TITLE:   GCN CIRCULAR
NUMBER:  28172
SUBJECT: GRB 200729A: Pan-STARRS pre-explosion source detection
DATE:    20/07/30 02:59:30 GMT
FROM:    Charles Kilpatrick at UC Santa Cruz  <cdkilpat@ucsc.edu>

C. D. Kilpatrick (UCSC), W. Fong, A. Hajela, R. Chornock, T. Laskar, R. Margutti, D. Coppejans, K. D. Alexander, (Northwestern), E. Berger (Harvard) report:

We examined the latest, enhanced XRT position of the long GRB 200729A detected by Swift (Evans et al., GCN 28165) in PAN-STARRS imaging. We downloaded the Pan-STARRS DR1 images and note the presence of a faint source detected within the XRT error circle (90% confidence Osborne et al., GCN 28168) at:

RA (J2000) = 12:17:30.32
Dec (J2000) = +45:35:41.4

The location of this source is consistent with the pre-burst counterpart detected in Spitzer/IRAC imaging (Laskar et al., GCN 28171).  Using a 1.5 arcsec aperture, we measure that the source has i=21.4+/-0.2 mag, y=21.0+/-0.2 mag, and z=21.0+/-0.1 mag. We note that our z-band photometry is 1 magnitude brighter than the limit reported by NOT (Malesani et al., GCN 28169). The source is not obviously detected in either Pan-STARRS g or r-band imaging. The source may be marginally extended, although it is too faint to definitively determine whether the source is intrinsically extended beyond the PSF of the Pan-STARRS DR1 imaging. 

Based on the column of neutral hydrogen inferred from the Swift/XRT X-ray spectrum (7.1e21 cm-2; https://www.swift.ac.uk/xrt_unenh_positions/00984929/), the total optical extinction to any counterpart is approximately A_V = 3.7 mag (Guver & Ozel, 2009, MNRAS, 400, 2050).  Assuming the Pan-STARRS source is in NGC 4242 with a TRGB distance of 5.3 Mpc (Sabbi et al. 2018, ApJS, 235, 23), any stellar counterpart would have M_i~-9.3 mag, which is extremely luminous for a star even with initial mass > 25 M_sun.  Thus the lack of an optical or near-infrared counterpart (Hu et al., GCN 28167; Lipunov et al., GCN 28166; Malesani et al., GCN 28169; D’Avanzo et al., GCN 28170) and large implied luminosity suggests that a supernova or non-terminal outburst from such a star in NGC 4242 is unlikely.

Alternatively, if the source is a background galaxy, and the i-z color represents a 4000 Angstrom break, this places the redshift of the galaxy at z~1.1, which would provide a natural explanation for the faint XRT flux (GCN 28165), and the lack of detected optical or NIR afterglow.