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GCN Circular 36286

LIGO/Virgo/KAGRA S240422ed: No significant counterpart candidates in HAWC observations
2024-04-24T16:02:42Z (3 months ago)
Hugo Ayala at Pennsylvania State University <>
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The HAWC Collaboration ( reports:

The HAWC Collaboration performed a follow-up of the gravitational wave
trigger S240422ed (GCN 36236). At the time of the trigger the HAWC
local zenith was oriented towards (RA, Dec) = (77.6 deg, 19.0 deg).
36% 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 49.8deg to 60deg for the area searched in this
analysis. The 5sigma detection sensitivity to a 1s (100s) burst in the
80-800GeV energy range goes from 9.9e-5 erg/cm^2 to 4.4e-4 erg/cm^2
(5.6e-4 erg/cm^2 to 2.5e-03 erg/cm^2), depending on the zenith

A list of some of the hotspots found in the search is shown below. We found
that one of the galaxies that are mentioned in GCN 36235 is close 
to one of the observed hotspots. 
No hotspot was close to the updated list in GCN 36243.
We mark with a “>” the hotspots that still show up in the updated localization
of the GW (GCN 36240). 

Timescale 3 seconds
  RA           Dec       Unc.      TS           Galaxy
  126.21    -8.61      0.83      10.97
> 126.91    -16.10    0.85      11.47

Timescale 10 seconds
  RA           Dec       Unc.      TS           Galaxy
  124.80    -9.97      0.49      12.31
> 126.65    -16.49    0.71       9.27

Timescale 30 seconds
  RA           Dec       Unc.      TS           Galaxy
  124.80    -9.97     0.49        9.01
> 127.53    -15.71    0.64       9.15

Timescale 100 seconds
  RA           Dec       Unc.      TS           Galaxy
  110.65    -27.95    0.50       9.23
  113.99    -20.42    0.37       10.23
  114.08    -19.87    0.50       9.29
  114.26    -21.94    0.52       11.29
  120.94    -13.32    0.59       13.33     WISEA J080028.14-134847.4 (0.95deg)
  128.23    -10.20    0.39       12.79
> 129.46    -16.49    0.55       12.06

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.

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