GCN Circular 13423

Daniel Kocevski (Stanford Univ.), Giacomo Vianello (CIFS/SLAC), and James Chiang (KIPAC/SLAC) report on behalf of the Fermi LAT Team:

Fermi-LAT has detected high energy emission from the long, hard and bright GRB 120709A in ground analysis. The GRB was triggered on by the GBM on July 9th, 2012 at 21:11:41 UTC (trigger 363561103).

The best GBM position was ~22 deg off-axis for the duration of the prompt emission (~35 seconds), and remained within the Fermi/LAT nominal field of view for an additional ~650 seconds.

Using a non-standard data selection most sensitive in the tens-of-MeV energy range and with a broader acceptance, we significantly detected the burst between ~T0 and ~T0+5 s. The significance of the excess corresponds to ~7 sigma. The light curve also shows a secondary peaks at ~11s of lesser significance, but coincident with the second of three peaks seen in the GBM light curve.

A preliminary maximum-likelihood analysis of the E>75MeV P7SOURCE_V6 LAT data generated during the interval T0, T0+100s revealed a significant transient source, with a spectrum well described by a power law of index -2.0 +/- 0.3 (68% C.L. statistical only). Integrating over a duration covering T0 to T0+200s, we obtained the best LAT on-ground localization of:

RA(J2000) = 318.41 deg
Dec(J2000) = -50.03 deg

with an error radius of 0.35 deg (90% containment, statistical error only), which is 6.3 deg from the best GBM localization.

The zenith angle for this source was ~42.3 deg at the time of the trigger, thus very far from the Earth limb.

A Swift/ToO request has been submitted.

The Fermi-LAT point of contact for this burst is Daniel Kocevski (kocevski@stanford.edu)