GCN Circular 24561

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

We have analysed 6.3 ks of XRT data for GRB 190515B, from 88 s to 27.5
ks after the  BAT trigger. The data comprise 66 s in Windowed Timing
(WT) mode with the remainder in Photon Counting (PC) mode. The enhanced
XRT position for this burst was given by Osborne et al. (GCN Circ.
24557
Not found
 
 
).

The light curve can be modelled with an initial power-law decay with an
index of alpha=3.85 (+0.26, -0.23), followed by a break at T+296 s to
an alpha of 0.38 (+0.11, -0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 3.26 (+0.33, -0.30). The
best-fitting absorption column is  2.1 (+0.6, -0.5) x 10^21 cm^-2, in
excess of the Galactic value of 1.3 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.06 (+0.24, -0.15)
and a best-fitting absorption column consistent with the Galactic
value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.3 x 10^-11 (4.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.3 (+0.6, -0.0) x 10^21 cm^-2
Galactic foreground: 1.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.06 (+0.24, -0.15)

If the light curve continues to decay with a power-law decay index of
0.38, the count rate at T+24 hours will be 0.027 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.0 x
10^-13 (1.2 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00903845.

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