W. Fong (Northwestern), C. D. Kilpatrick (UCSC), J. Rastinejad (Northwestern), T. Laskar (U. Bath), and E. Berger (Harvard) report on behalf of a larger collaboration:
���We report on new HST/WFC3 IR observations (PI: Berger; Program 15964) of the short GRB 200522A (Evans et al., GCN 27778) in the F125W and F160W filters at approximately 55 days post-burst. A total of 5223-s was obtained in both filters and the resulting images are deep enough to provide adequate templates for all previous epochs (F125W at 3.6 and 16.4 days, and F160W at 3.6 days; Fong et al., GCN 27826; Kilpatrick et al., GCN 27904). An independent analysis of the first two epochs of data resulted in a candidate afterglow (O���Connor et al., GCN 28100).
To constrain the presence of any transient sources, we first performed image subtraction between the observations in each filter taken at 3.6 days and the new 55-day templates using the HOTPANTS software (Becker 2015). In the difference images, we detect a high-significance residual within the XRT position (Beardmore et al., GCN 27780) and close to the host galaxy center in both filters. Given the contamination from residuals as a result of the core of the host galaxy, accurate photometry of the source will require further analysis.
We also perform image subtraction between the F125W observations at 16.4 days and 55 days. The source is no longer detected to a 3-sigma limit of m_F125W>27.3 AB mag. The fading behavior, coupled with the detection at 3.6 days in both filters, confirms the source as the counterpart to GRB 200522A. Based on our preliminary analysis, we cannot distinguish between whether the emission is resulting from the afterglow alone, or a combination of afterglow and kilonova emission.
We thank the HST staff for scheduling and implementing these observations.���