TITLE:   GCN CIRCULAR
NUMBER:  13379
SUBJECT: GRB 120624B: Fermi-LAT Detection
DATE:    12/06/25 10:18:47 GMT
FROM:    Giacomo Vianello at SLAC  <giacomov@slac.stanford.edu>

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

Fermi-LAT has detected high energy emission from the long, hard and
bright GRB 120624B in ground analysis. The GRB was triggered on by the
GBM on June 24, 2012 at 22:23:54.93 UTC, although the emission started
~250 seconds earlier (trigger 362269436, GCN 13377).

The best GBM position was \~70 deg off-axis for the whole duration of
the prompt emission (~270 seconds), outside of the Fermi/LAT nominal
field of view for the standard data analysis.

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 ~T0-250s and ~T0+20 s. The significance of the
excess corresponds to 10 sigma. The light curve shows 3 peaks, with a
total duration of \~270 s.

This burst was bright enough to result in a Fermi spacecraft
autonomous rapid repoint (ARR) maneuver, starting 100 s after the GBM
trigger. Thus, the GBM position entered the LAT field of view at
\~T0+100 s.

A preliminary maximum-likelihood analysis of the E>100MeV
P7TRANSIENT_V6 LAT data generated during the interval T0+100, T0+1.3
ks (until the GRB became occulted by the Earth) revealed a very
significant transient source, with a spectrum well described by a
power law of index -2.4 +/- 0.1 (68% c.l. statistical only). Using
this analysis, we obtained the best LAT on-ground localization of:

RA(J2000) = 170.73 deg
Dec(J2000) = 9.48 deg

with an error radius 0.45 deg (90% containment, statistical error
only), which is 3.6 deg from the best GBM localization.

The Zenith angle for this source was ~30 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)