GCN Circular 21732

I. Martinez-Castellanos (University of Maryland, College Park) and
A.J. Smith (University of Maryland, College Park) on behalf of the
HAWC Collaboration:

HAWC was operating and our real-time all-sky GRB monitoring analysis was
running at the time of the G298936 event. At the time of the event, the
HAWC detector was oriented at (��, ��) = (165.9��, 19.0��), local zenith. 60%
of the GW candidate location probability contour fell within our observable
field (0-45 deg zenith angle), including the location with highest
probability.

We perform a real-time search for counts above the steady-state cosmic-ray
background using 4 sliding time windows (0.1, 1, 10, and 100 seconds)
shifted forward in time by 10% of their width over the course of the entire
observing period. Within each time window, we search the HAWC sky within 45
degrees of zenith using 2.1 deg x 2.1 deg square bins shifted by ~0.1 deg
along the directions of Right Ascension and Declination. This analysis is
optimized for detecting ~100 GeV photons and is sensitive to the most
fluent GRBs. It did not report any significant post-trials events near the
time of the LIGO trigger.

After the LIGO trigger was reported, we re-analyzed the data collected
around the trigger time (T0-dt to T0+3dt) on 3 timescales (dt = 1, 10, 100
sec) with a reduced threshold to account for the reduced number of trials.
No significant candidates were identified.

Additionally, we searched for longer duration emission, integrating from
the time of the trigger to the time when the overlapping 90% containment
LIGO contour left our FOV. For reference, the maximum probability location
(RA = 66.7 deg, Dec = 49.7 deg) was observed from T0 to T0 + 8 ks. This
search covered the same 60% of the probability space as the prompt search.
No significant excess was observed. HAWC has a 5-sigma point-source
sensitivity to a flux >1 TeV of ~2x10^-11/cm^2/s, about ~1 ���crab unit���, for
a single transit observation.

HAWC is a TeV gamma ray water Cherenkov array located in the state of
Puebla, Mexico that monitors 2/3 of the sky every day with an instantaneous
field-of-view of ~2 sr.