GCN Circular 35780

A.P. Beardmore (U. Leicester), R. Brivio (INAF-OAB), A. D'Ai
(INAF-IASFPA), A. Melandri (INAF-OAR), S. Dichiara (PSU), J.A. Kennea
(PSU), A. Tohuvavohu (U. Toronto), J.P. Osborne (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 240222A, from 106 s to 41.2
ks after the  BAT 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 refined XRT position is RA, Dec = 143.2254,
-68.0305 which is equivalent to:

RA (J2000): 09 32 54.10
Dec(J2000): -68 01 49.7

with an uncertainty of 3.5 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.02 (+0.12, -0.11).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.52 (+0.24, -0.12). The
best-fitting absorption column is  consistent with the Galactic value
of 1.7 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 (5.3 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.7 (+/-0.9) x 10^21 cm^-2
Galactic foreground: 1.7 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.52 (+0.24, -0.12)

If the light curve continues to decay with a power-law decay index of
2.02, the count rate at T+24 hours will be 1.3 x 10^-5 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.8 x
10^-16 (6.7 x 10^-16) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01216804.

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