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GCN Circular 35600

Subject
GRB 240122A: Swift-XRT afterglow detection
Date
2024-01-23T11:25:05Z (a month ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
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, 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.


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