EP251229a

General information (1–2 sentences, single paragraph, no bullets)

EP251229a is a fast X-ray transient detected by the Einstein Probe Wide-field X-ray Telescope (WXT) on 2025-12-29, with an uncatalogued X-ray source confirmed by the Follow-up X-ray Telescope (FXT) and several rapid optical follow-up attempts reporting only upper limits (GCNs 43253, 43264, 43266, 43268, 43278).

Wavelength coverage (bullets by band + final sentence listing bands with no observations mentioned)

• X-ray: WXT trigger and refined temporal/spectral analysis; FXT detected an uncatalogued source and provided a 0.5–10 keV spectrum/flux (GCNs 43253, 43264).
• Optical: Multiple facilities reported non-detections/upper limits, including BOOTES-2 (clear >19.3), SLT/Lulin (r>22.4 at 0.32 hr), and NOT (i>24.0, z>23.1) (GCNs 43268, 43278, 43266); MASTER-Net reported shallower early limits (GCN 43254). No observations mentioned in: gamma-ray, UV, IR, radio, millimeter/submillimeter.

Lightcurve and spectrum (ONLY items explicitly reported: T90/duration, flux/fluence, spectral model+params, temporal decay; if nothing, leave empty)

The WXT event lasted 173 s with a single pulse (GCN 43264). The average WXT 0.5–4 keV spectrum is fit by an absorbed power law with photon index 2.3 (+0.77/-0.68) and Galactic NH 5.3 (+2.6/-2.2)×10^21 cm^-2; the derived average unabsorbed 0.5–4 keV flux is 1.62 (+0.87/-0.41)×10^-9 erg s^-1 cm^-2 (GCN 43264). The FXT spectrum is fit by an absorbed power law with fixed Galactic NH=6.0×10^20 cm^-2 and photon index 1.89 (+0.13/-0.13); the derived average unabsorbed 0.5–10 keV flux is 3.46 (+0.40/-0.37)×10^-11 erg s^-1 cm^-2 (GCN 43264).

Redshift (1–3 bullets; only if redshift mentioned; else leave empty)

What’s special vs typical (ONLY if there is at least one clearly special/atypical aspect; do not mention absence of non-EM; if nothing special, leave empty)

The optical follow-up includes rapid and relatively deep non-detections (e.g., r>22.4 at 0.32 hr and i>24.0 at 3.62 hr), described as consistent with a ‘dark’ fast X-ray transient population (GCNs 43278, 43266).