GCN Circular 41313
Subject
GRB 250809A: Swift-XRT refined Analysis
Event
Date
2025-08-10T09:44:16Z (13 days ago)
Edited On
2025-08-11T13:43:34Z (12 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 K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
J.P. Osborne (U. Leicester), A. D'Ai (INAF-IASFPA), S. Campana
(INAF-OAB), A. Melandri (INAF-OAR), M.A. Williams (PSU), S. Dichiara
(PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester) and P.A. Evans report on behalf of the Swift-XRT team:
We have analysed 6.7 ks of XRT data for GRB 250809A, from 113 s to 51.4
ks after the trigger. The data comprise 910 s in Windowed Timing (WT)
mode (the first 8 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. The best available XRT position
(using the promptly downlinked event data, the XRT-UVOT alignment and
matching UVOT field sources to the USNO-B1 catalogue) is RA, Dec =
233.00043, -53.29626 which is equivalent to:
RA (J2000): 15h 32m 00.10s
Dec(J2000): -53d 17' 46.5"
with an uncertainty of 3.0 arcsec (radius, 90% confidence).
The light curve can be modelled with an initial rise, with a power-law
index of alpha=-1.5 (NaN, NaN), followed by a break at T+281 s to an
alpha of 8.0 (NaN, NaN).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.41 (+/-0.13). The
best-fitting absorption column is 2.05 (+0.18, -0.17) x 10^22 cm^-2,
in excess of the Galactic value of 8.0 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 4.9 x 10^-11 (1.6 x 10^-10) erg
cm^-2 count^-1.
A summary of the WT-mode spectrum is thus:
Total column: 2.05 (+0.18, -0.17) x 10^22 cm^-2
Galactic foreground: 8.0 x 10^21 cm^-2
Excess significance: 11.9 sigma
Photon index: 2.41 (+/-0.13)
If the light curve continues to decay with a power-law decay index of
2.96, the count rate at T+24 hours will be 3.7 x 10^-8 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.8 x
10^-18 (6.1 x 10^-18) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01340954.
This circular is an official product of the Swift-XRT team.