TITLE: GCN CIRCULAR NUMBER: 36328 SUBJECT: LIGO/Virgo/KAGRA S240422ed: Swope Follow-up of AT2024hfq, AT2024hfr, AT2024hfs, and AT2024hfx DATE: 24/04/26 21:52:07 GMT FROM: Ryan Foley at UC Santa Cruz foley@ucsc.edu

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

report on behalf of the Carnegie Gravitational wave ElectroMagnetic follow-up (CGEM) collaboration, the One-Meter Two-Hemisphere (1M2H) collaboration, and the Swope Supernova Survey:

As part of our ongoing search for optical counterparts to the candidate gravitational wave event S240422ed (GCN 36236, GCN 36240), we have obtained imaging with the 1.0m Swope telescope at Las Campanas Observatory of the sources AT2024hfq (GCN 36298, GCN 36319), AT2024hfr (GCN 36298), AT2024hfs (GCN 36298), and AT2024hfx (GCN 36303, GCN 36304, GCN 36308).

Differential PSF photometry of the candidates and local sequence stars was obtained using DAOPHOT. The local sequence photometry was calibrated using the Refcat2 catalog, transformed to the Carnegie Supernova Project (CSP) natural system. The measured magnitudes are listed in the table below. AT2024hfq has a similar magnitude to that reported two nights earlier by DECam. AT2024hfr is likely somewhat brighter than measured by DECam on the previous night. AT2024hfs is consistent with being brighter and redder than the source was on the previous two nights. AT2024hfx is brighter by ~1 sigma than measured on the previous night by PS2.

+-----------+---------------+--------+--------------------+ | ID | JD | filter | magnitude | +-----------+---------------+--------+--------------------+ | AT2024hfq | 2460426.565 | i | 20.97 +/- 0.14 | | AT2024hfr | 2460426.590 | i | 19.69 +/- 0.19 | | AT2024hfs | 2460426.577 | g | 21.07 +/- 0.19 | | AT2024hfs | 2460426.582 | i | 20.53 +/- 0.25 | | AT2024hfx | 2460426.551 | g | 19.74 +/- 0.03 | | AT2024hfx | 2460426.557 | i | 19.88 +/- 0.05 | +-----------+---------------+--------+--------------------+

We thank Francesco di Mille and the rest of the LCO staff for making these observations possible.

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