GCN Circular 45730
GRB 260924A: Swift-XRT refined Analysis
From: Phil Evans at U of Leicester
Issued: 2026-09-24 18:40 UTC

S. Dichiara (PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P.
Beardmore (U. Leicester), C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB),
R. Brivio (INAF-OAB), D.N. Burrows (PSU) and P.A. Evans report on
behalf of the Swift-XRT team:

We have analysed 6.4 ks of XRT data for GRB 260924A, from 99 s to 43.5
ks after the trigger. The data comprise 365 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 late-time light curve (from T0+3.6 ks) can be modelled with an
initial power-law decay with an index of alpha=0.89 (+0.08, -0.07),
followed by a break at T+25.9 ks to an alpha of 2.4 (+/-0.7).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.491 (+0.017, -0.015). The
best-fitting absorption column is consistent with the Galactic value
of 4.1 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 2.03 (+/-0.09) and a best-fitting absorption column
of 6.9 (+2.2, -2.0) x 10^20 cm^-2. The counts to observed (unabsorbed)
0.3-10 keV flux conversion factor deduced from this spectrum is 3.3 x
10^-11 (3.9 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.9 (+2.2, -2.0) x 10^20 cm^-2
Galactic foreground: 4.1 x 10^20 cm^-2
Excess significance: 2.3 sigma
Photon index: 2.03 (+/-0.09)

If the light curve continues to decay with a power-law decay index of
2.4, the count rate at T+24 hours will be 0.050 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.6 x
10^-12 (1.9 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 260924A (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.