T.-W. Chen (NCUIA), S. Yang (HNAS), C.-S. Lin (NCUIA), S. Smartt (Oxford/QUB), M. Fraser (UCD), A.J. Levan (Radboud), M.-H. Lee, C.-C. Ngeow, Y.-C. Pan, H.-Y. Hsiao, W.-J. Hou, J.-K. Guo (NCUIA), Z.-N. Wang (HNAS), J. Gillanders (Oxford), M. Fulton, S. Srivastav, T. Moore, C. Angus, A. Aamer (QUB), and N. Tanvir (Leicester) report:
We observed the field of GRB 231117A (Laha et al., GCN 35071; Navaneeth et al., GCN 35072; Beardmore et al., GCN 35074; Cattaneo et al., GCN 35075; Svinkin et al., GCN 35079; Cheung et al., GCN 35081; Dafcikova et al. GCN 35095).
The afterglow candidate was initially identified in our previous report (Yang et al., GCN 35083) . Since it is a real astrophysical transient, but not yet confirmed as the afterglow, we registered it as AT2023yba (Chen et al., TNS 194096).
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 10:51 UT on 18 of November 2023 (MJD = 60266.452), 1.32 days after the Swift trigger. The r-band images were combined from 21 frames with a 300-second exposure time for the r band, taken under seeing conditions of an average of 1".87 and at a median airmass of 1.16. Then we continuously conducted observations of i, g, and z-band images, commencing from MJD = 60266.552, under seeing conditions of an average of 1".94 and at a median airmass of 1.83.
We used the Kinder pipeline (Yang et al. A&A 646, A22) to measure the PSF photometry of AT 2023yba after template subtraction using the SDSS images. We obtained the following magnitudes and a 3-sigma detection limit (in the AB system):
r = 20.86 +/- 0.13 mag (exposure time of 300sec*21),
i = 20.94 +/- 0.17 mag (300sec*6),
g = 21.37 +/- 0.13 mag (300sec*6) and
z > 20.02 mag (300sec*6).
The given magnitudes 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).
Since the various GCNs on this source have heterogeneous methods, we focus on comparing our own nightly template subtracted measurements. By comparing these data with measurements on MJD = 60265.435 (at the epoch of 7.39 hours after the Swift trigger), we found AT 2023yba has faded by more than 0.4 mag in g and i bands in one day, but no fading in the r band. No fading in r band was also noticed in Kumar et al. (GCN 35089) at the epoch of 10.56 hours. The redshift of the host galaxy, SDSS J220933.34+133119.5, has been spectroscopically confirmed at z = 0.257 (Ahumada et al., GCN 35093; Gonzalez-Bañuelos et al., GCN 35098), placing this transient at absolute magnitude M_r = -19.6 mag.