GRB 251202A

General information

GRB 251202A is a long gamma-ray burst detected by Fermi/GBM and followed by Einstein Probe, Swift, SVOM, AstroSat, Insight-HXMT and numerous ground-based facilities. A fading X-ray/optical counterpart was identified, and subsequent radio observations confirmed an afterglow.

Wavelength coverage

• Gamma-ray: Fermi/GBM prompt emission with spectral fit and fluence reported; detections/analyses also from SVOM/GRM, Insight-HXMT/HE, and AstroSat/CZTI.
• X-ray: Einstein Probe/WXT reports a ~400 s soft X-ray transient; Swift-XRT detects an uncatalogued X-ray afterglow with a power-law decay; EP/FXT reports an X-ray source in follow-up.
• UV/optical: Multiple detections including early bright optical (LCO) and UV detection (Swift/UVOT), plus multi-epoch optical photometry (e.g., SVOM/VT, KAIT, Liverpool Telescope, GOTO, LCO).
• Radio: VLA detections at 6, 10, and 15 GHz at ~3.45, 4.45, and 9.43 days post-burst. No observations were mentioned in the circulars provided for near-IR, mm/sub-mm, GeV/TeV, or polarization bands.

Lightcurve and spectrum

• Prompt duration: T90 ≈ 43 s (50–300 keV; Fermi/GBM); T90 = 16 +8/−4 s (15–5000 keV; SVOM/GRM); T90 = 25 +12/−11 s (Insight-HXMT/HE); T90 = 9.4 (+0.5, −1.3) s (20–200 keV; AstroSat/CZTI).
• Prompt spectral model (Fermi/GBM): cutoff power law with index −1.07 ± 0.06 and Epeak = 350 ± 50 keV; fluence (10–1000 keV) = (9.9 ± 0.6)×10−6 erg cm−2; 1 s peak photon flux (10–1000 keV) = 4.2 ± 0.2 ph s−1 cm−2.
• Soft X-ray transient (Einstein Probe/WXT): duration ∼400 s with three peaks; average 0.5–4 keV absorbed power-law photon index 1.23 (±0.19) and unabsorbed flux 5.40×10−10 erg s−1 cm−2.
• X-ray afterglow (Swift-XRT): absorbed power-law photon index 1.93 (+0.45, −0.22) and temporal decay index α = 1.08 (+0.33, −0.24).
• Optical temporal decay (KAIT): brightness from 18.7 ± 0.1 mag to 19.3 ± 0.2 mag between 6.30–10.55 hr with a power-law decay index of 1.07.
• Optical photometry examples: r ≈ 15.6 (calibrated to r; ~18 min; LCO discovery), r = 17.27 ± 0.01 at 1.05 hr (NOT).
• Radio detections (VLA): at 3.45 d, peak flux densities of 111±10 µJy (6 GHz), 169±7 µJy (10 GHz), 310±8 µJy (15 GHz); detections persisted at 4.45 d and 9.43 d with similar or higher fluxes.

Redshift

A spectroscopic redshift of z = 2.785 was measured with NOT/ALFOSC from an optical spectrum showing a break near 4610 Å and Lyα forest, with identified Lyα and C IV features at the burst redshift.

What’s special vs typical

• Firm spectroscopic redshift at z = 2.785 (high-redshift long GRB) reported very early, enabling physical interpretation beyond a purely photometric afterglow.
• Very bright early optical afterglow (∼15.6 mag calibrated to r at ~18 minutes) followed by extensive multi-facility coverage, including confirmed multi-epoch VLA radio detections.