GRB 260321A

General information

GRB 260321A was triggered and localized by SVOM/ECLAIRs at 2026-03-21 18:12:08 UTC, and was also reported as a sub-threshold event in Fermi/GBM targeted searches. Subsequent reports include refined ECLAIRs spectral analysis, optical upper limits, and EP-FXT X-ray follow-up with multiple uncataloged sources in the ECLAIRs error region.

Wavelength coverage

• Gamma-ray / hard X-ray: SVOM/ECLAIRs trigger/localization and refined spectrum/fluence; SVOM/GRM trigger and T90; Fermi/GBM targeted-search sub-threshold candidate.
• X-ray: Einstein Probe EP-FXT follow-up detects four uncataloged 0.5–10 keV sources within the ECLAIRs error circle; SVOM/MXT reported no X-ray observation at the time of the initial alert.
• Optical: SVOM/VT upper limits in VT_B and VT_R at ~1.1 hr; SVOM/COLIBRÍ (FM-GFT) upper limits in r and z (including deeper limits tied to EP-FXT source positions). Bands with no observations mentioned: radio, UV, IR (beyond VT_R), GeV/TeV, neutrinos, gravitational waves.

Lightcurve and spectrum

Redshift

A redshift is not measured; SVOM/ECLAIRs notes that the burst would be compatible with Type-II GRBs in an Amati-relation context if z > 3, and mentions the lack of an optical afterglow as potentially favoring a high-redshift origin.

What’s special vs typical

The combination of deep early optical non-detections and the explicit suggestion of a potentially high redshift (z > 3, not measured) makes this event more “dark/high-z candidate”-like than typical GRBs, though no counterpart confirmation is reported.