TITLE:   GCN CIRCULAR
NUMBER:  36267
SUBJECT: LIGO/Virgo/KAGRA S240422ed: Magellan Detection of a Potential Electromagnetic Counterpart
DATE:    24/04/24 01:48:38 GMT
FROM:    Ryan Foley at UC Santa Cruz <foley@ucsc.edu>

A. L. Piro, J. D. Simon (Carnegie), A. Polin (Carnegie/Purdue), D. A. Coulter (STScI), M. R. Drout (Toronto), R. J. Foley, C. Rojas-Bravo (UC Santa Cruz), and C. D. Kilpatrick (Northwestern)

report on behalf of the Carnegie Gravitational wave ElectroMagnetic follow-up (CGEM) and the One-Meter Two-Hemisphere (1M2H) collaborations:

As part of our ongoing search for optical counterparts to the candidate gravitational wave event S240422ed (GCN 36236) using the 6.5m Magellan Clay telescope with the LDSS-3 imager (GCN  36244), we have detected an apparent transient source in an i-band image that is not present in archival Pan-STARRS images (Chambers et al., 2016).  The transient is

Mag24a   07:56:05.6   -22:53:54  J2000

We have queried both the Minor Planet Checker and Transient Name Server, neither of which returned a known object.

We estimate the brightness of Mag24a is i = 23.3 mag at 20240423.98 UT.  It is offset 2.1” west and 8.6” north from its apparent host galaxy, WISEA J075605.75-225400.0, which has a photometric redshift of z = 0.049 (Bilicki et al., 2014, ApJS, 210, 9), corresponding to 213 Mpc (i.e., within the current Bilby measured volume; GCN 36240).  At this distance with a Milky Way extinction of A_i = 0.748 mag (Schlafly & Finkbeiner, 2011, ApJ, 737, 103) and no host-galaxy extinction, the absolute magnitude is M_i = -14.1 mag.  At this distance, Mag24a is offset by 9.2 kpc from the assumed host galaxy in projection.

We are undertaking additional observations at this time, and encourage additional follow-up observations.

We thank Marcelo Mora, Povilas Palunas, and the rest of the LCO staff for making these observations possible.