TITLE: GCN CIRCULAR NUMBER: 41214 SUBJECT: LIGO/Virgo/KAGRA S250727dc: DECam GW-MMADS candidates DATE: 25/07/31 20:21:13 GMT FROM: Tomas Cabrera at Carnegie Mellon University tcabrera@andrew.cmu.edu

Tomás Cabrera (CMU), Xander J. Hall (CMU), Lei Hu (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), Keerthi Kunnumkai (CMU), Brendan O’Connor (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team

DECam observed the southern high probability area of the LVK gravitational wave candidate S250727dc (GCN 41179) 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 2025-07-29 09:39 UTC and cover the 90% probability area (GCN 41188).

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. All candidates included in this GCN have been reported to the TNS. We report the novel candidates that pass our cuts at this stage in the first table; in the following sections we present genuine transients we reject as gravitational wave counterparts, and the confirmation of some previously reported candidates.

We report the discovery of the following 3 nuclear transients that we do not exclude from our search. For AT2025stf, the nucleus of the host is ambiguous, and so it is possible this transient is non-nuclear, which would eliminate it as a favored candidate.

| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | | -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- | | T202507290634429m374411 | AT 2025ssz | 98.678750 | -37.736389 | 2025-07-29 09:49:00.192 | 21.476 | 0.140 | 0.487 | | T202507290636497m363725 | AT 2025ste | 99.207083 | -36.623611 | 2025-07-29 09:49:00.192 | 22.437 | 0.408 | 1.627 | | T202507290626025m372843 | AT 2025stf | 96.510373 | -37.478478 | 2025-07-29 09:47:11.817 | 21.193 | 0.073 | -0.211 |

The following transients are coincident with objects in the quaia and Milliquas catalogs, and have redshifts that place them outside of the 90% localization volume for the gravitational wave event.

| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | possible_host | host_z | | -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- | ----------- | --------- | | T202507290621597m351731 | AT 2025ssx | 95.498727 | -35.291956 | 2025-07-29 10:26:17.504 | 20.945 | 0.061 | 0.661 | quaia 2891332298170436480 | 0.47 | | T202507290634416m350638 | AT 2025sta | 98.673141 | -35.110434 | 2025-07-29 09:52:36.137 | 19.783 | 0.047 | 0.288 | quaia 5581689916779289344 | 0.53 | | T202507290629125m373923 | AT 2025std | 97.302216 | -37.656276 | 2025-07-29 09:49:00.617 | 19.945 | 0.035 | 0.819 | milliquas 6dF J062912.5-373922 | 0.06 |

The following transients are nuclear, but are hosted by nearby galaxies (as determined by their large angular sizes), and so are unlikely to be associated with the gravitational wave event.

| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | | -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- | | T202507290623386m353830 | AT 2025ssy | 95.910743 | -35.641755 | 2025-07-29 09:45:22.413 | 19.100 | 0.013 | 0.454 | | T202507290627447m354045 | AT 2025stg | 96.936244 | -35.679174 | 2025-07-29 09:58:01.892 | 20.724 | 0.047 | 0.916 | | T202507290618452m351815 | AT 2025sth | 94.688504 | -35.304203 | 2025-07-29 09:45:22.413 | 19.080 | 0.014 | 0.441 | | T202507290627067m352915 | AT 2025sti | 96.778033 | -35.487632 | 2025-07-29 09:58:01.892 | 17.192 | 0.004 | 0.622 |

The following transients are non-nuclear, and appear to be supernovae.

| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | | -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- | | T202507290626444m373046 | AT 2025stb | 96.684801 | -37.512899 | 2025-07-29 09:47:11.817 | 21.174 | 0.072 | 0.028 | | T202507290625106m332028 | AT 2025stc | 96.294042 | -33.341084 | 2025-07-29 09:56:12.517 | 20.252 | 0.056 | -0.166 |

We additionally confirm the following candidates initially reported in GCN 41211, including the nuclear candidate with a Quaia host at a favorable redshift (AT 2025ssi).

| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | | -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- | | T202507290636054m360136 | AT 2025ssf | 99.022404 | -36.026624 | 2025-07-29 10:23:00.741 | 20.624 | 0.040 | [no mag_i] | | T202507290631153m360431 | AT 2025ssg | 97.813758 | -36.075317 | 2025-07-29 10:23:00.741 | 20.412 | 0.028 | [no mag_i] | | T202507290636352m372956 | AT 2025ssi | 99.146615 | -37.499005 | 2025-07-29 09:49:00.617 | 20.112 | 0.047 | 0.391 |

Further analysis is underway.

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

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