Radio Plus X-ray Equals Black Hole Mass
Contact:
Christopher Wanjek
wanjek@gsfc.nasa.gov
301-286-4453September 16, 1999
Portsmouth, N.H. -- Scientists have stumbled upon a simple way to deduce the mass of large black holes through a relationship between their radio and x-ray luminosity. The method is easy to apply and doesn't depend on detailed model calculations.
The results of this galactic weighing process are presented today at the 5th Compton Symposium, an international meeting of gamma-ray experts held this year from September 15-17 in Portsmouth, N.H. The meeting is hosted by the University of New Hampshire and NASA Goddard Space Flight Center.
Dr. Insu Yi of the Korea Institute for Advanced Study and Dr. Stephen Boughn of Haverford College have already weighed ten massive black holes by measuring radio and x-ray flux ratios. The masses are in reasonable agreement with previous measurements of these black holes using more complicated methods.
"By measuring radio and X-ray luminosities," Yi said, "we deduce the temperature and density of the emitting gas, which in turn give a unique combination of black hole mass and mass accretion rate for a given set of measured radio and X-ray fluxes. Therefore, we not only get estimates of black hole masses but also mass accretion rate estimates."
Black holes are regions in space where matter is so dense and the force of gravity so great that not even light can escape the pull of gravity. Yi and Boughn are weighing supermassive black holes, which contain the mass of millions to billions of suns compressed to regions the size of our solar system. Scientists believe these types of black holes likely formed from the rapid collapse of gas in the early Universe and are present in the centers of most galaxies, including our Milky Way.
Yi and Boughn's calculations rely on radio and x-ray fluxes emitted from small, central regions of x-ray bright galaxies. The radio and x-ray emission comes from a black hole's accretion disk, a stream of gas that spirals into the black hole like water down a drain. The hot, fast-moving gas is the source of both radio and x-ray radiation.
Yi said this new method to calculate mass is only applicable to black holes with advection-dominated accretion flows (ADAF). This is a specific type of gas flow into a black hole that radiates energy less efficiently than does accretion onto the black holes associated with powerful active galactic nuclei (AGN), popularly known as quasars, blazars and radio galaxies. It is likely that black holes that Yi and Boughn are studying -- the more modest x-ray bright galactic nuclei (XBGN) -- are much more common than their more flamboyant AGN cousins.
Yi said he could deduce a black hole mass because the radio and x-ray radiation from ADAF is produced from the same energetic electrons that are moving quickly around black holes. This two-tier emission can determine the temperature, density, and magnetic field strength of ADAFs, which are dependent almost entirely by black hole mass and mass accretion rate.
A common, current way to measure black hole mass is to observe the orbit of stars around the suspected black hole. The more massive the black hole, the greater its gravitational pull and the greater its effect on the star's orbit. This is called the stellar dynamical method, based on Johannes Kepler's centuries-old laws. Recently, scientists have also been able to deduce a mass by determining the orbits of diffuse gas via its "maser" emission. (Maser is an acronym for microwave amplification by stimulated emission of radiation and is a close relative of the laser). In addition, a theoretical method of obtaining an upper limit to black hole mass involves the measurement of short-term and long-term fluctuations of x-ray flux.
"The stellar dynamical method is widely used," Yi said, "but there are some serious uncertainties about how to interpret stellar motion and their orbits. Our method simply needs two relatively easy measurements, radio flux and hard X-ray flux. It can be used to independently check the existing mass estimates. One useful application of our method is to find black hole candidates and follow up with the conventional dynamical methods."
The only uncertainties to Yi and Boughn's method, Yi said, is the measurement of magnetic field strength, which is not a large issue, and the obtaining of high angular resolution observations, which ensures the x-ray and radio fluxes are being emitted from the very central regions of galaxies.
According to Boughn, the results are encouraging. "When we applied our method to a few nearby XBGN with previously measured black hole masses, we found fairly good agreement. Considering the simplicity of the ADAF model and the fact that the x-ray measurements had an angular resolution that was rather poor, the method appears to be quite promising. If the ADAF model turns out to be justified, then our weighing method certainly is useful to expand the knowledge about the masses of black holes that are currently thought to reside in the centers of most galaxies."
Yi said by accumulating mass estimates of many black holes, scientists could then concentrate on the physical origins of massive, non-stellar black holes and their mass distribution function.
Yi and Boughn used data attained from two radio telescope arrays, the VLBA and VLA; and several x-ray telescopes, including ASCA, ROSAT and, in a few cases, Einstein. Yi looks forward to higher resolution and sensitivity of Chandra, NASA's x-ray satellite launched in July, as well as Astro-E and XMM, two x-ray satellites to be launched by Japan and Europe respectively early next year. In addition, the routine operation of the National Radio Astronomy Observatory's VLBA will result in many high angular resolution radio observations of galactic nuclei.
Dr. Yi will be in Portsmouth to present his results. The Compton Symposium is the fifth in a series of international symposia dedicated to research in gamma-ray astronomy.
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UNH News Bureau
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