Einstein Probe discovery of EP J171159.4-333253: an eclipsing neutron star low-mass X-ray binary with clocked bursts

TL;DR

EP J171159-333253 is identified as a new clocked neutron-star LMXB exhibiting eclipses and dips, discovered in 2025 by the Einstein Probe and followed up with NuSTAR and ULTRACAM observations. The study combines timing analyses of 16 bursts (with a subset spanning 1.6 days around the NuSTAR epoch) and precise orbital measurements, obtaining P_orb ≈ 6.48301 hr and D_star,X ≈ 1245 s, along with a companion mass and radius consistent with a K-type donor. Broadband X-ray spectroscopy reveals a hard-state, disk-truncated accretion geometry with a hot corona and reflection features, while burst spectroscopy indicates helium-dominated burning with α ≈ 120–130, implying hydrogen burning is largely depleted between bursts. The optical data show a wavelength-dependent eclipse and a transient, eclipse-associated blue flare, pointing to an extended optical-emitting region and magnetic activity on the companion. Together, these findings place EP J171159-333253 among the small class of clocked bursters and provide valuable constraints on accretion geometry and thermonuclear burning in NS-LMXBs.

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 $t_{\rm rec}=8196 \pm 177\,$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 $P_{\rm orb}=6.48301 \pm 0.00003\,$hr and $D_{\star,X} = 1245.5^{+6.9}_{-6.5}\,$s, respectively. These enable an estimate of the mass and radius of the companion star and the binary inclination, which are $M_2\approx0.6-0.8\,M_\odot$, $R_2\approx0.7-0.8\,R_\odot$ and $i\approx73-75^\circ$, 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.

Figures (16)

• Figure 1: X-ray light curve of EP J1711$-$3332 extracted from EP/WXT data collected during the first 21 days of the outburst. Observed fluxes refer to the 0.5--4 keV energy range and were obtained by combining data from snapshots taken on the same day. The shaded gray area marks the epoch of the NuSTAR observation.
• Figure 2: X-ray light curve of EP J1711$-$3332 obtained in the NuSTAR observation. The dot-dashed gray lines mark the expected epochs of bursts, using the epoch of the fourth burst as a reference and assuming a constant $t_{\rm rec}$ of 8200 s. The expected ingress (dashed blue lines) and egress (dotted red lines) epochs of eclipses are calculated from ObsID 11900304129. The shaded gray areas are the observed eclipse periods. The orange hatched bands are the observed dips. A total of seven bursts, two eclipses and two dips were detected during this observation.
• Figure 3: Left: measured recurrence times $t_{\rm rec}$ (black) for the eight consecutive pairs of bursts with unambiguous cycle counts. The grey shaded regions denote the validity range of each measurement, extending from the start of one burst to the start of the subsequent burst. The first burst in the first $t_{\rm rec}$ measurement corresponds to burst No. 3 in Table \ref{['tab:bursts']}, while the final burst in the last measurement corresponds to burst No. 11. The blue solid and dashed lines show the weighted median recurrence time, $t_{\rm rec}=8196 \pm 177\,$s, which does not adequately describe the data ($\chi^2_{\rm red}=8.1$). The orange solid and dashed lines represent a linear fit to the $t_{\rm rec}$ measurements, indicating a decreasing trend of $\dot{t}_{\rm rec}=-270\pm110$ s day$^{-1}$, with an improved fit quality ($\chi^2_{\rm red}=3.2$). Right: NuSTAR light curves in 3--10 keV (blue) and 10--79 keV (red) bands and their respective linear fits. The NuSTAR data were binned over individual observational segments to improve the signal-to-noise ratio. The linear fit for $t_{\rm rec}$ (orange) from the left panel is also plotted on top of the light curves. The count rates in both bands show an increasing trend, with best-fit slopes of $0.40\,{\rm cts\,s^{-1}\,day^{-1}}$ (3--10 keV) and $0.097\,{\rm cts\,s^{-1}\,day^{-1}}$ (10--79 keV). Quoted errors are at 95% confidence level.
• Figure 4: X-ray light curve of EP J1711$-$3332 from ObsID 11900304129. This is the only EP/FXT observation where an entire eclipse event was captured. We use the ingress and egress epochs observed in this eclipse event as reference points to calculate the expected ingress and egress epochs in other observations. See Figure \ref{['fig:all_fxt_lc']} for all the FXT light curves.
• Figure 5: Phase-resolved light curves of four EP/FXT observations with dips (orange hatched bands). Each panel shows the data obtained over one orbit of the binary system. The orbit observed by the first FXT observation is defined as Orbit 1. The expected ingress (dashed blue lines) and egress (dotted red lines) epochs are calculated from ObsID 11900304129. The shaded gray area is the observed eclipse period. See Figure \ref{['fig:all_fxt_lc']} for all the FXT light curves.