GCN Circular 35052

T.-W. Chen, C.-S. Lin (both NCUIA), A.J. Levan (Radboud), S. Schulze (CIERA, NW), M. Fraser (UCD), P. D'Avanzo (INAF - OAB), J. Lyman (Warwick), Y.-C. Cheng, C.-H. Wang (both NTNU), S. Yang (HNAS), M.-H. Lee, Y.-C. Pan, C.-C. Ngeow, H.-Y. Hsiao, W.-J. Hou, J.-K. Guo (all NCUIA), M. Fulton, S. Srivastav, T. Moore, C. Angus, A. Aamer (all QUB) and S. Smartt (Oxford/QUB) report:

We observed the field of GRB 231115A, a short GRB discovered by the Fermi GBM (Fermi GBM Team, GCN 35035
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). The GRB position is consistent with M 82 galaxy (D’Avanzo et al., GCN 35036
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). This burst is also detected by IBAS (Mereghetti et al., GCN 35037
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), INTEGRAL (Burns, GCN 35038
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) and Glowbug on the ISS (Cheung et al., GCN 35045
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). The GROWTH-India discovery of a potential optical counterpart AT 2023xvj
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 with r~19.2 (Kumar et al., GCN 35041
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), taken on 1.19 hours after the Fermi trigger.

We used the 40cm SLT at Lulin Observatory, Taiwan to obtain r, i and z-band images of the field of M82, as part of the Kinder collaboration (Chen et al., AstroNote 2021-92).
The first SLT epoch of observations started at 16:46 UT on 15 of November 2023 (MJD = 60263.699), 1.15 hours after the Fermi GBM trigger. The images were combined from 7 frames with a 300-second exposure time for the r band, taken under seeing conditions of an average of 1.6" and at a median airmass of 2.14. We then took i-band images for 15 frames started at 18:56 UT, each with 300-second exposure time, taken under seeing conditions of an average of 2.2" and at a median airmass of 1.58. Finally, starting at 20:10 UT we observed z-band images for 13 frames, each with 300-second exposure time, taken under seeing conditions of an average of 2.0" and at a median airmass of 1.49.

We used the Kinder pipeline (Yang et al. A&A 646, A22) to measure the 3-sigma limits on the source location. We obtained the following preliminary magnitudes (in the AB system):

r > 19.23 mag,
i > 19.48 mag and,
z > 18.53 mag.

The given limit is derived based on calibrating against Pan-STARRS1 field stars and is not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.14 mag in the direction of the burst (Schlafly & Finkbeiner 2011).

We do not see the source AT 2023xvj
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 in the combined r, i and z-band images, nor in a subtraction against images obtained from the Legacy survey. However, the relatively poor conditions combined with high underlying surface brightness from the galaxy substantially reduce our sensitivity, and we would at best have marginally detected the transient identified by Kumar et al. (GCN 35041
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).