Fermi Observation of the transitional pulsar binary XSS J12270-4859

TL;DR

This study analyzes Fermi LAT data of the transitional pulsar binary XSS J12270-4859, revealing flux decrease, orbital modulation, and long-term variability associated with the disappearance of its accretion disk, providing insights into its emission mechanisms.

Contribution

First detailed Fermi LAT analysis of XSS J12270-4859 post-disk disappearance, identifying flux changes and orbital modulation linked to the pulsar's transitional state.

Findings

Detected a factor of 2 flux decrease after the transition.

Observed orbital modulation in gamma-ray emission post-transition.

Identified long-term flux variations possibly due to source contamination.

Abstract

Because of the disappearance of its accretion disk since the time period around 2012 November--December, XSS J12270-4859 has recently been identified as, in addition to PSR J1023+0038, another transitional millisecond pulsar binary. We have carried out detailed analysis of the Fermi Large Area Telescope data for the binary. While both spectra before and after the disk-disappearance transition are well described by an exponentially cut-off power law, typical for pulsars' emission in the Fermi's 0.2-300 GeV, a factor of 2 flux decrease related to the transition is detected. A weak orbital modulation is seen, but only detectable in the after-transition data, same to that found at X-rays. In the long-term light curve of the source before the transition, a factor of 3 flux variations are seen. Comparing to the properties of J1023+0038, we disucss the implications from these results. We suggest that since the modulation is aligned with that at X-rays in orbital phase, it possibly arises due to the occultation of the gamma-ray emitting region by the companion. The origin of the variations in the long-term light curve is not clear, because the source field also contains unidentified radio or X-ray sources and their contamination can not be excluded. Multi-wavelength observations of the source field will help identify the origin of the variations by detecting any related flux changes from the in-field sources.