GRB 251221A

General information (1–2 sentences, single paragraph, no bullets)

GRB 251221A is a long-duration gamma-ray burst detected by Swift/BAT and Fermi/GBM, with a fading Swift/XRT afterglow and reported optical/NIR counterpart detections. Multiple facilities reported follow-up photometry and upper limits, in a field with substantial foreground extinction.

Wavelength coverage (bullets by band + final sentence listing bands with no observations mentioned)

• Gamma-ray: Swift-BAT prompt emission and refined analysis; Fermi-GBM prompt detection and spectral fits.
• X-ray: Swift-XRT afterglow light curve and spectrum; (separately reported) Einstein Probe/WXT fast X-ray transient EP251221a at a different position.
• UV: Swift/UVOT upper limits in multiple filters.
• Optical: COLIBRÍ detection in z (and r nondetection); NOT and Liverpool Telescope upper limits; 7DT and MASTER-Net upper limits.
• Near-IR: PRIME detection in J (and later non-detection/upper limit); PRIME H mentioned for observations. No observations were mentioned in radio or millimeter/submillimeter.

Lightcurve and spectrum (ONLY items explicitly reported: T90/duration, flux/fluence, spectral model+params, temporal decay; if nothing, leave empty)

Swift-BAT reports T90(15–350 keV) = 56.86 ± 7.05 s and a time-averaged power-law index of 1.25 ± 0.13, with 15–150 keV fluence (2.0 ± 0.2)×10^-6 erg cm^-2 and 1-s peak photon flux (15–150 keV) 1.1 ± 0.2 ph cm^-2 s^-1 (GCN 43234). Fermi/GBM reports T90 ~ 28 s (50–300 keV) and a cutoff power-law with index −1.1 ± 0.1 and E_peak = 330 ± 100 keV (or Band fit with E_peak = 240 ± 130 keV, alpha = −1.0 ± 0.2, beta = −1.8 ± 0.2), with fluence (10–1000 keV) (4.2 ± 0.5)×10^-6 erg cm^-2 (GCN 43174). Swift-XRT reports a decay index alpha = 1.13 (+0.17/−0.14) and an absorbed power-law spectrum with photon index 1.49 (+0.19/−0.18), total N_H = 1.11 (+0.24/−0.22)×10^22 cm^-2, and an estimated 0.3–10 keV flux at T+24 hr of 1.3×10^-13 erg cm^-2 s^-1 observed (1.8×10^-13 unabsorbed) (GCN 43179).

Redshift (1–3 bullets; only if redshift mentioned; else leave empty)

What’s special vs typical (ONLY if there is at least one clearly special/atypical aspect; do not mention absence of non-EM; if nothing special, leave empty)

The field shows very high reported foreground extinction (E(B−V)=1.088; A_V≈3.1 mag), consistent with an apparent optical color drop (z detection with r non-detection) despite rapid early imaging (GCN 43177, GCN 43168).