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GCN Circular 36199

Subject
GRB240418A - Candidate afterglow from NIRES
Date
2024-04-20T19:35:35Z (3 months ago)
From
Viraj Karambelkar at Indian Inst of Tech,Bombay <karambelkarvraj21197@gmail.com>
Via
legacy email
Viraj Karambelkar (Caltech), Kaew Tinyanont (NARIT), Sam Rose (Caltech),
Tomas Ahumada (Caltech), Robert Stein (Caltech), Nathan Lourie (MIT),
Danielle Frostig (MIT), Geoffrey Mo (MIT), Robert Simcoe (MIT), Mansi
Kasliwal (Caltech)

We observed the field of the long GRB 240418A (GCN 36162; GCN 36166; GCN
36177; GCN 36184) in the near-infrared K-prime band with the NIRES
acquisition camera on the Keck II telescope starting at UTC
2024-04-19T09:56:00 (~14 hours since trigger). The observations lasted for
300 sec. (25 sec, 12 coadds).

We find an uncataloged source at RA = 239.3744, Dec = 23.5961 that is 3.1
arcsec from the centroid of the GRB position and within its 3.6 arcsec
localization (from GCN 36166). This source has m_Kp ~ 20.9 +/- 0.1 mag
(AB). There is no source at this location in an archival Ks-band image from
the UKIRT hemisphere survey (Lucas et al. 2008), and a deep optical image
from the Legacy Survey. However, we note that our NIRES observations are
more sensitive than the archival UKIRT observations, so it is possible that
the NIRES source is an unrelated background source.

We also observed this source in the J-band with the WINTER camera on the
Palomar 40-inch telescope (Lourie et al. 2021) on 2024-04-19T09:00:14 and
did not detect it to a limiting magnitude of m_J ~ 19 mag.

Further observations are encouraged.

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