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GCN Circular 29117

Subject
GRB 201221D: candidate optical counterpart from the NOT
Date
2020-12-22T01:34:37Z (3 years ago)
From
Daniele B Malesani at DTU Space <malesani@space.dtu.dk>
D. B. Malesani (DTU Space) and E. Knudstrup (NOT and Aarhus Univ.) 
report on behalf of a larger collaboration:

We observed the field of GRB 201221D (Page et al., GCN 29112) using the 
Nordic Optical Telescope (NOT) equipped with the ALFOSC imaging camera. 
Observations were carried out using the SDSS r (3x300 s) and z (5x200 s) 
filters. The data were taken as soon as the target rised, so at quite 
high airmass (2.9 to 2.3), yielding a seeing of ~1.5".

Inside or close to the XRT werror circle (current uncertainty 4.2": 
https://www.swift.ac.uk/xrt_positions/), a very faint object is detected 
in the stack of the r-band images. Calibrating against nearby stars from 
the Pan-STARRS catalog, we measure r = 23.1 +- 0.3 (AB), at a mean epoch 
2020 Dec 22.032 UT (1.67 hr after the GRB). Its coordinates are:

RA = 11:24:14.02
Dec = +42:08:39.97

No object is visible at this location in the Pan-STARRS r-band image, 
which has however a limiting magnitude (r ~ 23.3 AB) comparable to our 
measurement. Given the lack of variability and the still relatively 
large XRT error circle size, we cannot confirm at the present time 
whether this object is associated with GRB 201221D. Further observations 
are planned.
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