GRB 260516D

General information

GRB 260516D triggered SVOM/ECLAIRs at 2026-05-16T20:39:50 UTC, with subsequent identification of a fading counterpart in X-rays and optical, and later radio detection.

Wavelength coverage

• X-ray/soft gamma (SVOM/ECLAIRs): Detected via imaging trigger; multiple-peak light curve with most emission below 50 keV; duration ~152 s (4–20 keV) and power-law spectrum in 5–50 keV.
• X-ray (Einstein Probe/FXT): Uncatalogued fading 0.3–10 keV source localized to ~10 arcsec and spectrally fit by an absorbed power law.
• Optical (MASTER-Tavrida, FRAM-ORM): Early non-detections/upper limits (clear/unfiltered) starting ~0.95–0.47 hr post-trigger.
• Optical (SVOM/VT, GROWTH-India, GOTO-N, GTC/OSIRIS imaging, SVOM/COLIBRÍ, LCO 1m): Detection of an uncatalogued/fading optical source with multi-epoch photometry (including r/z).
• Radio (VLA): Detection at X-band (~10 GHz) at ~5.6 days post-trigger. No observations were mentioned in UV, IR, mm/sub-mm, or TeV/GeV bands.

Lightcurve and spectrum

• SVOM/ECLAIRs reports a multiple-peak prompt light curve with most emission below 50 keV and duration ~152 s (4–20 keV).
• SVOM/ECLAIRs time-averaged prompt spectrum (T0−78 s to T0+74 s, 5–50 keV) fit by a power law with alpha = −2.46(+0.32/−0.38); 4–120 keV fluence (5.3(+0.5/−2.4))×10^-7 erg cm^-2 and photon flux 0.22(+0.1/−1.1) ph cm^-2 s^-1.
• SVOM/ECLAIRs notes the spectrum implies Ep likely below 5 keV and classifies the event as an X-Ray Flash.
• Einstein Probe/FXT 0.3–10 keV spectrum fit by absorbed power law with intrinsic NH = 6.29(±1.71)×10^21 cm^-2 and photon index 2.43(±0.08); unabsorbed average flux 3.99(-0.14/+0.15)×10^-12 erg s^-1 cm^-2.
• GOTO-N reports optical fading consistent with a temporal decay index alpha_L = 1.30 ± 0.26.

Redshift

A redshift z = 1.801 is reported from GTC/OSIRIS spectra showing metal absorption lines interpreted at a common redshift.

What’s special vs typical

The prompt emission is explicitly classified as an X-Ray Flash (very soft spectrum with inferred Ep likely <5 keV), with a well-identified afterglow across X-ray, optical, and radio plus a spectroscopic redshift.