Continuing >100 MeV Activity from the PSR B1259-63/LS 2883 System 85 Days Post-periastron Detected with the Fermi-LAT

ATel #14612; T. J. Johnson (George Mason Univ, resident at US Naval Research Lab.), J. B. Coley (Howard Univ./NASA-GSFC CRESST), G. Marti-Devesa (Univ. of Innsbruck), R. H. D. Corbet (UMBC/NASA GSFC), C. C. Cheung (US Naval Research Lab.), M. Kerr (US Naval Research Lab.), A. B. Pearlman (McGill University, McGill Space Institute, Caltech) , J. Hare (NASA-GSFC/USRA), and Z. Wadiasingh (NASA-GSFC/USRA) , on behalf of the Fermi Large Area Telescope Collaboration
on 8 May 2021; 00:17 UT
Credential Certification: Tyrel Johnson (tyrel.j.johnson@gmail.com)

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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 associated with the PSR B1259-63/LS 2883 binary system, 85 days after periastron.

A preliminary analysis of LAT data in the time interval 2021-05-05 22:50:01 to 2021-05-06 21:47:02 UTC detects the source above the background with a signficance of ~8 sigma in the energy range 100 MeV to 300 GeV. Assuming a photon index of 2.8, based on previous periastron passages, we derive a flux in this energy range of (2.3 +/- 0.4) x 10^-6 ph/cm^2/s.

Following the previous report of a bright flare (ATel #14540), the source remained at a high flux level before gradually declining in brightness to a flux of (1.0 +/- 0.5) x 10^-6 ph/cm^2/s on 2021-04-29 UTC. Following this, a reduction in exposure towards B1259-63 due to the observatory's modified rocking profile prevented sensitive measurements between 2021-04-30 and 2021-05-05, UTC.

We also analyzed a Swift X-ray Telescope (XRT) observation (ObsID 00095731005) performed on MJD 59340.64 (2021 May 06 at 15:30:22 UTC), which is part of the Swift Guest Observer program 1619411 (PI: Coley). From a preliminary analysis of the Swift quick look data, the 0.7-7 keV spectrum is well-modeled as a power law with a photon index of 1.3+/-0.5 modified by a neutral absorption column of 0.6+/-(0.6,0.5)X10^22 cm^-2. Background was found to dominate at energies above 7 keV. The unabsorbed 0.7-10 keV flux was found to be (0.8+/-0.2)X10^-11 erg cm^-2 s^-1. The best fit resulted in a Cash statistic of 23.8 for 28 degrees of freedom. Monitoring of the X-ray flux of the source with Swift and NICER through this periastron passage suggests that the flux has dropped by a factor of four from a peak in mid March 2021. Preliminary X-ray spectral results can be seen at https://slac.stanford.edu/~tyrelj/5-7-2021_NICER_spectralparam.pdf.

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 contact for X-ray analysis is Joel Coley (joel.coley@Howard.edu).

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. We thank the Swift Operations, Software and Calibration teams for scheduling and the execution of these observations.