EP260727a

General information

EP260727a is an Einstein Probe fast X-ray transient with a confirmed optical counterpart and later radio emission. A spectroscopic follow-up measured a redshift of z = 2.817.

Wavelength coverage

• X-ray: detected by EP-WXT and followed by EP-FXT, with spectral fits and fluxes reported.
• Optical: detected and monitored in r, z, Ic, and Rc bands, with early g' and i' limits; optical spectroscopy was obtained with VLT/X-shooter.
• Radio: ATCA detected emission at 10 GHz. No observations are mentioned in gamma-ray, UV, infrared, or sub-mm bands.

Lightcurve and spectrum

• The WXT event started at T0 = 2026-07-27T20:36:47Z and lasted about 70 s.
• WXT 0.5-4 keV spectrum: absorbed power law with photon index 0.5 (-0.6/+1.2), intrinsic NH = 7.1 (-7.1/+240.0) x 10^20 cm^-2, and unabsorbed flux = 1.8 (-0.5/+0.8) x 10^-9 erg/s/cm^2.
• FXT 0.5-10 keV spectrum: absorbed power law with photon index 2.0 (-0.6/+1.1), intrinsic NH = 0.15 (-0.15/+0.87) x 10^22 cm^-2, and unabsorbed flux = 4.4 (-2.7/+0.2) x 10^-13 erg/s/cm^2.
• Optical detections reported: r = 21.98 +/- 0.08 at 4.18 hr, z = 21.66 +/- 0.17 at 5.9 hr, Ic = 21.76 +/- 0.26 at 3.24 hr, Rc = 21.64 +/- 0.17 at 3.19 hr, and r = 22.51 +/- 0.06 at 11.46 hr.
• Optical upper limits reported: g' > 20.3 and i' > 19.75 at 1.05 hr; r > 22.85 from Liverpool Telescope.
• Radio emission detected by ATCA at 10 GHz with a flux density of about 0.1 mJy at 31.76 days after trigger.

Redshift

• Spectroscopic redshift: z = 2.817.
• Method: optical spectroscopy with VLT/X-shooter.
• The redshift is supported by absorption features interpreted as H I Ly-alpha, Si II, O I, C II, Si IV, and Mg II, plus host-galaxy [O II], [O III], and Hgamma emission.

What’s special vs typical

• Secure high-redshift transient with a confirmed optical counterpart, broad X-ray-to-radio coverage, and a spectroscopic redshift.
• The combination of prompt X-ray transient behavior, optical spectroscopy, and late-time radio detection makes it notably richer than a routine single-band alert.