Haverford College professor of astronomy Stephen Boughn knows.
Boughn and Insu Yi, a colleague from the Korea Institute for Advanced Study, have found a simple way to deduce the mass of large black holes by analyzing a relationship between their radio and x-ray luminosity.
The results of their research were presented from September 15-17 at the 5th Compton Symposium, an international meeting of gamma-ray experts hosted in Portsmouth, N.H. by the University of New Hampshire's Space Science Center and NASA Goddard Space Flight Center.
Accreting matter--gas matter and all of its energy that fall into black holes--make black holes some of the largest sources of energy in the universe. The amount of energy released by black holes is enormous; our galaxy has a relatively small black hole that is a few million times the mass of the sun. Just as the sun emits radiation in the form of visible light and infrared light, objects in the universe emit their energy in x-ray and radio wave forms, two parts of the electromagnetic spectrum, which Boughn and his colleague use to measure the mass of black holes.
"There is a uniform x-ray background that is not perfectly understood. Some calculations have shown that the radio emission from these black holes was relatively insensitive to how much matter fell in them but was extremely sensitive to the mass of the black hole. By measuring the x-rays and the radio waves emitted, you can make a little correction to how much matter falls into a black hole and then deduce the mass of that black hole," says Stephen Boughn.
Using flux ratios of x-rays and radio waves, Boughn and Yi have weighed ten massive black holes. Their calculations of these masses are in reasonable agreement with previous measurements using more complicated methods of these black holes. "The estimates agreed within a factor of two, which is pretty good in this business," says Boughn.
Their method, according to Boughn and Yi, is easy to apply and doesn't depend on detailed model calculations, like some of the previous ways of weighing black holes. In the past, the mass of black holes has been measured by observing the orbit of stars around the suspected black hole. The more massive the black hole, the greater its gravitational pull and effect on the stars' orbit. Known as the stellar dynamical method, this way of measuring a black hole's mass is based on Johannes Kepler's centuries-old laws. Although the stellar dynamical method has been widely used, it includes some serious uncertainties about how to interpret stellar motion and their orbits.
The only uncertainties associated with Boughn and Yi's method is the measurement of magnetic field strength, which is not a large issue, and the obtaining of high angular resolution observations, which ensures that the x-ray and radio fluxes are being emitted from the very central regions of galaxies.
By using Boughn and Yi's method and accumulating mass estimates of many black holes, scientists can turn their astronomical focus to the physical origins of massive, non-stellar black holes and their role in the universe.
