TITLE:   GCN CIRCULAR
NUMBER:  27600
SUBJECT: GRB 200412B: Swift XRT confirmation of afterglow and UVOT upper limits
DATE:    20/04/17 17:02:07 GMT
FROM:    Boris Sbarufatti at PSU  <bxs60@psu.edu>

B. Sbarufatti (PSU), N. P. M. Kuin (UCL-MSSL), P. A. Evans (U. Leicester), and J. D. Gropp (PSU) report on behalf of the Swift team:

Swift XRT performed follow up observations on the afterglow candidate reported by Page et al. (GCN #27561).

XRT data now comprise 6.1 ks of Photon Counting data covering times from T+20 ks to T+370 ks.

The light curve is best fit by a power-law with decay index 1.6 +/- 0.3. We thus confirm that this is indeed the afterglow of GRB 200412B.
The latest results can be viewed via: https://www.swift.ac.uk/xrt_curves/00020981/

In the initial Swift observation about 20ks after the trigger, the source was not in the UVOT field of view, which is smaller than that of the XRT.  In the follow up observations around 360ks after the trigger we obtained the following 3-sigma upper limits to the source (Vega magnitudes, uncorrected for reddening):
filter exposure   UL (mag)
 v       374       >19.76
 b       374       >20.74
 u       374       >20.41
uvw1     749       >20.31
uvm2    1201      >20.48
uvw2    1499      >20.83 

This circular is an official product of the Swift team.