Detection of High-energy Gamma-ray Emission during the X-ray Flaring Activity in Grb 100728a

We present the simultaneous Swift and Fermi observations of the bright GRB 100728A and its afterglow. The early X-ray emission is dominated by a vigorous flaring activity continuing until 1 ks after the burst. In the same time interval high energy emission is significantly detected by the Fermi/LAT. Marginal evidence of GeV emission is observed up to later times. We discuss the broadband properties of this burst within both the internal and external shock scenarios, with a particular emphasis on the relation between X-ray flares, the GeV emission and a continued longduration central engine activity as their power source. Subject headings: gamma-ray burst: individual (GRB 100728A) 1 Corresponding authors: E. Troja, [email protected]; L. Piro, [email protected]; V. Vasileiou, [email protected]; S. Cutini, [email protected]; J. E. McEnery, [email protected]. 2 Center for Earth Observing and Space Research, College of Science, George Mason University, Fairfax, VA 22030, resident at Naval Research Laboratory, Washington, DC 20375 3 W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305, USA 4 Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, Italy 5 Laboratoire AIM, CEA-IRFU/CNRS/Université Paris Diderot, Service d’Astrophysique, CEA Saclay, 91191 Gif sur Yvette, France 6 Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, I-34127 Trieste, Italy 7 Dipartimento di Fisica, Università di Trieste, I-34127 Trieste, Italy 8 Rice University, Department of Physics and Astronomy, MS-108, P. O. Box 1892, Houston, TX 77251 9 Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova, Italy 10 Dipartimento di Fisica “G. Galilei”, Università di Padova, I-35131 Padova, Italy 11 Center for Space Plasma and Aeronomic Research (CSPAR), University of Alabama in Huntsville, Huntsville, AL 35899 12 Institut für Astround Teilchenphysik and Institut für Theoretische Physik, Leopold-Franzens-Universität Innsbruck, A-6020 Innsbruck, Austria 13 Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, I-06123 Perugia, Italy 14 Dipartimento di Fisica, Università degli Studi di Perugia, I-06123 Perugia, Italy 15 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA 16 Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742 17 Santa Cruz Institute for Particle Physics, Department of Physics and Department of Astronomy and Astrophysics, University of California at Santa Cruz, Santa Cruz, CA 95064, USA 18 Dipartimento di Fisica “M. Merlin” dell’Università e del Politecnico di Bari, I-70126 Bari, Italy 19 Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, Italy 20 Laboratoire Leprince-Ringuet, École polytechnique, CNRS/IN2P3, Palaiseau, France 21 Institut de Ciencies de l’Espai (IEEC-CSIC), Campus UAB, 08193 Barcelona, Spain 22 INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, I-20133 Milano, Italy 23 Artep Inc., 2922 Excelsior Springs Court, Ellicott City, MD 21042, resident at Naval Research Laboratory, Washington, DC 20375 24 Department of Physics, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden 25 The Oskar Klein Centre for Cosmoparticle Physics, Al2 The Fermi Collaboration