Black Hole Science
Iron Line Diagnostics
Nature has provided a direct probe of strong-field General Relativity in the vicinity of black holes. Relativistically broadened iron K lines have been detected from within 6 gravitational radii of black holes by ASCA, XMM-Newton, Chandra, and Suzaku. 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.
Simulation of iron lines observed by Constellation-X. Blue line shows the line profile for a Schwarzschild (non-spinning) black hole, and the red line shows the line profile for a Kerr (spinning) black hole.
Click on image for larger version
The line profiles currently available, however, are typically averaged over one day. The orbital timescale for a 106-7 solar mass black hole is measured in minutes to hours, so the currently observed line profiles are averaged over many times the relevant physical timescales. Constellation-X will finally allow studies on the much shorter timescales characteristic of local supermassive black holes.
Constellation-X will be able to observe and measure black hole spin. Theory predicts that accretion of matter onto a black hole will cause the black hole 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. Constellation-X 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 2003). 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.
Simulation of the iron line intensity as a function of energy and time for an accretion disk surrounding a black hole. These "streamers" show individual accretion disk hot spots moving around the disk.
Click on image for larger version
References
Armitage, P. J., and Reynolds, C. S. 2003, MNRAS, 341, 1041Blandford, R. D. and Znajek, R. L., 1977, MNRAS, 179, 433
Doviak, M., Karas, V., and Yaqoob, T. 2004, ApJS, 153, 205
