Fermi-LAT Detection of Continued Increasing Gamma-Ray Flux from the Binary System PSRB1259-63

ATel #3115; A. A. Abdo (GMU, resident at NRL), D. Parent (GMU, resident at NRL), R. Dubois (SLAC National Accelerator Laboratory) on behalf of the Fermi Large Area Telescope Collaboration; M. Roberts (Eureka Scientific)
on 19 Jan 2011; 22:29 UT
Credential Certification: A. A. Abdo (aous.abdo@nrl.navy.mil)

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We report on the increased activity from the binary system PSR B1259-63 with the Large Area Telescope (LAT), one of the two instruments on board the Fermi Gamma-ray Space Telescope.

After the initial detection of this source by the LAT over a period of 32 days (Atel # 3085) which happened during the first passage of the pulsar through the equatorial disk of the Be star, the source showed a decrease in flux. Preliminary analysis of Fermi-LAT data in the period 2010-12-20 to 2011-01-14 UTC shows no significant detection and we place a 95% confidence flux upper limit of 9 x 10^-8 ph/cm^2/s above 100 MeV assuming a spectral index of 2.2.

The second disk passage started in early January. An increase in the unpulsed flux in radio, X-ray, and gamma-rays are expected during this second disk passage. A recent Atel by Kong et al. (Atel 3111) reported an increase in the source activity in the energy range > 200 MeV in the period 2011-01-14 - 2011-01-16. Preliminary analysis of the Fermi-LAT data in the region around PSR B1259-63 in the time period 2011-01-17 to 2011-01-19 UTC shows that the source is still active and is now detectable on 6-hours time scale. Our preliminary analysis gives a flux (E >100 MeV) of 1.7 +/- 0.4 x 10^-6 ph/cm^2/s and a photon index of 3.0 +/- 0.3. This flux is considerably higher (x14) than the one reported for the first disk passage over a period of 32 days (Atel 3085). It is also worth noting the steepening of the spectral index from 2.2 for the first disk passage (Atel 3085) to 3.0 for the current passage. Preliminary analysis of the Fermi-LAT data since the start of the current flare (2011-01-14 to 2011-01-19 UTC) gives a flux (E >100 MeV) of 1.8 +/- 0.2 x 10^-6 ph/cm^2/s and a photon index of 3.1 +/- 0.2. A systematic uncertainty of 30% should be added in quadrature to the statistical uncertainties listed here.

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the ongoing activity of this source we strongly encourage multi-wavelength observations. For this source the Fermi LAT contact person is Aous Abdo (aous.abdo@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.