Detection of enhanced emission above 100 MeV, using Fermi-LAT data, associated with the PSR B1259-63/LS 2883 binary system approximately 60 days after periastron

ATel #14540; T. J. Johnson (GMU, resident at NRL), G. Marti-Devesa (Univ. of Innsbruck), and C. C. Cheung (NRL), on behalf of the Fermi Large Area Telescope Collaboration
on 12 Apr 2021; 05:38 UT
Credential Certification: Teddy Cheung (Teddy.Cheung@nrl.navy.mil)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed enhanced emission, on a one-day time scale, associated with the PSR B1259-63/LS 2883 binary system, 60 days after periastron.

Analyzing the LAT data in the region around PSR B1259-63 over the time interval 2021-04-04 00:00:00 to 2021-04-11 03:24:40 UTC, assuming a photon index of 2.8 based on previous periastron passages, we derived the 100 MeV to 300 GeV flux in one-day bins for PSR B1259-63. We find no significant detection from 2021-04-04, 05, 06, or 09, with 95% confidence level upper limits on the flux ranging from 0.7 to 0.8 x 10^-6 ph/cm^2/s. For 2021-04-07 and 08, we find low-significance detections (~3 sigma) with flux values of (0.8 +/- 0.4) x 10^-6 ph/cm^2/s and (0.8 +/- 0.6) x 10^-6 ph/cm^2/s, respectively. For the last bin in our analysis, 2021-04-10 to 2021-04-11 03:24:40 (spanning 1.14 days), we find a ~4 sigma detection with a flux of (1.2 +/- 0.3) x 10^-6 ph/cm^2/s, possibly indicative of the start of the expected intense post-periastron flaring period. All quoted uncertainties are statistical only.

The Fermi-LAT has observed enhanced emission following the three previous periastron passages of PSR B1259-63. The delay between periastron and onset of emission increased each time, from 30 days in 2010 to 32 days and 40 days in 2014 and 2017, respectively. If this passage follows the same pattern, the emission we have detected could be the start of a longer, flaring episode.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the expected ongoing activity of this source we strongly encourage multi-wavelength observations. PSR B1259-63 is included in the "LAT Monitored Sources" and consequently, a preliminary estimation of the daily gamma-ray flux observed by Fermi-LAT is publicly available ( https://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/PSR_B1259-63 ). For this source the Fermi LAT contacts are Tyrel Johnson (tyrel.johnson.ctr@nrl.navy.mil) and Kent Wood (kent.wood.ctr@nrl.navy.mil).

The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.