Einstein Probe Discovery of EP J171159.4–333253: An Eclipsing Neutron Star Low-mass X-Ray Binary with Clocked Bursts

Abstract EP J171159.4−333253 is a new neutron star low-mass X-ray binary discovered in outburst by the Einstein Probe (EP) on 2025 June 23, exhibiting clocked type I X-ray bursts, eclipses, and dips. In this paper, we report on the results of the X-ray spectral and timing analyses for EP J171159.4−333253 using data collected by EP and NuSTAR during the first 21 days of the outburst. The X-ray burst recurrence time can be characterized over a subset of nine bursts spanning 1.6 days around the NuSTAR observation, and the result is t rec = 8196 ± 177 s, with indications of a possible decreasing trend. From the X-ray eclipse events, the binary orbital period and the eclipse duration are estimated to be P orb = 6.48301 ± 0.00003 hr and D ⋆ , X = 1245 . 5 − 6.5 + 6.9 s, respectively. These enable an estimate of the mass and radius of the companion star and the binary inclination, which are M 2 ≈ 0.6–0.8 M ⊙ , R 2 ≈ 0.7–0.8 R ⊙ , and i ≈ 73°–75°, respectively. We also report on joint ULTRACAM and EP observations on 2025 July 21–22, detecting the source optical counterpart and covering an eclipse in both X-ray and optical bands. The optical eclipse is wavelength dependent and broader than in X-rays, indicating that part of the optical emission arises from an extended region in the accretion flow. Despite a moderate variation in the source flux, the properties of the persistent X-ray emission are typical of a hard spectral state. We further evaluated the ratio of the accretion energy to the thermonuclear energy to be 120–130, implying helium bursts with the accreted hydrogen being depleted in between bursts.