TITLE:   GCN CIRCULAR
NUMBER:  19423
SUBJECT: GRB 160509A: non-observation of VHE emission with HAWC
DATE:    16/05/11 17:27:37 GMT
FROM:    Dirk Lennarz at HAWC  <dirk.lennarz@gatech.edu>

D. Lennarz (Michigan State University), I. Taboada (Georgia Tech) report
on behalf of the HAWC collaboration
(http://www.hawc-observatory.org/collaboration/):

We used data from the HAWC detector to perform a search for VHE emission
in temporal coincidence with GRB 160509A (F.Longo et al., GCN 19403). At
the time of the LAT trigger, the elevation of the burst in HAWC's field
of view was only 27.98 degrees (it was rising, but culminated at an
elevation of 33 degrees). The sensitivity of HAWC at this elevation is
more than 2 orders of magnitude poorer than near the zenith.
Furthermore, the energy threshold towards the horizon is much higher.
Combined with the moderate redshift of z=1.17 (N. R. Tanvir et al., GCN
19419) it makes a detection by HAWC unlikely.

We used four search windows with respect to the LAT trigger time: one in
the range -5 s to 45 s, which covers the main GBM emission episode (O.J.
Roberts et al., GCN 19411) and appears to be correlated with the >100
MeV soft emission observed by the LAT (F.Longo et al., GCN 19413), a
window from -5 s to 375 s, which extends slightly beyond the T90
observed by GBM and a time window from 45 s to 375 s, where the LAT data
is fit with a power-law of index -2.0 +/- 0.1. We also searched -20 s to
20 s around the time of the highest-energy LAT photon (52 GeV) 77
seconds after the GBM trigger. A 2 degree angular bin is defined around
the position of the Swift-XRT afterglow position (J. A. Kennea et al.,
GCN 19408) and the number of background events is estimated using an
ON/OFF method. We find the counts in the search bin to deviate by 1.9 /
0.9 / 0.2 / -1.4 sigma from the background expectation. Our observations
are consistent with background only.

The search was conducted using the main data acquisition that
reconstructs the incident direction of showers. It uses data
reconstructed at the HAWC site, not applying gamma-hadron separation.
The implications of this non-detection with respect to the VHE fluence
of this GRB will be reported elsewhere.

HAWC is a very-high-energy gamma-ray observatory operating in Central
Mexico at a latitude of 19 deg north. HAWC has an instantaneous field of
view of 2 sr and surveys 2/3 of the sky every day. A detailed
description of the sensitivity of HAWC to GRBs can be found in A.U.
Abeysekara et al., Astroparticle Physics 35, 641-650 (2012).