Fermi -LAT Observations of Markarian 421: the Missing Piece of its Spectral Energy Distribution

We report on the γ-ray activity of the high-synchrotron-peaked BL Lacertae object Mrk 421 during the first 1.5 years of Fermi operation, from 2008 August 5 to 2010 March 12. We find that the Large Area Telescope (LAT) γ-ray spectrum above 0.3GeV can be well-described by a power-law function with photon index Γ = 1.78 ± 0.02 and average photon flux F (> 0.3GeV) = (7.23 ± 0.16) × 10 ph cm s. Over this time period, the Fermi -LAT spectrum above 0.3GeV was evaluated on 7-day-long time intervals, showing significant variations in the photon flux (up to a factor ∼ 3 from the minimum to the maximum flux), but mild spectral variations. The variability amplitude at X-ray frequencies measured by RXTE/ASM and Swift/BAT is substantially larger than that in γ-rays measured by Fermi -LAT, and these two energy ranges are not significantly correlated. We also present the first results from the 4.5month-long multifrequency campaign on Mrk 421, which included the VLBA, Swift, RXTE, MAGIC, the F-GAMMA, GASP-WEBT, and other collaborations and instruments which provided excellent temporal and energy coverage of the source throughout the entire campaign (2009 January 19 to 2009 June 1). During this campaign, Mrk 421 showed a low activity at all wavebands. The extensive multi-instrument (radio to TeV) data set provides an unprecedented, complete look at the quiescent spectral energy distribution (SED) for this source. The broad band SED was reproduced with a leptonic (one-zone Synchrotron SelfCompton) and a hadronic model (Synchrotron Proton Blazar). Both frameworks INAF Istituto di Radioastronomia, Sezione di Noto,Contrada Renna Bassa, 96017 Noto (SR), Italy Department of Physics, Purdue University, West Lafayette, IN 47907, USA Department of Physics and Astronomy, Brigham Young University, Provo, Utah 84602, USA Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK Department of Physics and Astronomy, Whittier College, Whittier, CA, USA INAF, Osservatorio Astronomico di Torino, I-10025 Pino Torinese (TO), Italy Universitat de València, 46010 València, Spain Department of Physics, University of Colorado, Denver, CO 80220, USA Department of Physics and Astronomy, Pomona College, Claremont CA 91711-6312, USA INAF Istituto di Radioastronomia, Stazione Radioastronomica di Medicina, I-40059 Medicina (Bologna), Italy