General information
EP260316a is a fast X-ray transient detected by Einstein Probe/EP-WXT with an autonomous EP/FXT follow-up that found an uncatalogued fading X-ray source consistent with the WXT localization. Several groups reported optical upper limits and a later faint optical source candidate close to the FXT position.
Wavelength coverage
- Gamma-ray: A double-peaked structure is reported to also be present in Fermi/GBM data.
- X-ray: EP-WXT (0.5–4 keV) detection with short (~22 s) observation; EP-FXT (0.5–10 keV) autonomous follow-up (5.4 ks) with a fading uncatalogued source.
- Optical: LOT/Lulin g-band upper limit; Xinglong 2.16m clear-band (reported as g~ limit) upper limit and Xinglong 0.8m g-band upper limit; COLIBRÍ simultaneous r and z imaging with a faint r-band source candidate and a z-band upper limit.
No observations were mentioned in radio, infrared, ultraviolet, TeV, neutrinos, or gravitational waves.
Lightcurve and spectrum
- WXT timing: observation started at T0=2026-03-16 12:32:52 and lasted ~22 s; light curve exhibits a double-peaked structure.
- WXT spectrum (0.5–4 keV): absorbed power-law fit with NH = 2.4 (-1.1/+1.4)×10^22 cm^-2 and photon index Γ = 2.9 (-1.4/+1.6); unabsorbed flux 3.1 (-2.1/+9.1)×10^-8 erg s^-1 cm^-2.
- FXT timing: started at 2026-03-16 12:35:40 UTC (T0+168 s), exposure 5.4 ks; on-ground analysis reports an uncatalogued fading source at RA,Dec = 226.9819, 27.4455 (10" radius, 90% CL).
- FXT spectrum (0.5–10 keV): absorbed power-law fit with NH = 7.01 (-0.55/+0.57)×10^21 cm^-2 and Γ = 2.39 (-0.10/+0.11); unabsorbed flux 9.51 (-0.41/+0.47)×10^-12 erg s^-1 cm^-2.
- Optical: LOT g > 23.2 (3σ) at t0+1.63 hr with no new source in the FXT 20" circle; Xinglong reports no transient to g~22.0 (3σ) within the WXT 3' circle at t0+3.76 hr, and g > 19.9 with the 0.8m; COLIBRÍ reports a faint uncatalogued source (r = 24.40 ± 0.19, SNR=5.7) at the edge of the FXT 10" circle and z > 23.38 (3σ).
Redshift
What’s special vs typical
- EP reports a prompt double-peaked X-ray light curve also stated to be present in Fermi/GBM data.
- A very faint optical counterpart candidate (r~24.4) is reported near the refined FXT localization, but it is not yet confirmed as transient.