TITLE:   GCN CIRCULAR
NUMBER:  36215
SUBJECT: GRB 240421B: J-band upper limits from WINTER
DATE:    24/04/21 19:08:39 GMT
FROM:    Viraj Karambelkar at Indian Inst of Tech,Bombay <karambelkarvraj21197@gmail.com>

Viraj Karambelkar (Caltech), Geoffrey Mo (MIT), Robert Stein (Caltech),
Danielle Frostig (MIT), Tomas Ahumada (Caltech), Nathan Lourie (MIT),
Robert Simcoe (MIT), Mansi Kasliwal (Caltech)

We observed the field of the long GRB 240421B (GCN 36208, 36213, 36214) in
the near-IR J-band with the WINTER camera on the Palomar 40-inch telescope.
Our observations started at UTC 2024-04-19T11:16:00 (~1.5 hours since
trigger) and lasted for an hour.

The images were processed through the WINTER data reduction pipeline
implemented using mirar (https://zenodo.org/records/10888437), with image
subtraction performed relative to reference images built from the VISTA
Hemisphere Survey (McMahon et al. 2013).



We do not detect any source at the location of the X-ray counterpart
(reported in GCN 36213) to a limit of m_J = 19.5 mag (AB, not corrected for
Galactic extinction). No other new sources are found within the 3 arcmin
localization of the GRB.

WINTER (Wide-field INfrared Transient ExploreR) is a partnership between
MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF
AAG, the David and Lucille Packard Foundation, and the MIT Kavli Institute
for Astrophysics and Space Research