GCN Circular 29942

D.N. Burrows (PSU), J. D. Gropp (PSU), J.A. Kennea (PSU), J.P. Osborne
(U. Leicester), K.L. Page (U. Leicester), A. Melandri (INAF-OAB), T.
Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB) and A.P. Beardmore report
on behalf of the Swift-XRT team:

We have analysed 6.7 ks of XRT data for GRB 210504A (Beardmore et al.
GCN Circ. 29929
Not found
 
 
), from 218 s to 24.9 ks after the  BAT trigger. The
data comprise 146 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. Using 2804 s of PC mode data and 3 UVOT
images, we find an enhanced XRT position (using the XRT-UVOT alignment
and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec =
222.39118, -30.53345 which is equivalent to:

RA (J2000): 14h 49m 33.88s
Dec(J2000): -30d 32' 00.4"

with an uncertainty of 2.0 arcsec (radius, 90% confidence).

The late-time light curve (from T0+11.6 ks) can be modelled with a
power-law decay with a decay index of alpha=1.9 (+/-0.8).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.00 (+0.13, -0.11). The
best-fitting absorption column is  1.50 (+0.41, -0.20) x 10^21 cm^-2,
consistent with the Galactic value of 1.3 x 10^21 cm^-2 (Willingale et
al. 2013). The PC mode spectrum has a photon index of 2.11 (+0.25,
-0.16) and a best-fitting absorption column of 1.35 (+0.75, -0.06) x
10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 3.1 x 10^-11 (4.2 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.35 (+0.75, -0.06) x 10^21 cm^-2
Galactic foreground: 1.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.11 (+0.25, -0.16)

If the light curve continues to decay with a power-law decay index of
1.9, the count rate at T+24 hours will be 1.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.3 x
10^-14 (4.4 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01046782.

This circular is an official product of the Swift-XRT team.