X-ray binaries (XRBs) are systems in which a neutron star or a black hole accretes matter from a companion secondary star. Several XRBs have mass functions greater than 3M_sun, the maximum mass of a neutron star. These are identified as black hole candidates purely on the basis of mass, but in a few cases there is additional evidence to suggest that the accreting stars actually do have event horizons (see below). Black hole XRBs display at least five spectral states: quiescent state, low state, intermediate state, high state, and very high state. The states are believed to correspond to increasing mass accretion rates. The phenomenology of XRBs can be explained by models which combine the following two basic modes of accretion. (1) Thin accretion disk: In this well-known accretion model, the gas radiates efficiently, is relatively cool, and is geometrically thin in the vertical direction. The spectrum is nearly blackbody. (2) Advection-dominated accretion flow (ADAF): Here the accreting gas is optically thin, radiates inefficiently, and is quasi-spherical. Because of the negligible loss of energy through radiation, the gas is extremely hot, with the ions approaching the virial temperature, T_i ~ 10(12) K /r, where r is the radius in Schwarzschild units. The electrons, however, level off at a temperature T_e ~ 10(9-10(10)) K. The spectrum consists of Comptonized synchrotron and bremsstrahlung emission. The quiescent state of black hole XRBs has been explained with a model in which the accreting gas is in the form of a thin disk at large radii, r>rtr ~ 10(3-10^4) , and an ADAF at smaller radii, r<rtr. Detailed models of two quiescent systems, V404 Cyg and A0620-00, fit the observed spectra well. Nearly all the observed radiation is from the ADAF. The models require the accreting star to have an event horizon through which the enormous thermal energy of the ADAF disappears. The success of the models is therefore a strong indication that these stars are true black holes. In addition, neutron star XRBs in the quiescent state are seen to be different from black hole XRBs. The data are consistent with neutron stars having solid surfaces and black holes having horizons. The interpretation of the other spectral states of black hole XRBs is less certain, but the following scenario explains most of the observations. The low state is similar to the quiescent state in geometry, but has a higher mass accretion rate. Comptonization is more important and the spectrum is a very hard power law (photon index ~ 1.5-2) extending out to >100 keV. In the intermediate state, the transition radius rtr moves in and the ADAF is limited to a smaller range of r. In the high state, the thin disk extends all the way down to the marginally stable orbit, and the spectrum is primarily an ultrasoft X-ray blackbody. Finally, in the very high state, the thin disk appears to develop an active corona which radiates a significant amount of hard radiation. The extreme ion temperature in ADAFs leads to two interesting phenomena. First, energetic helium ions produce substantial amounts of lithium via spallation. This could explain the puzzling detection of lithium in the secondaries of several XRBs. Second, the protons interact with each other and produce gamma-rays at ~ 100 MeV via pion production. Such gamma-rays have been detected from the Galactic Center source Sgr A(*) , which is a supermassive black hole in the quiescent state.
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 1996