TITLE:   GCN CIRCULAR
NUMBER:  32667
SUBJECT: GRB 221009A: Lulin SLT-40cm optical observations
DATE:    22/10/10 20:05:50 GMT
FROM:    Ting-Wan Chen at MPE  <janet.chen@astro.su.se>

T.-W. Chen (Stockholm), D. B. Malesani (Radboud Univ. and DAWN/NBI), S. Yang (Stockholm), W.-J. Hou, C.-C. Ngeow, Y.-C. Pan, H.-Y. Hsiao, C.-S. Lin, and J.-K. Guo (IANCU) report:

We observed the field of GRB 221009A (a.k.a. Swift J1913.1+1946; Dichiara et al., GCN 32632; Kennea & Williams, GCN 32635; Bissaldi et al., GCN 32637; Veres et al., GCN 32636; Lesage et al., GCN 32642; Gotz et al., GCN 32660), using the SLT-40cm at Lulin Observatory, Taiwan, to obtain g,r,i,z-band images as part of the Kinder collaboration (Chen et al., AstroNote 2021-92).

Observations started at 12:25 UT on 10 of October 2022 (MJD = 59862.518), 1.04 days after the Fermi GBM trigger time. The images were combined from 2 frames with 150 sec exposure time for each band, taken under variable seeing conditions (2".5 average) and at an airmass of 1.3.

We used aperture photometry to measure the transient brightness without template subtraction, and derived the following preliminary magnitudes and 3-sigma limits (all in the AB system):

g > 18.33 mag,
r = 18.67 +/- 0.16 mag,
i = 17.38 +/- 0.09 mag, and
z = 16.60 +/- 0.09 mag.

Given magnitudes are calibrated against Pan-STARRS1 field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 1.40 mag in the direction of the counterpart (Schlafly & Finkbeiner 2011).