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Active Galactic Nuclei (AGN)
The HEASARC archive provides a rich database of high-energy
observations of AGN, and these data have spawned numerous valuable
papers. In particular, the mature archives of ROSAT and ASCA
have yielded several important studies of large samples of AGN
and we highlight some of the most interesting and relevant ones.
The goal of current research on AGN is to understand the fundamental
physics of the central power source. A crucial piece of this puzzle is
understanding the distribution of gas and dust in the nucleus of an active
galaxy, and how this material is transported to and from the central
engine. The nature of the circumnuclear gas and dust is most directly
constrained when it is observed in absorption against the continuum and
line radiation emitted by the inner AGN. Malizia et al. (1997, ApJS 113,
311) combined data from the EXOSAT, Ginga, ROSAT, ASCA, HEAO-1, and BBXRT
archives within the HEASARC, to conduct a large-scale study of absorption
in AGN. The circumnuclear absorption in AGN often exceeds column
densities of NH ~ 1023cm-2 along the
line-of-sight (equivalent to extinctions of AV ~ 100) so only
X-ray and gamma-ray observations are able to provide a direct view
of the nucleus.
Conducting a systematic analysis of data from these combined archives
provides the only current method possible for studying column
variability because there are so few AGN which have been observed more
than once with a single satellite. By examining column densities and
axial ratios, Malizia et al. (1997) demonstrated that, in general, the
X-ray absorber is not dominated by the interstellar medium in the host
galaxy, but is a dense layer of gas whose orientation is unrelated to
the inclination of the host. This supports the so-called "Unified
Model" for AGN which postulates that the range of optical spectra
observed can be explained predominantly by differences in orientation
of a toroidal circumnuclear absorber. The Malizia et al. work was a
breakthrough study of variability of absorption in AGN, finding that
>70% of sources in the sample showed significant absorption
variability on time-scale of months to years. The importance of
variability studies is the insight gained into the location and
geometry of the absorbing gas. While some results had previously been
published for a few individual galaxies, the Malizia et al. study
provided the strongest evidence up to that point for a location of the
X-ray absorber in or near the broad-line-region, and for general
complexity of the absorber, which must be ionized or patchy in many
sources.
X-ray absorption column plotted against galaxy axial ratio
from Malizia et al. (1997).
Seyfert galaxies exhibit iron K-
emission lines in their X-ray spectra which are characteristic of
relativistic effects in an accretion disk surrounding a central black
hole. These lines can be used as a diagnostic of the innermost regions of
AGNs. Analysis of the Fe K- line in
a sample of quasars, taken from the ASCA archives (Nandra et al.
1997, ApJ 488, L91) revealed a decrease in the line strength with
increasing nuclear luminosity. While this effect had been suggested
previously, Nandra et al. went on to demonstrate, for the first time, that
this relation cannot be solely dependent on radio power because the effect
was observed within a radio-quiet subsample. They also noted that the red
wing of the line is reduced and the line peak energy increased with
nuclear luminosity. Nandra et al. suggested that the line evolution with
luminosity is driven by increasing ionization of the accretion disk. For a
given black hole mass, the higher luminosity sources must have higher
accretion rates and provide a more intense ionizing radiation field.
Both would tend to strip electrons from atoms in the accretion disk, which
would alter the line emission produced from those atoms in a way which
closely matches the observations.
Average profiles of relativistic iron lines from Nandra
et al. (1997).
One of the most important discoveries made using EGRET on CGRO was
the existence of gamma-ray luminous AGN. These all belong to a class
of AGN called "blazars", whose emission is believed to be dominated
by a jet pointed very close to our line-of-sight. The CGRO archive
is now used for systematic studies of such objects. Chiang &
Mukherjee (1998, ApJ 496, 752) have used the archive to show that pure
luminosity evolution explains the distributions of redshift and
luminosities observed. They also conclude that only 25% of the
diffuse emission measured by SAS 2 and EGRET can be attributed to
unresolved gamma-ray blazars and thus other sources of extra-galactic
gamma-ray emission must exist. The spatial distribution of sources
suspected to be associated with AGN is isotropic, as expected (Ozel &
Thompson 1996, ApJ 463, 105). Recently, Wallace et al. (2000, ApJ, in
press, astro-ph/0004252) have made a detailed study of the time
variability of unidentified EGRET sources on time-scales of days to
search for gamma-ray flares characteristic of AGNs; four unidentified
sources were found to flare.
Monitoring of such sources at energies of 20 - 100 keV can be
performed using CGRO BATSE archival data and measuring the
step in
flux as the source goes into and comes out of Earth occult. By
carefully considering systematic errors and using contemporaneous
measurements of selected objects with other instruments, notably
CGRO OSSE and BeppoSAX, Malizia et al. (2000, ApJ 531, 642) have been able
to study the variability of sources such as 4C 71.07 (20--100keV flux
~ 1.3 x 10-10 erg/cm2/s) on ~ 100-day
time-scales. These measurements, contemporaneous with those from
EGRET above 30 MeV, indicate a sharp high-energy turnover; perhaps more
severe than can be reasonably accounted for by standard synchrotron
self-Compton scenarios. The same team have recently improved on this
analysis method using detailed background modeling showing that the
BATSE archive will continue to be an important resource for studying
long-term variability.
Blazars, such as those detected by EGRET, cannot be found in the usual
optical surveys for AGN because they do not have strong emission
lines. They are usually identified by their combined X-ray and radio
emission so X-ray surveys are particularly important for building
large samples of blazars. Perlman et al. (1998, AJ 117, 2185; 2000,
astro-ph/9910321) have cross-correlated the ROSAT WGACAT with several
ground-based databases to find over 200 new blazars. Almost one
quarter of these are members of a previously unknown class of X-ray
bright, flat spectrum, radio-loud quasars.
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Last modified: Monday, 19-Jun-2006 11:24:57 EDT
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