GCN Circular 24067

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 (��, ��) = (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.