What caused the GeV flare of PSR B1259-63?

Abstract

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 ≈10 34 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.

Multi-wavelength observations of the binary system PSR B1259−63/LS 2883 around the 2014 periastron passage

“…Such a possibility was discussed by Pétri & Dubus (2011) and Dubus & Cerutti (2013). In Pétri & Dubus (2011) GeV emission is generated rather close to the pulsar, and the X-ray photons are scattered by the relativisitic pairs in the striped pulsar wind.…”

“…In this case the GeV flare was interpreted by the authors as a lucky combination of the geometry and a presence of additional seed photons coming from the shocked region. In the Dubus & Cerutti (2013) model the GeV emission is due to the IC scattering of the X-ray photons by the schocked relativistic wind. In this model the GeV flare is expected to peak near the inferior conjunction, but the reason of the delay between the X-ray and GeV peak in this model is not clear.…”

Pulsar-Wind Nebulae

“…The flare is about 10-times brighter than the predicted emission at periastron, which represents a gamma-ray luminosity comparable to the pulsar spin-down power. A radiative efficiency close to 100% is in principle achievable with inverse Compton scattering, but the density of stellar photons is far too low to explain the flux at these phases, unless there are extra sources of radiation close to the pulsar (see, e.g., Dubus and Cerutti 2013), or significant Doppler boosting of the emission towards the observer (Dubus et al 2010;Kong et al 2012). The flare peaks at 300 MeV and is seen only in the GeV band, which suggests that the particle energy distribution must be very narrow.…”

The Hour-timescale GeV Flares of PSR B1259–63 in 2017

“…This terminal shock model has explained the periodic light curve of the non-thermal X-ray emission very well; see, e.g., Takata et al (2012). In a previous modeling work on the GeV flare, Khangulyan et al (2012) used the IC scattering of the unshocked PW off soft photons from the Be star, while Dubus & Cerutti (2013) brought up an IC model using the X-ray synchrotron radiation from the shock as the IC target photons. Kong et al (2012) used the synchrotron model with relativistic flows beaming toward Earth along the bow shock tails.…”