The Yell of a Black Hole
How fast are
black holes at the centers of distant galaxies swallowing the matter around them? Using
observations from the Japanese/NASA Advanced Satellite for Cosmology and
Astrophysics (ASCA), Paul Nandra, Richard Mushotzky, and their
research team have begun to shed some light on this question.
Black holes are not directly visible, so learning about them can be
Scientists observing an X-ray
emission line (specifically, the K-alpha line of iron) found it was an indicator of
properties of the black holes found in the centers of
active galactic nuclei (AGN). The emission line is generated by
processes deep inside the
accretion disk swirling around the massive black hole.
"The indicator can be thought of as the final cry of doomed matter as it
slides down the throat of a black hole," said Nandra.
The scientists used the
Advanced Satellite for Cosmology and Astrophysics (ASCA) to observe
of almost 40 AGN. These objects, which emit strongly in X-rays, are
believed to harbor massive black holes at their cores, surrounded by rotating
disks of gas. Friction slows the material in these accretion disks causing
the material to spiral in closer and closer to the black hole. Eventually,
it crosses inside of the event horizon of the black hole, the point of no return, and disappears from the known Universe.
Data from ASCA's Solid-state Imaging Spectrometer (SIS) showed that
the iron line decreased in strength with increasing luminosity, and that the
line actually disappeared for the most powerful AGNs (with luminosities
than forty trillion times that of the Sun). ASCA's superior resolving
power revealed that the shape of the emission line changed significantly as
well: the lines had less of a red "tail" at higher luminosities. Both of these
observations offer clues about the nature of the AGNs producing the
Spectrum from a Faint (Low Luminosity) AGN
Spectrum from a Bright (High Luminosity) AGN
In the standard AGN model, the massive black hole is surrounded by an
accretion disk and, outside of that, a thick torus. All three are then
embedded inside a host galaxy of stars. One explanation for the results
Nandra and his team observed is that the shape and strength of the observed
emission line can vary depending on how much of the
emission comes from the accretion disk and how much comes from the torus.
Nandra and colleagues suggested a second explanation several years ago, that
the lack of iron line emission in high-luminosity AGNs might be due to the
fact that these galaxies have a high accretion rate, which would heat up the
disk more and ionize or partially ionize the iron. If this model is correct, the
changes observed in the iron line's strength and profile are ultimately
caused by different accretion rates. The more intense X-ray luminosity that a
higher accretion rate generates would tend to strip atoms in the accretion disk,
and more blue emission would be observed from the highly ionized atoms.
Currently, which model is correct remains a matter of speculation. Both
models fit the ASCA observations. Some progress will be made by studying a
larger, more complete sample of the AGN data within the ASCA archive.
Conclusive tests, however, will have to wait for instruments which can
more efficiently resolve high-energy photons, such as those aboard the Chandra X-ray Observatory.