Abstract
PSR B1259-63 is a gamma-ray binary system composed of a high spindown pulsar and a massive star. Non-thermal emission up to TeV energies is observed near periastron passage, attributed to emission from high energy e+e- pairs accelerated at the shock with the circumstellar material from the companion star, resulting in a small-scale pulsar wind nebula. Weak gamma-ray emission was detected by the Fermi/LAT at the last periastron passage, unexpectedly followed 30 days later by a strong flare, limited to the GeV band, during which the luminosity nearly reached the spindown power of the pulsar. The origin of this GeV flare remains mysterious. We investigate whether the flare could have been caused by pairs, located in the vicinity of the pulsar, up-scattering X-ray photons from the surrounding pulsar wind nebula rather than UV stellar photons, as usually assumed. Such a model is suggested by the geometry of the interaction region at the time of the flare. We compute the gamma-ray lightcurve for this scenario, based on a simplified description of the interaction region, and compare it to the observations. The GeV lightcurve peaks well after periastron with this geometry. The pairs are inferred to have a Lorentz factor ~500. They also produce an MeV flare with a luminosity ~1e34 erg/s prior to periastron passage. A significant drawback is the very high energy density of target photons required for efficient GeV emission. We propose to associate the GeV-emitting pairs with the Maxwellian expected at shock locations corresponding to high pulsar latitudes, while the rest of the non-thermal emission arises from pairs accelerated in the equatorial region of the pulsar wind termination shock.
Highlights
PSR B1259-63 is a 47.7 ms radio pulsar in a 3.5 year orbit around a Be star (Johnston et al 1992)
An outburst of very high energy gamma ray (VHE, >100 GeV), X-ray, and radio emission occurs at periastron passage (τ ≡ 0 d), when the pulsar wind interacts with the stellar wind and the equatorial disc surrounding the Be companion
We have explored the possibility to generate a HE gamma-ray flare by inverse Compton scattering X-ray photons emitted by the shocked pulsar wind instead of optical photons from the star
Summary
PSR B1259-63 is a 47.7 ms radio pulsar in a 3.5 year orbit around a Be star (Johnston et al 1992). The pulsar has a high spindown power E ≈ 8 × 1035 erg s−1. An outburst of very high energy gamma ray (VHE, >100 GeV), X-ray, and radio emission occurs at periastron passage (τ ≡ 0 d), when the pulsar wind interacts with the stellar wind and the equatorial disc surrounding the Be companion. A bow shock structure forms, whose appearance changes according to which component (stellar wind or Be disc) the pulsar wind interacts with. The e−e+ pairs present in the pulsar wind are isotropised and energised at the shock. The pairs radiate non-thermal emission as they flow away from the binary, producing a small-scale pulsar wind nebula (PWN)
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