GCN Circular 14795

G. Vianello (Stanford University), J. Racusin (NASA/GSFC), Nicola Omodei
(Stanford University) and M. Ohno (Hiroshima U.) report on behalf of the
Fermi-LAT team:

At 11:55:33 UT on 06 June 2013, Fermi LAT detected high energy emission
from GRB 130606B, which was also detected by Fermi-GBM (trigger 392212536 /
130606497). The brightness of the event in GBM triggered an autonomous
repoint of the spacecraft. Unfortunately, the GBM flight software
determined an unreliable position and the spacecraft slewed to that
position.

We were nevertheless able to detect a very clear excess at the position:

(RA, Dec) = 218.574, -22.131 (J2000) with an error radius of 0.1 deg (68%
containment radius, statistical errors only)

which was within the LAT field of view between T0 + 180 s and T0 + 1000 s.
Using SOURCE class we detected > 10 events, with 4 events above 1 GeV,
compatible with the position of the source during this time interval.

This source was about 91 deg from the LAT boresight at the time of the
trigger and during the whole prompt emission (~100 s long), well outside
the nominal field of view for the standard data analysis. However, we
detected a strong signal using the non-standard LLE selection, most
sensitive in the 10 MeV - 100 MeV energy range and featuring a broader
acceptance. We detected an excess of more than 17 sigma in a time interval
from 5 to 30 s after the trigger, composed of few hundreds events above the
background level. The effective area of the LLE class is small at such high
off-axis angle, therefore the strong excess indicates that this burst was
exceptionally bright and hard.

A Swift TOO request has been submitted.

A GBM circular is forthcoming.

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

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Giacomo Vianello

Stanford University,
Hansen Experimental Physics Lab,
452 Lomita Mall,
Stanford, CA 94305-4085

"A few observation and much reasoning lead
to error; many observations and a little
reasoning to truth." (A.Carrell)

ICQ: 566213964
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