TITLE:   GCN CIRCULAR
NUMBER:  21071
SUBJECT: LIGO/Virgo G284239: HAWC follow-up 
DATE:    17/05/05 20:01:03 GMT
FROM:    Andrew Smith at U Maryland  <asmith@umdgrb.umd.edu>

A.J. Smith (University of Maryland, College Park) and I. Martinez-Castellanos (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 G284239 event. At the time of the event, the HAWC detector was oriented at (α, δ) = (100.2°, 19.0°), local zenith. 28% of the LIGO/Virgo CWB probability contour fell within our observable field (0-45 deg zenith angle).

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 data the in the time the trigger time 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, integrating from the time of the trigger to the time when the overlapping 90% containment LIGO contour left our FOV. We did not find a significant excess in these ~1-2hr observations, which covered 28% of the probability space. Approximately 12 hr after the trigger, the high longitude portion of the contour transited through our field of view, and these locations were search integrating the whole ~6hr of their transit. No significant excess was observed. In total 63% of the of the LIGO contour was observed within 24hrs of the trigger. 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.