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

The paper investigates the role of infrared (IR) photons emitted by the circumstellar disc of the Be star LS 2883 in the high‑energy gamma‑ray production of the binary system PSR B1259‑63/LS 2883. The authors construct a fully anisotropic inverse Compton (IC) scattering model that incorporates both the stellar black‑body radiation (T_* ≈ 25 000 K) and the disc’s IR excess, the latter being derived via a curve‑of‑growth method that accounts for free‑free emission and the disc’s optical depth.

The electron population in the pulsar wind is assumed to follow a power‑law distribution nₑ(γ)=Kₑ γ⁻ᵖ with p≈2.4, extending from γ≈2.5×10⁵ to 2.5×10⁷. The synchrotron cooling time for electrons (t_sync≈770 s · (γ/10⁶)⁻¹ · (B/1 G)⁻²) exceeds the light‑crossing time of the disc, justifying the use of a time‑independent, adiabatically cooled electron spectrum. Radiative cooling, pair‑production absorption, and shock‑induced changes in the photon field are deliberately omitted to isolate the effect of the IR excess.

Two geometric configurations of the disc are explored: (1) a disc perpendicular to the orbital plane (inclination = 90°) with a half‑opening angle of 5° and temperature T_disc = 12 500 K, and (2) a disc tilted by 45° with a half‑opening angle of 0.7° and T_disc = 15 000 K. In both cases the disc radius is set to 50 R_* to match the binary separation at eclipse. The disc density follows a power‑law ρ(r)=ρ₀ (r/R_*)⁻ⁿ, calibrated against earlier optical/IR observations.

The anisotropic IC scattering rate is calculated from

dN_tot/dt dε₁ = ∫∫∫ n_ph(ε₀) nₑ(γ) (dN_γ,ε₀/dt dε₀) cosθ dε₀ dγ dΩ,

where θ is the angle between the incoming photon direction and the line of sight, and cosθ accounts for the anisotropy of the photon source (star or disc). Numerical integration over the full solid angle of the star and disc yields the energy‑dependent scattering rate and, consequently, the predicted gamma‑ray flux in the 0.1–100 GeV band.

Results show that the disc’s IR photons increase the IC scattering rate by roughly a factor of two at periastron compared with a model that includes only stellar photons. This enhancement is most pronounced at intermediate gamma‑ray energies (∼0.1–100 GeV), whereas the very‑high‑energy (>TeV) component remains dominated by stellar photons. The increase stems from the disc’s large solid angle and the ∼10³ boost in photon density at mid‑IR wavelengths. However, during the actual disc crossing the photon density drops sharply and the scattering geometry becomes unfavorable (head‑tail configuration), so the disc contribution diminishes and cannot account for the sharp GeV flare observed by Fermi after the second disc crossing.

The 45° tilted disc produces a slightly higher flux before periastron and shifts the flux peak away from periastron, but the overall effect remains modest. The authors acknowledge that their simplified treatment—static electron spectrum, approximate disc photon field, neglect of shock heating and pair‑production—likely underestimates the true disc photon density, especially at larger radii, and omits mechanisms that could boost the IC rate near disc crossings.

In conclusion, while the IR excess from the circumstellar disc can double the IC gamma‑ray flux at periastron, it alone does not explain the observed post‑crossing GeV flare. Future work should incorporate electron cooling, shock‑induced disc heating, and photon absorption processes to develop a more comprehensive model of high‑energy emission in PSR B1259‑63.