GCN Circular 35612

M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR), J. D. Gropp
(PSU), S. Dichiara (PSU), J.A. Kennea (PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), V.
D'Elia (SSDC & INAF-OAR) and P.A. Evans report on behalf of the
Swift-XRT team:

We have analysed 6.5 ks of XRT data for GRB 240123A, from 131 s to 23.7
ks after the  BAT trigger. The data comprise 164 s in Windowed Timing
(WT) mode (the first 8 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. The refined XRT position is RA,
Dec = 199.3473, +60.6184 which is equivalent to:

RA (J2000): 13 17 23.34
Dec(J2000): +60 37 06.1

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

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=7.2 (+0.8, -2.8). At T+145 s  the decay
flattens to an alpha of 0.3 (+1.0, -1.8). The light curve breaks again
at T+183 s to a decay with alpha=2.91 (+0.20, -0.19),  before a final
break at T+635 s s after which the decay index is 0.58 (+/-0.08).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.35 (+0.09, -0.07). The
best-fitting absorption column is  consistent with the Galactic value
of 1.4 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 2.01 (+0.17, -0.13) and a best-fitting absorption
column of 2.3 (+3.3, -0.9) x 10^20 cm^-2. The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 3.2 x 10^-11 (3.4 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.3 (+3.3, -0.9) x 10^20 cm^-2
Galactic foreground: 1.4 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.01 (+0.17, -0.13)

If the light curve continues to decay with a power-law decay index of
0.58, the count rate at T+24 hours will be 0.028 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.0 x
10^-13 (9.6 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/01210276.

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