GCN Circular 36217

J.P. Osborne (U. Leicester), B. Sbarufatti (INAF-OAB), C. Salvaggio
(INAF-OAB), M. Ferro (INAF-OAB), S. Dichiara (PSU), J.A. Kennea (PSU),
A. Tohuvavohu (U. Toronto), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.6 ks of XRT data for GRB 240421B, from 110 s to 36.0
ks after the  BAT trigger. The data comprise 42 s in Windowed Timing
(WT) mode (the first 9 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.50 (+/-0.07), followed by a break at T+2500 s to an
alpha of 1.20 (+/-0.10).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.87 (+/-0.13). The
best-fitting absorption column is  6.5 (+1.0, -0.9) x 10^21 cm^-2, in
excess of the Galactic value of 1.2 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 4.6 x 10^-11 (7.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.5 (+1.0, -0.9) x 10^21 cm^-2
Galactic foreground: 1.2 x 10^21 cm^-2
Excess significance: 9.6 sigma
Photon index:	     1.87 (+/-0.13)

If the light curve continues to decay with a power-law decay index of
1.20, the count rate at T+24 hours will be 0.023 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-12 (1.7 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/01223470.

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