We report on the \gamma -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 ) \gamma -ray spectrum above 0.3 GeV can be well-described by a power-law function with photon index \Gamma = 1.78 \pm 0.02 and average photon flux F ( > 0.3 GeV ) = ( 7.23 \pm 0.16 ) \times 10 ^ { -8 } ph cm ^ { -2 } s ^ { -1 } .
Over this time period , the Fermi -LAT spectrum above 0.3 GeV was evaluated on 7-day-long time intervals , showing significant variations in the photon flux ( up to a factor \sim 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 \gamma -rays measured by Fermi -LAT , and these two energy ranges are not significantly correlated .
We also present the first results from the 4.5-month-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 Self-Compton ) and a hadronic model ( Synchrotron Proton Blazar ) .
Both frameworks are able to describe the average SED reasonably well , implying comparable jet powers but very different characteristics for the blazar emission site .