Analysis of the 2021 GeV flare of PSR B1259-63

Abstract

PSR B1259-63/LS 2883 is a gamma-ray binary in which the compact object (a pulsar) crosses the disk of the massive companion twice during its 3.4 years orbit, when it is close to periastron. The GeV flare detected by Fermi-LAT around 30-40 days after the periastron passage in 2010, 2014 and 2017 was not detected until 60 days after the most recent periastron on 9th February 2021 and lasted more than the previous flares. An unbinned likelihood analysis was performed with the Python 3 version of the Fermi-Tools, HEAsoft 6.28 and making use of pass 8 Fermi-LAT data obtained up to 112 days after periastron (1st June 2021). Modelling PSR B1259-63 with a power law, a photon index of 2.76 ± 0.09 and a Test Statistics significance level of 170.3 were obtained during the flare, similar to the values in papers referring to the previous flares. Although several models have been proposed to explain this phenomenon that is likely related to the pulsar crossing the disk, the origin of the GeV is still unclear