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Ramesh Narayan

Publications and source records attributed to Ramesh Narayan.

319 records · Page 18Linked to original sources
arXiv1995

The lens parallax method: determining redshifts of faint blue galaxies through gravitational lensing

We propose a new technique, which we call the lens parallax method, to determine simultaneously the redshift distribution of the faint blue galaxies and the mass distributions of foreground clusters of galaxies. The method is based on gravitational lensing and makes use of the following: (1) the amplitude of lensing-induced distortions of background galaxies increases with redshift; (2) the surface brightnesses of galaxies decrease steeply with redshift. The distortions of galaxy images due to lensing are thus expected to be inversely correlated with surface brightness, allowing us to obtain relative distances to galaxies as a function of surface brightness. If the redshifts of the brightest galaxies are measured, then the relative distance scale can be converted to mean galaxy redshifts as a function of surface brightness. Further, by comparing the angular sizes of lensed galaxies with those of similar galaxies in empty control fields, it is possible to break the so-called mass sheet degeneracy inherent to cluster mass reconstruction techniques which are based purely on image ellipticities. This allows an unambiguous determination of the surface density of a lensing cluster. We describe an iterative algorithm based on these ideas and present numerical simulations which show that the proposed techniques are feasible with a sample of ~ 10 rich clusters at moderate redshifts ~ 0.3-0.4 and an equal number of control fields. The numerical tests show that the method can be used to determine the redshifts of galaxies with an accuracy of dz ~ 0.1-0.2 at z ~ 1-1.7, and to measure the masses of lensing clusters to about 5% accuracy.

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arXiv1995

Unified Description of Accretion Flows Around Black Holes

We provide a unified description of thermal equilibria of black hole accretion disks, including the newly-discovered advection-dominated solutions. We classify the solutions on the basis of optical depth and importance of advection cooling. We demonstrate that only four physically distinct topological types of equilibria exist. Two of the types correspond to optically thin and optically thick equilibria, while the other two types are distinguished by whether advection is negligible or dominant. A stable Shakura-Sunyaev disk exists only for accretion rates $\dot M$ below a certain maximum. However, there is a critical viscosity parameter $α_{\rm crit}$, which is a function of radius, such that for $α>α_{\rm crit}$ advection-dominated solutions exist for all $\dot M$. Even when $α< α_{\rm crit}$, the advection-dominated solutions are available for a wide range of $\dot M$ except for a gap around the Eddington rate. We therefore suggest that advection-dominated flows may be more common than standard thin disks in black hole systems. For certain ranges of radii and $\dot M$, no stable steady state solution is possible. In these cases, we suggest that limit cycle behavior may occur, leading to variability.

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arXiv1995

TURBULENCE IN MOLECULAR CLOUDS

We generate random Gaussian turbulent velocity fields with a Kolmogorov spectrum and use these to obtain synthetic line-of-sight velocity profiles. The profiles are found to be similar to line profiles observed in molecular clouds. We suggest methods for analysing measured line profiles to test whether they might arise from Gaussian Kolmogorov turbulence.

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arXiv1994

Advection-Dominated Accretion Model for Sagittarius A$^*$: Evidence for a $10^6\msun$ Black Hole at the Galactic Center

The enigmatic radio source Sagittarius A* at the centre of our Galaxy appears to be a low-luminosity version of active galactic nuclei in other galaxies. By analogy with active galactic nuclei models, it has been proposed that Sgr A* may be a massive accreting black holes. The black hole hypothesis is, however, problematic because no model of Sgr A* has been able to explain the observed spectrum in any self-consistent way, and there is no consensus on either the mass of the black hole or the mass accretion rate. Sgr A* has been observed in the radio, sub-mm, infrared, and X-ray bands, and the various various detections and flux upper limits cover more than ten decades of frequency, from $ν\sles\ 10^9$ Hz (meter wavelength radio bands) up to $ν \sgreat\ 10^{19}$ Hz (150 keV X-rays). We present a robust model of Sgr A* in which a million solar mass black hole accretes at the rate of a few $\times10^{-6}$ solar masses per year. This model fits the entire spectrum self-consistently. The unique feature of the model is that the flow is advection-dominated, i.e. most of the energy viscously dissipated in the differentially rotating flow is carried along with the gas and lost through the horizon. The apparent success of this model in explaining the data may be considered ``proof'' that horizons are real and that a massive black hole does exist at the Galactic center.

