GCN Circular 20542

A. Smith (UMD) and I. Martinez (UMD) 
report on behalf of the HAWC Collaboration

Subject: LIGO/Virgo G270580: HAWC follow-up observations

HAWC was operating and our real-time all-sky GRB monitoring analysis was running at the time of the G270580 event. However, the highest probability region of the LIGO/Virgo contour fell well outside the field-of-view of HAWC.

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 day. Within each time window, we search the HAWC sky within 50 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 tuned 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 trigger, we went back and re-analyzed the data within �� 60 seconds of the gravitational-wave trigger on 3 timescales (1, 10, 100 sec) to look for excesses consistent with the latest LIGO/Virgo map with a reduced threshold due to the reduced number of trials. None were found.

Additionally, we searched for long duration point sources in the 90% containment contour provided by LIGO during the following day when it transited through the HAWC field-of-view (~6hrs long transit). The highest probability region transited the HAWC field-of-view about 20hrs after the trigger. This analysis is optimized for ~0.5-100TeV. We found no evidence of emission.

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.