arXiv subjects

Ramesh Narayan

Publications and source records attributed to Ramesh Narayan.

At least 289 records · Page 16Linked to original sources

arXiv1999

Hydrodynamic Drag on a Compact Star Orbiting a Supermassive Black Hole

The proposed Laser Interferometer Space Antenna is expected to detect gravitational waves from neutron stars and stellar-mass black holes spiraling into supermassive black holes in distant galactic nuclei. Analysis of the inspiral events will require careful comparison of the observed signals with theoretical waveform templates. The comparison could be seriously compromised if non-gravitational torques modify the orbit of the star. This paper estimates the torque exerted on an orbiting star as a result of hydrodynamic interactions with an accretion flow around the supermassive black hole. It is argued that the majority of inspiral events will take place in low luminosity galactic nuclei in which the mass accretion rate is low and the accretion occurs via an advection-dominated flow. The hydrodynamic torque is negligibly small in such systems and will have no effect on gravitational wave experiments.

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arXiv1999

What is the Accretion Rate in Sgr A*?

The radio source Sgr A* at the center of our Galaxy is believed to be a 2.6 x 10^6 solar mass black hole which accretes gas from the winds of nearby stars. We show that limits on the X-ray and infrared emission from the Galactic Center provide an upper limit of ~ 8 x 10^{-5} solar masses per year on the mass accretion rate in Sgr A*. The advection-dominated accretion flow (ADAF) model favors a rate < 10^{-5} solar masses per year. In comparison, the Bondi accretion rate onto Sgr A*, estimated using the observed spatial distribution of mass losing stars and assuming non-interacting stellar winds, is ~ 3 x 10^{-5} solar masses per year. There is thus rough agreement between the Bondi, the ADAF, and the X-ray inferred accretion rates for Sgr A*. We discuss uncertainties in these estimates, emphasizing the importance of upcoming observations by the Chandra X-ray observatory (CXO) for tightening the X-ray derived limits.

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arXiv1999

Black Hole and Neutron Star Transients in Quiescence

We consider the X-ray luminosity difference between neutron star and black hole soft X-ray transients (NS and BH SXTs) in quiescence. The current observational data suggest that BH SXTs are significantly fainter than NS SXTs. The luminosities of quiescent BH SXTs are consistent with the predictions of binary evolution models for the mass transfer rate if (1) accretion occurs via an ADAF in these systems and (2) the accreting compact objects have event horizons. The luminosities of quiescent NS SXTs are not consistent with the predictions of ADAF models when combined with binary evolution models, unless most of the mass accreted in the ADAF is prevented from reaching the neutron star surface. We consider the possibility that mass accretion is reduced in quiescent NS SXTs because of an efficient propeller and develop a model of the propeller effect that accounts for the observed luminosities. We argue that modest winds from ADAFs are consistent with the observations while strong winds are probably not.

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arXiv1998

Thermal X-Ray Line Emission from Accreting Black Holes

We present model X-ray spectra of accreting black holes with advection-dominated accretion flows, paying attention to thermal emission lines from the hot plasma. We show that the Advanced X-ray Astrophysical Facility (AXAF) might be able to observe lines from X-ray binaries such as V404 Cyg in quiescence, the Galactic Center black hole Sagittarius A*, and the nuclei of nearby galaxies such as M87. Line intensities can provide new diagnostics to study the accreting plasma in these and related systems.

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arXiv1998

A Population of Faint Non-Transient Low Mass Black Hole Binaries

We study the thermal and viscous stability of accretion flows in Low Mass Black Hole Binaries (LMBHBs). We consider a model in which an inner advection-dominated accretion flow (ADAF) is surrounded by a geometrically thin accretion disk, the transition between the two zones occurring at a radius R_tr. In all the known LMBHBs, R_tr appears to be such that the outer disks could suffer from a global thermal-viscous instability. This instability is likely to cause the transient behavior of these systems. However, in most cases, if R_tr were slightly larger than the estimated values, the systems would be globally stable. This suggests that a population of faint persistent LMBHBs with globally stable outer disks could be present in the Galaxy. Such LMBHBs would be hard to detect because they would lack large amplitude outbursts, and because their ADAF zones would have very low radiative efficiencies, making the systems very dim. We present model spectra of such systems covering the optical and X-ray bands.

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arXiv1998

Spectral Models of Advection-Dominated Accretion Flows with Winds

We calculate spectral models of advection-dominated accretion flows, taking into account the possibility that significant mass may be lost to a wind. We apply the models to the soft X-ray transient V404 Cyg in quiescence and the Galactic center source Sgr A*. We show that there are qualitative degeneracies between the mass loss rate in the wind and parameters characterizing the microphysics of the accretion flow; of particular importance is $δ$, the fraction of the turbulent energy which heats the electrons. For small $δ$, current observations of soft X-ray transients and Sgr A* suggest that at least $\sim 10 %$ of the mass originating at large radii must reach the central object. For large $δ\sim 0.3$, however, models with significantly more mass loss are in agreement with the observations. We also discuss constraints on advection-dominated accretion flow models imposed by recent radio observations of NGC 4649 and other nearby elliptical galaxies. We conclude by highlighting future observations which may clarify the importance of mass loss in sub-Eddington accretion flows.