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arXiv1994

Advection-Dominated Accretion: Self-Similarity and Bipolar Outflows

We consider axisymmetric viscous accretion flows where a fraction f of the viscously dissipated energy is advected with the accreting gas as stored entropy and a fraction 1-f is radiated. When f is small (i.e. very little advection), our solutions resemble standard thin disks in many respects except that they have a hot tenuous corona above. In the opposite {\it advection-dominated} limit ($f\rightarrow1$), the solutions approach nearly spherical accretion. The gas is almost at virial temperature, rotates at much below the Keplerian rate, and the flow is much more akin to Bondi accretion than to disk accretion. We compare our exact self-similar solutions with approximate solutions previously obtained using a height-integrated system of equations. We conclude that the height- integration approximation is excellent for a wide range of conditions. We find that the Bernoulli parameter is positive in all our solutions, especially close to the rotation axis. This effect is produced by viscous transport of energy from small to large radii and from the equator to the poles. In addition, all the solutions are convectively unstable and the convection is especially important near the rotation axis. For both reasons we suggest that a bipolar outflow will develop along the axis of the flows, fed by material from the the surface layers of the equatorial inflow.

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arXiv1994

The lens parallax method: determining the redshifts of the faint blue galaxies through gravitational lensing

We propose a new technique, which we call the lens parallax method, to determine simultaneously the redshift distribution of the faint blue galaxies and the mass distribution of foreground clusters of galaxies. The method is based on gravitational lensing and uses the fact that the mean angular size of the faint blue galaxies is a well-determined function of surface brightness. The method requires first a calibration of the angular-size vs. surface brightness relation in unlensed blank fields and a determination of the mean redshift of the brightest of the faint blue galaxies. By combining this information with observations of the distorted images of background galaxies in the fields of foreground clusters, we show that it is possible to obtain the mean redshift of the galaxies as a function of their surface brightness. With a sample of about ten moderate redshift rich clusters and using ten bins in surface brightness, a bin-to-bin signal-to-noise ratio of $\sim3.5$ can be achieved. The method simultaneously allows a determination of the convergence and shear of the lensing cluster as a function of position, and through this the mass distribution of the lens can be obtained. The lens parallax method can be used in conjunction with, and will improve the accuracy of, other previously proposed techniques for mapping clusters.

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arXiv1994

Advection-Dominated Accretion: A Self-Similar Solution

We consider viscous rotating accretion flows in which most of the viscously dissipated energy is stored as entropy rather than being radiated. Such advection-dominated flows may occur when the optical depth is either very small or very large. We obtain a family of self-similar solutions where the temperature of the accreting gas is nearly virial and the flow is quasi-spherical. The gas rotates at much less than the Keplerian angular velocity; therefore, the central stars in such flows will cease to spin up long before they reach the break-up limit. Further, the Bernoulli parameter is positive, implying that advection-dominated flows are susceptible to producing outflows. Convection is likely in many of these flows and, if present, will tend to enhance the above effects. We suggest that advection-dominated accretion may provide an explanation for the slow spin rates of accreting stars and the widespread occurrence of outflows and jets in accreting systems.

