TITLE: GCN CIRCULAR NUMBER: 41177 SUBJECT: LIGO/Virgo/KAGRA S250725j: DECam GW-MMADS candidates DATE: 25/07/27 18:06:49 GMT FROM: xjh@andrew.cmu.edu

Xander Hall (CMU), Lei Hu (CMU), Tomás Cabrera (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 S250725j (GCN 41154) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope. Observations started at 2025-07-25 23:13:43 UTC (Prop ID: 2025A-227091; PI Soares-Santos) and covered the highest 90% probability region of the event (GCN 41168).

We run the SFFT difference imaging (Hu et al. 2022) on the available images, filter out likely stars and moving objects, visually inspect the remaining transients, and remove transients showing SNR>5 detections predating the gravitational wave alert from ATLAS forced photometry. We reported on TNS new transients within the LVK 99% CI area, and we report here those matched to a NED galaxy (GCN 41165):

| id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_r | NED separation | | ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- | ----- | | T202507251524174m373721 | AT 2025shz | 231.072434 | -37.622634 | 2025-07-25 23:29:54.630 | 22.63 | 0.06 | 0.37 | 17.0” | T202507251542117m350010* | AT 2025sib | 235.548892 | -35.002876 | 2025-07-25 23:15:43.009 | 22.5 | 0.1 | 0.7 | 2.1” | T202507251538092m361517 | AT 2025sia | 234.538140 | -36.254760 | 2025-07-25 23:19:38.393 | 21.74 | 0.03 | 1.06 | 1.1”

* - *We note that these transients may have tentative low SNR pre-detections from ATLAS forced photometry.*

We note that AT 2025shz cannot be excluded as a variable star and that it shows an extremely quick rise in the rz filters between two observations in the same night with a rise of several mag/day. We also note that AT 2025sib, shows a gradual decline in the grz filters between two observations with an average decline of ~0.5 mag/day.

Further analysis is underway.

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

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