GCN Circular 29750

P.A. Evans (U. Leicester), M. Capalbi (INAF-IASFPA), M. Perri (SSDC &
INAF-OAR), V. D'Elia (SSDC & INAF-OAR), J.A. Kennea (PSU), A.
Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), J.P. Osborne (U.
Leicester) and K.L. Page report on behalf of the Swift-XRT team:

We have analysed 6.0 ks of XRT data for GRB 210402A (Page et al. GCN
Circ. 29743
Not found
 
 
), from 160 s to 46.0 ks after the  BAT trigger. The data
comprise 1.1 ks in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. Using 1629 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 =
198.25927, +44.51454 which is equivalent to:

RA (J2000): 13h 13m 02.22s
Dec(J2000): +44d 30' 52.3"

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

The late-time light curve (from T0+5.4 ks) can be modelled with a
power-law decay with a decay index of alpha=1.03 (+0.21, -0.27).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.811 (+0.027, -0.026). The
best-fitting absorption column is  3.25 (+/-0.13) x 10^21 cm^-2, in
excess of the Galactic value of 1.5 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.09 (+0.20, -0.19)
and a best-fitting absorption column of 3.2 (+/-0.8) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.6 x 10^-11 (5.6 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.2 (+/-0.8) x 10^21 cm^-2
Galactic foreground: 1.5 x 10^20 cm^-2
Excess significance: 6.5 sigma
Photon index:	     2.09 (+0.20, -0.19)

If the light curve continues to decay with a power-law decay index of
1.03, the count rate at T+24 hours will be 0.017 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.0 x
10^-13 (9.4 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/01040123.

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