TITLE: GCN CIRCULAR NUMBER: 36281 SUBJECT: LIGO/Virgo/KAGRA S240422ed: MOSFIRE spectroscopy of the DECam GW-MMADS candidate AT 2024hdn DATE: 24/04/24 10:36:31 GMT FROM: Mansi Kasliwal at Caltech mansikasliwal@gmail.com

Samaporn Tinyanont (NARIT), Jesper Sollerman (OKC), Jacob Jencson (IPAC), Viraj Karambelkar (Caltech), Michael Lundquist (WMKO) and Mansi Kasliwal (Caltech)

report on behalf of a larger collaboration

We obtained a near-infrared spectrum of the DECam GW-MMADS source AT2024hdn (GCN 36273), reported as a candidate optical counterpart to the LVK gravitational wave event S240422ed (GCN 36236) in the near-IR y-band with MOSFIRE on the Keck-I telescope on UTC 2024-04-24. The signal-to-noise on the transient is low and analysis is underway. In the reduced 2D spectrum, offset from the transient location, we note two prominent emisson lines at approx. 10161 Angstrom and 10679 Angstrom from the possible star formation knot / background galaxy north of the putative host. If these two lines are [SIII], this corresponds to a galaxy redshift of z=0.12 (significantly higher than the photometric redshift estimate of 0.079 from Bilicki et al. 2014). If this knot is at the same redshift as the transient, it would imply that AT 2024hdn has an absolute magnitude that is consistent with a supernova. We thank the WMKO staff and Keck I scheduled observers for their co-operation with these Target of Opportunity observations.

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