The first Fermi multifrequency campaign on BL Lacertae: characterizing
  the low-activity state of the eponymous blazar
  We report on observations of BL Lacertae during the first 18 months of
Fermi-LAT science operations and present results from a 48-day multifrequency
coordinated campaign from 2008 August 19 to 2008 October 7. The radio to
gamma-ray behavior of BL Lac is unveiled during a low activity state thanks to
the coordinated observations of radio-band (Metsahovi and VLBA),
near-IR/optical (Tuorla, Steward, OAGH and MDM) and X-ray (RXTE and Swift)
observatories. No variability was resolved in gamma-rays during the campaign,
and the brightness level was 15 times lower than the level of the 1997 EGRET
outburst. Moderate and uncorrelated variability has been detected in UV and
X-rays. The X-ray spectrum is found to be concave indicating the transition
region between the low and high energy component of the spectral energy
distribution (SED). VLBA observation detected a synchrotron spectrum
self-absorption turnover in the innermost part of the radio jet appearing to be
elongated and inhomogeneous, and constrained the average magnetic field there
to be less than 3 G. Over the following months BL Lac appeared variable in
gamma-rays, showing flares (in 2009 April and 2010 January). There is no
evidence for correlation of the gamma-rays with the optical flux monitored from
the ground in 18 months. The SED may be described by a single zone or two zone
synchrotron self-Compton (SSC) model, but a hybrid SSC plus external radiation
Compton (ERC) model seems preferred based on the observed variability and the
fact that it provides a fit closest to equipartition.
