GCN 43606: GRB 260131A/AT2026bwg: Observations by the Zwicky Transient Facility

R. Jayaraman (Cornell), X. J. Hall (CMU), A. Y. Q. Ho (Cornell), I. Andreoni (UNC), M. Coughlin (UMN), A. Jacquesson (IJCLab), G. Schroeder (Cornell) report:

We report a serendipitous detection of the optical afterglow to GRB 260131A (Fermi/GBM Team, GCN 43579; Frederiks et al., GCN 43590; Luo et al., GCN 43591; Li et al., GCN 43605), which has been given the designation AT2026bwg (Gompertz et al., GCN 43586; Lee et al., GCN 43587; Malesani et al;, GCN 43595, García et al., GCN 43599; Shi et al., GCN 43601), as part of the Zwicky Transient Facility’s public survey. 

The transient ZTF26aaemwcb was first detected on MJD 61072.16995 (21.4h after the GRB trigger) with a magnitude of r = 18.08 ± 0.05 in ZTF alert photometry. Roughly a day later (MJD 61073.15126), the transient had faded to r = 19.13 ± 0.10. The last upper limit was approximately 2.5h before the initial trigger, with a limiting magnitude of r > 19.411. 

Separately, the transient was also observed at MJD 61072.13239 (20.5h after the GRB trigger) using multi-band imaging with the SED Machine on the Palomar P60 telescope. The table of photometry follows:

Time (MJD)  | Magnitude
61072.13239 | 18.21 ± 0.14 (sdss-g) 
61072.13405 | 18.03 ± 0.20 (sdss-r)
61072.13571 | 17.80 ± 0.48 (sdss-i) 

These measurements from both ZTF and SEDM are consistent with a decaying afterglow, when compared to those from Gompertz et al., Lee et al., Malesani et al., and García et al. The fast rise and decay was initially flagged by a pipeline for discovering afterglows in the ZTF data (e.g., Ho et al. 2020, ApJ, 905, 98), as well as the ZTFrest filter (Andreoni et al. 2021, ApJ, 918, 63). Upon a retrospective search, we identified ZTF26aaemwcb as the afterglow to GRB 260131A.

This Circular is 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; University of Wisconsin at Milwaukee, 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 OKC, University of Stockholm, Sweden. Operations are conducted by Caltech's Optical Observatory (COO), Caltech/IPAC, and the University of Washington at Seattle, USA.