GRB 251205A

General information

GRB 251205A was detected by Swift/BAT on 2025-12-05 23:39:47 UTC and rapidly followed up by Swift/XRT and Swift/UVOT, revealing a bright afterglow. Extensive ground-based observations detected the counterpart from optical to near-IR, and spectroscopy measured a redshift of z=1.100.

Wavelength coverage

• Gamma-rays: Swift/BAT detection and refined prompt analysis; Fermi/GBM targeted (sub-threshold) detection reported as spatially consistent.
• X-rays: Swift/XRT afterglow detections and refined spectral/temporal analysis.
• UV: Swift/UVOT detections in uvw1/uvw2 (and other UV/optical filters).
• Optical: Multiple facilities report detections in broad optical bands (e.g., r/i/z, GOTO L-band, survey photometry from ZTF/ATLAS).
• Near-IR: Detections reported in J and Ks bands (e.g., WINTER, FTW/3KK). No observations were mentioned in radio, sub-mm/mm, TeV, neutrinos, or gravitational waves.

Lightcurve and spectrum

• Prompt duration: BAT T90 (15–350 keV) = 128.00 ± 65.97 s; light curve shows a main pulse ending ~T+40 s with weak tail to ~T+140 s (GCN 43018).
• Prompt spectrum/fluence: BAT time-averaged spectrum fit by a power law with index 1.67 ± 0.27; fluence (15–150 keV) = (1.1 ± 0.2)×10^-6 erg cm^-2; 1-s peak photon flux (15–150 keV) = 0.5 ± 0.2 ph cm^-2 s^-1 (GCN 43018).
• X-ray afterglow: Late-time decay index alpha = 0.77 (+0.12, -0.11) (GCN 43017); XRT spectra fit with absorbed power law with photon index ~2.14 (WT) and ~1.95 (PC) (GCN 43017).
• Early X-ray flux: initial 0.2–10 keV flux reported as 1.44×10^-9 erg cm^-2 s^-1 in a 2.5 s image (GCN 43005).
• Optical/NIR photometry examples: UVOT white-band detections around 17–18 mag in early exposures (GCN 43016); ground-based detections around r16.8–18.3 mag within minutes–hours and fading to r/i21 mag by ~2.2 days (e.g., GCN 43019, GCN 43015, GCN 43038).

Redshift

• z = 1.100 measured from absorption features (Fe II, Mg II, Mg I) in GTC/OSIRIS+ spectroscopy, suggested as the GRB redshift (GCN 43008).

What’s special vs typical

• A secure spectroscopic redshift (z=1.100) was obtained within hours, and the afterglow was detected across a broad optical-to-NIR filter set with very dense follow-up coverage (GCN 43008 and numerous optical/NIR follow-ups).
• Fermi/GBM did not trigger onboard, but a targeted search reported a weak, temporally consistent signal with a hard spectral template and localization consistent with Swift (GCN 43059).