GRB 260809A

General information

GRB 260809A was detected by SVOM/ECLAIRs at 2026-08-09T14:57:33 UTC as a faint gamma-ray burst, followed promptly by SVOM/MXT X-ray and SVOM/VT optical counterpart detections. Subsequent circulars report extensive optical follow-up, an Einstein Probe FXT X-ray detection, and a Fermi/GBM sub-threshold GRB-like signal consistent with the SVOM localization.

Wavelength coverage

• Gamma-ray: SVOM/ECLAIRs triggered with image-trigger SNR 7.20 in 8–120 keV, and refined analysis reports emission from T0-24 s to T0+19 s with 4–120 keV fluence (1.4 +/- 0.3)e-6 erg/cm^2; Fermi/GBM found a sub-threshold targeted-search candidate spatially consistent with ECLAIRs.
• X-ray: SVOM/MXT found an uncatalogued X-ray source 170 s after trigger, and EP-FXT detected a consistent X-ray source with 0.5–10 keV flux about 4.5 × 10^-12 erg s^-1 cm^-2.
• Optical: SVOM/VT detected the counterpart at VT_B = 21.0 +/- 0.2 and VT_R = 18.8 +/- 0.1 at 263 s, with later detections by OHP/T193, MAO/AZT-22, NOT, COLIBRÍ, and SAO RAS in R/r/i/z/Rc bands.
• Near-infrared: OHP/T193 did not detect the source in Y to a preliminary 3-sigma limit of Y = 21.2, and WINTER did not detect it in J to a 5-sigma upper limit of J = 18.6 AB. No radio, millimeter, UV, neutrino, or gravitational-wave observations are mentioned.

Lightcurve and spectrum

• SVOM/ECLAIRs refined analysis reports a multiple-peak light curve detected from T0-24 s to T0+19 s in 4–120 keV.
• The ECLAIRs time-averaged 5–120 keV spectrum is fit by a power law with photon index -1.3 +/- 0.2.
• ECLAIRs reports a 4–120 keV photon flux of 0.76 +0.06/-0.20 photons/cm^2/s and fluence of (1.4 +/- 0.3)e-6 erg/cm^2.
• SVOM/VT reports a slow brightening trend in both optical channels during the first 600 s, followed by fading with temporal decay slope -1.2 since 1.02 hr.
• Optical measurements show continued fading from early VT_R = 18.8 +/- 0.1 at 263 s to r/R/Rc/z-band detections around 22-24 mag over the following hours.

Redshift

What’s special vs typical

• The burst is notable for being faint in SVOM/ECLAIRs and only sub-threshold in Fermi/GBM, while still having a secure X-ray/optical afterglow and extensive follow-up coverage.