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

F. Aharonian A. G. Akhperjanian A. R. Bazer-Bachi 1 M. Beilicke W. Benbow D. Berge K. Bernlöhr C. Boisson 2 O. Bolz V. Borrel 1 I. Braun F. Breitling A. M. Brown R. Bühler I. Büsching S. Carrigan P. M. Chadwick L.-M. Chounet 3 R. Cornils L. Costamante B. Degrange 3 H. J. Dickinson A. Djannati-Ataï 4 L. O'C. Drury G. Dubus 3 K. Egberts D. Emmanoulopoulos P. Espigat 5 F. Feinstein 6 E. Ferrero G. Fontaine 3 Seb. Funk S. Funk Y.A. Gallant 6 B. Giebels 3 J. F. Glicenstein 7 P. Goret 7 C. Hadjichristidis D. Hauser M. Hauser G. Heinzelmann G. Henri 8 G. Hermann J. A. Hinton W. Hofmann M. Holleran D. Horns A. Jacholkowska 6 O. C. de Jager B. Khélifi 3 Nu. Komin A. Konopelko I. J. Latham R. Le Gallou A. Lemière 5 M. Lemoine-Goumard 3 T. Lohse J. M. Martin O. Martineau-Huynh A. Marcowith 1 C. Masterson T. J. L. Mccomb M. de Naurois D. Nedbal S. J. Nolan A. Noutsos K. J. Orford J. L. Osborne M. Ouchrif M. Panter G. Pelletier 8 S. Pita 5 G. Pühlhofer M. Punch 5 B. C. Raubenheimer M. Raue S. M. Rayner A. Reimer O. Reimer J. Ripken L. Rob L. Rolland G. Rowell V. Sahakian L. Saugé 8 S. Schlenker R. Schlickeiser U. Schwanke H. Sol 2 D. Spangler F. Spanier R. Steenkamp C. Stegmann G. Superina 3 J.-P. Tavernet R. Terrier 5 C.G. Théoret 5 M. Tluczykont 3 C. van Eldik G. Vasileiadis 6 C. Venter P. Vincent H. J. Völk S. J. Wagner M. Ward
Abstract : New observations of LS 5039, a High Mass X-ray Binary comprising a massive star and compact object, were carried out with the High Energy Stereoscopic System of Cherenkov Telescopes (H.E.S.S.) in 2005 at very high energy (VHE) gamma-ray energies. These observations reveal that its flux and energy spectrum are modulated with the 3.9 day orbital period of the binary system. This is the first time in gamma-ray astronomy that orbital modulation has been observed, and periodicity clearly established using ground-based gamma-ray detectors. The VHE gamma-ray emission is largely confined to half of the orbit, peaking around the inferior conjunction epoch of the compact object. For this epoch, there is also a hardening of the energy spectrum in the energy range between 0.2 TeV and a few TeV. The flux vs. orbital phase profile provides the first clear indication of gamma-ray absorption via pair production within an astrophysical source, a process which is expected to occur if the gamma-ray production site is situated within ~1 AU of the compact object. Moreover the production region size must be not significantly greater than the binary separation (~0.15 AU). Notably, these constraints are also considerably smaller than the collimated outflows or jets (extending out to ~1000 AU) observed in LS 5039. The spectral hardening could arise from variations with phase in the maximum electron energies, and/or the dominant VHE gamma-ray production mechanism.
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F. Aharonian, A. G. Akhperjanian, A. R. Bazer-Bachi, M. Beilicke, W. Benbow, et al.. 3.9 day orbital modulation in the TeV gamma-ray flux and spectrum from the X-ray binary LS 5039. Astronomy and Astrophysics - A&A, EDP Sciences, 2006, 460, pp.743-749. ⟨10.1051/0004-6361:20065940⟩. ⟨in2p3-00124857⟩

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