TITLE:   GCN CIRCULAR
NUMBER:  28178
SUBJECT: GRB 200729A: further analysis of the NOT data
DATE:    20/07/30 15:36:00 GMT
FROM:    Daniele B Malesani at DTU Space  <malesani@space.dtu.dk>

Daniele B. Malesani (DTU Space), Antonio de Ugarte Postigo (HET, 
IAA-CSIC), Luca Izzo (DARK/NBI), Jens Hjorth (DARK/NBI), D. Alexander 
Kann (HET, IAA-CSIC), Christina C. Thoene (HET, IAA-CSIC), Martin A. 
Guerrero (IAA-CSIC), report on behalf of a larger collaboration:

With the availability of the full calibration set, we inspected again 
the data from our observation of GRB 200729A (Malesani et al., GCN 
28169) taken with the Nordic Optical Telescope (NOT). The seeing in our 
images is 1".

The object mentioned by Kilpatrick et al. (GCN 28172; see also Laskar et 
al., GCN 28171; Anand et al., GCN 28175) is marginally seen in our 
z-band image (a stack of 3x200 s exposures, with mean time 2.5 hr after 
the GRB trigger). Computing forced photometry against nearby stars from 
the Pan-STARRS catalog (and using a 1.5" radius aperture), we measure z 
= 22.1 +- 0.3 AB. We also note that the photometry value is sensitive to 
the chosen aperture size. This measurement is consistent with the 
preliminary upper limit we reported in GCN 28169.

No clear detection is present in the r-band image, though there is 
clearly a background of diffuse, patchy emission from NGC 4242 (as also 
visible in the HST image; Anand et al. GCN 28175).

We downloaded the Pan-STARRS images from 
https://ps1images.stsci.edu/cgi-bin/ps1cutouts . Using a 1.5" radius 
aperture, we measure for the object z = 22.5 +- 0.5 AB, which is 
consistent with our measurement. As a check, we also measure i = 22.45 
+- 0.18 AB. Both values are fainter than reported by Kilpatrick et al. 
(GCN 28172).

In summary, our NOT images provide confirmation for the optical source 
mentioned by Kilpatrick et al. (GCN 28172), but we do not find any 
indication for variability compared to the archival magnitudes from the 
Pan-STARRS survey. As other groups have noted (Kilpatrick et al., GCN 
28172; Anand et al., GCN 28175), this object could be a background 
galaxy unrelated to NGC 4242.