TITLE: GCN CIRCULAR NUMBER: 20811 SUBJECT: LIGO/Virgo G275404: HAWC follow-up DATE: 17/03/04 02:08:55 GMT FROM: Andrew Smith at U Maryland HAWC was operating and our real-time all-sky GRB monitoring analysis was running at the time of the G275404 event. At the time of the event, the HAWC detector was oriented at (α, δ) = (336.1°, 19.0°), local zenith. 33% of the LIGO/Virgo CWB probability contour fell within our observable field (0-45 deg zenith angle). We continuously 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 gravitational-wave trigger. After the GW trigger was reported, we re-analyzed the data within ± 60 seconds of the gravitational-wave trigger contour on 3 timescales (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. Since the most probable region of the contour was setting at the time of the trigger, we observed this region during the following transit, 17-25 hrs after the GW event. We did not find a significant excess. 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. 67% of the LIGO/Virgo contour is within HAWC's observable declination range. 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.