GCN Circular 21904

B. Sbarufatti (INAF-OAB/PSU), S.L. Gibson (U. Leicester), J.P. Osborne
(U. Leicester), K.L. Page (U. Leicester), V. D'Elia (ASDC), A. Melandri
(INAF-OAB), D.N. Burrows (PSU), A. Tohuvavohu (PSU) and A. D'Ai report
on behalf of the Swift-XRT team:

We have analysed 6.5 ks of XRT data for GRB 170921A (D'Ai et al. GCN
Circ. 21901
Not found
 
 
), from 82 s to 16.5 ks after the  BAT trigger. The data
comprise 8 s in Windowed Timing (WT) mode (taken while Swift was
slewing), with the remainder in Photon Counting (PC) mode. The best
available XRT position	(using the promptly downlinked event data, the
XRT-UVOT alignment and matching UVOT field sources to the USNO-B1
catalogue) is RA, Dec = 26.8487, 30.9067 which is equivalent to:

RA (J2000): 01 47 23.70
Dec(J2000): +30 54 24.3

with an uncertainty of 2.1 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=3.3 (+0.5, -0.4), followed by a break at T+238 s to an
alpha of 0.85 (+0.10, -0.09).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.7 (+0.4, -0.3). The
best-fitting absorption column is  1.00 (+0.27, -0.23) x 10^22 cm^-2,
in excess of the Galactic value of 5.5 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 (1.2 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.00 (+0.27, -0.23) x 10^22 cm^-2
Galactic foreground: 5.5 x 10^20 cm^-2
Excess significance: 6.7 sigma
Photon index:	     2.7 (+0.4, -0.3)

If the light curve continues to decay with a power-law decay index of
0.85, the count rate at T+24 hours will be 2.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.3 x
10^-14 (2.5 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00773509.

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