GCN Circular 35055
Subject
GRB 231115A / AT2023xvj: Updated GIT analysis
Date
2023-11-16T06:07:34Z (a year ago)
From
Varun Bhalerao at IIT Bombay <varunb@iitb.ac.in>
Via
Web form
R. Kumar (IIT Bombay), V. Karambelkar (Caltech), V. Swain, V. Bhalerao, A. Salgundi, Y. Wagh (IIT Bombay), G. C. Anupama, S. Barway (IIA), R. Norboo (IAO), T. Ahumada, M. M. Kasliwal (Caltech) report on behalf of a larger collaboration
We undertook refined analysis of GIT231115AA / AT2023xvj (Kumar et al, GCN 35041). The source is detected in 25 individual r-band images obtained by GIT (https://sites.google.com/view/growthindia/results/at2023xvj), as well as in a stacked image.
We investigated the reference images from PanSTARRS used for image subtraction and find a small, fuzzy, "negative" artifact at the location of the source (see link above), which may boost the counts at the source location. This had been considered in the original report, and excluded due to three reasons: first, the measured source flux is varying despite fixed exposure times, which makes it unlikely that they are all caused due to the same artifact. Second, there are similar artifacts at other locations in the reference images which do not create such bright spurious sources. Third, the source shows a PSF-like profile while the reference image artifact does not.
Other groups have reported non-detections which are broadly consistent with our detection: Balanutsa et al, (GCN 35046) report an upper limit of 21.3 which is comparable to our image sensitivity of 21.45 (5-sigma). However, the candidate is in a high background region which will result in shallower upper limits. Upper limits reported by Iskandar et al (GCN 35051 - 18.9 mag), and Chen et al (GCN 35052 - 19.2 mag) are consistent with our measurement.
Given the complex nature of background in the M82 field and the potential artifact, we undertook manual image subtraction using SDSS images as a reference. In this scenario, the subtraction is noisier, and we do not detect any source at the location of GIT231115AA / AT2023xvj, to a limiting magnitude of 19.3 (5-sigma).
Given this analysis, we caution observers that AT2023xvj be treated as a candidate, not a secure counterpart.