GCN Circular 28300

A. D'Ai (INAF-IASFPA), B. Sbarufatti (PSU), S. R. Oates (U.
Birmingham), D.N. Burrows (PSU), J. D. Gropp (PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), A.
Melandri (INAF-OAB), T. Sbarrato (INAF-OAB) and P.A. Evans (U.
Leicester) report on behalf of the Swift team:

Swift-XRT has performed follow-up observations of theFermi/GBM and IPN
detected burst GRB 200826A (Gupta et al. GCN Circ. 28288
Not found
 
 
, Hurley et
al., GCN Circ. 28291
Not found
 
 
) in a series of observations tiled on the sky. The
total exposure time is 7.4 ks, distributed over 4 tiles; the maximum
exposure at a single sky location was 5.2 ks. The data were collected
between T0+59.9 ks and T0+169.6 ks, and are entirely in Photon Counting
(PC) mode. 

Seven uncatalogued X-ray sources are detected, of which one ("Source
3") is consistent with ZTF20abwysqy, reported by ZTF (Ahumada et al.
GCN Circ. 28293
Not found
 
 
 and 28295
Not found
 
 
). 
Source 3 is fading with 3-sigma significance, and is therefore likely
the GRB afterglow. Using 1504 s of PC mode data and 1 UVOT image, we
find an enhanced XRT position (using the XRT-UVOT alignment and
matching UVOT field sources to the USNO-B1 catalogue): RA, Dec =
6.78466, +34.02670 which is equivalent to:

RA (J2000): 00h 27m 08.32s
Dec(J2000): +34d 01' 36.1"

with an uncertainty of 3.3 arcsec (radius, 90% confidence). This
position is 104 arcsec from the Fermi/GBM position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.3 (+0.7, -0.5).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.8 (+0.6, -0.5). The
best-fitting absorption column is  2.2 (+2.5, -1.6) x 10^21 cm^-2,
consistent with the Galactic value of 6.0 x 10^20 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 3.5 x 10^-11 (4.6 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.2 (+2.5, -1.6) x 10^21 cm^-2
Galactic foreground: 6.0 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.8 (+0.6, -0.5)

The results of the XRT-team automatic analysis of the likely afterglow
are at https://www.swift.ac.uk/xrt_products/TILED_GRB00093/Source3.php.

The results of the full analysis of the tiled XRT observations are
available at https://www.swift.ac.uk/xrt_products/TILED_GRB00093.

The Swift/UVOT began observations of the optical counterpart
ZTF20abwysqy, 140ks after the Fermi/GBM trigger (Fermi GBM Team, GCN
Circ. 28284
Not found
 
 
). A source consistent with ZTF20abwysqy is detected in the
UVOT summed exposures. ZTF20abwysqy is consistent with XRT source 3,
however no earlier UVOT photometry is avaliable since XRT source 3 is
outside of the field of view for all earlier tiles. The UVOT detection
is possibly explained by the underlying galaxy emission. 

The preliminary detection and 3-sigma upper limits using the UVOT
photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373)
for the early exposures are:

Filter	       T_start(s)   T_stop(s)	   Exp(s)	    Mag
white		140782	   169578	5032	       21.86 +/- 0.13

The magnitudes in the table are not corrected for the Galactic
extinction due to the reddening of E(B-V) = 0.07 in the direction of
the burst (Schlegel et al. 1998).

This circular is an official product of the Swift-XRT and Swift-UVOT
teams.