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Published February 1, 2011 | Published
Journal Article Open

Insights into the High-energy γ-ray Emission of Markarian 501 from Extensive Multifrequency Observations in the Fermi Era


We report on the γ-ray activity of the blazar Mrk 501 during the first 480 days of Fermi operation. We find that the average Large Area Telescope (LAT) γ-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 ≾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 (≃10^(44) erg s^(−1)) constitutes only a small fraction (~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.

Additional Information

© 2011 The American Astronomical Society. Received 2010 August 18; accepted 2010 November 19; published 2011 January 12. The Fermi-LAT Collaboration acknowledges the generous support of a number of agencies and institutes that have supported the Fermi-LAT Collaboration. These include theNational Aeronautics and Space Administration and the Department of Energy in the United States, the Commissariat `a l'Energie Atomique and the Centre National de la Recherche Scientifique / Institut National de Physique Nucl´eaire et de Physique des Particules in France, the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), High Energy Accelerator Research Organization (KEK) and Japan Aerospace Exploration Agency (JAXA) in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council and the Swedish National Space Board in Sweden. Additional support for science analysis during the operations phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica in Italy and the Centre National d' ´ Etudes Spatiales in France. The MAGIC collaboration would like to thank the Instituto de Astrof´ısica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The support of the German BMBF and MPG, the Italian INFN, the Swiss National Fund SNF, and the Spanish MICINN is gratefully acknowledged. This work was also supported by the Marie Curie program, by the CPAN CSD2007-00042 and MultiDark CSD2009-00064 projects of the Spanish Consolider- Ingenio 2010 programme, by grant DO02-353 of the Bulgarian NSF, by grant 127740 of the Academy of Finland, by the YIP of the Helmholtz Gemeinschaft, by the DFG Cluster of Excellence "Origin and Structure of the Universe," and by the Polish MNiSzW Grant N N203 390834. VERITAS is supported by grants from the US Department of Energy, the US National Science Foundation, and the Smithsonian Institution, by NSERC in Canada, by Science Foundation Ireland, and by STFC in the UK. The VERITAS Collaboration also acknowledges the support of the Fermi/LAT team through the Guest Investigator Program Grant NNX09AT86G. 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 theMedicina 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 opticalto- 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).

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