TITLE: GCN CIRCULAR NUMBER: 24144 SUBJECT: LIGO/Virgo S190421ar: HAWC follow-up DATE: 19/04/22 18:59:11 GMT FROM: Andrew Smith at UMD/HAWC The HAWC Collaboration performed a follow-up of the gravitational wave trigger S190421ar (GCN #24141). At the time of the trigger the HAWC local zenith was oriented towards (RA, Dec) = (76.8 deg, 19.0 deg), 6% of the GW candidate sky location probability fell within our observable field of view (0-45 deg zenith angle). We performed a search for a short timescale emission using 6 sliding time windows (dt = 0.3s, 1s, 3s, 10s, 30s and 100s), shifted forward in time by 20% of their width. We searched the 95% probability containment area in a timescale-dependent time period, from t0-5dt to t0+10dt, where t0 is the time of the GW trigger. No significant gamma-ray detection above the background was observed. The sensitivity of this analysis is greatly dependent on zenith angle, ranging from 11deg to 31deg for the area searched in this analysis. The 5sigma detection sensitivity to a 1s (100s) burst in the 80-800GeV energy range goes from 1.4e-6 erg/cm^2 to 8.0e-6 erg/cm^2 (7.5e-6 ergs/cm2 to 3.9e-5 erg/cm^2), depending on the zenith angle. HAWC is a TeV gamma ray water Cherenkov array located in the state of Puebla, Mexico. It is sensitive to the energy range ~0.1-100TeV, and monitors 2/3 of the sky every day with an instantaneous field-of-view of ~2 sr.