What caused the GeV flare of PSR B1259-63?
Résumé
Context. 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.
Aims: 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.
Methods: We compute the gamma-ray lightcurve for this scenario, based on a simplified description of the interaction region, and compare it to the observations.
Results: 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 ≈ 1034 erg s-1 prior to periastron passage. A significant drawback is the very high energy density of target photons required for efficient GeV emission.
Conclusions: 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.
Aims: 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.
Methods: We compute the gamma-ray lightcurve for this scenario, based on a simplified description of the interaction region, and compare it to the observations.
Results: 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 ≈ 1034 erg s-1 prior to periastron passage. A significant drawback is the very high energy density of target photons required for efficient GeV emission.
Conclusions: 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.
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