GCN 44172: GRB 260310A / AT2026fgk: Luminance-band optical follow-up with a 28cm telescope in Daocheng

2026-04-01T07:07:05.602Z | rev 0 | event: GRB 260310A
Zhuokai Liu, Zexuan Wu, Chenxi Bao, Yacheng Kang (PKU) report on behalf of the Interestar Collaboration and the PKU HiTF (High-energy Transients Follow-up) group:

We observed the field of GRB 260310A, detected by Fermi/GBM (GCN 43951; T0 = 2026-03-10T04:57:10.81) and AstroSat/CZTI (Salunke et al., GCN 43958), using a 28cm optical telescope located in Daocheng, Sichuan, China. Observations began on 2026 Mar 17, approximately ~ 7.4 days after the GRB trigger, and spanned six epochs in the Luminance filter.

In the stacked Luminance-band images, we detect the optical counterpart at a position consistent with AT2026fgk, the reported GRB afterglow candidate at z = 0.153 (O'Neill et al. 2026, TNS Discovery Report 294132; Hinds et al., AstroNote 2026-65; Konno et al., GCN 43974; Hinds et al., GCN 43977), later found to show Type Ic-BL SN features (de Ugarte Postigo et al., GCN 44124; Guelfand et al., GCN 44125; O'Connor et al., GCN 44137).  Preliminary photometry was calibrated using nearby catalogued field stars and is reported as approximate AB-equivalent magnitudes, without correction for Galactic extinction.

Our measurements show that the optical counterpart continues to fade, from ~ 19.0 mag to ~ 19.8 mag between 7.4 and 14.3 days after the GRB trigger. The observation log is summarized below:

|    Date    | Start_UT | T_mid - T0 (days) |  Filter   | Exposure (s) | Magnitude (AB) |
| :--------: | :------: | :---------------: | :-------: | :----------: | :------------: |
| 2026-03-17 | 13:28:40 |       7.40        | Luminance |     7400     | 19.02 +/- 0.04 |
| 2026-03-18 | 13:52:51 |       8.42        | Luminance |     7600     | 19.10 +/- 0.04 |
| 2026-03-19 | 14:14:50 |       9.42        | Luminance |     6000     | 19.18 +/- 0.04 |
| 2026-03-20 | 12:37:02 |       10.36       | Luminance |     7400     | 19.39 +/- 0.04 |
| 2026-03-21 | 12:39:41 |       11.35       | Luminance |     5000     | 19.65 +/- 0.05 |
| 2026-03-24 | 12:33:31 |       14.33       | Luminance |     2200     | 19.84 +/- 0.08 |

Further analysis and follow-up observations are ongoing.

The 28cm optical telescope is located in Daocheng, Sichuan, China, and is operated by the Interestar Collaboration. The PKU HiTF group is dedicated to rapid follow-up observations of high-energy transients.