GRB 260818B

General information

GRB 260818B was triggered by SVOM/ECLAIRs on 2026-08-18T20:11:12 UTC and localized to RA=44.9407 deg, Dec=7.4547 deg with a 90% error radius of 4.60 arcmin. It was classified as a short GRB, independently detected by SVOM/GRM and Insight-HXMT/HE, with extensive follow-up but no firmly confirmed counterpart.

Wavelength coverage

• Gamma-ray: SVOM/ECLAIRs, SVOM/GRM, and Insight-HXMT/HE detected the burst; ECLAIRs measured T90=0.22 s for the main pulse, while GRM measured T90=0.16 s.
• X-ray: EP-FXT observed at about 1.84 hr and again at 9.5 d, finding several uncataloged sources initially but no fading X-ray source within the 90% ECLAIRs localization region.
• Optical: LCO, COLIBRÍ, NOT, SVOM/VT, and C-GFT/LATIOS obtained observations; early observations mostly gave nondetections, while later COLIBRÍ subtraction found a possible optical transient inside the ECLAIRs region.
• Near-infrared: TNG/NICS and VLT/HAWKI observed the COLIBRÍ candidate position; TNG did not detect it in J, and VLT also reported no J-band detection.
• Radio: VLA observed at 6 GHz over multiple epochs, detecting multiple radio sources in the ECLAIRs region, including S10 close to the COLIBRÍ candidate, but without a secure radio-counterpart identification. Bands with no observations mentioned: gravitational-wave and neutrino.

Lightcurve and spectrum

• SVOM/ECLAIRs initially reported a single-peaked light curve with duration about 1 s.
• Refined SVOM/ECLAIRs analysis gave a main FRED-like pulse with T90(4-120 keV)=0.22 +0.07/-0.02 s.
• SVOM/ECLAIRs reported possible extended emission in 4-20 keV with T90=0.71 +0.40/-0.39 s.
• The ECLAIRs main-pulse spectrum from T0 to T0+0.22 s was fit by a cutoff power law with photon index -0.61 +/- 0.14 and Epeak=56 +16/-12 keV.
• The possible ECLAIRs extended-emission spectrum from T0+0.22 to T0+0.71 s was fit by a blackbody with kT=1.6 +0.4/-0.3 keV.
• SVOM/GRM fit the time-averaged spectrum from T0-0.08 to T0+0.12 s with a cutoff power law, index -1.00 +0.22/-0.23 and Epeak=70 +7/-6 keV.
• COLIBRÍ measured the possible optical transient at r=24.24±0.15 AB and z>23.81 AB at 13.20 hr, then r=24.60±0.29 AB and z=22.84±0.15 AB at 157.38 hr.
• VLT reported a faint z-band source at about 24.7 AB mag and fading by at least about 2 mag between 6.5 d and 9.5 d, corresponding to a power-law decay slope of about -4.6.

Redshift

What’s special vs typical

• This is a short GRB with possible soft extended emission, which makes it more notable than a routine single-instrument short-burst alert.
• The possible optical transient is intriguing because it is close to VLA source S10 and near EPF_J025955.0+072535, shows late red behavior, and appears to fade strongly by the VLT epoch.
• The main caveat is that the association remains unconfirmed: there is possible host/template emission at the candidate position, no variable X-ray source at the candidate, and the radio field contains multiple persistent or catalogued sources.