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Published April 10, 2012 | Published
Journal Article Open

Long-term Monitoring of the High-energy γ-Ray Emission from LS I +61°303 and LS 5039


The Fermi Large Area Telescope (LAT) reported the first definitive GeV detections of the binaries LS I +61°303 and LS 5039 in the first year after its launch in 2008 June. These detections were unambiguous as a consequence of the reduced positional uncertainty and the detection of modulated γ-ray emission on the corresponding orbital periods. An analysis of new data from the LAT, comprising 30 months of observations, identifies a change in the γ-ray behavior of LS I +61°303. An increase in flux is detected in 2009 March and a steady decline in the orbital flux modulation is observed. Significant emission up to 30 GeV is detected by the LAT; prior data sets led to upper limits only. Contemporaneous TeV observations no longer detected the source, or found it—in one orbit—close to periastron, far from the phases at which the source previously appeared at TeV energies. The detailed numerical simulations and models that exist within the literature do not predict or explain many of these features now observed at GeV and TeV energies. New ideas and models are needed to fully explain and understand this behavior. A detailed phase-resolved analysis of the spectral characterization of LS I +61°303 in the GeV regime ascribes a power law with an exponential cutoff spectrum along each analyzed portion of the system's orbit. The on-source exposure of LS 5039 is also substantially increased with respect to our prior publication. In this case, whereas the general γ-ray properties remain consistent, the increased statistics of the current data set allows for a deeper investigation of its orbital and spectral evolution.

Additional Information

© 2012 American Astronomical Society. Received 2011 November 11; accepted 2012 February 6; published 2012 March 22. The Fermi LAT Collaboration acknowledges generous ongoing support from a number of agencies and institutes that have supported both the development and the operation of the LAT as well as scientific data analysis. These include the National Aeronautics and Space Administration and the Department of Energy in the United States, the Commissariat à l'Energie Atomique and the Centre National de la Recherche Scientifique/Institut National de Physique Nucléaire 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'Études Spatiales in France. This work has been additionally supported by the Spanish CSIC and MICINN and the Generalitat de Catalunya, through grants AYA2009-07391 and SGR2009-811, as well as the Formosa Program TW2010005. S.Z. acknowledges supports from National Natural Science Foundation of China (via NSFC-10325313, 10521001, 10733010, 10821061, 11073021, and 11133002), and 973 program 2009CB824800. G.D. acknowledges support from the European Community via contract ERC-StG-200911. A.B.H. acknowledges funding via an EU Marie Curie International Outgoing Fellowship under contract no. 2010-275861. The AMI arrays are supported by STFC and the University of Cambridge.

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