Mon. Not. R. Astron. Soc. 000, 1–8 (2020) Printed 26 June 2020 (MN L A T E X style file v2.2) New insight into the origin of the GeV flare in the binary system PSR B1259-63 / LS 2883 from the 2017 periastron passage. M. Chernyakova, 1,2⋆ D. Malyshev, 3 S. Mc Keague, 1 B. van Soelen, 4 J.P. Marais, 4 A. Martin-Carrillo, 5 and D. Murphy 5 1 School of Physical Sciences and CfAR, Dublin City University, Dublin 9, Ireland 2 Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland 3 Institut f ¨ ur Astronomie und Astrophysik T¨ ubingen, Universit¨ at T¨ ubingen, Sand 1, D-72076 T ¨ ubingen, Germany 4 University of the Free State Department of Physics PO Box 339 9300 Bloemfontein South Africa 5 Space Science Group, School of Physics, University College Dublin, Belfield, Dublin 4, Ireland Received <date> ; in original form <date> ABSTRACT PSR B1259-63 is a gamma-ray binary system hosting a radio pulsar orbiting around a O9.5Ve star, LS 2883, with a period of ∼ 3.4 years. The interaction of the pulsar wind with the LS 2883 outflow leads to unpulsed broad band emission in the radio, X-rays, GeV and TeV domains. While the radio, X-ray and TeV light curves show rather similar behaviour, the GeV light curve appears very different with a huge outburst about a month after a periastron. The energy release during this outburst seems to significantly exceed the spin down luminosity of the pulsar and both the GeV light curve and the energy release varies from one orbit to the next. In this paper, we present for the first time the results of optical observations of the sys- tem in 2017, and also reanalyze the available X-ray and GeV data. We present a new model in which the GeV data are explained as a combination of the bremsstrahlung and inverse Compton emission from the unshocked and weakly shocked electrons of the pulsar wind. The X-ray and TeV emission is produced by synchrotron and inverse Compton emission of en- ergetic electrons accelerated on a strong shock arising due to stellar/pulsar winds collision. The brightness of the GeV flare is explained in our model as a beaming effect of the energy released in a cone oriented, during the time of the flare, in the direction of the observer. 1 INTRODUCTION Gamma-ray binary systems are composed of a compact object, ei- ther a black hole or a neutron star, orbiting a massive O or B type star. They are distinguished from X-ray binaries of a similar nature by non-thermal emission that peaks at energies  1 MeV (Dubus et al. 2017). Of all γ-ray binaries the only systems where the na- ture of the compact object is known are PSR B1259-63 and PSR J2032+4127, both of which are radio pulsars. In PSR B1259-63 the pulsar has a spin period of 47.76 ms and is orbiting a O9.5Ve star (LS 2883) with a period of ∼ 1236.7 days in a highly eccentric orbit (e ∼ 0.87) (Johnston et al. 1992; Negueruela et al. 2001; Shan- non, Johnston & Manchester 2014). Based on the parallax data in the Gaia DR2 Archive (Gaia Collaboration et al. 2018) the distance to the system is 2.39 ± 0.19 kpc, which is consistent with the value of 2.6 +0.4 −0.3 kpc is reported by Miller-Jones et al. (2018). The optical spectrum of the companion shows evidence of an equatorial disc, which is thought to be inclined with respect to the orbital plane by ∼ 10 − 40 ◦ (Melatos, Johnston & Melrose 1995), which causes the pulsar to cross the disc twice during the perias- tron passage. The interaction of the pulsar wind with the compan- ion’s outflow leads to the generation of the unpulsed non-thermal emission in radio, X-ray, GeV and TeV energies. X-ray emission is observed throughout the orbit but the unpulsed radio, GeV and TeV radiation occurs only within a few months before and after the peri- astron (e.g. Johnston et al. 1999; Chernyakova et al. 2015; Johnson et al. 2018; H. E. S. S. Collaboration et al. 2020). The unpulsed radio and X-ray emission exhibits a similar two peak light curve with the peaks occurring during the time when the pulsar crosses the disc of the companion. Current H.E.S.S. obser- vations indicate that TeV emission can have a similar behaviour (H. E. S. S. Collaboration et al. 2020), but more sensitive observa- tions are needed to confirm this. Hopefully CTA will address this issue in the very near future (Chernyakova et al. 2019). The GeV emission, however, shows a very different behaviour and is charac- terised by a strong flare, which started about ∼ 30 days after the 2010 and 2014 periastron passages (Abdo et al. 2011; Caliandro et al. 2015) and has no obvious flaring counterparts at other wave- length. The only visible effect coinciding in time with a GeV flare is a rapid decrease of the H α equivalent width (Chernyakova et al. 2015), usually interpreted as a measure of the companion’s star disc. The destruction of the disc is also evident in Chandra obser- vations of the source far away from the periastron (Pavlov, Chang & Kargaltsev 2011; Pavlov et al. 2015; Pavlov, Hare & Kargaltsev 2019; Hare et al. 2019). These data demonstrates the presence of X-ray emitting clumps moving away from the binary with speeds of about 0.1 of the speed of light. The clumps are being ejected at least once per binary period, 3.4 years, presumably around binary arXiv:2005.14060v2 [astro-ph.HE] 25 Jun 2020