TITLE:   GCN CIRCULAR
NUMBER:  39318
SUBJECT: LIGO/Virgo/KAGRA S250206dm: DECam GW-MMADS candidates
DATE:    25/02/14 01:40:50 GMT
FROM:    Tomas Cabrera <tcabrera@andrew.cmu.edu>

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

​​We observed the southern high probability area of the LVK gravitational wave candidate S250206dm (GCN 39175) 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 2025-02-13 06:15 UTC and covered 9.7% of the total event probability in the Bilby.offline1 S250206dm skymap (GCN 39231), a majority of the probability observable from the Blanco Telescope. These observations were started at the earliest possible time following the GW alert, since DECam was not available on the telescope at the time of the alert. The median 5sigma depths of our exposures are r\~22.6 mag and i\~23.1 mag.  Our pointings have been posted on TreasureMap at [https://treasuremap.space/alerts?graceids=S250206dm](https://treasuremap.space/alerts?graceids=S250206dm).

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 alert from ATLAS forced photometry. We reported on TNS new transients within the LVK 90% CI area, and we report here those matched to a NED galaxy in the catalog reported by GCN 39235:

| id | AT name | ra | dec | discovery_date (UT) | mag_i | mag_i_err | mag_r-mag_i |
| ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- |
| T202502131633495m700257* | AT 2025bnt | 248.456112 | -70.049160 | 2025-02-13 06:52:20.818 | 21.359 | 0.062 | 0.886 |
| T202502131558164m675652* | AT 2025bob | 239.568537 | -67.947727 | 2025-02-13 06:26:32.375 | 21.656 | 0.084 | 1.360 |
| T202502131622522m690124* | AT 2025bnm | 245.717365 | -69.023307 | 2025-02-13 06:46:37.789 | 22.104 | 0.165 | -0.496 |

We also report other potentially interesting transients showing a red color:

| id | AT name | ra | dec | discovery_date (UT) | mag_i | mag_i_err | mag_r-mag_i |
| ----------------------- | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ------ |
| T202502131559510m665547 | AT 2025bnj | 239.962646 | -66.929715 | 2025-02-13 06:35:06.754 | 20.869 | 0.042 | >1.238 |
| T202502131552222m700443 | AT 2025bnp | 238.092561 | -70.078637 | 2025-02-13 06:32:14.412 | 21.141 | 0.057 | 0.540 |
| T202502131632382m683103 | AT 2025bnh | 248.159200 | -68.517481 | 2025-02-13 07:06:47.575 | 21.446 | 0.063 | 2.151 |
| T202502131628244m704858 | AT 2025bms | 247.101545 | -70.816131 | 2025-02-13 06:55:13.894 | 21.375 | 0.062 | 1.003 |

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

Further analysis is underway.

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