GCN Circular 45775
GRB 260924A: Swift/UVOT UV grism spectrum results
From: Paul Kuin at MSSL
Issued: 2026-09-28 13:34 UTC

N. P. M. Kuin (UCL-MSSL), L. Izzo (INAF/OACN and DARK/NBI), D. B.
Malesani (DAWN/NBI and Radboud Univ.), Noel Klingler (NASA) and
V. D'Elia (SSDC and INAF-OAR) report on behalf of the Swift/UVOT
team:

Swift UVOT observed the bright GRB 260924A (D'Elia et al., GCN Circ.
45713) with an exposure time of 50 s in the UV grism, starting 258 s
after the trigger.

The data rate was large enough to cause the data buffers to overflow,
causing a drop at wavelengths longer than 4500 AA. The inherent
accuracy of the wavelength scale is limited by a 3 pixel uncertainty
and is typically 20 AA in the UV, but closer to 40 AA at 4000 AA.

The spectrum shows clear detection of continuum at wavelengths redder
than 4110 AA, but no flux at all to the blue. This is consistent with
a drop-out due to the Lyman limit at a redshift of z = 3.51 +- 0.04.
The unreliability of the spectrum above 4500 AA does not allow to look
for further features to the red. Our findings are thus overall fully
consistent with the higher-resolution spectroscopic measurement
z = 3.51 provided by Castro-Tirado (GCN Circ. 45740).

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.