GCN Circular 40589

M.A. Williams (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), M. Ferro (INAF-OAB), R.
Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), S. Dichiara (PSU), J.A.
Kennea (PSU) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 7.7 ks of XRT data for GRB 250530C, from 90 s to 52.0
ks after the   trigger. The data comprise 9 s in Windowed Timing (WT)
mode (taken while Swift was slewing), with the remainder in Photon
Counting (PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.80 (+/-0.04).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.55 (+0.20, -0.19). The
best-fitting absorption column is  2.8 (+1.1, -0.9) x 10^21 cm^-2, in
excess of the Galactic value of 1.7 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 5.0 x 10^-11 (6.1 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.8 (+1.1, -0.9) x 10^21 cm^-2
Galactic foreground: 1.7 x 10^20 cm^-2
Excess significance: 4.7 sigma
Photon index:	     1.55 (+0.20, -0.19)

If the light curve continues to decay with a power-law decay index of
0.80, the count rate at T+24 hours will be 0.012 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.0 x
10^-13 (7.3 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/01319125.

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