Exploring Black Holes

Two NASA astrophysicists at the Goddard Space Flight Center at Greenbelt, Md., Nikolai Shaposhnikov and Lev Titarchuk, have found an alternate way to ascertain the masses of black holes. The method, first suggested in 1998 by Titarchuk, was used to determine the mass of a black hole located in the Cygnus X-1 system, which is comprised of a blue supergiant star accompanied by a massive invisible object. The wobble experienced by the star, observed with optical instruments, indicated the invisible presence to be a black hole containing the mass of about 10 of our Suns. Titarchuk’s method indicates that the black hole in the binary system has 8.7 times the mass of our Sun, with a margin of error of .8 solar mass The similar results obtained from the two methods of determining mass gave the scientists assurance that the new system is accurate and can reveal the mass of a black hole when other methods cannot.

Titarchuk’s technique uses the connections between the black hole and the accretion disk, or material spiraling around it. Gas circling in these disks will at some point enter the black hole. When the amount of gas entering the black hole increases to a high level, the material gets backed up into a hot region similar to a traffic jam. Titarchuk has demonstrated that the more distant this hot region lies from the black hole and the longer the orbital period, the larger the mass of the black hole.

The gas jamming up in the hot region is connected to the observation that variations in x-ray intensity occur on a nearly, but not exactly, periodic basis. These quasi-periodic oscillations (QPOs) are seen in many systems containing black holes and show simple, predictable changes in the spectrum of the system as the gases heat and cool in response to variations in the rate of accretion. Observations from NASA’s Rossi X-Ray Timing Explorer (RXTE) satellite reveal a very close correlation between QPO frequencies and the spectrum of the system, which indicate the efficiency of the black hole at generating x-rays.

With information from the European Space Agency’s XMM-Newton X-Ray Observatory, two QPOs were detected in NGC-5408 X- 1, the brightest source of x-rays in the irregular galaxy NGC -5408, sixteen light years away from Earth in the Centaurus constellation. The QPO frequencies as well as luminosity and spectrum features suggest the presence of an intermediate mass black hole with the mass of 2,000 Suns. Although the existence of these black holes is controversial because of the lack of an explanation for their formation, two other methods of estimating the mass of the black hole were employed, and all three agreed within a factor of two.