Fermi LAT detection of a flaring activity from PSR B1259-63/LS 2883

ATel #6216; P. H. Thomas Tam, Albert K. H. Kong (National Tsing Hua University, Taiwan), and Gene C. K. Leung (The University of Hong Kong)
on 9 Jun 2014; 15:51 UT
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Credential Certification: P.H.Thomas Tam (grbtom@gmail.com)

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We report on the first clear GeV flaring activity of the binary system PSR B1259-63 as seen by the Fermi Large Area Telescope (LAT) after the recent periastron passage that occurred on May 4, 2014.

Using 100 MeV to 300 GeV data obtained between 2014-6-6 00:00:00 UTC and 2014-6-9 00:30:21 UTC (i.e., MET 423705603 to 423966624), we performed a binned likelihood analysis of the 21x21 square degrees region-of-interest (ROI) centered at the position of PSR B1259-63, for which a simple power law was assumed. We obtained a test-statistics (TS) value of ~28 for the binary, which corresponds to a detection at the ~ 5-sigma significance level. This is the first clear detection in the gamma-ray band in our current analysis (see our monitoring campaign as described in Atel #6198). The analysis also returns a photon index of 2.9+-0.2, consistent with that found for the post-periastron flares in early 2011, and a photon flux of (8.0+-2.0)e-7 ph/s/cm^2, more than an order of magnitude increase in flux than that measured over a two-month period around the recent periastron passage (Atel #6198).

The reprocessed pass 7 source data were used in the above analysis. The background photon level was estimated by including the sources in the second LAT source catalog and the two diffuse background sources: the Galactic and isotropic components, while fixing all spectral parameters except those of PSR B1259-63, the normalization of the 2FGL sources in the ROI, and the diffuse sources. The same was done in Atel #6198.

We also repeated the analysis of the data in the time range as described in Atel #6204 (i.e., the 2.3-day period between June 3 and June 5), using reprocessed pass 7 source data and the same background source model as they describe. However, we obtained a TS value of ~4 only for these data, much less than the TS values reported in Atel #6204. Therefore we do not confirm the flaring activity during the June 3-5 period.

We note that the post-periastron flaring activity was first reported in Atel #3111 just after the last periastron passage in 2010-2011.

We thank the LAT team to schedule the current ToO observations of this source and acknowledge the use of public data available at the Fermi Science Support Center.