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

Aharonian, F. and Akhperjanian, A. G. and Bazer-Bachi, A. R. and Beilicke, M. and Benbow, W. and Berge, D. and Bernlöhr, K. and Boisson, C. and Bolz, O. and Borrel, V. and Braun, I. and Brown, A. M. and Bühler, R. and Büsching, I. and Carrigan, S. and Chadwick, P. M. and Chounet, L.-M. and Cornils, R. and Costamante, L. and Degrange, B. and Dickinson, H. J. and Djannati-Ataï, A. and O'C. Drury, L. and Dubus, G. and Egberts, K. and Emmanoulopoulos, D. and Espigat, P. and Feinstein, F. and Ferrero, E. and Fiasson, A. and Fontaine, G. and Funk, S. and Funk, S. and Füßling, M. and Gallant, Y. A. and Giebels, B. and Glicenstein, J. F. and Goret, P. and Hadjichristidis, C. and Hauser, D. and Hauser, M. and Heinzelmann, G. and Henri, G. and Hermann, G. and Hinton, J. A. and Hoffmann, A. and Hofmann, W. and Holleran, M. and Horns, D. and Jacholkowska, A. and de Jager, O. C. and Kendziorra, E. and Khélifi, B. and Komin, N. and Konopelko, A. and Kosack, K. and Latham, I. J. and Le Gallou, R. and Lemière, A. and Lemoine-Goumard, M. and Lohse, T. and Martin, J. M. and Martineau-Huynh, O. and Marcowith, A. and Masterson, C. and Maurin, G. and McComb, T. J. L. and Moulin, E. and de Naurois, M. and Nedbal, D. and Nolan, S. J. and Noutsos, A. and Orford, K. J. and Osborne, J. L. and Ouchrif, M. and Panter, M. and Pelletier, G. and Pita, S. and Pühlhofer, G. and Punch, M. and Raubenheimer, B. C. and Raue, M. and Rayner, S. M. and Reimer, A. and Reimer, O. and Ripken, J. and Rob, L. and Rolland, L. and Rowell, G. and Sahakian, V. and Santangelo, A. and Saugé, L. and Schlenker, S. and Schlickeiser, R. and Schröder, R. and Schwanke, U. and Schwarzburg, S. and Shalchi, A. and Sol, H. and Spangler, D. and Spanier, F. and Steenkamp, R. and Stegmann, C. and Superina, G. and Tavernet, J.-P. and Terrier, R. and Tluczykont, M. and van Eldik, C. and Vasileiadis, G. and Venter, C. and Vincent, P. and Völk, H. J. and Wagner, S. J. and Ward, M. J. (2007) '3.9 day orbital modulation in the TeV gamma ray flux and spectrum from the X-ray binary LS 5039.', Astronomy & astrophysics., 460 (3). pp. 743-749.

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.

Item Type:Article
Additional Information:
Keywords:Gamma rays, Binary systems.
Full text:Full text not available from this repository.
Publisher Web site:http://dx.doi.org/10.1051/0004-6361:20065940
Record Created:25 Mar 2008
Last Modified:08 Apr 2009 16:34

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