Role of the disk environment in the gamma-ray emission from the binary system PSR B1259-63/LS 2883

TL;DR

This paper investigates how the circumstellar disk in the PSR B1259-63/LS 2883 binary affects gamma-ray emission, focusing on inverse Compton scattering, pair cascades, and gamma-gamma absorption, explaining observed flux modulation.

Contribution

It introduces a detailed analysis of gamma-ray interactions within the Be star's disk, highlighting the disk's role in shaping high-energy emission features.

Findings

Gamma-ray absorption in the disk explains flux modulation at periastron.

Pair cascades contribute to the GeV flux in the system.

Disk photon fields significantly influence the observed gamma-ray spectrum.

Abstract

PSR B1259-63/LS 2883 is a very high energy (VHE; E > 100 GeV) gamma-ray emitting binary consisting of a 48 ms pulsar orbiting around a Be star with a period of 3.4 years. The Be star features a circumstellar disk which is inclined with respect to the orbit in such a way that the pulsar crosses it twice every orbit. The circumstellar disk provides an additional field of target photons which may contribute to inverse Compton scattering and gamma-gamma absorption, leaving a characteristic imprint in the observed spectrum and light curve of the high energy emission. We study the signatures of Compton-supported, VHE gamma-ray induced pair cascades in the circumstellar disc of the Be star and their possible contribution to the GeV flux. We also study a possible impact of the gamma-gamma absorption in the disk on the observed TeV light curve. We show that the cumulative absorption of VHE gamma-rays in stellar and disk photon fields can explain the modulation of the flux at the periastron passage.