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

Why Do AGN Lighthouses Switch Off?

Nearby galactic nuclei are observed to be very much dimmer than active galactic nuclei in distant galaxies. The Chandra X-ray Observatory has provided a definitive explanation for why this is so. With its excellent angular resolution, Chandra has imaged hot X-ray-emitting gas close to the gravitational capture radius of a handful of supermassive black holes, including Sgr A* in the nucleus of our own Galaxy. These observations provide direct and reliable estimates of the Bondi mass accretion rate in these nuclei. It is found that the Bondi accretion rate is significantly below the Eddington mass accretion rate, but this alone does not explain the dimness of the accretion flows. In all the systems observed so far, the accretion luminosity is orders of magnitude less than Mdot_{Bondi} times c^2, which means that the accretion must occur via a radiatively inefficient mode. This conclusion, which was strongly suspected for many years, is now inescapable. Furthermore, if the accretion in these nuclei occurs via either a Bondi flow or an advection-dominated accretion flow, the accreting plasma must be two-temperature at small radii, and the central mass must have an event horizon. Convection, winds and jets may play a role, but observations do not yet permit definite conclusions.Comment: Invited review, to appear in "Lighthouses of the Universe" eds. M. Gilfanov, R. Sunyaev et al., Springer-Verlag; 25 pages, 8 figure

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

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

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