Super-orbital variability of LS I +61°303 at TeV energies

¹ ETH Zurich, 8093 Zurich, Switzerland
² Università di Udine, and INFN Trieste, 33100 Udine, Italy
³ INAF National Institute for Astrophysics, 00136 Rome, Italy
⁴ Università di Siena, and INFN Pisa, 53100 Siena, Italy
⁵ Croatian MAGIC Consortium, Rudjer Boskovic Institute, University of Rijeka, University of Split and University of Zagreb, Croatia
⁶ Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Salt Lake, Sector-1, 700064 Kolkata, India
⁷ Max-Planck-Institut für Physik, 80805 München, Germany
⁸ Universidad Complutense, 28040 Madrid, Spain
⁹ Inst. de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
¹⁰ Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
¹¹ University of Łódź, 90236 Lodz, Poland
¹² Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹³ Institut de Fisica d’Altes Energies, The Barcelona Institute of Science and Technology (IFAE-BIST), Campus UAB, 08193 Bellaterra ( Barcelona), Spain
¹⁴ Universität Würzburg, 97074 Würzburg, Germany
¹⁵ Università di Padova and INFN, 35131 Padova, Italy
¹⁶ Institute for Space Sciences (CSIC/IEEC), 08193 Barcelona, Spain
¹⁷ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, 28040 Madrid, Spain
¹⁸ Technische Universität Dortmund, 44221 Dortmund, Germany
¹⁹ Unitat de Física de les Radiacions, Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
²⁰ Universitat de Barcelona, ICC, IEEC-UB, 08028 Barcelona, Spain
²¹ Japanese MAGIC Consortium, ICRR, The University of Tokyo, Department of Physics and Hakubi Center, Kyoto University, Tokai University, The University of Tokushima, KEK, Japan
²² Finnish MAGIC Consortium, Tuorla Observatory, University of Turku and Department of Physics, University of Oulu, Finland
²³ Inst. for Nucl. Research and Nucl. Energy, 1784 Sofia, Bulgaria
²⁴ Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
²⁵ ICREA and Institute for Space Sciences (CSIC/IEEC), 08193 Barcelona, Spain
²⁶ Centro Brasileiro de Pesquisas Físicas (CBPF/MCTI), R. Dr. Xavier Sigaud, 150 - Urca, 22290-180 Rio de Janeiro – RJ, Brazil
²⁷ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA and Department of Physics and Department of Astronomy, University of Maryland, College Park, MD 20742, USA
²⁸ Humboldt University of Berlin, Institut für Physik, Newtonstr. 15, 12489 Berlin Germany
²⁹ École polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland
³⁰ Department of Physics & Astronomy, UC Riverside, CA 92521, USA
³¹ Japanese MAGIC Consortium, Japan
³² Finnish Centre for Astronomy with ESO (FINCA), Turku, Finland
³³ INAF-Trieste, Italy
³⁴ ISDC – Science Data Center for Astrophysics, 1290 Versoix ( Geneva), Switzerland
³⁵ Laboratoire AIM, Service d’Astrophysique, DSM\IRFU, CEA\Saclay, 91191 Gif-sur-Yvette Cedex, France

Received: 15 December 2015
Accepted: 21 March 2016

Abstract

Context. The gamma-ray binary LS I +61°303 is a well-established source from centimeter radio up to very high energy (VHE; E> 100 GeV). The broadband emission shows a periodicity of ~26.5 days, coincident with the orbital period. A longer (super-orbital) period of 1667 ± 8 days was proposed from radio variability and confirmed using optical and high-energy (HE; E> 100 MeV) gamma-ray observations. In this paper, we report on a four-year campaign performed by MAGIC together with archival data concentrating on a search for a long-timescale signature in the VHE emission from LS I +61°303.

Aims. We focus on the search for super-orbital modulation of the VHE emission, similar to that observed at other energies, and on the search for correlations between TeV emission and an optical determination of the extension of the circumstellar disk.

Methods. A four-year campaign has been carried out using the MAGIC telescopes. The source was observed during the orbital phases when the periodic VHE outbursts have occurred (φ = 0.55–0.75, one orbit = 26.496 days). Additionally, we included archival MAGIC observations and data published by the VERITAS collaboration in these studies. For the correlation studies, LS I +61°303 has also been observed during the orbital phases where sporadic VHE emission had been detected in the past (φ = 0.75–1.0). These MAGIC observations were simultaneous with optical spectroscopy from the LIVERPOOL telescope.

Results. The TeV flux of the periodical outburst in orbital phases φ = 0.5–0.75 was found to show yearly variability consistent with the long-term modulation of ~4.5 years found in the radio band. This modulation of the TeV flux can be well described by a sine function with a best-fit period of 1610 ± 58 days. The complete data, including archival observations, span two super-orbital periods. There is no evidence for a correlation between the TeV emission and the mass-loss rate of the Be star, but this may be affected by the strong, short-timescale (as short as intra-day) variation displayed by the Hα fluxes.