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We report on observations of BL Lacertae during the first 18 months ofFermi-LAT science operations and present results from a 48-day multifrequencycoordinated campaign from 2008 August 19 to 2008 October 7. The radio togamma-ray behavior of BL Lac is unveiled during a low activity state thanks tothe 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 EGREToutburst. Moderate and uncorrelated variability has been detected in UV andX-rays. The X-ray spectrum is found to be concave indicating the transitionregion between the low and high energy component of the spectral energydistribution (SED). VLBA observation detected a synchrotron spectrumself-absorption turnover in the innermost part of the radio jet appearing to beelongated and inhomogeneous, and constrained the average magnetic field thereto be less than 3 G. Over the following months BL Lac appeared variable ingamma-rays, showing flares (in 2009 April and 2010 January). There is noevidence for correlation of the gamma-rays with the optical flux monitored fromthe ground in 18 months. The SED may be described by a single zone or two zonesynchrotron self-Compton (SSC) model, but a hybrid SSC plus external radiationCompton (ERC) model seems preferred based on the observed variability and thefact that it provides a fit closest to equipartition.

The first Fermi multifrequency campaign on BL Lacertae: characterizing the low-activity state of the eponymous blazar