General information
EP260131a is a fast X-ray transient discovered by Einstein Probe/WXT with an EP/FXT X-ray counterpart and a coincident fading optical source reported by multiple facilities. VLT/X-shooter spectroscopy reports absorption features consistent with z=0.937 (with stated association caveats).
Wavelength coverage
- X-ray: EP/WXT discovery and refined spectral/flux analysis; EP/FXT autonomous follow-up detection with spectrum/flux.
- Optical: Detections and photometry in g/r/z (Las Cumbres, COLIBRÍ, NOT), plus later upper limits (Liverpool Telescope) and a reported Xinglong non-detection noting the target was not observed.
- Optical/NIR spectroscopy: VLT/X-shooter spectrum covering 3000–21000 Å with continuum and absorption lines.
No observations were mentioned in gamma-ray, radio, mm/submm, gravitational waves, or neutrinos.
Lightcurve and spectrum
- EP/WXT: event start T0=2026-01-31T03:28:00.15 UTC; duration ~340 s; absorbed power-law fit with photon index 1.9 (-0.8/+0.9), intrinsic NH = 5 (-3/+3)×10^21 cm^-2; unabsorbed 0.5–4 keV flux 6.3 (-1.5/+3.6)×10^-10 erg s^-1 cm^-2.
- EP/FXT: detection starting at T0+2.7 ks with ~3 ks effective exposure; absorbed power-law photon index 2.2 ±0.4, intrinsic NH = 1.5 (-1.1/+1.3)×10^21 cm^-2; unabsorbed 0.5–10 keV flux 4.4 ±0.6×10^-13 erg s^-1 cm^-2.
- Optical photometry: r~20.9 at ~1 hr; r≈21.05, z≈20.69 at ~1.1–2.0 hr; NOT r'≈21.18 at 53.5 min and z'≈20.75 at 75.2 min; VLT acquisition r=21.20 at 3.79 hr.
- Optical evolution: COLIBRÍ reports slow fading from r≈21.1 to r≈21.4 and z≈20.8 to z≈21.2 over 1.14–8.35 hr with temporal index -0.16±0.07; later COLIBRÍ reports r~22 at 27–33 hr with average temporal index ~-0.5; color g-r=0.14±0.06.
- Late limits: Liverpool Telescope at 19.9 hr gives r>22.05, z>21.90 (3σ).
Redshift
VLT/X-shooter reports absorption features (Al II, Mn II, Fe II, Mg II doublet, Ca II) at a common redshift z=0.937; continuum detected down to 3090 Å and lack of hydrogen absorption implies z<1.54. The team notes the lack of fine-structure transitions means the z=0.937 system cannot be firmly associated with the transient, but they consider it the most likely redshift.
What’s special vs typical
The event was discovered as a fast X-ray transient by Einstein Probe with rapid autonomous X-ray follow-up and a promptly identified fading optical counterpart, and it has an early-time broad-band spectrum yielding a likely redshift (z=0.937) with explicit association caveats.