GCN Circular 40113
Subject
GRB 250407A: Swift-XRT afterglow detection
Date
2025-04-08T00:14:46Z (7 days ago)
Edited On
2025-04-08T14:57:19Z (7 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Simone Dichiara at Pennsylvania State University <sbd5667@psu.edu>
Via
email
S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR), B.
Sbarufatti (INAF-OAB), D.N. Burrows (PSU), M. A. Williams (PSU) and
P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the
Fermi/GBM-detected burst GRB 250407A (GCN circ. 40104, GCN circ.
40105). We searched for X-ray sources in 293 s of Photon Counting (PC)
mode data. The total exposure at the position of the afterglow (see
below) is 293 s, obtained between T0+18.8 ks and T0+19.1 ks.
An uncatalogued X-ray source is detected within the estimated 1-sigma
(statistical) SRG/ART-XC error region (20 arcsec; GCN circ. 40110) and
is above the RASS 3-sigma upper limit at this position, and is therefore
likely the GRB afterglow. Using 293 s of PC mode data and 1 UVOT image,
we find an enhanced XRT position (using the XRT-UVOT alignment and
matching UVOT field sources to the USNO-B1 catalogue): RA, Dec =
110.75876, +36.79827 which is equivalent to:
RA (J2000): 07h 23m 02.10s
Dec(J2000): +36d 47' 53.8"
with an uncertainty of 2.8 arcsec (radius, 90% confidence). This
position is 6.9 arcsec from the SRG/ART-XC position reported by Molkov
et al (GCN circ. 40110). We note that the source has faded since the
SRG/ART-XC observation.
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.8 (+0.4, -0.3). The
best-fitting absorption column is 3.0 (+1.7, -1.4) x 10^21 cm^-2, in
excess of the Galactic value of 7.0 x 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 4.3 x 10^-11 (5.8 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 3.0 (+1.7, -1.4) x 10^21 cm^-2
Galactic foreground: 7.0 x 10^20 cm^-2
Excess significance: 2.8 sigma
Photon index: 1.8 (+0.4, -0.3)
The results of the XRT-team automatic analysis of the likely afterglow
are at https://www.swift.ac.uk/ToO_GRBs/00021826/Source1.php.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021826.
This circular is an official product of the Swift-XRT team.