Radio
+ X-rays = How to weigh black holes
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.
Dr. Insu Yi of the Korea Institute
for Advanced Study and Dr. Stephen Boughn of Haverford College
have already weighed 10 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 temperature
and density, 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
on 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 super-massive black holes, which contain the mass
of millions to billions of suns compressed to region 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 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,
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
AGN with previously measured black hole masses, we found fairly
good agreement. And this is considering the simplicity of the
ADAF model and the fact that the x-ray measurements had an angular
resolution that was rather poor. If the ADAF model turns out
to be justified, then our [weighing method] certainly be 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 VLBI 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 Astronomical Observatories' VLBA
will result in many high angular resolution radio observations
of galactic nuclei.
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