Black Hole Science
Accretion Power in Supermassive Black Holes
Accretion-powered emission onto supermassive black holes (SMBHs) constitutes a huge, if not dominant, component of radiative energy in the observable Universe (e.g., Brandt and Hasinger 2005). The signal we receive from accreted matter comes predominatently in the X-ray band, so Constellation-X is well-suited to studies of the accretion power of SMBHs.
The observation of broad iron fluorescence lines seen in the X-ray spectrum of many accreting black holes (e.g., Tanaka et al. 1995, Reynolds and Nowak 2003) is the most powerful tool for inner accretion disk studies. This line is emitted by the surface layers of the thin, Keplerian accretion disks believed to extend nearly down to the last stable orbit, and possesses a highly broadened and skewed energy profile sculpted by the effects of relativistic Doppler shifts and gravitational redshifts.
XMM-Newton and Chandra iron line spectra have revealed robust examples of extremely broadened iron lines in both stellar mass and supermassive black hole systems. In fact, the highest quality spectra show evidence of extreme gravitational redshifts indicative of rapidly rotating black holes (Fabian et al. 2002, Miller et al. 2004). In addition, some systems have shown time-variable, quasi-periodic substructure in their iron line emission, which could be a first glimpse at orbiting, non-axisymmectric features in the inner accretion disk (Iwasawa et al. 2004).
Despite progress, the line profiles available are temporally averaged over several orbits of the emitting material in the accretion disk, so several models (e.g. with different black hole spin parameters) can fit the same line profile (Dovciak, Karas and Yaqoob 2004). The existing data can not constrain both the space-time geometry and the accretion disk physics.
The extremely high effective area and spectral resolution of Constellation-X will produce iron line profiles of the brightest active galactic nuclei (AGN) in less than one orbit of the emitting material. With such short timescales, we can determine the line profile over the light-crossing time of the system for the more massive AGN systems. This will allow observations of reverberation effects due to dramatic X-ray flares tp probe the nature of space-time around the black hole (Young and Reynolds 2000).
Constellation-X will also observe and measure black hole spin. Theory predicts that accretion of matter onto a black hole will cause it to spin up (to a maximum spin), spinning space-time along with it. Constellation-X will observe these effects and study the relationship between spin and other properties of the black hole system (e.g., between spin and the existence of a relativistic jet). Iron line studies by Constellation-X of the brightest AGN will allow a calibration of time-averaged line profiles for measuring black hole spin. Then further observations of fainter sources exhibiting the calibrated spectral feature will allow determination of the spin down to a very faint flux level.
In addition to probing space-time around the black hole, Constellation-X observations of the innermost region of accretion disks will probe the behavior of material as it undergoes the "final plunge" into the black hole's event horizon and will investigate the possibility that the spin energy of the black hole is energizing the inner accretion disk and/or relativistic jet (Blandford and Znajek 1977).
X-ray view of an accretion disk (Armitage and Reynolds). View of the disk as seen by a distant observer at an inclination angle of 30° (left) and 80° (right). The inset in each panel shows the corresponding Iron K-shell spectral line profile that will be observed by Constellation-X.
The Beyond Einstein observatories, LISA and Constellation-X will allow precise measurments of the two critical black hole parameters, mass and angular momentum, in complementary ways. LISA will provide exquisite precision for a number of very special systems (sellar-mass black holes spiraling into 106 MSun black holes and mergers of SMBHs with masses < 107 MSun). Constellation-X will measure mass and spin for a large number of accreting black holes, from stellar-mass systems to multi-billion solar-mass black holes at the centers of galaxies. In addition, Constellation-X will further our knowledge of how matter accretes onto black holes, a process that is critical to the total radiant energy of the observed Universe.
References
Armitage, P. J. and Reynolds, C. S., 2003, MNRAS, 341, 1041Blandford, R. D. and Znajek, R. L., 1977, MNRAS, 179, 433
Brandt, W. N., and Hasinger, G. 2005, astro-ph/050105
Doviak, M., Karas, V., and Yaqoob, T. 2004, ApJS, 153, 205
Fabian, A. C. et al., 2002, MNRAS, 331, L35
Iwasawa, K., Miniutti, G. and Fabian, A. C., 2004, MNRAS, 355, 1073
Miller, J. M., et al., 2004, ApJ, 606, L131
Reynolds, C. S. and Nowak, M. A., 2003, Phys Reports, 377, 389
Tanaka, Y., et al., 1995, Nature, 375, 659
Young, A. J. and Reynolds, C. S., 2000, ApJ, 529, 101
