arXiv subjects

Ramesh Narayan

Publications and source records attributed to Ramesh Narayan.

At least 307 records · Page 17Linked to original sources

arXiv1997

Advection-Dominated Accretion Model of the Black Hole V404 Cyg in Quiescence

We have analyzed archival ASCA data on the soft X-ray transient source V404 Cyg in quiescence. We find that in the energy range 0.7 to 8.5 keV the spectrum is a hard power-law with a photon spectral index between 1.8 and 2.6 (90% confidence limits). We present a model of V404 Cyg in which the accretion flow has two components: (1) an outer thin disk with a small annular extent, and (2) a large interior region where the flow is advection-dominated. Nearly all the radiation in the infrared, optical, UV and X-ray bands is from the advection-dominated zone; the thin disk radiates primarily in the infrared where it contributes about ten percent of the observed flux. The spectrum we calculate with this model is in excellent agreement with the ASCA X-ray data presented here, as well as with previous optical data. Moreover, the fit is very insensitive to the choice of parameters such as black hole mass, orbital inclination, viscosity coefficient $α$, and magnetic field strength. We consider the success of the model to be strong support for the advection-dominated accretion paradigm, and further evidence of the black hole nature of V404 Cyg. We discuss strategies whereby systems with advection-dominated accretion could be used to prove the reality of event horizons in black holes.

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arXiv1996

Advective Disks

Recent work on advection-dominated accretion flows (ADAFs) is reviewed. The article concentrates on an optically thin branch of ADAFs which is present at mass accretion rates below a critical value $\sim(10^{-2}-10^{-1})$ the Eddington rate. Models based on this branch have been quite successful at explaining a number of low-luminosity X-ray binaries and galactic nuclei, and some brighter systems. Some progress has also been made toward understanding the various spectral states of accreting black holes. It is argued that ADAFs may provide one of the best techniques for demonstrating the reality of event horizons in black holes.

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arXiv1996

Global Structure and Dynamics of Advection-Dominated Accretion Flows Around Black Holes

We present global solutions that describe advection-dominated accretion flows around black holes. The solutions are obtained by numerically solving a set of coupled ordinary differential equations corresponding to a steady axisymmetric height-integrated flow. The solutions satisfy consistent boundary conditions at both ends. We obtain well-behaved transonic solutions for a wide range of values of the viscosity parameter $α$, from 0.001 to 0.3. We do not find any need for shocks in our solutions, and disagree with previous claims that viscous accretion flows with low values of $α$ must have shocks. We see a qualitative difference between solutions with low values of the viscosity parameter, $α \sles\ 0.01$, and those with large values, $α\sgreat\ 0.01$. The solutions with low $α$ have their sonic transitions occurring close to the radius of the marginally bound orbit. These flows are characterized by regions of super-Keplerian rotation, and have pressure maxima outside the sonic point. The solutions are similar in many respects to the hydrostatic thick tori developed previously as models of active galactic nuclei. In contrast, the solutions with large $α$ have sonic transitions farther out, close to or beyond the marginally stable orbit, and have no super-Keplerian rotation or pressure maxima. We believe these flows will be nearly quasi-spherical down to the sonic radius and will not have empty funnels along the rotation axis. The large $α$ solutions are more likely to be representative of real systems.

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arXiv1996

Spectra and Line Profiles of FU Orionis Objects: Comparisons Between Boundary Layer Models and Observations

We present solutions for the accretion disks and boundary layers in pre-main-sequence stars undergoing FU Orionis outbursts. These solutions differ from earlier disk solutions in that they include a self-consistent treatment of the boundary layer region. In a previous paper (Popham 1996), we showed that these stars should stop accreting angular momentum once they spin up to modest rotation rates. Here we show that for reasonable values of $α$, these low angular momentum accretion rate solutions fit the spectra and line profiles observed in FU Orionis objects better than solutions with high rates of angular momentum accretion. We find solutions which fit the observations of FU Orionis and V1057 Cygni. These solutions have mass accretion rates of 2 and $1 \times 10^{-4} \msyr$, stellar masses of 0.7 and $0.5 \msun$, and stellar radii of 5.75 and $5.03 \rsun$, respectively. They also have modest stellar rotation rates $8-9 \xss$, comparable to the observed rotation rates of T Tauri stars, and angular momentum accretion rates of zero. This supports our earlier suggestion that FU Orionis outbursts may regulate the rotation rates of T Tauri stars.

