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Fermi LAT detection and Swift/XRT observations of PSR B1259-63 before its 2017 periastron passage

ATel #10775; Xinbo He, P. H. Thomas Tam, Partha Sarathi Pal (Sun Yat-sen University)
on 25 Sep 2017; 04:22 UT
Credential Certification: P.H.Thomas Tam (grbtom@gmail.com)

Subjects: X-ray, Gamma Ray, Binary, Transient, Pulsar

Referred to by ATel #: 10818, 10918, 10925, 10972, 10973

PSR B1259-63, orbiting the massive star LS 2883, has just passed the periastron on September 22, 2017. During past passages, high-energy enhancement has been seen and is expected for this passage. Here we report, that the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope, has observed an increasing gamma-ray emission from PSR B1259-63 over the last month.

We analyzed a circular region-of-interest of 20 deg radius centered on PSR B1259-63 using binned maximum likelihood analysis. Using data obtained from August 23 00:00:00 to Sept. 23 UT 06:02:39, PSR B1259-63 was significantly detected with a TS value of 44.7 (corresponding to detection significance of about 6.7 sigma), an index of 3.3+-0.4 and an average photon flux of (2.9 +/- 0.9)e-7 photons/cm^2/s. We also selected the Fermi LAT data from July 23 to Sept. 23. and five-day time bins are used in our analysis. No significant emission was found in any time bins in late July and August, i.e., TS value is below 4. From August 28 to Sept. 2, the TS value raised above 6, indicating some emission. Then the TS value further increased to 8.5 (13.5) on Sept. 12-17 (Sept. 17-22), respectively.

The pre-periastron enhancement/brightening in gamma-rays was first seen in 2010 (Tam et al. 2011, Abdo et al. 2011). Its origin remains elusive.

Swift/XRT also observed PSR B1259-63 with a high cadence since 2017-08-22 (ToO PIs: Chernyakova & Bordas). It saw a peak flux of 4.4e-11 erg/s/cm^2 (count rate 0.56+/-0.02) on the September 6, decreasing to 3.0e-11 erg/s/cm^2 (count rate 0.27+/-0.02) on the September 12. This is similar to what was previously seen over last passages.

Continuous observations of the source are strongly encouraged. GeV flares were seen about a month after the periastron both in 2011 and 2014.

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 also thank the Swift team to react to the respective ToO requests.