TITLE:   GCN CIRCULAR
NUMBER:  32965
SUBJECT: GRB 221120A: Archival PS1 Imaging of Faint Source near XRT position
DATE:    22/11/21 19:18:20 GMT
FROM:    Jillian Rastinejad at Northwestern Univ.  <jillianrastinejad2024@u.northwestern.edu>

J. Rastinejad, C. D. Kilpatrick, A. E. Nugent and W. Fong  (Northwestern) report:

We analyzed stacked images from the Pan-STARRS 3pi Data Release 1 image archive (Flewelling et al. 2016) of the location of the short-duration GRB 221120A (Eyles-Ferris et al., GCN 32955, Tohuvavohu et al., GCN 32959).  We detect a faint source in multiple bands centered at

RA (J2000): 02:45:21.31
Dec (J2000): +43:14:35.35

This source is just outside the enhanced XRT position (90% confidence, Goad et al., GCN 32962) and marginally consistent with the optical source reported by O'Connor et al. (GCN 32957) using their reported 0.5'' uncertainty. Performing forced aperture photometry of the source with an aperture approximately equal to the FWHM size (0.9 - 2.0''), we measure the following magnitudes:

g > 23.8 mag
r = 23.2 +/- 0.5 mag
i = 23.1 +/- 0.3 mag
z = 23.5 +/- 0.5 mag
y = 22.0 +/- 0.4 mag

All magnitudes are in the AB system. The source appears extended although the low S/N precludes a firm conclusion. Using the i-band magnitude and an offset of 2.7'' between the centers of this possible galaxy and the XRT position, we calculate a probability of chance coincidence (Pcc; Bloom et al. 2002) of 0.05. The bright, offset galaxy previously reported by O'Connor et al. (GCN 32957), PSO J041.3414+43.242, is also apparent in the PS1 imaging, and for which we measure r_AB = 17.6 mag (similar to the tabulated SDSS magnitude for this source). At an angular offset of 13.2'', we find a Pcc of 0.02.

Thus, it is possible that the optical afterglow candidate reported by GCN 32957 is actually the galaxy apparent in PS1 imaging. Given the comparable Pcc values of both sources, it is difficult to make a clear host association with present information. Confirmation of a fading counterpart is critical to refining the position of the counterpart and determining the true host.