TITLE:   GCN CIRCULAR
NUMBER:  45904
SUBJECT: GRB 261011A: Optical follow-up observations with Kinder
DATE:    26/10/11 16:42:15 GMT
FROM:    Janet Chen at National Central University <janetstars@gmail.com>

A. Aryan, Z.-Y. Chen, T.-W. Chen, W.-J. Hou, Y.-H. Lee (all NCU), A. K. H. Kong (NTHU), S. Yang (HNAS), S. J. Smartt, J. H. Gillanders (both Oxford), A. Sankar.K, K. N.-T. Ho, A. Dutta, M.-H. Lee, C.-S. Lin, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), K. W. Smith, H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report:

As part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86,doi:10.3847/1538-4357/adb428), we conducted multi-band optical follow-up observations of GRB 261011A, detected by SVOM (Huang et al., GCN 45895) and Swift (Cenko et al., GCN 45896).

Imaging was acquired in the g-band using the 1m Lulin One-meter Telescope (LOT), and in the r- and i-bands using the 40cm Seisi Lulin Telescope (SLT). Our observations started at MJD 61324.6155, approximately 0.99 hr after the Swift/BAT trigger.

The optical counterpart reported by Swift/UVOT (Cenko et al., GCN 45896), GOTO (O'Neill et al., GCN 45897), and more instruments (Fu et al., GCN 45898; Izzo et al., GCN 45899; Wu et al., GCN 45900; Yin et al., GCN 45901; Shi et al., GCN 45902) was clearly detected in all three bands.

We used AutoPhOT (Brennan & Fraser 2022, A&A, 667, A62) to perform PSF photometry. Preliminary measurements are as follows:

Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude
LOT       | g      | 61324.6155  | 0.99      | 60 * 1       | 20.47 +/- 0.10
SLT       | r      | 61324.6157  | 1.04      | 300 * 1      | 19.91 +/- 0.07
SLT       | i      | 61324.6194  | 1.09      | 300 * 1      | 19.69 +/- 0.07

Using the nearly simultaneous g- and r-band measurements at approximately 1.04 hr after the trigger, and correcting for Galactic foreground extinction, we obtain (g-r) = 0.64 +/- 0.11 mag. Using the earliest r- and i-band measurements, we obtain (r-i) = 0.20 +/- 0.10 mag. Thus, the source shows a relatively red (g-r) colour while remaining considerably flatter between r and i.

The presented magnitudes are calibrated using field stars from the ATLAS-RefCat2 catalog from MAST (Tonry et al. 2018, ApJ, 867, 105) and are not corrected for Galactic foreground extinction of A_g = 0.07, A_r = 0.05 and A_i = 0.04 mag in the direction of the transient (Schlafly & Finkbeiner 2011).  

The methodology, details on the Lulin Observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein Probe mission are presented in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.