General information
GRB 251209A is a likely long GRB detected by Fermi/GBM on 2025-12-09 13:24:03 UT, with prompt emission reported by several high-energy missions and a GBM on-ground localization at RA 214.2°, Dec 43.1° (J2000). The prompt light curve shows multiple emission episodes/pulses with reported T90 values of order ~33–57 s depending on instrument and band.
Wavelength coverage
- Gamma-rays / hard X-rays: Detected by Fermi/GBM (localization, Band fit, fluence, peak flux), GECAM-B (70–6000 keV light curves), Insight-HXMT/HE (300–3000 keV light curve and peak rate), AstroSat/CZTI (20–200 keV and 100–500 keV veto), and NuSTAR CsI shield rates (SINGS prompt-emission detection).
- Optical: MASTER-Net reported only upper limits (clear filter) to ~15.0–15.1 mag at ~4.9 ks after T0.
No observations were mentioned here for: radio, infrared, ultraviolet, GeV (Fermi/LAT), TeV, neutrinos, or gravitational waves.
Lightcurve and spectrum
- Duration: Fermi/GBM T90 ~57 s (50–300 keV); GECAM-B T90 = 40.5 ± 1.7 s (70–6000 keV); Insight-HXMT/HE T90 = 43.0 (+13.5/-10.0) s; AstroSat/CZTI T90 = 38 (+4/-3) s (20–200 keV) and 33 (+5/-3) s (100–500 keV veto).
- Structure: Multiple pulses/structured emission episodes; NuSTAR notes two ~30-s peaks and a possible fainter, narrower initial burst near 13:24:03 UT.
- Spectrum (prompt, Fermi/GBM): Band function with Epeak = 105 ± 4 keV, alpha = -1.63 ± 0.01, beta = -2.37 ± 0.06 (time-averaged T0-4 s to T0+81 s).
- Fluence/flux (Fermi/GBM): Fluence (10–1000 keV) = (6.89 ± 0.05)×10^-5 erg cm^-2; 1-s peak photon flux (10–1000 keV) = 38.4 ± 0.6 ph s^-1 cm^-2 (starting T0+49.7 s).
Redshift
What’s special vs typical
- Unusually broad prompt high-energy coverage including a blind-search detection in NuSTAR’s anti-coincidence shield rates (SINGS), alongside multiple dedicated GRB monitors reporting consistent long-duration behavior.