\Gamma$ factors of $10^{2-3}$). We...