Black Hole Science
A driving Constellation-X science objective is to test General Relativity through observations of material falling into black holes, close to the event horizon where the strong field will dominate the observed properties. In addition Constellation-X will utilize the observed properties of accreting black holes to constrain the growth of black holes by measuring the fundamental parameter of black hole spin.
On macroscopic scales, General Relativity (GR) remains our best theory of gravity. For weak gravitational fields, GR has passed precision tests but strong-field tests of GR are more difficult. The lack of a single parameterization/theory for alternatives to GR highlights the need to probe strong-field gravity in as many independent and unbiased ways as possible. Of the known inventory of astrophysical objects, only neutron stars and black holes are strong-gravity entities. Black holes are observationally much simpler objects, having only two parameters: mass and spin. Constraint of black hole spin is a difficult measurement that has been achieved in very few cases.
On the right we show a 300 ks Constellation-X simulation of the broad line expected for an AGN having the parameters measured for MCG -6-30-15 (a nearly maximally spinning black hole). The simulation on the left shows how different the iron line shape appears when the black hole is not spinning.
Ever since the detection of rapid X-ray variability over 20 years ago, it has been clear that X-ray observations of accreting black holes provide a window on the immediate vicinity of the black hole event horizon. The most powerful technique for inner accretion disk studies to date is the study of the broad iron fluorescence line seen in the X-ray spectrum of many accreting black holes (see Figure). This line is emitted by the surface layers of the thin, Keplerian accretion disks believed to extend nearly down to the event horizon, and possesses a highly broadened and skewed energy profile sculpted by the effects of relativistic Doppler shifts and gravitational redshifts.
The most powerful technique for inner accretion disk studies to date is the study of the broad iron fluorescence line seen in the X-ray spectrum of many accreting black holes. Observations by the current generation of X-ray observatories have demonstrated the X-rays are highly variable, indicating that the accretion process is composed of hot spots generated by magnetic reconnection instabilities in the disk, probably analogous to the coronal loops seen above the solar surface. It is expected that the broad iron K lines that are seen with current observatories are made up of many narrower features that are currently smeared out because of insufficient collecting area and spectral resolving power.
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