GCN Circular 45697
GRB 260922A: Swift-XRT refined Analysis
From: Phil Evans at U of Leicester
Issued: 2026-09-23 02:13 UTC

V. D'Elia (SSDC & INAF-OAR), D.N. Burrows (PSU), S. Lanava (PSU), S.
Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester),
B. Sbarufatti (INAF-OAB), C. Salvaggio (INAF-OAB) and P.A. Evans report
on behalf of the Swift-XRT team:

We have analysed 6.9 ks of XRT data for GRB 260922A, from 97 s to 33.8
ks after the trigger. The data comprise 192 s in Windowed Timing (WT)
mode (the first 10 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode.

The light curve can be modelled with an initial power-law decay with an
index of alpha=2.80 (+0.11, -0.10), followed by a break at T+463 s to
an alpha of 0.77 (+0.06, -0.07).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.18 (+0.11, -0.10). The
best-fitting absorption column is 5.1 (+2.0, -1.8) x 10^20 cm^-2, in
excess of the Galactic value of 2.9 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.70 (+0.15, -0.10)
and a best-fitting absorption column consistent with the Galactic
value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 3.9 x 10^-11 (4.1 x 10^-11) erg
cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.9 (+3.3, -0.0) x 10^20 cm^-2
Galactic foreground: 2.9 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index: 1.70 (+0.15, -0.10)

If the light curve continues to decay with a power-law decay index of
0.77, the count rate at T+24 hours will be 0.025 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.6 x
10^-13 (1.0 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
UKSSDC | Swift-XRT high-level data products for GRB 260922A (swift.ac.uk).

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

Filter: gamma-ray bursts are posted only if in the top 10% of the past year's — by fluence (total energy received per cm², from Fermi GBM, Konus-Wind, GECAM and similar circulars; now ≥ 3.9×10^-5 erg/cm²) or by the number of GCN Circulars about the burst (now ≥ 14). Thresholds are recalculated monthly.
Relayed automatically from NASA's General Coordinates Network.