TITLE: GCN CIRCULAR NUMBER: 14549 SUBJECT: GRB 130427A: Non-observation of VHE emission with HAWC DATE: 13/05/04 01:24:00 GMT FROM: Dirk Lennarz at HAWC D. Lennarz, I. Taboada (Georgia Tech) report on behalf of the HAWC collaboration (http://www.hawc-observatory.org/collaboration/): 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, 641-–650 (2012).