EP260321a

General information

EP260321a is an Einstein Probe/WXT X-ray transient (trigger ID 01709259023) detected on 2026-03-21, with EP/FXT follow-up finding a fading uncatalogued X-ray source and extensive optical spectroscopy later identifying the counterpart as SN 2026gzf (Type Ic-BL) at z≈0.0343.

Wavelength coverage

• X-ray: EP/WXT detection with soft spectrum; EP/FXT follow-up detected a fading source with very soft blackbody spectrum.
• Optical: Multiple facilities reported detections, upper limits, imaging, and spectroscopy (including VLT/MUSE, VLT/X-shooter, VLT/FORS2, Gemini/GMOS) leading to SN classification.
• Near-IR: J-band detection reported (SYSU 80 cm IR telescope).
• Neutrinos: IceCube search found no significant coincident track-like muon neutrinos; upper limits reported. No observations were mentioned in radio, UV, or gamma-ray bands.

Lightcurve and spectrum

• X-ray (EP/WXT): 0.5–4 keV absorbed blackbody fit (NH fixed to Galactic 2.7e20 cm^-2) with kT=164 eV (90% CL) and average unabsorbed flux 8.0e-11 erg s^-1 cm^-2 over ~432 s.
• X-ray (EP/FXT): fading during initial observation; 0.5–10 keV absorbed blackbody with kT=121 eV (90% CL) and average unabsorbed flux 9.8e-12 erg s^-1 cm^-2.
• Optical photometry: r=20.97±0.04 at 1.13 hr; ZTF g=19.7±0.2 at ~19 hr (with r non-detection r>19.6 at ~18.8 hr); reported brightening to r=18.67±0.02 at 43.61 hr and g=18.30±0.03 at 48.20 hr; additional R-band measurements (OT+host) brightened from R=18.09 at 0.118 d to R=17.29 at 2.116 d.
• Spectroscopy: VLT/X-shooter showed broad features below 5000 Å consistent with early core-collapse SN-like spectra and narrow Ca II absorption at z=0.0344; VLT/FORS2 spectrum matched SN 2006aj (pre-peak) and was used to identify a broad-lined Type Ic SN; Gemini/GMOS found broad absorption features consistent with Type Ic-BL SNe and host emission lines at z=0.0343.

Redshift

Redshift z=0.0343 was reported for the associated galaxy/source from 2dF and DESI spectroscopy and supported by VLT/MUSE and Gemini/GMOS host emission lines; VLT/X-shooter reports narrow Ca II absorption at z=0.0344.

What’s special vs typical

The event combines an unusually early, very soft and rapidly fading X-ray transient with a subsequently rising optical counterpart that is spectroscopically classified as a nearby broad-lined Type Ic supernova, making it consistent with very early SN emission/shock-breakout-candidate behavior discussed in the circulars.