GCN Circular 40962
Subject
EP250704a / GRB 250704B : GROWTH-India Telescope optical observations
Event
Date
2025-07-05T00:09:42Z (4 months ago)
From
V. Swain at IIT Bombay <vishwajeet.s@iitb.ac.in>
Via
Web form
T. Mohan, V. Swain, A.P. Saikia, V. Bhalerao (IITB), G.C. Anupama, S. Barway (IIA) and K. Angail (IAO) report on behalf of the GIT team:
We observed the field of EP250704a (Li et al., GCN 40941) also detected by SVOM (GRB 250704B; Wang et al., GCN 40940), with 0.7m GROWTH-India Telescope (GIT). We obtained two epochs of multiple exposures in r' and i' filters. The first and second epochs started at 8.05 hours and 14.2 hours after the SVOM trigger. We detected a source at position reported by COLIBRÍ (Schneider et al., GCN 40942) and is within the uncertainty region of EP FXT localization (Li et al., GCN 40956). The photometry results are as follows:
| MJD (mid) | Filter | t-t0 (in hours) | Total Exposure Time (sec) | Magnitude (AB) |
| ------------ | ------ | --------------- | ------------------------- | -------------- |
| 60860.739443 | r' | 9.47 | 11 x 300 | 20.12 +/- 0.08 |
| 60860.936365 | r' | 14.2 | 5 x 300 | 19.80 +/- 0.09 |
| 60860.919386 | i' | 13.79 | 4 x 300 | 19.81 +/- 0.09 |
The measurement is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.
We see that the source has brightned in the r' band between our observations at 9.5 to 14.2 hours. The r'-i' colour is consistent with zero at 14.2 hours. The same trend is also seen when considering other reported values (Schneider et al., GCN 40942, Malesani et al., GCN 40945, Brivio et al., GCN 40947, Perez-Garcia et al., GCN 40957, Gillanders et al., GCN 40958, Xin et al., GCN 40960).
Given the proximity of the source to the galactic plane (latitude -10.4 deg), we cannot rule out the possibility that this may be a galactic source rather than a distant cosmological afterglow. Spectroscopic observations are strongly encouraged to ascertain the nature of this source.
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/.