TITLE: GCN CIRCULAR NUMBER: 34780 SUBJECT: AT2023sva / GRB230916B: GIT observations of the afterglow DATE: 23/09/28 18:27:31 GMT FROM: Varun Bhalerao at IIT Bombay varunb@iitb.ac.in

A. Salgundi, V. Swain, R. Kumar, G. Waratkar, R. Sharma, V. Bhalerao (IITB), G.C. Anupama, S. Barway (IIA) and K. Angail (IAO) report on behalf of the GIT team:

We observed AT2023sva (Vail et al., GCN 34730; Tonry et al. 2023, TNS report 188327) which is confirmed as an orphan afterglow (Roberts et al., GCN Circ. 34748; GCN Circ. 34749) with the 0.7m GROWTH-India Telescope (GIT). The observations started at 16:42:17 UT on 2023-09-19, roughly 2.3 days after the first ZTF report. We obtained 10 images of 300s exposure in the r' filter. We clearly detect the candidate in our stacked image at the position reported by Vail et al., GCN Circ. 34730. It was not detected in deep observations on later days. The photometry of the candidate is reported below.

---------------------------------------------------------------- | JD (mid) | Filter | Total Exposure (s) | Magnitude (AB) | Limiting Mag (AB) | | ----------------- | ------ | ------------------ | -------------- | ----------------- | | 2460207.214532005 | r' | 10x300 | 21.89 +/- 0.07 | 22.63 | | 2460209.289487210 | r' | 14x300 | -- | 22.28 | | 2460210.459151377 | r' | 14x300 | -- | 22.36 |

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The measurement is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.

Based on the publicly avialable photometry (Vail et al., GCN Circ. 34730; Malesani et al., GCN Circ. 34731; Parra-Ramos et al., GCN Circ. 34744) and our measurements, we find that the source is consistently fading as a power-law with flux proportional to (t−t0)^−alpha where t0 is MJD 60203.08 +/- 0.25 (2023-09-16 01:55:12) and alpha = 3.79 +/- 0.48. This decline is much faster than typical afterglows. This t0 is consistent with Fermi GBM trigger 716527670/230916144, which is also spatially coincident with the afterglow as reported by Roberts et al. (GCN 34748). The t0 fit confirms the association between the two events, which we name GRB 230916B.

The t0 is not consistent with GRB 230916A (Grefenstette et al., GCN 34721). We note that Ruffini et al (GCN 34779) also are referring to this GRB rather than the NuSTAR-reported trigger.

Further imaging of the field is encouraged to find any potential host galaxy.

The GROWTH India Telescope (GIT; Kumar et al. 2022) is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/.

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