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arXiv1994

Causality in Strong Shear Flows

It is well known that the standard transport equations violate causality when gradients are large or when temporal variations are rapid. We derive a modified set of transport equations that satisfy causality. These equations are obtained from the underlying Boltzmann equation. We use a simple model for particle collisions which enables us to derive moment equations non-perturbatively, i.e. without making the usual assumption that the distribution function deviates only slightly from its equilibrium value. We apply the model to two problems: particle diffusion and viscous transport. In both cases we show that signals propagate at a finite speed and therefore that the formalism obeys causality. When the velocity gradient is large on the scale of a mean free path, the viscous shear stress is suppressed relative to the prediction of the standard diffusion approximation. The shear stress reaches a maximum at a finite value of the shear amplitude and then decreases as the velocity gradient increases. In the case of a steady Keplerian accretion disk with hydrodynamic turbulent viscosity, the stress-limit translates to an upper bound on the Shakura-Sunyaev $α$-parameter, namely $α<0.07$. The limit on $α$ is much stronger in narrow boundary layers where the velocity shear is larger than Keplerian.

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arXiv1993

Do Gamma-Ray Burst Sources Repeat

Following the discovery by Quashnock and Lamb (1993) of an apparent excess of $γ$-ray burst pairs with small angular separations, we reanalyze the angular distribution of the bursts in the BATSE catalogue. We find that in addition to an excess of close pairs, there is also a comparable excess of antipodal bursts, i.e pairs of bursts separated by about 180 degrees in the sky. Both excesses have only modest statistical significance. We reject the hypothesis put forward by Quashnock and Lamb that burst sources are repeaters, since it is obvious that this hypothesis does not predict an excess of antipodal coincidences. Lacking any physical model of bursts that can explain the antipodal pairs, we suggest that the two excesses seen in the data are either due to an unusual statistical fluctuation or caused by some unknown selection effect.

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arXiv1993

On the Bimodal Distribution of Gamma-Ray Bursts

Kouveliotou et al. (1993) recently confirmed that gamma-ray bursts are bimodal in duration. In this paper we compute the statistical properties of the short ($\le 2$~s) and long ($>2$~s) bursts using a method of analysis that makes no assumption regarding the location of the bursts, whether in the Galaxy or at a cosmological distance. We find the 64 ms channel on BATSE to be more sensitive to short bursts and the 1024 ms channel is more sensitive to long bursts. We show that all the currently available data are consistent with the simple hypothesis that both short and long bursts have the same spatial distribution and that within each population the sources are standard candles. The rate of short bursts is $\sim 0.4$ of the rate of long bursts. Although the durations of short and long gamma-ray bursts span several orders of magnitude and the total energy of a typical short burst is smaller than that of a typical long burst by a factor of $\sim 20$, surprisingly the peak luminosities of the two kinds of bursts are equal to within a factor of $\sim 2$.

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arXiv1993

Hydrodynamics of Relativistic Fireballs

We examine analytically and numerically the evolution of a relativistic fireball. We show that, after an early rearrangement phase, most of the matter and energy in the fireball is concentrated within a narrow shell. The shell propagates at nearly the speed of light, with a frozen radial profile and according to a simple set of scaling laws. The spectrum of the escaping radiation is harder at early times and softer later on. The results are applicable to models of $γ$-ray bursts. Submitted to Monthly Notices of the RAS Figures available on request from the authors

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arXiv1992

Gamma-Ray Bursts as the Death Throes of Massive Binary Stars

It is proposed that gamma-ray bursts are created in the mergers of double neutron star binaries and black hole neutron star binaries at cosmological distances. Bursts with complex profiles and relatively long durations are the result of magnetic flares generated by the Parker instability in a post-merger differentially-rotating disk. Some bursts may also be produced through neutrino-antineutrino annihilation into electrons and positrons. In both cases, an optically thick fireball of size $\sles\ 100$ km is initially created, which expands ultrarelativistically to large radii before radiating. Several previous objections to the cosmological merger model are eliminated. It is predicted that $γ$-ray bursts will be accompanied by a burst of gravitational radiation from the spiraling-in binary which could be detected by LIGO.

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