New Probe of Black Holes Discovered?

These oscillations are trapped close to the black hole by the same relativistic property of general relativity that produces the famous precession of the perihelion of Mercury. They produce a potentially powerful probe for two reasons: (a) they do not exist within Newtonian gravity and (b) the frequency of the most observable mode depends almost entirely upon only the mass and angular momentum of the black hole (and only very weakly upon the properties of the disk).

These two binary systems also happen to be the two "microquasars" that have been detected in our galaxy, producing relativistic jets of particles like quasars (but on a much smaller scale consistent with their smaller black holes). The power spectrum of X-ray fluctuations sometimes contains a narrow feature whose frequency does not vary as the luminosity of the source changes (unlike the other features that are observed in such systems), as predicted. The observations have been carried out with the RXTE satellite by a group at M.I.T.

In one object, GRS 1915+105, the 67 Hz frequency yields a black hole mass from 10.6 solar masses if it is nonrotating to 36.3 solar masses if it is maximally rotating. The mass of the suspected black hole in the other system, GRO J1655-40, has been determined from spectroscopic observations of its companion star to be 7.2 solar masses. The 300 Hz frequency of the stable feature in its power spectrum then indicates that the black hole is rotating at 93 percent of its maximum value. This is the first precise determination of the spin of a black hole, and agrees with an approximate value obtained from an analysis of the X-ray energy spectrum by a group headed by Nan Zhang at NASA Marshall Space Flight Center.