Pair Cascades in the Disk Environment of the Binary System PSR B1259-63/LS 2883

TL;DR

This paper investigates how pair cascades and gamma-ray absorption in the circumstellar disk of PSR B1259-63/LS 2883 influence observed high-energy emissions, explaining features in GeV and TeV light curves.

Contribution

It provides a detailed analysis of pair cascade signatures and gamma-ray absorption effects in the disk, clarifying their roles in observed high-energy phenomena.

Findings

Cascade emission does not cause the GeV flare.

Gamma-gamma absorption in the disk may explain the TeV light curve.

Cascade processes could account for GeV emission near periastron.

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 $\sim3.4$ 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 of the high energy emission. At GeV energies, the source was detected for the first time during the previous periastron passage which took place on December 15, 2010. The Fermi Large Area Telescope (LAT) reported a spectacular and unexpected {\gamma}-ray flare occurring around 30 days after periastron and lasting for about 7 weeks. In this paper, 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 show that cascade emission generated in the disk cannot be responsible for the GeV flare, but it might explain the GeV emission observed close to periastron. We also show that the {\gamma}{\gamma}-absorption in the disk might explain the observed TeV light curve.