GRB 251230A

General information

GRB 251230A (also associated with EP251230a) is a long-duration GRB detected by Swift/BAT and Einstein Probe, with a bright X-ray and optical/UV counterpart reported by multiple facilities. Extensive follow-up spans gamma-ray through optical/NIR, including a tentative redshift measurement from Swift/UVOT grism.

Wavelength coverage

• Gamma-ray / hard X-ray: Swift-BAT, Konus-Wind, Insight-HXMT/HE, GECAM-B reported detections and timing/spectral properties.
• X-ray: Swift-XRT follow-up with a multi-segment power-law decay; Einstein Probe WXT/FXT provided soft X-ray detection and refined localization/flux.
• UV: Swift/UVOT detections in white/v/b/u/uvw1 and UV grism spectroscopy.
• Optical: Multiple facilities reported detections, limits, and decay behavior (e.g., TAROT, SVOM/VT, NOT, LCO, MASTER, Virtual Telescope, 7DT, GOTO, DFOT, Mephisto, Mondy).
• NIR: FTW/3KK reported a J-band detection (alongside r and i). No observations are mentioned for radio, mm/sub-mm, or GeV–TeV gamma-ray bands.

Lightcurve and spectrum

• Duration: BAT duration ~80 s (Swift); WXT duration ~72 s (EP); T90 ~71.2 s (GECAM-B) and ~54.4 s (Insight-HXMT/HE); Konus-Wind total duration ~73 s with multi-peaked structure starting ~T0-25 s.
• XRT temporal behavior: initial steep decay alpha=7.1, flattening to alpha~0.2 at T+129 s, then breaking to alpha=1.51 at T+194 s (Swift-XRT).
• X-ray spectrum: absorbed power law with photon index ~1.80–1.82 (Swift-XRT); EP-FXT photon index ~1.83; EP-WXT photon index ~2.16 (0.5–4 keV).
• Flux/fluence: Konus-Wind fluence 2.03e-5 erg/cm^2 (20 keV–10 MeV) and 64-ms peak flux 2.51e-6 erg/cm^2/s; EP-FXT unabsorbed 0.5–10 keV flux 4.02e-11 erg/s/cm^2; EP-WXT unabsorbed 0.5–4 keV flux 6.56e-9 erg/s/cm^2.
• Prompt spectral shape (Konus-Wind): cutoff power law with alpha ~ -1.04 and Ep ~ 354 keV (time-averaged), and alpha ~ -0.70 with Ep ~ 350 keV near the peak; Band-model fits give similar alpha/Ep with beta limits.
• Optical evolution: very bright early optical (e.g., TAROT r~13.1 at ~36 s; UVOT white ~13.6 at ~76–226 s) followed by rapid decay (NOT suggests optical decay index ~1.95) and later behavior indicating a shallower decay or possible rebrightening (NOT i-band at ~1.15 d).

Redshift

Swift/UVOT UV grism reports a tentative redshift z=1.73 (with noted contamination), while NOT spectroscopy provides a redshift upper limit z < 2.2 based on blue continuum detection down to ~3900 Å and lack of hydrogen absorption.

What’s special vs typical

• Exceptionally bright early optical/UV emission compared to many long GRBs (mag ~13–14 within minutes).
• A tentative UVOT-grism redshift (z≈1.73) reported very early, plus an independent spectroscopic upper limit from NOT.
• Optical decay behavior may deviate from a simple monotonic fade (shallower decay/possible rebrightening reported at ~1 day).