GCN Circular 35600

E. Ambrosi  (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC &
INAF-OAR), D.N. Burrows (PSU), J. D. Gropp (PSU), S. Dichiara (PSU),
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester) and P.A. Evans
(U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the MAXI-detected
burst GRB 240122A in a series of observations tiled on the sky. The
total exposure time is 3.6 ks, distributed over 7 tiles; the maximum
exposure at a single sky location was 1.3 ks. The data were collected
between T0+29.5 ks and T0+35.6 ks, and are entirely in Photon Counting
(PC) mode. 

Four uncatalogued X-ray sources are detected, of which one ("Source 3")
is above the RASS 3-sigma upper limit at this position, and is
therefore likely the GRB afterglow. Using 930 s of PC mode data and 2
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 = 93.05391, -19.14418 which is equivalent to:

RA (J2000): 06h 12m 12.94s
Dec(J2000): -19d 08' 39.0"

with an uncertainty of 2.9 arcsec (radius, 90% confidence). This
position is 14.5 arcmin from the MAXI position, and it is consistent
with the GOTO optical detection (GCN 35596
Loading...
 
 , Kumar et al. 2024).

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.5 (+0.7, -2.1).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.82 (+0.33, -0.25). The
best-fitting absorption column is  consistent with the Galactic value
of 1.0 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 3.3 x 10^-11 (3.6 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.0 (+/-0.8) x 10^21 cm^-2
Galactic foreground: 1.0 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.82 (+0.33, -0.25)

If the light curve continues to decay with a power-law decay index of
2.5, 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 3.8 x
10^-13 (4.2 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/00021645.
The results of the full analysis of the tiled XRT observations are
available at https://www.swift.ac.uk/xrt_products/TILED_GRB00119.

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