Black Holes in Astrophysics

This article reviews the current status of black hole astrophysics, focusing on topics of interest to a physics audience. Astronomers have discovered dozens of compact objects with masses greater than 3 solar masses, the likely maximum mass of a neutron star. These objects are identified as black hole candidates. Some of the candidates have masses of 5 to 20 solar masses and are found in X-ray binaries, while the rest have masses from a million to a billion solar masses and are found in galactic nuclei. A variety of methods are being tried to estimate the spin parameters of the candidate black holes. There is strong circumstantial evidence that many of the objects have event horizons. Recent MHD simulations of magnetized plasma accreting on rotating black holes seem to hint that relativistic jets may be produced by a magnetic analog of the Penrose process.Comment: To appear in a forthcoming Special Focus Issue on "Spacetime 100 Years Later" published by the New Journal of Physics (http://www.iop.org/EJ/njp) The article, finalized in October, 2004, consists of 21 pages of text, 3 figures and 6 movies (found at http://www.cfa.harvard.edu/~narayan/NJP

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.Comment: 8 pages, LaTeX, submitted to The Astrophysical Journa

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.Comment: 18 pages, with 2 figures included. Postscript. ApJ, in pres

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.Comment: 22 Pages, 5 Figures are available by request to [email protected], Plain Tex, CfA Preprint No. 3931, To Appear in Astrophysical Journal 5/1/9

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.Comment: 31 pages, uuencoded compressed postscript file containing 10 figures, to be published in the Sep. 20 issue of Ap

Self-Similar Hot Accretion Flow onto a Rotating Neutron Star: Structure and Stability

We present analytical and numerical solutions which describe a hot, viscous, two-temperature accretion flow onto a rotating neutron star or any other rotating compact star with a surface. We assume Coulomb coupling between the protons and electrons, and free-free cooling from the electrons. Outside a thin boundary layer, where the accretion flow meets the star, we show that there is an extended settling region which is well-described by two self-similar solutions: (i) a two-temperature solution which is valid in an inner zone $r\le10^{2.5}$ ($r$ is in Schwarzchild units), and (ii) a one-temperature solution at larger radii. In both zones, $\rho\propto r^{-2}, \Omega\propto r^{-3/2}, v\propto r^0, T_p\propto r^{-1}$; in the two-temperature zone, $T_e\propto r^{-1/2}$. The luminosity of the settling zone arises from the rotational energy of the star as the star is braked by viscosity. Hence the luminosity and the flow parameters (density, temperature, angular velocity) are independent of $\dot M$. The settling solution described here is not advection-dominated, and is thus different from the self-similar ADAF found around black holes. When the spin of the star is small enough, however, the present solution transforms smoothly to a (settling) ADAF. We carried out a stability analysis of the settling flow. The flow is convectively and viscously stable and is unlikely to have strong winds or outflows. Unlike another cooling-dominated system --- the SLE disk, --- the settling flow is thermally stable provided that thermal conduction is taken into account. This strong saturated-like thermoconduction does not change the structure of the flow.Comment: 5 pages, 2 figures. To appear in proceedings of the Gamma 2001 symposiu

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.Comment: revised version: references updated, some new results included; 53 pages without any figures; complete versions can be found at http://www.mpa-garching.mpg.de/Lenses/Preprints/JeruLect.htm

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 $p(H)d \log H \propto H^\gamma d \log H$. 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 $\gamma$. 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 $\gamma \approx 0$ or even $\gamma =0.5$. 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 $\gamma\ \sgreat\ 0$, then it would be interesting to look for MeV bursts in the future.Comment: 5 pages, Latex using aps macros including one figure. Also available at ftp://shemesh.fiz.huji.ac.il or at http://shemesh.fiz.huji.ac.il/mev.ps To appear in Gamma-Ray Bursts, third workshop, Huntsville Oct-199