TITLE: GCN CIRCULAR NUMBER: 34763 SUBJECT: LIGO/Virgo/KAGRA S230922g: DECam GW-MMADS candidates DATE: 23/09/25 04:04:03 GMT FROM: Antonella Palmese at Carnegie Mellon University apalmese@andrew.cmu.edu

Tomás Cabrera (CMU), Lei Hu (CMU), Igor Andreoni (UMD), Keerthi Kunnumkai (CMU), Brendan O’Connor (CMU), Antonella Palmese (CMU), on behalf of the GW-MMADS team

We observed the high probability area of the LVK gravitational wave candidate S230922g (GCN 34757) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope, as part of the Gravitational Wave Multi-Messenger Astronomy DECam Survey (GW-MMADS; PI: Andreoni & Palmese). Observations started at 2023-09-23 00:37 UTC on the first night and 2023-09-23 23:42 UTC on the second night. The first night’s observations were impacted by clouds and only covered a fraction of the planned observations in g band. The second night’s observations covered 70% CI of the Bilby S230922g skymap (GCN 34758) spatial probability in g and i band. The median 5sigma depths of our exposures are g~22.8 mag and i~22.9 mag.

We run the SFFT difference imaging (Hu et al. 2022) on the available images, filter out likely stars and moving objects, and visually inspect the remaining transients. We report on TNS transients within the LVK 90% CI area, and report here those that we do not currently exclude as candidate counterparts based on available redshift information:

| TNS Name | RA | dec | Internal Name | Discovery Mag | Filter | Discovery Date (UT)| | AT 2023tou | 22:18:20.254 | -17:46:08.13 | T202309242218203m174608 | 21.61 | g | 2023-09-24 04:40:35.040 | | AT 2023tos | 22:30:44.167 | -22:17:27.51 | T202309232230442m221728 | 21.49 | g | 2023-09-24 02:49:27.552 | | AT 2023toq | 22:14:29.326 | -26:37:43.27 | T202309242214293m263743 | 20.24 | g | 2023-09-24 04:00:04.608 | | AT 2023top | 22:19:56.773 | -25:01:51.54 | T202309242219568m250152 | 21.74 | g | 2023-09-24 03:08:02.112 | | AT 2023too | 22:34:53.892 | -30:29:31.00 | T202309242234539m302931 | 21.38 | g | 2023-09-24 04:04:53.184 | | AT 2023tom | 22:18:37.979 | -21:40:53.30 | T202309242218380m214053 | 21.91 | g | 2023-09-24 03:44:43.584 | | AT 2023toj | 22:47:55.439 | -14:06:41.45 | T202309242247554m140641 | 20.46 | g | 2023-09-24 04:31:36.768 | | AT 2023toi | 22:43:50.303 | -29:10:48.27 | T202309242243503m291048 | 21.17 | g | 2023-09-24 04:06:22.176 | | AT 2023toh | 22:58:08.145 | -20:11:39.61 | T202309242258081m201140 | 20.67 | g | 2023-09-24 05:31:32.736 | | AT 2023tog | 22:04:20.455 | -24:41:16.83 | T202309242204205m244117 | 21.35 | g | 2023-09-24 04:03:06.912 | | AT 2023tof | 22:48:36.866| | -28:29:48.33 | T202309242248369m282948 | 20.62 | g | 2023-09-24 04:06:22.176 | | AT 2023toe | 22:42:25.888 | -16:27:41.76 | T202309242242259m162742 | 20.83 | g | 2023-09-24 03:46:26.400 | | AT 2023toc | 23:03:05.323 | -22:47:53.46 | T202309242303053m224753 | 21.11 | g | 2023-09-24 04:26:57.696 |

Further inspection of candidate counterparts is underway.

We thank the CTIO and NOIRLab staff for supporting these observations and the data calibration.

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