3.9 day orbital modulation in the TeV -ray flux and spectrum from the X-ray binary LS 5039

M. Tluczykont^{9, 10}, C. van Eldik¹, G. Vasileiadis¹⁴, C. Venter⁸, P. Vincent¹⁸, H. J. Völk¹, S. J. Wagner¹³, and M. Ward<sup>7

1</sup>  Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany
²  Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
³  Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, 9 Av. du Colonel Roche, BP 4346, 31029 Toulouse Cedex 4, France
⁴  Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
⁵  Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
⁶  LUTH, UMR 8102 du CNRS, Observatoire de Paris, Section de Meudon, 92195 Meudon Cedex, France
⁷  University of Durham, Department of Physics, South Road, Durham DH1 3LE, UK
⁸  Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
⁹  Laboratoire Leprince-Ringuet, IN2P3/CNRS, École Polytechnique, 91128 Palaiseau, France
¹⁰  European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
¹¹  APC, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France
¹²  Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
¹³  Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
¹⁴  Laboratoire de Physique Théorique et Astroparticules, IN2P3/CNRS, Université Montpellier II, CC 70, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
¹⁵  DAPNIA/DSM/CEA, CE Saclay, 91191 Gif-sur-Yvette Cedex, France
¹⁶  Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France
¹⁷  Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
¹⁸  Laboratoire de Physique Nucléaire et de Hautes Énergies, IN2P3/CNRS, Universités Paris VI & VII, 4 place Jussieu, 75252 Paris Cedex 5, France
¹⁹  Institute of Particle and Nuclear Physics, Charles University, V Holesovickach 2, 180 00 Prague 8, Czech Republic
²⁰  Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
²¹  University of Namibia, Private Bag 13301, Windhoek, Namibia
²²  Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany

(Received 30 June 2006 / Accepted 15 September 2006)

Abstract
Aims. LS 5039 is a High Mass X-ray Binary (HMXRB) comprising a compact object in an eccentric 3.9 day orbit around a massive O6.5V star. Observations at energies above 0.1 TeV (10¹¹ eV) by the High Energy Stereoscopic System (HESS) in 2004 revealed that LS 5039 is a source of Very High Energy (VHE) -rays and hence, is able to accelerate particles to multi-TeV energies. Deeper observations by HESS were carried out in 2005 in an effort to probe further the high energy astrophysics taking place. In particular, we have searched for orbital modulation of the VHE -ray flux, which if detected, would yield new information about the complex variation in -ray absorption and production within X-ray binary systems.
Methods. Observations at energies above 0.1 TeV (10¹¹ eV), were carried out with the High Energy Stereoscopic System (HESS) of Cherenkov Telescopes in 2005. A timing analysis was performed on the dataset employing the Lomb-Scargle and Normalised Rayleigh statistics, and orbital phase-resolved energy spectra were obtained.
Results. The timing analysis reveals a highly significant (post-trial chance probability <10^-15) peak in the TeV emission periodogram at a frequency matching that of the 3.9 day orbital motion of the compact object around the massive stellar companion. This is the first time in -ray astronomy that orbital modulation has been observed, and periodicity clearly established using ground-based -ray detectors. The -ray emission is largely confined to half of the orbit, peaking around the inferior conjunction epoch of the compact object. Around this epoch, there is also a hardening of the energy spectrum in the energy range between 0.2 TeV and a few TeV.
Conclusions. The -ray flux vs. orbital phase profile suggests the presence of -ray absorption via pair production, which would imply that a large fraction of the -ray production region is situated within ~1 AU of the compact object. This source size constraint can be compared to the collimated outflows or jets observed in LS 5039 resolved down to scales of a few AU. The spectral hardening is however not explained exclusively by the absorption effect, indicating that other effects are present, perhaps related to the -ray production mechanism(s). If the -ray emission arises from accelerated electrons, the hardening may arise from variations with phase in the maximum electron energies, the dominant radiative mechanism, and/or the angular dependence in the inverse-Compton scattering cross-section. Overall, these results provide new insights into the competing -ray absorption and production processes in X-ray binaries.