EP260116a

General information

EP260116a is a soft X-ray transient detected by Einstein Probe/WXT at 2026-01-16T00:08:51 UTC, with rapid EP/FXT follow-up and extensive ground-based optical observations reported in multiple GCN circulars. Optical counterpart claims are mixed, with later analysis suggesting the faint optical source seen in some observations is likely unrelated to the FXT localization.

Wavelength coverage

• X-ray: EP/WXT detected the outburst; EP/FXT observed 24 min later and detected an uncatalogued X-ray source in the WXT region (GCN 43424).
• Optical: Multiple teams reported early upper limits (Xinglong, MASTER-Net, DFOT, Liverpool Telescope), and COLIBRÍ later reported a faint fading source in r/z, with subsequent discussion of a tentative rebrightening that was not confirmed (GCN 43425, 43427, 43430, 43432, 43436, 43459, 43480, 43491). No observations were mentioned for gamma-ray, UV, infrared, radio, or gravitational waves.

Lightcurve and spectrum

• X-ray spectrum (WXT, 0.5–4 keV): absorbed power law with NH = 7.65×10^20 cm^-2 and photon index Γ = 1.77; unabsorbed flux 7.16×10^-11 erg s^-1 cm^-2 (GCN 43424).
• X-ray spectrum (FXT, 0.5–10 keV): absorbed power law with photon index Γ = 1.98 (NH tied to WXT value); unabsorbed flux 9.52×10^-14 erg s^-1 cm^-2 (GCN 43424).
• Optical limits: e.g., r > 23.52 and z > 22.44 (3σ) at ~30.6 hr post-trigger (GCN 43436); r > 23.0 (R band, 3σ) at ~18.9 hr (GCN 43430); r > 22.5 and i > 21.9 (3σ) at ~21.7 hr (GCN 43432).
• Optical detections/decay (COLIBRÍ): faint source reported at RA = 07:35:14.14, Dec = +06:20:14.3 with r = 23.22±0.06 and z = 22.81±0.14 at ~78.7 hr, with power-law temporal evolution Flux_r ∝ t^(−0.33±0.12) and Flux_z ∝ t^(−0.53±0.23) (GCN 43459).
• Optical variability: a tentative rebrightening/slowing decay was reported at ~100–104 hr (Δr = −0.17±0.11, Δz = −0.30±0.21) but later observations did not confirm a rebrightening and favored flattening; the same source was argued to be outside the FXT error region and likely not associated (GCN 43480, 43491).

Redshift

What’s special vs typical

• The EP/FXT follow-up began only ~24 minutes after trigger but the WXT outburst had already ended, leaving a low-flux X-ray detection and complicating counterpart interpretation (GCN 43424).
• The main faint optical candidate shows a slow decay and was initially presented as a counterpart, but later analysis disputes the association based on its location relative to the FXT localization (GCN 43459, 43491).