GRB 251208B

General information

GRB 251208B triggered Fermi/GBM at 2025-12-08 10:17:17.29 UTC and was detected by Fermi-LAT and Swift/BAT-GUANO, with Swift-XRT follow-up reporting a likely X-ray afterglow candidate. Prompt emission properties and multiple follow-up upper limits are reported in the cited circulars.

Wavelength coverage

• Gamma-rays (keV–MeV): Fermi/GBM reports T90 ~62.5 s and time-averaged spectral fit parameters; Swift/BAT-GUANO recovered event-mode data and provided an arcminute localization (GCN 43028, 43033, 43046).
• Gamma-rays (GeV): Fermi-LAT reports significant >100 MeV emission, a >100 MeV photon flux for 0–1 ks, and a highest-energy photon of 17 GeV at 419 s (GCN 43034).
• X-rays: Swift-XRT detects an uncatalogued source within the LAT region, interpreted as the afterglow candidate, with spectral fit parameters reported (GCN 43050).
• Optical: GIT and COLIBRÍ/DDRAGO report non-detections/upper limits in r/i/z bands at ~13–27 hr post-trigger (GCN 43045, 43057). No observations were mentioned in the provided circulars for radio, mm/sub-mm, infrared, ultraviolet, or neutrinos/GWs.

Lightcurve and spectrum

• Duration: GBM T90 ≈ 62.5 s (50–300 keV) (GCN 43046).
• Prompt spectrum (GBM): cutoff power law with index −0.67 ± 0.08 and Epeak = 250 ± 20 keV (time interval T0−4.1 to T0+61.4 s) (GCN 43046).
• Prompt fluence (GBM): (9.5 ± 0.5)×10−6 erg cm−2 (10–1000 keV) (GCN 43046).
• 1-s peak photon flux (GBM): 2 ± 0.2 ph s−1 cm−2 (10–1000 keV), starting at T0+55 s (GCN 43046).
• LAT >100 MeV: photon flux (0–1 ks) = (3.13 ± 0.09)×10−6 ph cm−2 s−1; photon index = −2.01 ± 0.23; highest-energy photon 17 GeV at 419 s (GCN 43034).
• XRT light curve: mean count rate 5.0×10−2 ct s−1; described as consistent with constant; a power-law fit gives temporal index 1.2 ± 1.2 (GCN 43050).
• XRT spectrum: absorbed power law with photon index 2.2 (+0.4, −0.3) and total NH = 1.4 (+1.1, −0.8)×10^21 cm−2 (Galactic 2.6×10^20 cm−2) (GCN 43050).

Redshift

What’s special vs typical

• Strong high-energy (Fermi-LAT) behavior is explicitly reported, including a highest-energy photon of 17 GeV and a measured >100 MeV flux over 0–1 ks (GCN 43034).
• Swift/BAT did not localize onboard but GUANO recovered BAT event-mode data and NITRATES provided an arcminute localization with sqrt(TS)=17.6 (GCN 43033).