General information
GRB 260818A was detected by SVOM/ECLAIRs at 2026-08-18T01:04:36 UTC as a possible long, faint GRB with image-trigger SNR 6.86 and an initial 90% error radius of 11.41 arcmin. EP-FXT later confirmed a fading X-ray afterglow, EPF_J232025.8-320549, at RA 350.1064 deg, Dec -32.0978 deg, and ATCA reported a candidate radio counterpart consistent with that position.
Wavelength coverage
- Gamma-ray / hard X-ray: SVOM/ECLAIRs detected the event in 8–120 keV over an 81.92 s image-trigger window, with refined analysis in 5–120 keV and 4–120 keV spectral/fluence reporting.
- X-ray: EP-FXT observed at 1.67 hr, 0.76 d, and 2.75 d after trigger; EPF_J232025.8-320549 faded and was confirmed as the X-ray afterglow.
- Optical: SVOM/COLIBRÍ and LCO reported no new source in r/z bands, with limits r > 22.3, z > 20.4 and r > 21.8 AB, z > 20.5 AB.
- Near-infrared: WINTER J-band imaging reported no counterparts at the remaining relevant FXT source positions, with J > 19.14 AB.
- Radio: ATCA detected a >~7 sigma source at both 6.53 and 10.36 GHz, consistent with the confirmed fading X-ray afterglow and described as a candidate radio counterpart.
Bands with no observations mentioned: UV, millimeter/submillimeter, GeV/TeV.
Lightcurve and spectrum
- ECLAIRs refined analysis describes a featureless light curve detected from T0-82 s to T0 in the 8–120 keV band.
- The time-averaged 5–120 keV spectrum from T0-82 s to T0 is well fitted by a blackbody model with temperature 7(+2/-1) keV.
- The reported 4–120 keV photon flux is 0.26(+0.06/-0.05) photons/cm^2/s.
- The reported 4–120 keV fluence is (7 +/- 2)e-7 erg/cm^2.
- EPF_J232025.8-320549 faded from (13.1 +/- 2.6)e-14 to (6.7 +/- 1.9)e-14 erg/s/cm^2 in 0.3–10 keV between the first and second FXT epochs, and was not detected in the third FXT observation.
Redshift
A redshift z=0.1898 is reported from the GAMA survey for EPF_J231953.3-320838, which WINTER notes is classified as a quasar in Gaia. The circulars do not report this as the redshift of the confirmed fading afterglow EPF_J232025.8-320549.
What’s special vs typical
- The burst is described as long and faint, with an ECLAIRs detection below the slew threshold.
- The refined prompt spectrum is reported as a blackbody with kT about 7 keV.
- Despite optical/NIR nondetections, the event has a confirmed fading X-ray afterglow and a later candidate radio counterpart.