General information
EP260610b is an Einstein Probe fast X-ray transient detected by WXT on 2026-06-10, with an uncatalogued EP-FXT X-ray source at RA 267.6268 deg, Dec 46.5388 deg. A candidate optical counterpart was reported by Gemini and subsequently followed by several optical facilities.
Wavelength coverage
- X-ray: EP-WXT detected the transient and EP-FXT detected an uncatalogued X-ray source; refined analysis reports absorbed power-law spectra and unabsorbed fluxes in 0.5–4 keV and 0.5–10 keV bands.
- Optical: Gemini, NOT, MAO/AZT-22, COLIBRI, GTC, and Liverpool Telescope observed the counterpart or its position in r, z, R, g, and i filters, with detections, marginal detections, and upper limits reported.
- Spectroscopy: GTC/OSIRIS+ obtained 3x1200 s spectra covering 3600–7790 AA and detected weak continuum with likely absorption features.
No radio, gamma-ray, neutrino, gravitational-wave, infrared, ultraviolet, or sub-mm observations are mentioned.
Lightcurve and spectrum
- WXT light curve shows an increase in flux during the last 30 s of the approximately 230 s WXT observation.
- WXT spectrum: absorbed power law with Galactic NH fixed at 2.43 x 10^20 cm^-2, photon index 1.48 (+0.81/-0.70), and average unabsorbed 0.5–4 keV flux 2.39 (+1.35/-0.90) x 10^-10 erg s^-1 cm^-2.
- FXT spectrum: absorbed power law with photon index 2.19 (+0.1/-0.1) and average unabsorbed 0.5–10 keV flux 7.36 (+0.4/-0.4) x 10^-12 erg s^-1 cm^-2.
- Optical photometry evolved from Gemini r = 22.23 ± 0.03 and z = 22.19 ± 0.05 at about 0.8–1.1 hr to later measurements including R = 22.85 ± 0.15 at 0.317 d, r ≈ 23.7 at 16.5–17.7 hr, and Liverpool limits of g > 23.2, r > 23.5, i > 22.9 at about 1.46 d.
Redshift
GTC/OSIRIS+ spectroscopy reports weak continuum across the wavelength range, a firm upper limit z < 2.1 from the lack of hydrogen absorption, and weak Mg II and Fe II absorption features with line-stacking support at z = 1.10, suggested as the likely redshift of EP260610b.
What’s special vs typical
- The event has unusually strong follow-up coverage for an X-ray transient alert, including rapid X-ray follow-up, multi-telescope optical detections, late upper limits, and spectroscopy.
- A likely spectroscopic redshift for the optical counterpart substantially increases its scientific value, although the classification is not explicitly confirmed in the provided circulars.