Onset and Rapid Increase of Gamma-Ray activity from the Binary System PSR B1259-63 detected by Fermi LAT

ATel #6225; K. S. Wood (NRL), G. A. Caliandro (CIFS/SLAC), C. C. Cheung (NRL), J. Li (IEEC-CSIC), D. F. Torres (IEEC-CSIC), for the Fermi LAT Collaboration
on 10 Jun 2014; 23:52 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Teddy Cheung (ccheung@milkyway.gsfc.nasa.gov)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, is measuring rapid brightening of the binary system PSR B1259-63/SS 2883 in the GeV energy range. The rise is consistent with repetition of the GeV light curve observed at the 2010-2011 periastron. Up to at least 2014-06-05 there were rare marginal detections and no detection at all over 2014-05-25 to 2014-06-05, corresponding to a broad minimum at the same phase of the previous cycle (see Tam et al. 2011, ApJ 736, L10; Abdo et al. 2011, ApJ 736, L11; Chernyakova et al. 2014, MNRAS 439, 432).

A difference from analysis published in Atels #6216, #6204, and #6198 is that we model the region around PSR B1259-63 using an internal source list based on 4 years of LAT data. A likelihood binned analysis was performed for a time window of 4 days, shifted in time with steps of 3 hours. The profile of test statistic (TS) values obtained shows a sharp rise, from which we determine onset of GeV activity to be on 2014-06-06 03:00:00 UTC +/- 1.5 h, i.e., 33 days after periastron (2014-05-04). On 2014-06-09 we measure high significance detections at timescales of 1 day and 12 hours. Preliminary analysis of the Fermi LAT data in the one-day time period of 2014-06-09 00:10:04 to 2014-06-10 00:10:04 UTC gives a detection with significance above 5 sigma (or TS ~61, depending slightly on model details), corresponding to flux (E > 100 MeV) of (1.8 +/- 0.3) x 10^-6 ph cm^-2 s^-1, and a photon index of 3.1 +/- 0.2. In a shorter interval of 12 hours starting 2014-06-09 12:10:04 we measure TS = 33, flux (E > 100 MeV) of (2.1 +/- 0.6) x 10^-6 ph cm^-2 s^-1, and a photon index of 3.3 +/- 0.3. If the light curve of this cycle continues to track that of 2010-2011, then the highest intensity of the GeV flaring is expected on June 12.

Fermi will continue to operate in a pointed mode to enhance exposure. We strongly encourage observations in other wavelengths. Spectroscopic observations to track evolution of H-alpha equivalent width over coming days to weeks would be particularly valuable because there was a gap in coverage near this phase in the previous cycle. X-ray observations during the next few days (the anticipated GeV maximum) should also be high priority. Observers planning coordinated observations are encouraged to list them at the following site: http://fermi.gsfc.nasa.gov/ssc/observations/multi/reporting/ because this will assist the Fermi project in planning.

The Fermi LAT contact persons for this source are Kent S. Wood (kent.wood@nrl.navy.mil) and G. Andrea Caliandro (caliandr@slac.stanford.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.