[vsnet-grb-info 12916] GBM trigger 383712005 (GRB 130228A?) : Possible high-energy photon emission in Fermi/LAT data

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  14258
SUBJECT: GBM trigger 383712005 (GRB 130228A?) : Possible high-energy photon emission in Fermi/LAT data
DATE:    13/03/05 18:55:53 GMT
FROM:    Weikang Zheng at U.of Michigan  <zwk at umich.edu>

WeiKang Zheng (UC Berkeley) and Carl Akerlof (Umich), report on behalf
of the ROTSE collaboration (the authors are not affiliated with the
Fermi Collaboration),

We report the possible high-energy photon emission in Fermi/LAT data
correlated with GBM trigger 383712005 (02:40:02 UT on Feb. 28, 2013,
probably GRB 130228A) as a follow-up of a routine search in GBM trigger
catalogs using our LAT data processing pipeline. The GBM trigger
location (RA = 265.830, Dec = 55.930) was about 24 degree from the LAT
boresight, and the zenith angle was about 45 degrees.

A standard matched filter technique analysis (Akerlof et al. 2010, 2011,
2013, Zheng et al. 2012a,b), which uses a duration data of 47.5s
after the trigger, did not reveal confident signal in LAT data. However,
if extending the duration to 150s, the matched filter analysis gives a
weight value of 10.53, not high enough to claim a firm detection
compared to standard case, but is clearly higher than background. This
kind of delayed LAT high energy photon emission is quite common in GRB.
The new GRB location estimated by the matched filter technique is Ra =
255.46, Dec = 55.02 with uncertainty ~0.5 degree, this is ~6 degrees
away from the GBM trigger location. ~7 photons above 100 MeV were
detected within 150s, with the highest energy of ~692 MeV at 133s after
the burst.

A maximum likelihood analysis focusing on the new GRB location gives a
TS value of 31.0 over the same time window of 150s after the trigger.
This supports the above finding from matched filter analysis.

The photon information can be found in the following link including the
skymap:
http://www.rotse.net/LAT/GBMTriggers/383712005/383712005_res.html