TITLE:   GCN CIRCULAR
NUMBER:  29133
SUBJECT: GRB 201221D: Archival PS1 Imaging of the Putative Host Galaxy
DATE:    20/12/22 15:34:10 GMT
FROM:    Charles Kilpatrick at UC Santa Cruz  <cdkilpat@ucsc.edu>

C. D. Kilpatrick (Northwestern), D. B. Malesani (DTU Space), and W. Fong (Northwestern), report:

“We analyzed stacked images from the PS1 Data Release 1 image archive (Flewelling et al., 2016, arXiv:1612.05243) of the location of the short-duration GRB 201221D (Page et al., GCN 29112).  We detected a single source centered at:

RA (J2000): 11:24:14.01
Dec (J2000): +42:08:39.69

in PS1 g-band, 4.6 arcseconds from the latest enhanced XRT position of GRB 201221D (Evans et al., GCN 29119).  We note that this places the centroid of the candidate host galaxy just outside the enhanced XRT error circle reported by GCN 29119, which has a radius of 3.9 arcsec (90% confidence), although consistent with the real-time XRT error circle at https://www.swift.ac.uk/xrt_positions/01014037/ <https://www.swift.ac.uk/xrt_positions/01014037/>. Performing forced aperture photometry on this position with an aperture fixed to the FWHM size in all PS1 bands (approximately 1.1 arcseconds), we found that this source has a brightness:

g = 23.2 +/- 0.2 mag
r > 23.2 mag
i > 23.3 mag
z > 22.2 mag
y = 22.6+/-0.2 mag

All magnitudes are given in AB.  We note that this source is coincident with emission identified in Malesani & Knudstrup (GCN 29117) who find a source with r = 23.1+/-0.3 mag (AB) in NOT follow up imaging.  Similarly, Dichiara et al. (GCN 29128) find r=23.9 mag and i=23.7 mag (AB), all of which are consistent with our limiting magnitudes.  Based on this photometry, it is fully plausible that the reported optical candidate (GCN 29117, GCN 29128) is dominated by the host galaxy.  However, the detection of absorption features in the spectrum (de Ugarte Postigo et al., GCN 29132), coupled with the possible r-band fading between NOT and DCT observations (reported in GCN 29128) may indicate the presence of an optical afterglow on top of the host emission. Image subtraction will be required to test for fading, and further, deep optical observations are encouraged.