GCN Circular 28178
Subject
GRB 200729A: further analysis of the NOT data
Date
2020-07-30T15:36:00Z (4 years ago)
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.