Black holes get new way to be weighed

William Atkins
Monday, 21 May 2007 21:02

Science - Space

Nikolai Shaposhnikov, a scientist at GSFC’s Universities Space Research Association, and Lev Titarchuk, a research scientist at George Mason University (Arlington, Virginia) and the Naval Research Laboratory (Washington D.C.), have successfully tested a way to determine the masses of black holes, even when current methods fail to come up with a value.

They tested their system themselves and also had other scientists try out the process.

Shaposhnikov and Titarchuk measured the black hole in a binary system called Cygnus X-1 (often abbreviated Cyg X-1, and located in the constellation Cygnus, about 10,000 light-years from the Earth). It contains a blue supergiant star and huge but invisible companion that is believed to be a black hole.

Previous calculations show that the black hole has about ten solar masses (that is, it is ten times more massive than our Sun). When the two scientists used their method, they found the black hole to be 8.7 solar masses, with a margin of error of ± 0.8 solar masses.

Independently, GSFC scientists Tod Strohymayer and Richard Mushotzhy, along with four associates, estimated an ultra-luminous x-ray (ULX) source that is located in the irregular NGC 5408 galaxy, which is about 20 million light-years from the Earth.

(A ULX is thought to be a medium-sized black hole that takes in matter from a nearby companion star. One light-year is the distance that light travels in vacuum over a one year period—light travels at about 186,000 miles per second.)

Using the XMM-Newton (X-ray Multi-Mirror Mission) orbiting x-ray observatory, from the European Space Agency (ESA), the research group found the black hole to have a mass of about 2,000 solar masses.

Titarchuk—who first suggested this particular black hole weighing method back in 1998—and Shaposhnikov measure the mass of black holes by knowing that gaseous matter within accretion disks, which surround black holes, eventually spiral into black holes. Oftentimes these gases, as they flow into black holes, spiral in so fast that some of it is unable to make the journey unimpeded.

(A accretion disk is a structure formed by diffuse material in orbital motion around a central body. In the case of black holes, an accretion disk contains high density gases that are located on the boundary around the black hole, its event horizon.)

We can compare this situation with troublesome road construction: Four lanes of traffic at rush hour are reduced down to one lane so that vehicles begin to back up and must wait their turn.

With matter going into a black hole, the faster that gases flow into the black hole, the more likely for it to become backlogged. Titarchuk has calculated that the distance from the black hole where this backlog occurs can directly determine how massive the black hole has become. Further out from (closer to) the black hole where the matter begins to clog up means the more (less) massive is the black hole.

The reason for this linking of distance-of-the-backlog and mass-of-the-black-hole is believed to occur because of quasi-periodic oscillations (QPOs)—what are x-ray intensity variations (flickering of x-ray radiation) that generally repeat their pattern on a regular basis. The QPOs cause changes in the x-ray spectrum as the gases heat up and cool down. The gases are hotter in these areas where they are clogged up and cooler in areas where they are freely flowing.

The tests performed by Titarachuk and Shaposhnikov and the tests conducted by Tod Strohymayer, Richard Mushotzhy, and their collaborators produced results that were similar to other methods already in use for measuring masses of black holes.

The results of Titarachuk and Shaposhnikov (“Determination of Black Hole Mass in Cyg X-1 by Scaling of Spectral Index-QPO Frequency Correlation”) will appear in the July 1, 2007 issue of Astrophysical Journal.

The results of Strohmayer and Mushotzky, along with co-authors L. Winter, R. Soria, P. Uttley, and M. Cropper, will appear in the Astrophysical Journal under the title “Quasi-Periodic Variability in NGC 5408 X-1”.