Black Hole Science
Galactic Black Holes
Stellar-mass black holes in binary systems become X-ray bright when matter is transferred to the black hole from its companion. These objects are the nearest X-ray bright black holes that Constellation-X can study (McClintock and Remillard 2004). Many of these systems have been extensively observed at optical and other wavelengths, and their masses have been measured using the methods of dynamical astronomy. These precise mass measurements makes constraining the black hole's spin easier. Perhaps most important, these are the only black holes for which X-ray spectroscopic and timing signatures of matter moving in the curved space-time near the event horizon have been detected. Constellation-X will be the first X-ray observatory with the capability to study both of these strong gravity signatures simultaneously.
Recent X-ray observations have shown that, like their supermassive cousins, stellar-mass black holes show broad, relativistic iron K-shell emission lines (Miller et al. 2002, in 't Zand et al. 2002, Miniutti, Fabian and Miller 2004; Miller et al. 2004). Since the line profiles can be very similar to those seen from the supermassive black holes, they are also likely to be produced in the innermost accretion flow near the black hole. Because these objects are bright, Constellation-X will be able to measure their relativistic line profiles with exquisite precision. Comparisons between lines detected in a number of stellar-mass black holes and Seyfert AGN will test the relative importance of mass accretion versus black hole formation in driving spin.
Observations of some of these same black hole binaries with NASA's Rossi X-ray Timing Explorer (RXTE) resulted in the discovery of high frequency quasi-periodic intensity oscillations (QPOs) with frequencies as high as 450 Hz (Remillard et al. 1999; Strohmayer 2001). These QPOs are the highest frequency flux variations seen from any black holes, and have frequencies characteristic of the orbital timescale near the event horizon. Four of these objects show a pair of QPOs with frequencies in a 3:2 ratio, which could be direct evidence of a resonance phenomenon associated with General Relativistic effects near the black hole (Abramowicz and Kluzniak 2001; Remillard et al. 2002). Interestingly, very recent work on the Galactic microquasar GRS 1915+105 using RXTE suggests that the strength of the broad iron K-shell line and the phase of maximum intensity of a lower frequency (1-2 Hz) QPO are correlated (Miller and Homan 2005). This suggests that simultaneous X-ray spectroscopy and fast timing measurements can provide new diagnostics of the space-time in the immediate vicinity of these black holes, particularly if Constellation-X can find a direct link between properties of the relativistic iron K-shell line and the high-frequency QPOs.
The presence of two different diagnostics provides determination of both the radial location of the X-ray emitting matter and its orbital frequency. If, for example, the black hole mass is known from optical observations, and the spin can be constrained using the observed relativistic line profile, then measurements of QPO frequencies could be used to test whether the orbital frequency at that radius is in agreement with the value predicted by General Relativity. Although such inferences require an understanding of the QPO production mechanism and this problem may not be fully solved by the time of the Constellation-X launch, the new data provided by Constellation-X will, at a minimum, greatly increase our understanding of how both the relativistic lines and QPOs are generated, and thus bring us closer to the goal of using them as fundamental probes of strong gravity.
References
Abramowicz, M. A., and Kluzniak, W., 2001, A&A, 374, L19in't Zand, J. J. M., et al., 2002, A&A, 390, 597
McClintock, J. E., and Remillard, R. A., 2004, to appear in "Compact Stellar X-ray Sources," eds. W. H. G. Lewin and M. van der Klis, Cambridge University Press (astro-ph/0306213)
Miller, J. M., et al., 2002, ApJ, 578, 348
Miller, J. M., et al., 2004, ApJ, 606, L131
Miller, J. M., and Homan, J., 2005, ApJ, 613, L107
Miniutti, G., Fabian, A. C., and Miller, J. M., 2004, MNRAS, 351, 466
Remillard, R. A., Morgan, E. H., McClintock, J. E., Bailyn, C. D., and Orosz, J. A., 1999, ApJ, 522, 1030
Remillard, R. A., Muno, M. P., McClintock, J. E., Orosz, J. A., 2002, ApJ, 580, 1030
Strohmayer, T. E., 2001, ApJ, 552, L49
