Gamma-ray production in the Be-type star-pulsar binary system PSR B1259-63

[摘要] English: The high-mass binary system PSR B1259-63/LS 2883 is one of only six known gamma-ray binaries, and the only one where the compact object is known from the direct observation of a pulsed radio signal to be a 48 ms pulsar. During it's eccentric 3.4 year orbit, the pulsar moves through the circumstellar disc of the optical companion, approximately twenty days before and after periastron. This results in conditions for complicated interactions between the material in the disc, the fast rotating pulsar, the pulsar wind, and the radiation field from the star and disc. The system has been the object of multi-wavelength campaigns with telescopes such as the VLT, H.E.S.S. and Fermi. The interaction between the stellar and pulsar wind results in the formation of a radiating pulsar wind nebula within the binary system, which has been detected from radio to TeV gamma-ray energies. The spectral energy distribution is dominated by the emission at gamma-ray energies, classifying this system as a gammaray binary. The interaction between the stars is greater near periastron where the pulsar passes closest to the optical companion. Approximately twenty days from periastron the pulsar passes through or behind the Be star's circumstellar disc, obscuring the pulsed radio signal. During this period there is a corresponding increase in the unpulsed emission from the system. The TeV gamma-rays are believed to be produced by electrons in the pulsar wind which cool via the inverse Compton up-scattering of stellar photons from the optical companion. The circumstellar disc associated with the Be star produces an infrared flux below ! 1015 Hz, which is greater than that expected from the blackbody distribution associated with star, providing additional target photons which could increase the inverse Compton scattering rate. The scattering of infrared photons can occur in the Thomson limit with its significantly larger cross-section and should produce GeV energy gamma-rays in the energy range observed by the Fermi telescope. A curve of growth method is presented to model the infrared free-free and free-bound emission from the circumstellar disc, taking into account the changing viewing angle as observed from the pulsar. The curve of growth model is fitted to archive near-infrared and optical data and mid-infrared data obtained with the Very Large Telescope during January 2011. The effect of this infrared excess on the inverse Compton scattering rate is considered for an isotropic and anisotropic photon distribution, considering pre�?and post–shock electron distributions. The anisotropic modelling considers the effects of the changing size and orientation of the circumstellar disc relative to the pulsar, as well as the change in the inverse Compton scattering angle during the orbit. The inverse Compton scattering rate for three disc orientations is modelled over a period of approximately 160 days around periastron, including the disc crossing epochs before and after periastron. The maximum disc contribution is found to occur close to periastron and not near the disc–crossing where the low infrared flux from the disc, at a radius of ! 45 stellar radii, has a less significant effect. It is found that the inclusion of the infrared flux from the circumstellar disc can increase the GeV flux from the system by a factor ! 2 near periastron, for favourable disc orientations. The predicted increase is, however, less than was detected with Fermi during the 2011 periastron passage. The observations showed a flare which cannot be explained by this, or any current model.

[发布机构] University of the Free State