GCN Circular 30023

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), E. Ambrosi 
(INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR),
B. Sbarufatti (PSU), D.N. Burrows (PSU), J. D. Gropp (PSU), A.P.
Beardmore (U. Leicester) and A. D'Ai report on behalf of the Swift-XRT
team:

We have analysed 7.0 ks of XRT data for GRB 210515C (D'Ai et al. GCN
Circ. 30020
Not found
 
 
), from 76 s to 29.8 ks after the  BAT trigger. The data
comprise 98 s in Windowed Timing (WT) mode with the remainder in Photon
Counting (PC) mode. Using 1614 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 =
1.09417, -14.95334 which is equivalent to:

RA (J2000): 00h 04m 22.60s
Dec(J2000): -14d 57' 12.0"

with an uncertainty of 2.3 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.63 (+0.13, -0.12), followed by a break at T+853 s to
an alpha of 0.57 (+/-0.14).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 3.06 (+0.24, -0.21). The
best-fitting absorption column is  1.7 (+/-0.4) x 10^21 cm^-2, in
excess of the Galactic value of 2.6 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.11 (+0.26, -0.23)
and a best-fitting absorption column of 6.4 (+5.7, -3.8) x 10^20 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.1 x 10^-11 (3.7 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.4 (+5.7, -3.8) x 10^20 cm^-2
Galactic foreground: 2.6 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.11 (+0.26, -0.23)

If the light curve continues to decay with a power-law decay index of
0.57, the count rate at T+24 hours will be 3.8 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-13 (1.4 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/01048618.

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