AGN emit roughly equal
amounts of energy throughout the major part of the electromagnetic
spectrum, extending from > 100µm in the FIR to
> 10 keV in the hard X-ray. Roughly 10% also emit strongly at
radio wavelengths and a significant fraction of these are strong
-ray sources. Thus, to
obtain a complete picture of these
powerful sources, observations must be made at all wavelengths.
To fully observe a single AGN requires obtaining time and observing on
many different telescopes and satellites, often using more than one
instrument in each case. In many parts of the spectrum these objects are
sufficiently faint to push the limits of current technology. Given that
many of them are variable on fairly short timescales,
obtaining a single snapshot of a AGN SED is a daunting task and
one that has not been completed for many AGN to date.
The number of compiled AGN SEDs in the literature
has increased rapidly in the past decade, including many for individual
objects (eg.
Treves et al.
1988,
Kolman et al. 1991,
Kuhn et al. 1995,
Puchnarewicz et al.
1995)
and larger compilations at both low-redshift
(Kriss 1988,
Sanders et al.
1989,
Barvainis 1990,
Masnou et al. 1992,
Elvis et al. 1994
(EWM94),
Fiore et al. 1995,
Laor et al. 1997)
and higher redshift
(Bechtold et al.
1994,
Tripp, Bechtold &
Green 1994).
While many of these incorporate data observed over many years and often
by many
different groups in order to produce an SED, the majority are
partially contemporaneous and
provide a reasonable approximation to an instantaneous SED for
relatively non-variable (ts
1 year) objects in at
least the near-IR-optical range.
When multiple epochs are available, it is also possible to
estimate the level of variability and thus the uncertainty in the final SED.
Examples of typical SEDs in the literature are given in
Figure 1.
|
Figure 1. Rest-frame radio-X-ray spectral
energy distributions (SEDs) for
low-redshift radio-loud (upper) and radio-quiet (lower) quasars (from
EWM94,
their Figure 1). These are typical of the quality of SEDs in the
current literature. The display of
|
A small number of AGN known to be strongly variable have been the subjects of coordinated campaigns to monitor and relate their variability at a number of wavelengths simultaneously. The AGN which has the most complete, simultaneous coverage is the brightest nearby quasar: 3C273 (Tuerler et al. 1998, Courvoisier 1998) for which fairly complete SEDs are available at a number of epochs.
There remain parts of the SED which are not well-observed in more than a
handful of objects. The FIR and sub-mm have very limited data which
recent new capabilities, such as the photometer on the Infrared Space
Observatory (ISO) and SCUBA (Submm Common-User Bolometer Array)
on the James Clerk Maxwell Telescope (JCMT),
should begin to rectify over the next few years. The extreme
ultra-violet (EUV)
region was observed by the wide field camera (WFC) on ROSAT and by the
Extreme UV Explorer (EUVE) but only a handful of the brightest, low-redshift
sources have been detected. In the
-ray, the Gamma-Ray
Observatory (GRO) has made an industry of
observing core-dominated, radio-loud quasars
(Fichtel et al.
1994).
The lack of strong -ray
emission from other classes of AGN
implies that beaming is an important factor and results in a
lack of knowledge on -ray
emission from the general AGN population.
In the high-energy -ray,
only three BL Lac objects (e.g. MKN421,
Zweerink et al.
1997,
MKN501,
Catanese et al.
1997)
have been detected to date, two of which
show flaring behaviour.
The radio region is also relatively unknown for radio-quiet AGN, which represent 90% of the class. This situation is rapidly improving as deeper surveys (FIRST: Gregg et al. 1996, NVSS: Condon et al. 1998) and directed studies (Kukula et al. 1998, Blundell & Beasley 1998) are completed.
L