GRB 251220A

General information

GRB 251220A / EP251220a was reported as a ~170 s fast soft X-ray transient by Einstein Probe, with an uncatalogued X-ray source and an optical afterglow identified within the follow-up localization. Subsequent extensive optical follow-up and spectroscopy established the afterglow nature and a redshift measurement.

Wavelength coverage

• X-ray: Einstein Probe WXT detection with spectral fit and flux estimates; EP-FXT ToO follow-up detected an uncatalogued X-ray source and reported a power-law fit and flux.
• Optical: Multiple detections in g/r/i/z/y (COLIBRÍ) plus r/R-band detections from several facilities (e.g., GOTO, NOT, DOT, CrAO).
• Near-IR: J-band observations reported, with a non-detection limit (PRIME). No observations were mentioned in: radio, mm/sub-mm, UV, high-energy (GeV/TeV) gamma-ray.

Lightcurve and spectrum

• Duration: transient lasts ~170 s (Einstein Probe WXT).
• X-ray spectrum (EP-WXT, 0.5–4 keV): absorbed power law with photon index ~2.2 (+1.5/-1.1); unabsorbed average flux ~1.2 (+1.2/-0.4)×10^-9 erg s^-1 cm^-2; peak flux ~4.4×10^-9 erg s^-1 cm^-2.
• X-ray spectrum (EP-FXT, 0.5–10 keV): absorbed power law with photon index ~1.7 (+0.5/-0.5) (NH fixed to Galactic); unabsorbed flux ~3.6 (+2.4/-1.2)×10^-13 erg s^-1 cm^-2.
• Optical photometry examples: r=21.15±0.02 and z=20.72±0.04 at ~19.7–20.1 hr; r≈21.79–21.80 at ~35–40 hr; r=21.64±0.02 at ~1.36 d; R=22.55±0.20 at ~2.52 d; J>20.5 (3σ) at ~35 hr.
• Temporal decay: fading confirmed with temporal index ~-1.38±0.25 (COLIBRÍ).

Redshift

• Spectroscopic redshift: z = 2.375 from VLT/FORS2 absorption features (Lyα trough and metal lines such as Si II, C II, Al II).
• Photometric constraint: extinction-corrected SED fit yielded an upper limit z < 3.4 from grizy photometry.

What’s special vs typical

• A firm high-redshift (z=2.375) afterglow identification for an Einstein Probe-discovered fast soft X-ray transient, supported by many independent follow-up reports.
• Reported optical behavior includes modest change around ~35–40 hr in r-band in at least one dataset (noted as “little decay” relative to other optical reports).