GCN Circular 28355
V. Shenoy, V. Bhalerao (IITB), I. Andreoni, M. Kasliwal (Caltech), D. Bhattacharya (IUCAA), A. R. Rao (TIFR), S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration and GROWTH collaborations: We fit the optical emission ZTF20abtxwfx/AT2020sev (Andreoni et al., GCN #28305) with a (t-t0)^-alpha power law decay, we obtain a decay rate alpha=0.68+-0.12. The best-fit explosion time is MJD 59078.77 +- 0.17 (2020-08-17 18:34:32 UT +- 4.08 hours), 1-sigma errors. We searched AstroSat CZTI data for a coincident transient within the 3-sigma time span, and do not find any significant candidates. The closest candidate is GRB200817B (Shenoy et al., GCN #28354) which is 3.01 sigma from the calculated T0. While CZTI has limited localisation abilities for GRBs, the illumination of the detectors (Detector Plane Histogram) is inconsistent with this GRB arising from the location of AT2020sev. Hence we rule out an association between the long GRB 200817B and ZTF20abtxwfx/ AT2020sev.