TITLE: GCN CIRCULAR NUMBER: 24067 SUBJECT: LIGO-Virgo S190408an: HAWC follow-up DATE: 19/04/08 21:08:25 GMT FROM: Israel Martinez-Castellanos at UMD/HAWC I. Martinez-Castellanos (University of Maryland, College Park) report on behalf of the HAWC Collaboration: The HAWC Collaboration performed a follow-up of the gravitational wave trigger S190408an. At the time of the trigger the HAWC local zenith was oriented toward (α, δ) = (13.9°, 19.0°). 80% of the GW candidate sky location probability fell within our observable field (0-45 deg zenith angle). We performed a search for a short timescale emission using 6 sliding time windows (dt = 0.3s, 1s, 3s, 10, 30s and 100s), shifted forward in time by 20% of their width. We searched 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 steady-state cosmic-ray background was observed. The sensitivity of this analysis is greatly dependent on energy and zenith angle. For reference, it has a 5sigma detection sensitivity to a 1s (100s) burst with a fluence of 3x10^-7 erg/cm^2 to 2x10^-5 erg/cm^2 (2x10^-6 erg/cm^2 to 8x10^-5 erg/cm^2) in the 0.3-1TeV energy range, 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 ~0.3-100TeV, and monitors 2/3 of the sky every day with an instantaneous field-of-view of ~2 sr.