TITLE: GCN CIRCULAR NUMBER: 39311 SUBJECT: LIGO/Virgo/KAGRA S250206dm: FTW observations of counterpart candidates AT2025bbo and AT2025bcx, and of NED galaxies DATE: 25/02/13 20:19:59 GMT FROM: Malte Busmann at Ludwig-Maximilians-Universität München m.busmann@physik.lmu.de
Malte Busmann (LMU), Daniel Gruen (LMU), Antonella Palmese (Carnegie Mellon U.), Brendan O’Connor (Carnegie Mellon U.), Yajie Zhang (LMU), Julius Gassert (LMU), Lei Hu (Carnegie Mellon U.), Arno Riffeser (LMU/MPE), Julian Sommer (LMU), Lena Schnappinger (LMU), Franziska Krause (LMU) and Elia Engelhard (LMU) report:
We observed transients discovered in the search for a counterpart of S250206dm (LIGO Scientific Collaboration et al., GCN 39175; LIGO Scientific Collaboration et al., GCN 39184) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i, and J bands simultaneously.
### AT2025bbo We observed AT2025bbo (Smartt et al., GCN 39244; Liu et al., GCN 39249; Lee et al., GCN 39252; Ducoin et al., GCN 39258; Huber et al., GCN 39272; Karambelkar et al., GCN 39273; Chen et al., GCN 39285; Xu et al., GCN 39293) in riJ bands on 2025-02-09 and on 2025-02-10. Difference imaging of our data, between the two epochs (riJ) and against PS1 (ri) and UKIDSS (J) archival images, reveals no confident detection of a source. The depths of our observations are as follows:
| Time (UT) | Band | Exposures | Depth (3 sigma, AB mag) | |---------------------|------|-----------|-------------------------| | 2025-02-09T21:26:34 | r | 10x 180s | 22.9 | | 2025-02-09T21:26:34 | i | 10x 180s | 22.7 | | 2025-02-09T21:26:48 | J | 10x 180s | 21.6 | | 2025-02-10T20:18:10 | r | 5x 180s | 22.3 | | 2025-02-10T20:18:10 | i | 5x 180s | 22.2 | | 2025-02-10T20:18:25 | J | 5x 180s | 21.2 |
### AT2025bcx We observed AT2025bcx for 8x 180s starting on 2025-02-10T18:40:45 and performed difference imaging in the r and i band with templates from PS1. In the difference images, we detect a source at the position of AT2025bcx at r = (20.5 +/- 0.2) mag and i = (20.9 +/- 0.3) mag.
### AT2025bmq Additionally, we have observed a list of NED galaxies selected from the list reported by Cook et al. (GCN 39235). We observed the first 11 candidate hosts as ranked by `P_3D_LumW1` in the night starting 2025-02-08. Typical depths in r,i bands are 21.5. Difference imaging w.r.t. PS1 reveals one candidate (AT2025bmq), consistent with being a nuclear transient in a host galaxy that shows previous activity.
The r- and i-band magnitudes are calibrated against the PS1 catalog and the J-band is calibrated with the 2MASS Catalog. All magnitudes are provided in the AB system and are not corrected for Galactic extinction.
We thank Christoph Ries and Michael Schmid from the Wendelstein Observatory staff for obtaining these observations.
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