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arXiv1996

Cooling Time Scales and Temporal Structure of Gamma-Ray Bursts

A leading mechanism for producing cosmological gamma-ray bursts (GRBs) is via ultra-relativistic particles in an expanding fireball. The kinetic energy of the particles is converted into thermal energy in a forward shock and a reverse shock, when the outward flowing particles encounter the interstellar medium. The thermal energy is radiated via synchrotron emission and Comptonization. We estimate the synchrotron cooling time scale of the shocked regions for electrons of various Lorentz factors, focusing in particular on those electrons whose radiation falls within the energy detection range of the BATSE detectors. We find that in order to produce the rapid variability observed in most bursts the magnetic energy density in the shocked material must be greater than about 1\% of the thermal energy density. Additionally, the electrons must be nearly in equipartition with the protons, otherwise the radiative efficiency is low. Inverse Compton scattering can increase the cooling rate of the relevant electrons but the Comptonized emission itself is never within the BATSE range. These arguments allow us to pinpoint the conditions within the radiating regions in GRBs and to determine the important radiation processes. In addition, the model predicts that the duty cycle of intensity variations in GRB light curves should be nearly independent of burst duration, and should scale inversely as the square root of the observed photon energy. Both correlations are in agreement with observations. The model also provides a plausible explanation for the bimodal distribution of burst durations. There is no explanation, however, for the characteristic break energy in GRB spectra.

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arXiv1996

Deep radio observation of the gravitational lens candidate QSO2345+007

The double QSO2345+007 comprises two optical components separated by 7.1\ arcseconds and is the most prominent `dark matter' gravitational lens candidate. Despite being known for more than a decade, optical spectroscopy and imaging have been unable to determine whether this double QSO is a binary QSO or a gravitational lens system. In this note we report a deep VLA observation of this system, yielding a map with a noise level of 8.5\ts $μ$Jy per beam. We have a $4σ$ detection of a radio source within one arcsecond of the optical position of the brighter A-component of the QSO, but no significant detection of any radio counterpart of the B component. Given that the flux ratio in the optical waveband is $\sim$3--4, the gravitational lens hypothesis for this system would predict a radio flux of image B of $\lesssim 10$\ts $μ$Jy. The non-detection of the B component is thus consistent with, but does not prove, the lens interpretation.

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arXiv1996

Harmony in Electrons: Cyclotron and Synchrotron Emission by Thermal Electrons in a Magnetic Field

We present a complete solution to the cyclotron-synchrotron radiation due to an isotropic distribution of electrons moving in a magnetic field. We make no approximations in the calculations other than artificially broadening the harmonics by a small amount in order to facilitate the numerics. In contrast to previous calculations, we sum over all relevant harmonics and integrate over all particle and observer angles relative to the magnetic field. We present emission spectra for electron temperatures $T=5\times10^8$ K, $10^9$ K, $2\times10^9$ K to $3.2\times10^{10}$ K, and provide simple fitting formulae which give a fairly accurate representation of the detailed results. For $T\geq3.2\times10^{10}$ K, the spectrum is represented well by the asymptotic synchrotron formula, which is obtained by assuming that the radiating electrons have Lorentz factors large compared to unity. We give an improved fitting formula also for this asymptotic case.

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arXiv1996

Maximum-Likelihood Cluster Recontruction

We present a novel method to recontruct the mass distribution of galaxy clusters from their gravitational lens effect on background galaxies. The method is based on a least-chisquare fit of the two-dimensional gravitational cluster potential. The method combines information from shear and magnification by the cluster lens and is designed to easily incorporate possible additional information. We describe the technique and demonstrate its feasibility with simulated data. Both the cluster morphology and the total cluster mass are well reproduced.

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arXiv1995

A New Model of the Gravitational Lens 0957+561 and a Limit on the Hubble Constant

We present a simple mass model for the lensing galaxy in the gravitationally lensed quasar 0957+561. The model is a generalization of the singular isothermal sphere and includes a core radius, $r_c$, and a power-law index, $η$, defined such that mass increases as $r^η$ at large radius. We approximate the galaxy cluster surrounding the lensing galaxy with a quadratic potential described by its convergence $κ$ and shear $γ$. We fit the model to a recent high resolution VLBI map of the two images of 0957+561. We obtain a tight constraint on the radial index, $1.07 < η<1.18$, which means that the lens galaxy is nearly isothermal with increasing mass-to-light ratio out to $15 h^{-1}$ kpc. We also obtain an upper limit on the core radius, $r_c < 330 h^{-1}$ pc. We use the model to calculate the Hubble constant $H_0$ as a function of the time delay $Δτ_{BA}$ between the two images: $H_0 = \left({ 82.5^{+5.9} _{-3.0} }\right) (1 - κ) \left({ Δτ_{BA}/1.1 \,{\rm yr} }\right)^{-1}$ km/s/Mpc. Once $Δτ_{BA}$ is measured, this will provide an upper bound on $H_0$ since $κ$ cannot be negative. In addition, the model degeneracy due to $κ$ can be eliminated if the one-dimensional velocity dispersion $σ$ of the lensing galaxy is measured. We then have $H_0 = \left({ 82.5^{+8.7} _{-5.6} }\right) (σ/ 322\,{\rm km/s})^2 \left({ Δτ_{BA} /1.1 \,{\rm yr} }\right)^{-1}$ km/s/Mpc. We investigate the effects of ellipticity in the lensing galaxy and clumpiness in the lensing cluster and find that these cause little change in our results.

