General information
GRB 251222B is a long-duration gamma-ray burst temporally and spatially consistent with the EP soft X-ray transient EP251222b, with subsequent EP/FXT detection of an uncatalogued X-ray source at the burst position.
Wavelength coverage
- Gamma-rays / hard X-rays: SVOM/GRM reports multiple pulses with T90 = 85.5 s and a cutoff power-law spectrum (GCN 43202).
- X-rays (soft): Einstein Probe WXT detected EP251222b; Einstein Probe FXT later detected an uncatalogued X-ray source consistent with the event position (GCN 43201, 43215).
No observations are mentioned in the provided circular text for: UV/optical/NIR, radio.
Lightcurve and spectrum
- SVOM/GRM duration: T90 = 85.5 +3.0/-3.5 s (15–5000 keV) (GCN 43202).
- SVOM/GRM time-averaged prompt spectrum (T0-20 to T0+100 s): cutoff power law, index = -1.49 ± 0.12, Epeak = 221 (+233/-60) keV; fluence (10–1000 keV) = (1.37 +0.17/-0.14)×10^-5 erg cm^-2 (GCN 43202).
- SVOM/GRM 1 s peak (T0+53 to T0+54 s): cutoff power law, index = -0.95 ± 0.12, Epeak = 209 (+40/-28) keV; flux (10–1000 keV) = (1.05 +0.09/-0.08)×10^-6 erg cm^-2 s^-1 (GCN 43202).
- EP/WXT refined analysis: soft X-ray activity starting 2025-12-22T17:04:52 UTC, lasting ~350 s; peak flux 1.4×10^-8 erg s^-1 cm^-2 (0.5–4 keV); absorbed power law with photon index 1.1 ± 0.15; average unabsorbed flux (0.5–4 keV) 2.8 (-0.2/+0.3)×10^-9 erg s^-1 cm^-2 (GCN 43215).
- EP/FXT follow-up at ~12.11 h after T0: absorbed power law with photon index 1.86 (-0.25/+0.26); average unabsorbed flux (0.5–10 keV) 1.8 (-0.3/+0.4)×10^-12 erg s^-1 cm^-2 (GCN 43215).
Redshift
What’s special vs typical
- The event was discovered as an EP/WXT fast X-ray transient and has both wide-field soft X-ray detection and pointed EP/FXT X-ray follow-up, providing unusually rich soft X-ray coverage reported directly in the circulars (GCN 43201, 43215).