GCN Circular 39258
Jean-Grégoire Ducoin (CPPM), Rosa L. Becerra (U Rome), Dalya Akl (AUS), Alan M. Watson (UNAM), Ny Avo Rakotondrainibe (LAM), Sarah Antier (OCA), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Camila Angulo (UNAM), Jean-Luc Atteia (IRAP), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Noémie Globus (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), and Benjamin Schneider (LAM) report:
We imaged the field of the candidate reported by the Pan-STARRS Collaboration (Young et al. GCN 39210) of the optical transient AT 2025bbo reported as a possible counterpart of the GW transient S250206dm (LVK Collaboration, GCN Circ. 39175, GCN Circ. 39184) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico.
We observed AT 2025bbo from 2025-02-09 03:47 to 04:44 (from T+2.27 to T+2.30 days after the GW transient), obtaining 600 seconds of exposure in both g and r. The data were reduced and stacked using custom software and then calibrated against the PS1 catalog and analysed using STDPipe (Karpov 2021). We carried out image subtraction against PS DR2 images. At the position of the source reported by Young et al., we detect the source at:
r = 19.85 +/- 0.05
In agreement to the increase in the brightness reported by the Kinder (Lee et al., GCN Circ. 39252) and WFST collaborations (Smartt et al., GCN Circ. 39249).
In g, the source is fainter than our 3-sigma limit of
g > 21.1
This suggests that the source is red. Further observations are planned.
Furthermore, we observed 10 galaxies in the MANGROVE catalog (Ducoin et al., 2020) in the 50% region of probability of S250206dm, consistent with the 90% containment volume. We observed between 2025-02-09 02:35 and 06:15 (from T+2.2 to T+2.4 days after the GW transient) and obtained 600 seconds of exposure in r on each galaxy. The data were reduced and stacked using custom software and then calibrated against the PS1 catalog and analysed using STDPipe (Karpov 2021). We carried out image subtraction against PS DR2 images. With one exception, we detect no possible candidates to the 10-sigma limits quoted below:
| Name | RA | Dec | 10-sigma limit | |
|---|---|---|---|---|
| 1 | 2MASS 02242513+5242101 | 36.105 | 52.703 | r > 21.2 |
| 2 | 2MASS 02353482+5315284 | 38.895 | 53.258 | r > 20.9 |
| 3 | 2MASS 02292273+5329140 | 37.345 | 53.487 | r > 20.2 |
| 4 | HyperLEDA 166437 | 40.104 | 54.253 | r > 20.8 |
| 5 | 2MASS 02234887+5338542 | 35.954 | 53.648 | r > 20.8 |
| 6 | 2MASS 02363571+5351138 | 39.149 | 53.854 | r > 20.6 |
| 7 | 2MASS 02363578+5325108 | 39.149 | 53.420 | r > 20.9 |
| 8 | 2MASS 02261049+5307282 | 36.544 | 53.125 | r > 19.8 |
| 9 | 2MASS 02243083+5251021 | 36.128 | 52.851 | r > 20.7 |
| 10 | 2MASS 02371899+5401539 | 39.329 | 54.032 | r > 20.6 |
The one exception is that in the field of 2MASS 02363571+5351138 (galaxy 6 above), we detect a transient source 9-3 arcmin from the galaxy at RA = 39.27171 and Dec = 53.99079 degrees with a magnitude of r = 19.98 +/- 0.05. However, a source at the same position appears in the Pan-STARRS catalogs with g = 21.1 in DR1 and at several epochs at g = 22.5 in DR2. We interpret this as a variable star.
Our photometry is not corrected for Galactic extinction.
We warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.