TITLE:   GCN CIRCULAR
NUMBER:  35859
SUBJECT: GRB 240225B: Swift-XRT afterglow detection
DATE:    24/03/04 13:18:03 GMT
FROM:    Chiara Salvaggio at INAF OABrera <chiara.salvaggio@inaf.it>

C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), A. A. Breeveld (UCL-MSSL),
P.A. Evans and M.R. Goad (U. Leicester) report on behalf of the Swift-XRT
team:

Swift-XRT has performed further follow-up observations of the MAXI-detected
burst GRB 240225B (Nakajima et al., GCN Circ. 35796; Joshi et al., GCN
Circ. 35798; Kawakubo et al., GCN Circ. 35811; Frederiks et al., GCN Circ.
35835; Cheung et al., GCN Circ. 35848). The data were collected between
T0+461.7 ks and T0+508.9 ks, and are entirely in Photon Counting (PC) mode
for a total exposure time of 1.4 ks.

The uncatalogued X-ray source reported by D'Ai et al. ("Source 1"; GCN
Circ. 35810), is still detected at an average count rate of ~ 1.3e-2 ct/s
and shows signs of fading with >3-sigma significance, and is therefore
likely the GRB afterglow.
Using 1113 s of XRT Photon Counting mode data and 1 UVOT
image, we find an astrometrically corrected X-ray position (using the
XRT-UVOT alignment and matching UVOT field sources to the USNO-B1
catalogue): RA, Dec =
128.36159, 27.07601 which is equivalent to:

RA (J2000):  08 33 26.78
Dec (J2000): +27 04 33.6

with an uncertainty of 3.9 arcsec (radius, 90% confidence). This position
is consistent with the reported optical afterglow position (Gompertz et
el., GCN Circ. 35805; Liu et al., GCN Circ. 35812; Malesani et al., GCN
Circ. 35819; Wise et al., GCN Circ. 35820; Gompertz et el., GCN Circ.
35824; Pankov et al., GCN Circ. 35826; Moskvitin et al., GCN Circ. 35828;
Amit et al., GCN Circ. 35030; Sasada et al., GCN Circ. 35831; Moskvitin et
al., GCN Circ. 35839).

This position may be improved as more data are received.  The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

The X-ray afterglow light curve can be modelled with a power-law with no
breaks and decay index alpha = 1.09 (+0.16, -0.13).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.1 (+0.8, -0.6). The
best-fitting absorption column is 1.0 (+2.0, -1.0) × 10^21 cm^-2, at a
redshift of 0.946 (Schneider et al., GCN Circ. 35832), in addition to the
Galactic value of 4.0 × 10^20 cm ^-2 (Willingale et al. 2013). The counts
to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 2.72 x 10^-11 (3.7 x 10^-11) erg cm^-2 count^-1.

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/00021675.

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