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arXiv1995

Are There MeV Gamma-Ray Bursts?

It is often stated that gamma-ray bursts (GRBs) have typical energies of several hundred keV. Is this a real feature of GRBs or is it due to an observational bias? We consider the possibility that bursts of a given bolometric luminosity occur with a hardness distribution $p(H)d \log H \propto H^γd \log H$. We model the detection efficiency of BATSE as a function of $H$ and calculate the expected distribution of $H$ in the observed sample for various values of $γ$. We show that because the detection efficiency of BATSE falls steeply with increasing $H$, the paucity of hard bursts need not be real. We find that the observed sample is consistent with a distribution above $H = 100$ keV with $γ\approx 0$ or even $γ=0.5$. Thus, a large population of unobserved hard gamma-ray bursts may exist. It is important to extend the present analysis to a larger sample of BATSE bursts and to include the OSSE and COMPTEL limits. If the full sample is consistent with $γ \sgreat\ 0$, then it would be interesting to look for MeV bursts in the future.

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arXiv1995

Advection-Dominated Accretion Flows: Optically Thin Solutions

General properties of advection-dominated accretion flows are discussed. Special emphasis is given to the optically thin branch of solutions, which has very high ion and electron temperatures and is thermally stable. This solution branch has been applied to a number of low-luminosity accreting black holes. The models have resolved some puzzles and have provided a straightforward explanation of the observed spectra. The success of the models confirms that the central objects in these low-luminosity sources are black holes. There is some indication that advection-dominated models may be relevant also for higher luminosity systems. The properties of the Low state of accreting black holes, the transition from the Low state to the High state, and the similarity of hard X-ray/gamma-ray spectra of black hole X-ray binaries and active galactic nuclei, are explained.

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arXiv1995

Are Gamma Ray Bursts due to Rotation Powered High Velocity Pulsars in the Halo ?

The BATSE experiment has now observed more than 1100 gamma-ray bursts. The observed angular distribution is isotropic, while the brightness distribution of bursts shows a reduced number of faint events. These observations favor a cosmological burst origin. Alternatively, very extended Galactic Halo (EGH) models have been considered. In the latter scenario, the currently favored source of gamma-ray bursts involves high velocity pulsars ejected from the Galactic disk. To be compatible with the observed isotropy, most models invoke a sampling distance of 300 kpc, a turn-on delay of 30 Myrs, and a source life time of about 1 Gyr. We consider the global energy requirements of such models and show that the largest known resource. rotational kinetic energy, is insufficient by orders of magnitude to provide the observed burst rate. More exotic energy sources or differently tuned pulsar models may be able to get around the global energy constraint but at the cost of becoming contrived. Thus, while extended halo models are not ruled out, our arguments place a severe obstacle for such models and we encourage proponents of EGH models to clearly address the issue of global energetics.

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arXiv1995

Advection-Dominated Models of Luminous Accreting Black Holes

It has been found that a class of optically-thin two-temperature advection-dominated accretion solutions explains many observations of low-luminosity accreting black holes. Here it is shown that these models give a satisfactory description also of higher luminosity systems, provided the viscosity parameter $α$ is large. The models reproduce the spectra of black hole X-ray binaries in the Low State, and explain the transition from the Low State to the High State at a critical mass accretion rate. The models also show that the X-ray/$γ$-ray spectra of X-ray binaries and active galactic nuclei should be similar, as confirmed by observations.

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arXiv1995

Models of Quiescent Black Hole Neutron Star Soft X-Ray Transients

When the mass accretion rate onto a black hole (BH) falls below a critical rate, ${\dot M}_{crit}\sim α^2 {\dot M}_{Edd}$, accretion can occur via a hot optically thin flow where most of the dissipated energy is advected inward. We present such an advection-dominated model for the soft X-ray transient (SXT) A0620-00. This source has a puzzlingly low X-ray luminosity in quiescence, $\sim 6\times 10^{30} ~{\rm erg\,s^{-1}}$, despite a relatively high mass accretion rate $\sim10^{-10} ~M_{\odot}{\rm yr^{-1}}$ deduced from its optical flux. The accreting gas makes a transition from a standard thin disc at large radii to an advection-dominated flow at small radii. The transition occurs when the effective temperature of the thin disc is $\sim 10^4$ K. Because of the very low accretion efficiency, $\sim 10^{-3}-10^{-4}$, in the inner flow, the model fits both the optical and X-ray data. We also present models for V404 Cyg and Nova Mus 1991 in quiescence. Quiescent neutron star (NS) transients are expected to appear very different from BH systems because the advected energy is re-radiated from the NS surface whereas a BH swallows the advected energy. We discuss models for NS SXTs.

