First clues about jet physics are revealed by blazars Spectral Energy
Distributions (SEDs). These are typically double-humped (see left
figure, from my PhD Thesis work). The first component peaks anywhere
from IR to optical in the so-called "red blazars" (including classical
Flat Spectrum Radio Quasars and Low-energy peaked BL Lacs, LBLs) and
at UV/X-rays in the so-called "blue blazars" (including High-energy
peaked BL Lacs, HBLs). This component is polarized and rapidly
variable, especially above the peak, so there is little doubt that its
origin is synchrotron emission from high-energy electrons in the
jet. The second component extends up to gamma-rays, peaking at GeV
energies in red blazars and at TeV energies in blue blazars, and its
origin is less well understood. A popular model is that it originates
from Inverse Compton (IC) scattering of seed photons off the jet's
electrons. The origin of the seed photons is under debate, and could
be different in red and blue blazars. It is current belief that the
seed photons are external to the jet (thermal emission from disk,
Broad Line Regions, or torus) in red blazars, and internal to the jet
(synchrotron-self Compton, SSC) in their blue counterparts. Interestingly, there seems to be a continuity of SED shapes with luminosity going from red to blue jets. As several selection effects could be at play, this "luminosity sequence" is currently under investigation. It is important to verify luminosity trends - if confirmed, they could have crucial ripercussions on unification theories for blazars and on the origin of radio-loud activity.
Correlated multiwavelength variability is the key to probe the structure of the inner jet in blazars and the origin of the seed photons for the IC process. In the context of a simple, homogeneous scenario, the emission at the synchrotron and IC peaks is produced by the same electron population. Thus, if the synchrotron flux varies (due, for example, to a variation of the electron density), the IC flux should also vary simultaneously, with no time delays. (Other factors may come into play to cause variability - such as beaming, external radiation density, etc. - but to a zeroth order this argument provides a reasonable working frame.) Thus, one expects correlated variability between IR/optical and GeV flux in red blazars, and correlated variability between X-rays and TeV in blue blazars.
To properly test this scenario one needs continuously sampled light
curves at various wavelengths, which are notoriously difficult to
obtain. It is no wonder, then, that good quality light curves were
obtained only for a few sources of both the red and blue type. The
Figure on the left shows a collection of contemporaneous SEDs at
various epochs for the red jet in 3C279 (from Wehrle et al. 1998). It
shows that, generally, when the optical varies, the GeV flux varies
too, although not always with a precisely predictable relationship
between the two wavelengths. The Figure on the right shows the X-ray
and TeV light curves of the blue blazar Mrk501, one of only a handful
bright TeV sources, during the X-ray/TeV outburst of June 1998 (from
Sambruna et al. 2000). The X-ray and TeV flux vary together, with no
delays larger than 1 day (the sampling of the TeV observations). My
undergraduate student Emily Chapman is working on probing the
X-ray/TeV correlation on longer timescales (months to years) using
archival RXTE, HEGRA, Whipple observations (Chapman et
al. 2002).
Thus, to a zeroth order the current body of data supports the synchrotron/IC scenario. Multiwavelength campaigns of blazars are currently undermined by the death of many crucial experiments - such as CGRO and SAX - but will be soon revamped by the advent of more powerful telescopes, such as GLAST at GeV energies and HESS and VERITAS at TeV energies. Hundreds more gamma-ray blazars will be discovered at both energies, allowing the parameter space to expand and new constraints on the theoretical models.
This work is supported by NASA's ADP funds.