A Luminous and Highly-variable Gamma-ray Flare Following the 2017 Periastron of PSR B1259-63/LS 2883

TL;DR

The 2017 gamma-ray flare of PSR B1259-63/LS 2883 was highly variable, with short timescale fluctuations and spectral curvature, indicating beamed emission and providing new insights into the system's post-periastron high-energy activity.

Contribution

This study reports the first detection of a luminous, highly-variable gamma-ray flare with spectral curvature and rapid variability following the 2017 periastron, revealing new emission characteristics.

Findings

2017 flare showed spectral curvature and short-term variability.

Flare flux exceeded the pulsar's spin-down power.

Flares occurred long after periastron, constraining emission models.

Abstract

Three periastron passages of the PSR B1259$-$63/LS 2883 binary system, consisting of a 48 ms rotation-powered pulsar and a $\sim30$ M$_{\odot}$ Be star, have been observed by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope, in 2010, 2014, and 2017. During the most-recent periastron passage, sustained low-level gamma-ray emission was observed over a $\sim3$-week long interval immediately after periastron, which was followed by an interval of no emission. Sporadic flares were detected starting 40 days post-periastron and lasted approximately 50 days, during which the emission displayed significant spectral curvature, variability on timescales as short as 1.5 minutes, and peak flux levels well in excess of the pulsar spin-down power. By contrast, during the 2010 and 2014 periastron passages, significant gamma-ray emission was not observed with the LAT until 30 and 32 days post-periastron, respectively. The previous flares did not exhibit spectral curvature, showed no short term variability, and did not exceed the pulsar spin-down power. The high flux and short timescales observed in 2017 suggest significant beaming of the emission is required and constrain the size of the emission region. The flares occur long enough after periastron that the neutron star should already have passed through the extended disk-like outflow, thus constraining options for target material and seed photon sources for inverse Compton models.