Detection of Vhe Γ-rays from Hess J0632+057 during the 2011 February X-ray Outburst with the Magic Telescopes

The very high energy (VHE) γ-ray source HESS J0632+057 has recently been confirmed to be a γ-ray binary. The optical counterpart is the Be star MWC 148, and a compact object of unknown nature orbits it every ∼321 d with a high eccentricity of ∼0.8. We monitored HESS J0632+057 with the stereoscopic MAGIC telescopes from 2010 October to 2011 March and detected significant VHE γ-ray emission during 2011 February, when the system exhibited an X-ray outburst. We find no γ-ray signal in the other observation periods when the system did not show increased X-ray flux. Thus HESS J0632+057 exhibits γ-ray variability on timescales of the order of one to two months possibly linked to the X-ray outburst that takes place about 100 days after the periastron passage. Furthermore our measurements provide for the first time the γ-ray spectrum down to about 140 GeV and indicate no turnover of the spectrum at low energies. We compare the properties of HESS J0632+057 with the similar γ-ray binary LS I +61303, and discuss on the possible origin of the multiwavelength emission of the source. Subject headings: binaries: general — gamma rays: general — stars: individual (MWC 148) — X-rays: binaries — X-rays: individual (HESS J0632+057) 1 IFAE, Edifici Cn., Campus UAB, E-08193 Bellaterra, Spain 2 Universidad Complutense, E-28040 Madrid, Spain 3 INAF National Institute for Astrophysics, I-00136 Rome, Italy 4 Università di Siena, and INFN Pisa, I-53100 Siena, Italy 5 Technische Universität Dortmund, D-44221 Dortmund, Germany 6 Max-Planck-Institut für Physik, D-80805 München, Germany 7 Università di Padova and INFN, I-35131 Padova, Italy 8 Inst. de Astrofı́sica de Canarias, E-38200 La Laguna, Tenerife, Spain 9 University of Łódź, PL-90236 Lodz, Poland 10 Depto. de Astrofı́sica, Universidad de La Laguna, E-38206 La Laguna, Spain 11 Deutsches Elektronen-Synchrotron (DESY), D-15738 Zeuthen, Germany 12 ETH Zurich, CH-8093 Zurich, Switzerland 13 Universität Würzburg, D-97074 Würzburg, Germany 14 Universitat de Barcelona (ICC/IEEC), E-08028 Barcelona, Spain 15 Università di Udine, and INFN Trieste, I-33100 Udine, Italy 16 Institut de Ciències de l’Espai (IEEC-CSIC), E-08193 Bellaterra, Spain 17 Inst. de Astrofı́sica de Andalucı́a (CSIC), E-18080 Granada, Spain 18 Croatian MAGIC Consortium, Rudjer Boskovic Institute, University of Rijeka and University of Split, HR-10000 Zagreb, Croatia 19 Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain 20 Tuorla Observatory, University of Turku, FI-21500 Piikkiö, Finland 21 Inst. for Nucl. Research and Nucl. Energy, BG-1784 Sofia, Bulgaria 22 Japanese MAGIC Consortium, Division of Physics and Astronomy, Kyoto University, Japan 23 INAF/Osservatorio Astronomico and INFN, I-34143 Trieste, Italy 24 Università dell’Insubria, Como, I-22100 Como, Italy 25 Università di Pisa, and INFN Pisa, I-56126 Pisa, Italy 26 ICREA, E-08010 Barcelona, Spain 27 now at: Ecole polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland 28 supported by INFN Padova 29 now at: Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain 30 now at: KIPAC, SLAC National Accelerator Laboratory, USA 31 now at: Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Finland * Corresponding authors: T. Jogler, [email protected], P. MunarAdrover, [email protected]