TITLE:   GCN CIRCULAR
NUMBER:  30918
SUBJECT: GRB 210928A: Kinder SLT-40cm optical limits
DATE:    21/10/01 20:24:10 GMT
FROM:    Ting-Wan Chen at MPE  <janet.chen@astro.su.se>

T.-W. Chen, S. Yang (Stockholm), P. Evans (U. Leicester), W.-J. Hou, C.-S. Lin, Y.-C. Pan, C.-C. Ngeow, H.-Y. Hsiao, and J.-K. Guo (IANCU) report:

We observed the field of the GRB 210928A (Axelsson et al., GCN Circ. 30912), as part of the Kinder survey (Chen et al., AstroNote 2021-92). We used the SLT-40cm at Lulin Observatory, Taiwan, to obtain r-band images.

Observations started at 10:18 UT on 01 of October 2021 (MJD = 59488.430), 3.3 days after the GBM trigger time (Fletcher et al., GCN Circ. 30913), and were centred on the Swift X-ray source J162315.0-175233 (Page et al., ATel #14945), which had been identified as the likely counterpart to the Fermi source (first reported in Rani et al., ATel #14939). The images were combined from 12 frames with 300 sec exposure time for each (two additional frames removed due to poor image quality), taken under a various seeing conditions 2".7 and at an average airmass of 2.4.

We subtracted the template of this field with our combined image, and derived the following preliminary 2.5-sigma limit (in the AB system):
r > 21.22 mag.

Given magnitudes are calibrated against PanSTARRS1 field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.29 mag in the direction of the counterpart (Schlafly & Finkbeiner 2011).