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arXiv1998

On the Energetics of Advection-Dominated Accretion Flows

Using mean field MHD, we discuss the energetics of optically thin, two temperature, advection-dominated accretion flows (ADAFs). If the magnetic field is tangled and roughly isotropic, flux freezing is insufficient to maintain the field in equipartition with the gas. In this case, we expect a fraction of the energy generated by shear in the flow to be used to build up the magnetic field strength as the gas flows in; the remaining energy heats the particles. We argue that strictly equipartition magnetic fields are incompatible with a priori reasonable levels of particle heating; instead, the plasma $β$ in ADAFs (defined to be the gas pressure divided by magnetic/turbulent pressure) is likely to be $\gsim 5$; correspondingly, the viscosity parameter $α$ is likely to be $\lsim 0.2$

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arXiv1998

What is the Accretion Rate in NGC 4258?

We consider the implications of recent infrared and radio observations of the nucleus of NGC 4258. There is no direct evidence that the nucleus has been steadily accreting on the viscous timescale of the outer masing disk, which is >~ 10^9 \yr. Thus the mass accretion rate in the outer disk need not be the same as in the inner accretion flow where most of the gravitational binding energy is released. We show that an advection-dominated flow model with a transition radius of \sim (10-100) M (where M is the mass of the hole) and \mdot \approx 10^{-2}\msun \yr^{-1} is consistent with the observed spectrum from radio to X-rays. We also show that a thin (flat or warped) disk can fit the observed fluxes outside the X-ray band. The X-rays can be explained by means of a corona in such a model, but the absence of radio emission from the location of the putative central black hole provides a serious constraint on the properties of the corona. A wide range of accretion rates, 10^{-4} <~ \mdot <~ 10^{-2} \msun\yr^{-1}, can be made to fit the data, but the most ``natural'' models have 10^{-3} <~ \mdot <~ 10^{-2} \msun\yr^{-1}. Physical conditions in the observed VLBI jet features can also be related to conditions in the inner accretion flow. We conclude with a list of future observations that might help to constrain the accretion rate.

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arXiv1998

Astrophysical Evidence for Black Hole Event Horizons

Astronomers have discovered many potential black holes in X-ray binaries and galactic nuclei. These black holes are usually identified by the fact that they are too massive to be neutron stars. Until recently, however, there was no convincing evidence that the objects identified as black hole candidates actually have event horizons. This has changed with extensive applications of a class of accretion models for describing the flow of gas onto compact objects; for these solutions, called advection-dominated accretion flows (ADAFs), the black hole nature of the accreting star, specifically its event horizon, plays an important role. We review the evidence that, at low luminosities, accreting black holes in both X-ray binaries and galactic nuclei contain ADAFs rather than the standard thin accretion disk.

gr-qc↗

arXiv1998

Advection-Dominated Accretion around Black Holes

This article reviews the physics of advection-dominated accretion flows (ADAFs) and describes applications to several black hole X-ray binaries and galactic nuclei. The possibility of using ADAFs to explore the event horizons of black holes is highlighted.

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arXiv1998

The Hardness Distribution of Gamma-Ray Bursts

It is often stated that gamma-ray bursts (GRBs) have typical energies of several hundreds $\keV$, where the typical energy may be characterized by the hardness H, the photon energy corresponding to the peak of $νF_ν$. Among the 54 BATSE bursts analyzed by Band et al. (1993), and 136 analyzed by us, more then 60% have 50 keV < H < 300 keV. Is the narrow range of H a real feature of GRBs or is it due to an observational difficulty to detect harder bursts? We consider a population of standard candle bursts with a hardness distribution: rho(H) d log H \propto H^gamma d log H and no luminosity - hardness correlation. We model the detection algorithm of BATSE as a function of H, including cosmological effects, detector characteristics and triggering procedure, and we calculate the expected distribution of H in the observed sample for various values of gamma. Both samples shows a paucity of soft (X-ray) bursts, which may be real. However, we find that the observed samples are consistent with a distribution above H=120 keV with gamma \sim -0.5 (a slowly decreasing numbers of GRBs per decade of hardness). Thus, we suggest that a large population of unobserved hard gamma-ray bursts may exist.

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arXiv1997

Spectra and Light Curves of Gamma-Ray Burst Afterglows

The recently discovered GRB afterglow is believed to be described reasonably well by synchrotron emission from a slowing down relativistic shell that collides with an external medium. To compare theoretical models with afterglow observations we calculate here the broad band spectrum and corresponding light curve of synchrotron radiation from a power-law distribution of electrons in an expanding relativistic shock. Both the spectrum and the light curve consist of several power-law segments. The light curve is constructed under two limiting models for the hydrodynamical evolution of the shock: fully adiabatic and fully radiative. We compare the results with observations of $γ$-ray burst afterglows.

