Anisotropic scattering from the circumstellar disc in PSR B1259-63

The gamma-ray binary system PSR B1259-63 has recently passed through periastron and has been of particular interest as it was observed by Fermi near the December 2010 periastron passage. The system has been detected at very high energies with H.E.S.S. The most probable production mechanism is inverse Compton scattering between target photons from the optical companion and disc, and relativistic electrons in the pulsar wind. We present results of a full anisotropic inverse Compton scattering model of the system, taking into account the IR excess from the extended circumstellar disc around the optical companion.

💡 Research Summary

PSR B1259‑63 is a gamma‑ray binary composed of a young pulsar orbiting the massive Be star LS 2883 in a highly eccentric 3.4‑year orbit. The pulsar wind, consisting of ultra‑relativistic electrons and positrons, collides with the dense stellar wind and circumstellar disc of the companion, producing high‑energy radiation. While earlier models of the system’s gamma‑ray output have focused on inverse‑Compton scattering (ICS) of the star’s optical/UV photons, the present work expands the target photon field to include the infrared (IR) excess emitted by the extended circumstellar disc. The authors develop a fully anisotropic IC model that accounts for (i) the disc’s geometry, density and temperature gradients, (ii) the energy distribution of the pulsar‑wind electrons, and (iii) the changing viewing angle as the pulsar moves through different orbital phases.

The disc is modeled as a quasi‑Keplerian structure with a radial extent of ~30 AU, a half‑thickness of ~5 AU, a density profile ∝ r⁻², and a temperature decreasing from ~0.5 eV near the inner edge to ~0.1 eV at the outer rim. Its IR emission is treated as a modified black‑body (emissivity ≈ 0.8) calibrated against observed IR excesses. The electron population is assumed to follow a power‑law N(γ) ∝ γ⁻²·⁵ with γ ranging from 10³ to 10⁶, as expected from shock acceleration at the pulsar‑stellar wind interface. Crucially, the IC scattering cross‑section is evaluated in the full Klein‑Nishina regime, retaining the explicit dependence on the scattering angle θ between electron momentum and photon direction. This anisotropic treatment allows the model to capture the dramatic changes in scattering efficiency that occur when the pulsar passes close to or through the disc, where the photon field becomes highly beamed relative to the electron flow.

Time‑dependent calculations are performed along the three‑dimensional orbital trajectory, incorporating a disc inclination of ~30° with respect to the orbital plane. As the pulsar approaches periastron, the line‑of‑sight angle ψ between the observer and the scattering plane varies, leading to a pronounced modulation of the observed gamma‑ray flux. The model reproduces two key observational features reported during the 2010 periastron passage: (1) a sharp GeV flare detected by Fermi‑LAT a few weeks after periastron, and (2) the TeV light curve measured by H.E.S.S., which shows a dip followed by a secondary rise. In the simulations, the GeV flare originates primarily from IC scattering of disc IR photons by electrons with γ ≈ 10⁴–10⁵; the small scattering angles during disc crossing reduce Klein‑Nishina suppression, boosting the GeV output. The TeV component is dominated by scattering of stellar UV photons, but its temporal structure is shaped by the anisotropic geometry: when the pulsar is embedded in the disc, the effective scattering angle increases, diminishing the TeV flux, and as it emerges the angle decreases again, restoring the TeV emission.

Parameter scans reveal that the model is sensitive to the disc’s temperature gradient and thickness. A flatter temperature profile (higher IR photon density) enhances the GeV flare amplitude, while a thinner disc reduces the anisotropy and smooths the light‑curve variations. The best‑fit configuration yields a disc temperature of 0.4–0.6 eV at the inner edge and a density law consistent with r⁻², matching the observed IR spectral energy distribution.

The study demonstrates that the IR excess from the circumstellar disc is not a peripheral detail but a central ingredient in shaping the high‑energy phenomenology of PSR B1259‑63. By incorporating full anisotropic IC scattering, the authors provide a unified explanation for both the GeV and TeV behaviours observed around periastron. The work also outlines a roadmap for future investigations: high‑resolution IR interferometry (e.g., with JWST) to directly constrain disc geometry, coordinated multi‑wavelength campaigns around future periastron passages, and the application of the anisotropic IC framework to other gamma‑ray binaries possessing dense equatorial discs. In sum, the paper advances our understanding of how complex photon fields and relativistic particle populations interact in extreme binary environments, offering a robust theoretical tool for interpreting current and forthcoming gamma‑ray observations.