GCN Circular 32965

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
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, Tohuvavohu et al., GCN 32959
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).  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
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) and marginally consistent with the optical source reported by O'Connor et al. (GCN 32957
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) 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
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), 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
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 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.