General information
GRB 260131A triggered Fermi/GBM at 2026-01-31 06:39:30 UT and was also reported by Konus-Wind, Insight-HXMT/HE, and GRID, with discussion that MAXI GRB 260131B likely refers to the same event. An optical/X-ray/IR fading counterpart (AT2026bwg) was reported at RA=24.7137 deg, Dec=34.3284 deg.
Wavelength coverage
- Gamma-ray: Fermi/GBM prompt emission with Band spectral fit and reported fluence/peak flux; Konus-Wind prompt emission with Band fits and fluence/peak flux; Insight-HXMT/HE detection with reported T90 and counts; GRID detection with reported T90.
- X-ray: Einstein Probe/FXT detected an uncatalogued X-ray source consistent with AT2026bwg and reported an absorbed power-law fit and unabsorbed 0.5–10 keV flux.
- Optical: Afterglow detections and fading reported by GOTO, COLIBRÍ/DDRAGO, ZTF (including pre-burst limit), NOT, FTW/3KK; SEDM provided multi-band photometry.
- Infrared: SYSU 80cm detected the counterpart in J band; FTW/3KK reported detection including J.
Bands with no observations mentioned: radio, sub-mm/mm, UV, very-high-energy (TeV), gravitational waves, neutrinos.
Lightcurve and spectrum
- Fermi/GBM: two separated FRED-like emission episodes; T90 ~257 s (50–300 keV); time-averaged Band fit (Epeak=148±39 keV, alpha=-0.90±0.16, beta=-1.76±0.06); fluence (2.82±0.09)×10^-5 erg/cm^2 (10–1000 keV); 1-s peak photon flux 8.3±0.4 ph/s/cm^2.
- Konus-Wind: two separated pulses (near T0 and T0+215 s), total duration ~250 s; fluence (4.14±0.91)×10^-5 erg/cm^2 (20 keV–10 MeV); 64-ms peak energy flux (4.33±0.52)×10^-6 erg/cm^2/s; time-integrated and peak Band spectral parameters reported.
- Insight-HXMT/HE: multiple pulses; T90=7.0 (+2.5/-1.0) s (instrument band); 1-s peak rate 982 counts/s; total 5366 counts.
- GRID: T90 = 10.5 ± 1.4 s (30–2000 keV) for the first trigger.
- Optical/IR: GOTO L-band fading (17.41±0.03 to 17.59±0.04 over ~1.1 h); COLIBRÍ r and z fading with temporal indices -2.01±0.26 (r) and -1.50±0.33 (z); ZTF r-band fading between ~21.4 h and ~45.0 h; SYSU J-band ~16.9–17.1 (Vega) at ~32 h; NOT r=18.98±0.02 at ~37.1 h.
- X-ray: EP/FXT spectrum fit with photon index ~2.01 and unabsorbed 0.5–10 keV flux ~3.35×10^-12 erg/s/cm^2; intrinsic NH upper limit reported at z=1.434.
Redshift
NOT/ALFOSC spectroscopy reports absorption features consistent with z=1.434 (also noting continuum down to 3600 Å implying z<1.95), and GTC/OSIRIS spectroscopy finds a consistent absorption system at z=1.435.
What’s special vs typical
A secure spectroscopic redshift (confirmed by two facilities) together with a well-sampled fading afterglow across optical/IR/X-ray makes this event better characterized than many routine GRB alerts. The circular facts also include a notable coordinate discrepancy for an SVOM/VT-reported optical counterpart relative to the AT2026bwg afterglow position.