GRB 251118A

General information

GRB 251118A is a long-duration gamma-ray burst detected by multiple high-energy missions, with a well-identified fading X-ray/UV/optical counterpart and extensive follow-up. Spectroscopy confirms a redshift of about z=1.217 for the burst/host system.

Wavelength coverage

• Gamma-ray: Detected by Fermi/GBM (T90 ~46.6 s; bright prompt emission) and Konus-Wind, with additional reports from GECAM-B, Glowbug, and NuSTAR shields; high-energy emission detected by Fermi-LAT.
• X-ray: Einstein Probe WXT/FXT reported the transient and an uncatalogued X-ray source; Swift/XRT detected an X-ray afterglow with a decaying light curve.
• UV/optical: Swift/UVOT detected a fading UV/optical source; numerous ground-based optical detections (e.g., NOT, GOTO, LCO, MASTER, DOT/DFOT, LOT, Xinglong).
• Near-IR: VLT/HAWK-I reported a J-band detection; FTW reported simultaneous r/i/J detections.
• Radio: VLA detected a 6 GHz counterpart (~74 μJy). No explicit observations were mentioned for TeV/VHE gamma-rays, sub-mm/mm, or polarization.

Lightcurve and spectrum

• Prompt duration: T90 ~46.6 s (Fermi/GBM, 50–300 keV) and T90 ~46.2 s (GECAM-B); Konus-Wind reports a soft precursor followed by a main episode, with total duration ~48 s.
• Prompt fluence/flux: Konus-Wind fluence 1.07×10^-4 erg/cm^2 (20 keV–10 MeV) and 64-ms peak flux 2.03×10^-5 erg/cm^2/s; Fermi/GBM fluence (10–1000 keV) ≈1.03×10^-4 erg/cm^2.
• Prompt spectral shape: Konus-Wind main episode fit with a Band model (alpha≈-1.29, beta≈-2.56, Ep≈154 keV); Fermi/GBM reports Comptonized/Band fits with Epeak ≈149 keV and alpha ≈-1.40 (Band fit also gives beta ≈-2.56).
• High-energy: Fermi-LAT reports emission above 100 MeV and a highest-energy photon of 6.8 GeV ~1400 s after trigger.
• X-ray decay/spectrum: Swift/XRT light curve decay index alpha≈1.08; absorbed power-law spectrum with photon index ≈1.73 and excess absorption.
• Soft X-ray spectrum (EP/WXT): absorbed power-law with photon index ≈1.32 and unabsorbed average 0.5–4.0 keV flux ≈2.5×10^-8 erg/s/cm^2 (peak ~1×10^-7).

Redshift

A spectroscopic redshift of z=1.216 was reported from NOT afterglow absorption features, and VLT/MUSE spectroscopy confirms z=1.217 with multiple absorption lines and [O II] emission at z=1.218.

What’s special vs typical

The combination of very bright prompt emission (fluence ~10^-4 erg/cm^2) and confirmed GeV (LAT) detection, together with a secure spectroscopic redshift confirmed by VLT/MUSE, makes this a particularly well-characterized bright long GRB.