Insights into the high-energy gamma-ray emission of Markarian 501 from extensive multifrequency observations in the Fermi era

© The American Astronomical Society. Royal Swedish Academy of Sciences Research Fellow, funded by a grant from the K. A. Wallenberg Foundation.Partially supported by the International Doctorate on Astroparticle Physics (IDAPP) program.Supported by INFN Padova.We acknowledge the use of public data from the Swift and RXTE data archive. The Metsahovi team acknowledges the support from the Academy of Finland to the observing projects (numbers 212656, 210338, among others). This research has made use of data obtained from the National Radio Astronomy Observatory's Very Long Baseline Array (VLBA), projects BK150, BP143, and MOJAVE. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. St. Petersburg University team acknowledges support from Russian RFBR foundation via grant 09-02-00092. AZT-24 observations are made within an agreement between Pulkovo, Rome and Teramo observatories. This research is partly based on observations with the 100-m telescope of the MPIfR (Max-Planck-Institut fur Radioastronomie) at Effelsberg, as well as with the Medicina and Noto telescopes operated by INAF-Istituto di Radioastronomia. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. M. Villata organized the optical-to-radio observations by GASP-WEBT as the president of the collaboration. Abastumani Observatory team acknowledges financial support by the Georgian National Science Foundation through grant GNSF/ST07/4-180. The OVRO 40 m program was funded in part by NASA (NNX08AW31G) and the NSF (AST-0808050).
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 Large Area Telescope (LAT) gamma-ray spectrum of Mrk 501 can be well described by a single power-law function with a photon index of 1.78 +/- 0.03. While we observe relatively mild flux variations with the Fermi-LAT (within less than a factor of two), we detect remarkable spectral variability where the hardest observed spectral index within the LAT energy range is 1.52 +/- 0.14, and the softest one is 2.51 +/- 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 Very Long Baseline Array (VLBA), Swift, RXTE, MAGIC, and VERITAS, the F-GAMMA, GASP-WEBT, and other collaborations and instruments which provided excellent temporal 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 (SSC) model. In the framework of this model, we find that the dominant emission region is characterized by a size less than or similar to 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 (similar or equal to 10(44) erg s(-1)) constitutes only a small fraction (similar to 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 su
K. A. Wallenberg Foundation
International Doctorate on Astroparticle Physics (IDAPP) program
INFN Padova
Academy of Finland
National Radio Astronomy Observatory's Very Long Baseline Array (VLBA)
Russian RFBR foundation
Georgian National Science Foundation
NASA
NSF
Depto. de Estructura de la Materia, Física Térmica y Electrónica
Fac. de Ciencias Físicas
TRUE
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