GRB 260120B

General information

GRB 260120B is a long-duration gamma-ray burst detected by multiple high-energy instruments, with a localized afterglow identified in X-rays and an early optical/UV counterpart reported. The event shows a single-pulse prompt light curve and a rapidly fading optical transient.

Wavelength coverage

• Gamma-ray: Fermi/GBM (prompt detection and spectral fits), Swift/BAT (trigger and refined analysis), Konus-Wind (fluence/peak flux and CPL fit), SVOM/GRM (T90 and CPL fit), AstroSat/CZTI (20–200 keV detection and T90).
• X-ray: Swift/XRT afterglow detection and refined temporal/spectral analysis.
• Optical/UV: SVOM/C-GFT reports an uncatalogued optical counterpart with rapid fading; Swift/UVOT detects a fading source in early exposures; COLIBRÍ and MASTER-Net report optical upper limits at later times. No observations are mentioned in the cited circulars for radio, infrared, sub-mm/mm, very-high-energy (TeV), or polarization.

Lightcurve and spectrum

• Prompt duration: single pulse; total duration ~5.9 s in Konus-Wind, T90 = 4.48 ± 0.94 s (Swift/BAT, 15–350 keV), T90 ~7 s (Fermi/GBM, 50–300 keV), T90 = 8.5 (+5.0/-3.5) s (SVOM/GRM, 15–5000 keV), T90 ~5.0 ± 1.5 s (AstroSat/CZTI, 20–200 keV).
• Prompt energetics/spectrum: Konus-Wind fluence 3.41(-0.52,+0.58)×10^-6 erg cm^-2 (20 keV–10 MeV) and 64-ms peak flux 1.97(-0.58,+0.60)×10^-6 erg cm^-2 s^-1; Konus-Wind CPL fit with alpha = -0.56(-0.23,+0.30), Ep = 246(-53,+67) keV. Fermi/GBM reports fluence (3.1 ± 0.1)×10^-6 erg cm^-2 (10–1000 keV) and CPL index -0.19 ± 0.01 with Epeak 214 ± 4 keV (Band fit also reported). Swift/BAT refined CPL fit gives Epeak 103.9 ± 38.6 keV with 15–150 keV fluence 1.2 ± 0.1×10^-6 erg cm^-2.
• X-ray afterglow: Swift/XRT light curve modeled as a power-law decay with index alpha = 0.84 ± 0.06; PC-mode spectrum fits an absorbed power law with photon index 1.97 (+0.29, -0.27) and total column 2.1 (+1.2, -1.0)×10^21 cm^-2.
• Optical/UV behavior: SVOM/C-GFT reports i=15.74±0.04 at ~T0+68 s and a fade by ~3.4 mag in i over ~1000 s; Swift/UVOT detects white=18.38±0.06 (149–299 s) and u=18.80±0.14 (307–557 s), followed by upper limits in later exposures; COLIBRÍ reports r>22.87 and z>22.12 at ~8.0–8.3 hr post-trigger.

Redshift

What’s special vs typical

The optical counterpart is reported very early and is initially bright, then fades extremely rapidly (multi-magnitude decline within ~1000 s), standing out relative to more slowly fading afterglows.