S. Yang (HNAS), T.-W. Chen, C.-S. Lin, C.-C. Ngeow (NCUIA), Z.-N. Wang (HNAS), S. Smartt (Oxford/QUB), M. Fraser (UCD), J. Gillanders (Oxford), M.-H. Lee, Y.-C. Pan, H.-Y. Hsiao, W.-J. Hou, J.-K. Guo (NCUIA), M. Fulton, S. Srivastav, T. Moore, C. Angus, and A. Aamer (QUB) report:
We observed the field of GRB 231117A, a bright, short GRB discovered by the Swift BAT (Laha et al., GCN 35071). This burst is also detected by AstroSat (Navaneeth et al., GCN 35072), AGILE (Cattaneo et al., GCN 35075), Konus-Wind (Svinkin et al., GCN 35079), and Glowbug (Cheung et al., GCN 35081).
We used the 40cm SLT at Lulin Observatory, Taiwan to obtain r-band images of the field of GRB 231117A, as part of the Kinder collaboration (Chen et al., AstroNote 2021-92). The first SLT epoch of observations started at 10:26 UT on 17 of November 2023 (MJD = 60265.435), 7.39 hours after the Swift trigger. The images were combined from 14 frames with a 300-second exposure time for the r band, taken under seeing conditions of an average of 1".93 and at a median airmass of 1.02. We then used the Lulin One-meter Telescope (LOT) to obtain g, r, i, and z-band images of this field starting on MJD = 60265.520, under seeing conditions of an average of 1".39 and at a median airmass of 1.21.
We found a GRB afterglow candidate at the position:
RA (J2000): 22h 09m 33.37s
Dec (J2000): +13d 31' 20.2”
which is about 3" away from the enhanced Swift-XRT position (Beardmore et al., GCN 35074). This afterglow condidate is right on the top of the faint galaxy, SDSS J220933.34+133119.5, with a petroMag_r = 21.32 +/- 0.22 mag. The likely host galaxy has a SDSS photo-z of 0.481 +/- 0.103 and a Legacy Survey photo-z of 0.342 +/- 0.131.
We used the Kinder pipeline (Yang et al. A&A 646, A22) to measure the PSF photometry of this afterglow candidate after template subtraction using the SDSS images. We obtained the following magnitudes (in the AB system):
SLT r = 20.94 +/- 0.10 mag (exposure time of 300sec*14)
LOT g = 20.90 +/- 0.10 mag (300sec*1),
LOT r = 20.77 +/- 0.07 mag (300sec*7),
LOT i = 20.52 +/- 0.11 mag (300sec*1) and,
LOT z = 20.00 +/- 0.18 mag (SNR=2; 300sec*1).
The given limit is derived based on calibrating against SDSS field stars and is not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.06 mag in the direction of the burst (Schlafly & Finkbeiner 2011).
Our detection magnitudes are deeper than those non-detection reports from the GOTO (Godson et al., GCN 35073), MITSuME (Takei et al., GCN 35076) and ATLAS (Gillanders et al., GCN 35080).