GCN Circular 45843
Nabeel Rehemtulla, Gokul Srinivasaragavan, Adam A. Miller, Ved G. Shah (Northwestern, CIERA), Tomas Ahumada (NOIRLab), Antoine Le Calloch, Michael W. Coughlin, Michael C. Davis (UMN), Igor Andreoni, Anirudh Salgundi, and James Freeburn (UNC) report on behalf of the ZTF collaboration:
We report on ZTF and P60/SEDM follow-up of the optical counterpart (Zheng et al., GCN 45825; Saccardi et al., GCN 45829; Mohan et al., GCN 45832; Saccardi et al., GCN 45837, Shi et al., GCN 45838, Arya et al., GCN 45841) to GRB 261007A (Goto et al., GCN 45824).
The counterpart was automatically discovered in ZTF data by the BTSbot machine learning model (Rehemtulla et al. 2024) at 2026-10-07 08:14, independent of the GRB trigger. BTSbot autonomously requested P60/SEDM follow-up, which was carried out 34 minutes later (2026-10-07 8:48). The SEDM spectrum is at ultra low-resolution (R~100) and shows only a continuum. ZTFReST (Andreoni et al. 2021) and other fast transient filters developed within the ZTF Collaboration also identified ZTF26abzxgsr as a candidate fast transient independent of the GRB trigger.
The ZTF observations were conducted as part of the 1-day cadence public gri survey overlapping the Rubin-LSST Wide-Fast-Deep footprint. The source (ZTF26abzxgsr / AT 2026aepi) was detected in ZTF gri images and was not detected the previous night.
| Obs time (UTC) | Time since SVOM detection (2026-10-07 05:42:03) (hh:mm:ss) | Band | Alert mag (AB) | Mag unc |
|---|---|---|---|---|
| 2026-10-06 06:02:24 | – | r | 19.96 | non-det |
| 2026-10-06 08:35:41 | – | i | 19.75 | non-det |
| 2026-10-06 09:23:14 | – | g | 19.92 | non-det |
| 2026-10-07 06:02:02 | 00:19:59 | r | 17.16 | 0.032 |
| 2026-10-07 06:34:56 | 00:52:53 | i | 17.83 | 0.067 |
| 2026-10-07 07:33:02 | 01:50:59 | g | 18.84 | 0.088 |
BTSbot processes alerts in the BOOM alert broker (Jegou du Laz et al. 2026) and interfaces with follow-up facilities through the SkyPortal marshal (Coughlin et al. 2023).
Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan, and the German Center for Astrophysics (DZA), Germany. Operations are conducted by Caltech's Optical Observatory (COO), Caltech/IPAC, and the University of Washington at Seattle, USA.
SED Machine is based upon work supported by the National Science Foundation under Grant No. 1106171
The Gordon and Betty Moore Foundation, through both the Data-Driven Investigator Program and a dedicated grant, provided critical funding for SkyPortal.