D. Lennarz, I. Taboada (Georgia Tech) report on behalf of the HAWC
We used data from the partially built HAWC detector to perform a search
for VHE emission in temporal coincidence with GRB 130427A (A. Maselli et
al., GCN 14448). This search was conducted using the scaler data
acquisition only, as the main data acquisition was not operational at
the time. At the time of the GBM trigger, the elevation of the burst in
HAWC's field of view, was only 33.13 degrees and setting. The
sensitivity of HAWC at this elevation is more than 2 orders of magnitude
poorer than near the zenith. Furthermore, while near zenith the nominal
threshold of the scaler system is a few GeV, towards the horizon the
nominal threshold is much higher.
We used six search windows with respect to the GBM trigger time: one in
the range 0 s to 20 s, which covers the bright structured peak seen by
Fermi-GBM that seems to be correlated with the Fermi-LAT emission (S.
Zhu et al., GCN 14508), an extended window from -5 s to 55 s and a
window from -5 s to 145 s, which is slightly larger than the T90
reported by GBM. We also searched around the second peak in the GBM
light curve (120 s to 300 s), -10 s to 10 s around the time of the
highest energy LAT photon and in an extended window from -10 s to 290 s.
We find a deviation of +38960 / -77884 / -337877 / -165991 / -519485 /
-1036 of the global PMT count with respect to a moving average in the
six time windows mentioned above. The p-value for these deviations
assuming background hypothesis are 17 % / 78 % / 95 % / 71 % / 90 % and
50 % respectively. Our observations are consistent with background only.
The implications of this non-detection with respect to the VHE fluence
of this GRB will be reported elsewhere.
HAWC is a gamma-ray detector under construction in Central Mexico. It
currently consists of 29 operational Water Cherenkov Detectors out of
300 planned. A detailed description of the sensitivity of HAWC to GRBs
can be found in A.U. Abeysekara et al., Astroparticle Physics 35,