GCN Circular 27617

Hugo Ayala (Penn State) reports on behalf of the HAWC
collaboration (http://www.hawc-observatory.org/collaboration):

On 2020/04/21 00:35:24 UTC, the IceCube collaboration reported a
track-like very-high-energy event  that has a high probability of
being an astrophysical neutrino, IceCube-200421A. Location is at
RA: 87.93 (+3.49/-2.83 deg  90% PSF containment) J2000
Dec: 8.23 (+2.09/-1.84 deg 90% PSF containment) J2000
(GCN circular 27612
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 ).

We performed two types of analyses for the follow-up. The first is for
a steady source in archival data and the second is a search for a
transient source. We assume a power-law spectrum with an index of -2.3
for both analyses.

Search for a steady source in archival data:
The archival data spans from November 2014 to May 2018. We searched
inside the reported IceCube error region.
The highest significance, 2.83 sigma (-0.43 post-trials),
is at RA 87.19 deg, Dec +7.37 deg (+-0.21 deg 68% containment) J2000.
We set a time-integrated 95% CL  upper limit on gamma rays at the
maximum position of:

E^2 dN/dE = 2.82e-13 (E/TeV)^-0.3 TeV.cm^-2.s^-1

Search for a transient source.

Since the IceCube event fall inside the HAWC field of view,
we report on the result during the transit of the IceCube
position.

Data acquisition started on 2020/04/20 19:23:55 UTC and ended
2020/04/21 01:24:32 UTC.
The most significant location, with 3.28 sigma (1.53 post-trials),
is at RA 85.05 deg, Dec +8.5 (+-0.23 deg 68% containment) deg (J2000).
We set a time-integrated 95% CL upper limit at the position of
maximum significance of:

E^2 dN/dE = 1.3.e-11 (E/TeV)^-0.3 TeV.cm^-2.s^-1

HAWC is a very-high-energy gamma-ray observatory operating in Central
Mexico at latitude 19 deg. north. Operating day and night with over
95% duty cycle, HAWC has an instantaneous field of view of 2 sr and
surveys 2/3 of the sky every day. It is sensitive to gamma rays from
300 GeV to 100 TeV.