GCN 44096: GRB260310A: Palomar 60-inch photometry of an extremely bright late-time afterglow with a temporal break at 8 days

GRB260310A: Palomar 60-inch photometry of an extremely bright late-time afterglow with a temporal break at 8 days

D. A. Perley (LJMU), Y. Wagh (LJMU), R. Jayaraman (Cornell), J. Sollerman (Stockholm), A. Y. Q. Ho (Cornell), K.-R. Hinds (Caltech), and A. Bochenek (LJMU) report:

We used the SED Machine Rainbow Camera optical imager mounted on the Palomar 60-inch telescope to obtain observations of AT2026fgk (Hinds et al., TNS AstroNote 2026-65), the optical counterpart (Konno et al., GCN 43974) of the Fermi/GBM gamma-ray burst GRB260310A (Fermi GBM Team, GCN 43951; Hamburg & Meegan, GCN 43975).   Observations began on the night of 2026-03-12 at 06:03 UT and have continued every night since in the SDSS g, r, and i filters; u-band imaging was also taken on certain nights.  Photometry was performed following image subtraction of template images taken from Pan-STARRS survey imaging.

For the first seven days after the GRB, the fading is generally consistent with a power-law evolution with a shallow temporal index of approximately -0.6.  Beginning at around 7-8 days, the afterglow begins falling more sharply, with a decay index closer to -1.35.  (A break in the X-ray light curve was seen at a similar time; Jayaraman et al., GCN 44095 ).   Select photometry (AB, not corrected for extinction) is provided below:

| UT date             | dt (d) | filter |  mag  +/- unc  |
|---------------------|--------|--------|----------------|
| 2026-03-12 06:03:41 |  2.047 |      r | 18.14 +/- 0.04 |
| 2026-03-13 06:33:38 |  3.067 |      r | 18.19 +/- 0.03 |  
| 2026-03-15 06:08:19 |  5.050 |      r | 18.59 +/- 0.03 |
| 2026-03-18 06:01:43 |  8.045 |      r | 18.83 +/- 0.03 |
| 2026-03-23 05:20:09 | 13.016 |      r | 19.58 +/- 0.04 |

The behavior is consistent with other recent reports noting the continuation of the optical decay with no SN-like rebrightening yet (e.g., Volnova et al., GCN 44060; Bussman et al., GCN 44061).

Accounting for Galactic extinction and converting to the equivalent host-frame bandpass, the absolute magnitude of the optical transient as of the most recent observation is M_V ~ -19.7.  The fact that the light curve is still decaying steeply suggests that any associated supernova is likely to be significantly dimmer than the benchmark SN1998bw, which peaked at a similar magnitude and phase (e.g., Galama et al. 1998, Nature, 395, 670).  The color of the afterglow indicates only modest dust extinction in the host galaxy, so the subluminous nature of the SN is likely intrinsic.  A fainter SN comparable to SN2006aj is not yet ruled out, but will be tightly constrained by observations during the next 1-2 weeks.

Additionally, we note that the optical afterglow is both extremely bright and very luminous at late times.  To our knowledge only one[1] previous GRB (GRB030329 at z=0.168) has remained brighter than r=20 mag more than 10 observer-frame days after the event, excluding events dominated by the rebrightening of the SN on this timescale (see e.g., Kann et al. 2023, ApJL, 948, 12).  The luminosity is also notable:  despite its relatively weak prompt emission (E_iso ~ 10^51 erg: Minaev et al., GCN 44053), the afterglow luminosity of GRB260310A at the current time is at the upper end of the afterglow luminosity distribution at this time post-burst, comparable to many high-luminosity (E_iso ~ 10^54 erg) GRBs such as 080319B, 130427A, and 221009A.  

We encourage continued monitoring of this exceptional GRB afterglow (and any potential associated SN), which is likely to remain accessible to small- to medium-size optical facilities for many more weeks and possibly months.

[1] GRB221009A would have been observed to have a similar apparent magnitude at this time but was heavily obscured by Galactic dust.


Based on observations obtained with 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. 

DAP acknowledges the work, legacy, and friendship of D. Alexander Kann, who doubtless would have been tremendously excited by this GRB afterglow.