We report on the \gamma -ray activity of the blazar Mrk 501 during the first 480 days of \Fermi operation .
We find that the average LAT \gamma -ray spectrum of Mrk 501 can be well described by a single power-law function with a photon index of 1.78 \pm 0.03 .
While we observe relatively mild flux variations with the Fermi -LAT ( within less than a factor of 2 ) , we detect remarkable spectral variability where the hardest observed spectral index within the LAT energy range is 1.52 \pm 0.14 , and the softest one is 2.51 \pm 0.20 .
These unexpected spectral changes do not correlate with the measured flux variations above 0.3 GeV .
In this paper , we also present the first results from the 4.5-month-long multifrequency campaign ( 2009 March 15 – August 1 ) on Mrk 501 , which included the VLBA , Swift , RXTE , MAGIC and VERITAS , the F-GAMMA , GASP-WEBT , and other collaborations and instruments which provided excellent time and energy coverage of the source throughout the entire campaign .
The extensive radio to TeV data set from this campaign provides us with the most detailed spectral energy distribution yet collected for this source during its relatively low activity .
The average spectral energy distribution of Mrk 501 is well described by the standard one-zone synchrotron self-Compton model .
In the framework of this model , we find that the dominant emission region is characterized by a size \lesssim 0.1 pc ( comparable within a factor of few to the size of the partially-resolved VLBA core at 15-43 GHz ) , and that the total jet power ( \simeq 10 ^ { 44 } erg s ^ { -1 } ) constitutes only a small fraction ( \sim 10 ^ { -3 } ) of the Eddington luminosity .
The energy distribution of the freshly-accelerated radiating electrons required to fit the time-averaged data has a broken power-law form in the energy range 0.3 GeV -10 TeV , with spectral indices 2.2 and 2.7 below and above the break energy of 20 GeV .
We argue that such a form is consistent with a scenario in which the bulk of the energy dissipation within the dominant emission zone of Mrk 501 is due to relativistic , proton-mediated shocks .
We find that the ultrarelativistic electrons and mildly relativistic protons within the blazar zone , if comparable in number , are in approximate energy equipartition , with their energy dominating the jet magnetic field energy by about two orders of magnitude .