TITLE:   GCN CIRCULAR
NUMBER:  27457
SUBJECT: GRB 200325B: Swift-XRT and UVOT observations
DATE:    20/03/27 17:35:01 GMT
FROM:    Paolo D'Avanzo at INAF-OAB  <pda.davanzo@gmail.com>

P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), A. Melandri (INAF-OAB), 
T. Sbarrato (INAF-OAB), S. R. Oates (U. of Birmingham), M. H. Siegel (PSU), 
F. E. Marshall (NASA/GSFC), S. Dichiara (NASA/GSFC/UMCP) and P.A. Evans 
(U.Leicester) report on behalf of the Swift-XRT and UVOT teams:


Swift-XRT has performed follow-up observations of the Fermi-detected
burst GRB 200325B (Fermi GBM team, GCN Cir. 27441), collecting 1.1
ks of Photon Counting (PC) mode data between T0+102.4 ks and T0+124.8 ks 
in a single pointing covering the region of the candidate GRB optical 
counterpart initially reported by Lipunov et al. (GCN Circ. 27446).

An uncatalogued X-ray source has been detected: 

Source 1:
 RA (J2000.0):  167.55381  =  11:10:12.91
 Dec (J2000.0):  +27.8196  =  +27:49:10.5
 Error: 4.9 arcsec (radius, 90% conf. [Enhanced position])
 Count-rate: 0.020 [+/-0.005] ct s^-1   
 Flux: (7.2 [+/-1.8])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

The source is below the RASS limit and, due to the limited amount of
data available so far, shows no signs of fading.

The position of this source is consistent with the one reported for the 
GRB optical afterglow (Lipunov et al., GCN Circ. 27446; Pozanenko et al., 
GCN Circ. 27452; Belkin et al., GCN Circ. 27455; Strausbaugh & Cucchiara, 
GCN Circ. 27456). We therefore conclude that this is the X-ray afterglow of 
GRB 200325B.

The results of the XRT-team automatic analysis of the XRT observations,
including a position-specific upper limit calculator, are available at
https://www.swift.ac.uk/ToO_GRBs/00020978 <https://www.swift.ac.uk/ToO_GRBs/00020978>.



The Swift/UVOT began settled observations of the field of GRB 200325B
102ks after the Fermi/GBM trigger. An optical source consistent with the 
GRB optical afterglow (Lipunov et al., GCN Circ. 27446; Pozanenko et al., 
GCN Circ. 27452; Belkin et al., GCN Circ. 27455; Strausbaugh & Cucchiara, 
GCN Circ. 27456) is detected in the initial UVOT white exposure. 
A preliminary magnitude in white using the UVOT photometric system 
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) is:

Filter         T_start(ks)   T_stop(ks)      Exp(s)         Mag

white             102.39       124.75         1044        21.27 ± 0.24

This magnitude is not corrected for the Galactic extinction due to the reddening 
of E(B-V) = 0.03 in the direction of the burst (Schlegel et al. 1998).