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arXiv1995

Advection-Dominated Accretion Model of Sagittarius A$^*$ and Other Accreting Black Holes

Viscous rotating accretion flows around black holes become advection-dominated when the accretion rate $\dot M$ is sufficiently low. Most of the accretion energy in such flows is stored within the gas and advected radially inward. The temperature is therefore very high, and much of the radiation comes out in hard X-rays and $γ$-rays. We have constructed an advection-dominated accretion flow model for the Galactic Center source Sagittarius A$^*$. The model consists of a $7\times10^5M_\odot$ black hole accreting at $\dot M=1.2\times10^{-5}α\, M_\odot{\rm yr^{-1}}$, where $α$ is the usual viscosity parameter. The model spectrum fits the observations from radio to $γ$-rays quite well and explains the unusually low luminosity of the source. Since the model explicitly makes use of a horizon at the inner edge to swallow the advected energy, the success of the model strongly suggests that the central object in Sgr A$^*$ is a black hole. We further show that, if $α$ is not much smaller than unity, then advection-dominated models can be applied even to higher luminosity black holes. The existence of Low and High States in black hole X-ray binaries, and the abrupt transition between the two states, find a natural explanation. The models also explain the close similarity in the hard X-ray/$γ$-ray spectra of black hole X-ray binaries and active galactic nuclei.

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arXiv1995

Accretion Flows Around Black Holes

Most accreting black holes emit a substantial fraction of their luminosity in hard X-rays/soft $γ$-rays of up to $\sim{\rm few}\times100$ keV. The observations suggest that the radiating gas is very hot, $T>10^9$ K, and optically thin. Theoretical attempts to explain the observations are reviewed and unresolved issues are highlighted.

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arXiv1995

A New Model for Black Hole Soft X-ray Transients in Quiescence

We present models of the soft X-ray transients, A0620-00, V404 Cyg, and X-ray Nova Mus 1991, in quiescence. In each source, we postulate that there is an outer region, extending outward from about 3000 Schwarzschild radii, where the accretion flow is in the form of a standard thin disk. The outer disk produces most of the radiation we observe in the infrared, optical and UV bands. We propose that the disk undergoes an instability at its inner edge, perhaps by the mechanism described recently by Meyer \& Meyer-Hofmeister for cataclysmic variables. The accreting gas is thereby converted into a hot corona which flows into the black hole as a nearly virial two-temperature flow. We describe the hot inner flow by means of a recently discovered stable solution of optically thin advection-dominated accretion. In this flow, most of the thermal energy released by viscous dissipation is advected into the black hole and only a small fraction, $\sim10^{-4}-10^{-3}$, of the energy is radiated. The radiation is in the form of Comptonized synchrotron and bremsstrahlung emission, and has a broad spectrum extending from optical to soft gamma-rays. The models we present are consistent with all the available data in the three sources. In particular, the X-ray emission from the hot inner flow fits the observed flux and spectral index of A0620-00. We derive a mass accretion rate of $\sim10^{-11}\msyr$ in A0620-00 and Nova Mus, and $\sim{\rm few}\times10^{-10}\msyr$ in V404 Cyg. The best fit to the data is obtained for a viscosity parameter $α\sim0.1-0.3$ in the hot flow. The models predict that all three sources must have substantial flux in hard X-rays and soft $γ$-rays. This prediction is testable in the case of V404 Cyg with current instruments. A necessary feature of our proposal is that most of the viscous

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arXiv1995

Advection-Dominated Accretion: Underfed Black Holes and Neutron Stars

We describe new optically thin solutions for rotating accretion flows around black holes and neutron stars. These solutions are advection dominated, so that most of the viscously dissipated energy is advected radially with the flow. We model the accreting gas as a two temperature plasma and include cooling by bremsstrahlung, synchrotron, and Comptonization. We obtain electron temperatures $T_e\sim 10^{8.5}-10^{10}$K. The new solutions are present only for mass accretion rates $\dot M$ less than a critical rate $\dot M_{crit}$ which we calculate as a function of radius $R$ and viscosity parameter $α$. For $\dot M<\dot M_{crit}$ we show that there are three equilibrium branches of solutions. One of the branches corresponds to a cool optically thick flow which is the well-known thin disk solution of Shakura \& Sunyaev (1973). Another branch corresponds to a hot optically thin flow, discovered originally by Shapiro, Lightman \& Eardley (1976, SLE). This solution is thermally unstable. The third branch corresponds to our new advection-dominated solution. This solution is hotter and more optically thin than the SLE solution, but is viscously and thermally stable. It is related to the ion torus model of Rees et al. (1982) and may potentially explain the hard X-ray and $γ$-ray emission from X-ray binaries and active galactic nuclei.

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