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GCN Circular 35531

Subject
GRB 240112C: Swift-XRT refined Analysis
Date
2024-01-13T10:19:51Z (a month ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), P. D'Avanzo
(INAF-OAB), M.G. Bernardini (INAF-OAB), E. Ambrosi  (INAF-IASFPA) ,
J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), D.N. Burrows (PSU) and
P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.3 ks of XRT data for GRB 240112C, from 97 s to 45.8
ks after the  BAT trigger. The data comprise 39 s in Windowed Timing
(WT) mode (the first 10 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. Using 2660 s of PC mode data
and 6 UVOT images, we find an enhanced XRT position (using the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue):
RA, Dec = 270.64402, +24.48893 which is equivalent to:

RA (J2000): 18h 02m 34.56s
Dec(J2000): +24d 29' 20.2"

with an uncertainty of 2.0 arcsec (radius, 90% confidence).

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.61 (+0.04, -0.48), followed by a break at T+5970 s to
an alpha of 1.30 (+0.18, -0.20).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.92 (+0.14, -0.13). The
best-fitting absorption column is  2.0 (+0.5, -0.4) x 10^21 cm^-2, in
excess of the Galactic value of 1.3 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 3.6 x 10^-11 (4.8 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.0 (+0.5, -0.4) x 10^21 cm^-2
Galactic foreground: 1.3 x 10^21 cm^-2
Excess significance: 2.5 sigma
Photon index:	     1.92 (+0.14, -0.13)

If the light curve continues to decay with a power-law decay index of
1.30, the count rate at T+24 hours will be 0.015 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.3 x
10^-13 (7.0 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/01208389.

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

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