GRB 090926

Elisabetta Bissaldi (MPE) reports on behalf of the Fermi GBM Team:


"At 04:20:26.99 UT on 26 September 2009, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 090926 (trigger 275631628 / 090926181).
The on-ground calculated location, using the GBM trigger
data, is RA = 354.5, DEC = -64.2 (J2000 degrees,
equivalent to 23h 38m, -64d 12'), with an uncertainty
of 1.0 degree (radius, 1-sigma containment,
statistical only; there is additionally a systematic
error which is currently estimated to be 2 to 3 degrees).
 
The angle from the Fermi LAT boresight is 48 degrees.
Moreover, this burst was bright enough to result in
a Fermi spacecraft repointing maneuver.

The burst was also independently detected by INTEGRAL SPI-ACS.

The GBM light curve consists of single pulse
with a duration (T90) of 20 +/- 2 s (8-1000 keV).
The time-averaged spectrum from T0 to T0+21 s is
best fit by a Band function with Epeak = 314 (+/- 4) keV,
alpha = -0.75 (+/-0.01) and beta = -2.59 (+0.04/-0.05)
(C-stat 1136 for 478 d.o.f.).

The event fluence (8-1000 keV) in this time interval is
(1.45 +/- 0.04)E-04 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+3 s in the 8-1000 keV band
is 80.8 +/- 0.4 ph/s/cm^2.


The spectral analysis results presented above are preliminary; 
final results will be published in the GBM GRB Catalog."

GRB 090926: Fermi LAT detection

Takeshi Uehara, Hiromitsu Takahashi (Hiroshima University) and
Julie McEnery (NASA/GSFC) report on behalf of the Fermi LAT team:

At 04:20:26.99 (UT) on 26 Sep 2009, the Fermi Large Area Telescope
(LAT) detected gamma rays from the long GRB 090926, which was
triggered and located by the Fermi Gamma-ray Burst Monitor (GBM)
(trigger 275631628 / 090926181, GCN9933
Loading...
 
 ). The angle of the GBM best
position (RA, Dec= 354.5, -64.2) with respect to the LAT boresight was
~52 degrees at the time of the trigger, which is close the edge of our
field of view.

The data from the Fermi LAT shows a significant increase in the event
rate within 2.5 degrees of the GBM location 7 s after the GBM trigger
that is spatially and temporally correlated with the GBM emission with
high significance. More than 150 photons above 100 MeV and more
than 20 photons above 1 GeV are observed up to more than 200 s after
the GBM trigger. The highest energy photon is a 19.6 GeV event which is
observed 26 seconds after the GBM trigger.

The best LAT on-ground localization is found to be (RA, Dec = 353.56,
-66.34) with a 90% containment radius of 0.07 deg (statistical; 68%
containment radius: 0.04 deg, preliminary systematic
error is less than 0.1 deg) which is consistent with the GBM localization.
A Swift TOO request has been issued.

Further analysis is ongoing.

The points of contact for this burst is

Takeshi Uehara : uehara@hep01.hepl.hiroshima-u.ac.jp

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.

This message can be cited.