GRB 120727A/Galactic source: Fermi-LAT detection of a transient source

ATel #4285; G. Vianello (CIFS/SLAC), J. Chiang (SLAC/KIPAC), V. Pelassa (UAH), V. Connaughton (UAH), report on behalf of the Fermi-LAT and GBM team:
on 29 Jul 2012; 05:21 UT
Credential Certification: Giacomo Vianello (giacomov@slac.stanford.edu)

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At 07:21:05 on July 27, 2012, the Automated Science Processing of Fermi LAT data detected high energy emission from a transient during an on-ground blind search analysis.

The transient has been detected during one orbit. The duration is then between half an hour and 2 hours.

The mean spectrum is well described by a power law with a photon index -2.3 +/- 0.2. The mean flux of the source was 3.0 +/- 1.3 x 10^-9 erg/cm2/s (100 MeV - 10 GeV). All errors are statistical only.

The best LAT on-ground location is found to be RA, DEC = 250.86, -45.97 (J2000) with an error radius of 0.32 deg (68% containment, statistical error only).

The found position corresponds to Galactic coordinates l,b = 339.05, -0.003. No concurrent low-energy emission from this source has been detected by Fermi/GBM. Both these facts and the long duration could point to a Galactic origin for the transient.

There are two known Fermi-LAT gamma-ray sources close to the position of the transient: 2FGL J1640.5-4633 and 2FGL J1648.4-4612, respectively ~0.8 and ~0.9 deg away. 2FGL J1640.5-4633 has been classified as potentially associated with an SNR or PWN in the Fermi 2FGL catalog (Nolan et al., 2012, ApJS, 199, 31). The second 2FGL source (2FGL J1648.4-4612) is a pulsar. Both the sources are formally outside the error circle for the transient, but they are well within the Region of Interest used for this analysis (a circle with a 12 deg radius). Thus, given also the limits of our knowledge of the background especially close to the Galactic plane, we cannot completely rule out the presence of confusion effects or other problems in the analysis.

In conclusion, we cannot provide at this stage a secure classification for this source, which could be either a long/weak GRB or a Galactic transient. Because of the potential uniqueness of this event, we encourage further observations.

The Fermi LAT point of contact for this burst is Giacomo Vianello (giacomov@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.