General information
EP260509a is an Einstein Probe (EP) X-ray transient detected by EP/WXT starting 2026-05-09T13:44:02 UTC with a reported duration of ~250 s and a 2.78 arcmin (90% CL) localization at RA=239.4813 deg, Dec=-18.4328 deg.
Wavelength coverage
- X-ray: EP/WXT detection with spectral fit and flux estimate; EP/FXT follow-up ~20.5 hr later reports a non-detection with a 0.5–10 keV upper limit.
- Optical: MASTER-Net and LCO 1m (SINISTRO) report non-detections with limiting magnitudes (including deep LCO r and z limits).
No observations were mentioned in gamma-ray, UV, IR, radio, or neutrinos.
Lightcurve and spectrum
- Duration: ~250 s (EP/WXT).
- Spectrum (EP/WXT): absorbed power-law with Galactic NH fixed at 1.05 x 10^21 cm^-2 and photon index 2.0 (-0.8, +0.9).
- Flux (EP/WXT): unabsorbed 0.5–4 keV flux 6.0 (-2.0, +3.1) x 10^-11 erg/s/cm^2.
- X-ray follow-up (EP/FXT): 5475 s observation starting 2026-05-10T10:15:44 finds no significant source; 0.5–10 keV upper limit 3e-14 erg/cm^2/s.
- Optical limits: LCO stacked limits r > 22.8 and z > 21.4 (AB, not extinction-corrected); MASTER clear-band limits up to ~17.9 mag.
Redshift
What’s special vs typical
The source appears to have faded below EP/FXT detectability by ~20.5 hours after the EP/WXT detection, with follow-up optical imaging also yielding only upper limits.