GCN Circular 35105

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
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; Navaneeth et al., GCN 35072
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; Beardmore et al., GCN 35074
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; Cattaneo et al., GCN 35075
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; Svinkin et al., GCN 35079
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; Cheung et al., GCN 35081
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; Dafcikova et al. GCN 35095
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).

The afterglow candidate was initially identified in our previous report (Yang et al., GCN 35083
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) . Since it is a real astrophysical transient, but not yet confirmed as the afterglow, we registered it as AT2023yba
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 (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
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 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
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 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
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) 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
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; Gonzalez-Bañuelos et al., GCN 35098
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), placing this transient at absolute magnitude M_r = -19.6 mag.