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arXiv1997

Spectral Transitions in Cyg X-1 and Other Black Hole X-Ray Binaries

We show that the model proposed by Esin, McClintock & Narayan (1997) for the low state, intermediate state and high state of the black hole soft X-ray transient, Nova Muscae 1991, is consistent with the spectral evolution of the black hole X-ray binary, Cyg X-1, during the hard-to-soft state transition observed in 1996. We also apply the model to the outbursts of two other black hole X-ray transients, GRO J0422+32 and GRO J1719-24.

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arXiv1997

Lectures on Gravitational Lensing

These lectures give an introduction to Gravitational Lensing. We discuss lensing by point masses, lensing by galaxies, and lensing by clusters and larger-scale structures in the Universe. The relevant theory is developed and applications to astrophysical problems are discussed.

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arXiv1997

Advection-Dominated Accretion Model of Sagittarius A*: Evidence for a Black Hole at the Galactic Center

Sgr A* at the Galactic Center is a puzzling source. It has a mass M=(2.5+/-0.4) x 10^6 solar masses which makes it an excellent black hole candidate. Observations of stellar winds and other gas flows in its vicinity suggest a mass accretion rate approximately few x 10^{-6} solar masses per year. However, such an accretion rate would imply a luminosity > 10^{40} erg/s if the radiative efficiency is the usual 10 percent, whereas observations indicate a bolometric luminosity <10^{37} erg/s. The spectrum of Sgr A* is unusual, with emission extending over many decades of wavelength. We present a model of Sgr A* which is based on a two-temperature optically-thin advection-dominated accretion flow. The model is consistent with the estimated mass and accretion rate, and fits the observed fluxes in the cm/mm and X-ray bands as well as upper limits in the sub-mm and infrared bands; the fit is less good in the radio below 86 GHz and in gamma-rays above 100 MeV. The very low luminosity of Sgr A* is explained naturally in the model by means of advection. Most of the viscously dissipated energy is advected into the central mass by the accreting gas, and therefore the radiative efficiency is extremely low, approximately 5 x 10^{-6}. A critical element of the model is the presence of an event horizon at the center which swallows the advected energy. The success of the model could thus be viewed as confirmation that Sgr A* is a black hole.

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arXiv1997

Gamma-ray Emission From Advection-Dominated Accretion Flows Around Black Holes: Application to the Galactic Center

We calculate the flux and spectrum of γ-rays emitted by a two-temperature advection-dominated accretion flow (ADAF) around a black hole. The γ-rays are from the decay of neutral pions produced through proton-proton collisions. We discuss both thermal and power-law distributions of proton energies and show that the γ-ray spectra in the two cases are very different. We apply the calculations to the γ-ray source, 2EG J1746-2852, detected by EGRET from the direction of the Galactic Center. We show that the flux and spectrum of this source are consistent with emission from an ADAF around the supermassive accreting black hole Sgr A^* if the proton distribution is a power-law. The model uses accretion parameters within the range made likely by other considerations. If this model is correct, it provides evidence for the presence of a two temperature plasma in Sgr A^*, and predicts γ-ray fluxes from other accreting black holes which could be observed with more sensitive detectors.

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arXiv1997

Lithium Production in Hot Advection-Dominated Accretion Flows in Soft X-Ray Transients

High Li abundances have been reported in the late type secondaries of five soft X-ray transients (SXTs), V404 Cyg, A0620-00, GS2000+25, Nova Mus 1991, and Cen X-4. Since Li is likely to be depleted in stars of this type, the origin of the Li is puzzling. Li has not been seen in similar secondaries of cataclysmic variables, which suggests that the high Li abundance is not due to an anomalous suppression of Li depletion in close binaries. SXTs in the quiescent state have hot advection-dominated accretion flows (ADAFs) in which the ions are essentially at virial temperature. At such temperatures, Li production via $α-α$ spallation is possible. We show that quiescent SXTs can produce sufficient Li via spallation to explain the observations in V404 Cyg, A0620-00, GS2000+25, and Nova Mus 1991. Depending on the Li depletion time scale in the secondary, which may range between $10^7-10^9$ yr, the model requires $\sim 10^{-4}-10^{-6}$ of the accreted mass to be intercepted by the secondary after undergoing Li production and being ejected. In the case of Cen X-4, we can explain the observed Li only if the mass accretion rate is $\sim 10^{-3}$ times the Eddington rate and if there is enhanced ejection due to a propeller effect. We discuss possible observational tests of this proposal. Li production during outbursts could be quite important and may even dominate over the production during quiescence, but the estimate of the Li yield is uncertain. We calculate the expected luminosity in gamma-ray lines due to the production of excited Li and Be nuclei, but conclude that the line cannot be detected with current instruments.

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arXiv1997

Advection-Dominated Accretion and Black Hole Event Horizons

The defining characteristic of a black hole is that it possesses an event horizon through which matter and energy can fall in but from which nothing escapes. Soft X-ray transients (SXTs), a class of X-ray binaries, appear to confirm this fundamental property of black holes. SXTs that are thought to contain accreting black holes display a large variation of luminosity between their bright and faint states, while SXTs with accreting neutron stars have a smaller variation. This difference is predicted if the former stars have horizons and the latter have normal surfaces.

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