Einstein Probe and NuSTAR follow-up observations of the new NS LMXB EP J171159.4-333253: clocked type-I X-ray bursts and strong reflection features

ATel #17271; A. Marino (ICE-CSIC), F. Coti Zelati (ICE-CSIC), Y. L. Wang (NAOC; ICE-CSIC), N. Rea , E. Parent (ICE-CSIC), J. H. Wu (GZHU), K.-R Ni (CCNU), W. Yuan, H. Cheng, H. Sun, D. Y. Li, W. Zhang, Y. Liu (NAOC), H. Feng, L. Tao (IHEP), A. K. H. Kong (NTHU), S. Guillot (IRAP), J.-U. Ness (ESA) on behalf of the Einstein Probe team.
on 5 Jul 2025; 08:09 UT
Credential Certification: Alessio Marino (marino@ice.csic.es)

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EP J171159.4-333253 (previously identified as EP250623a) is a new X-ray transient source discovered by the Wide-field X-ray Telescope (WXT) (Ni et al., 2025, Atel #17247) on board Einstein Probe (EP, Yuan et al., 2025). Immediate follow-up X-ray observations with Swift/XRT and the Follow-up X-ray Telescope (FXT) on board EP provided a first refinement of the source position and reported the potential presence of type-I X-ray bursts and eclipses (Wu et al., 2025, Atel #17255). Further refinement on the position of the source and a clear detection of a radio counterpart have been reported by Cowie et al., 2025, Atel #17258 using MeerKAT data.

Here we report on a ToO observation performed with NuSTAR and on the ongoing EP/FXT campaign on the source. NuSTAR observed the source starting on 2025-06-27 at 20:31:09 UTC for an on-source exposure of 51 ks. The source is clearly detected at a count-rate of about 4 cts/s. The NuSTAR light curve exhibited seven type-I X-ray bursts and two eclipses (although with only partial coverage). We discovered that the bursts occur regularly, with an estimated recurrence period of around 8.2 ks. This discovery makes EP J171159.4-333253 one of the few “clocked” bursters (e.g., Di Salvo et al., 2023 and refs. therein, Fu et al., 2024), a regime made possible by stable mass-accretion rate and low metallicity of the accreted material (e.g. Galloway et al., 2004).

The X-ray spectrum of the persistent emission is characterised by a cut-off power-law model with prominent residuals in the 6-7 keV energy range. A preliminary broadband spectral analysis performed by fitting jointly the NuSTAR spectrum with a simultaneous EP/FXT spectrum (obsID: 06800000708) using an absorbed Comptonization plus disk blackbody plus reflection component, yielded an acceptable fit (reduced chi-squared of 1.1, 478 d.o.f.). The measured broadband (0.5-80 keV) unabsorbed flux is of about 5e-10 erg/cm2/s. The main physical parameters derived from the fit are: absorption column density of N_H of (0.44+/0.05)e22 cm-2, power-law photon index of 1.78+/-0.02, electron temperature of the corona kTe of 18+/-2 keV, seed photons temperature of the Comptonization spectrum of 0.80+/-0.02 keV, disk blackbody temperature of 0.34+/-0.05 keV, disk inner radius of 20-40 Rg. The parameters are typical of a neutron star low-mass X-ray binary in a hard state. In addition, an absorption line at an energy of about 6.8-7.2 keV is clearly detected.

Preliminary searches for periodicities in both persistent and burst emission on all the NuSTAR and EP/FXT data collected so far have not revealed any significant candidates for pulsations or burst oscillations. We have also performed a preliminary analysis of the source aperiodic variability. The power spectrum of the persistent emission is substantially flat, with a fractional rms variability amplitude of only around 3% over the full 3-79 keV energy range.

According to our EP/WXT monitoring, the source has been slowly rising since discovery, doubling its measured X-ray flux in about one week. Follow-up X-rays observations with Swift/XRT and EP/FXT have been planned. Further multi-band observations are encouraged.

We kindly thank the NuSTAR SOC for rapidly scheduling and executing the ToO observation.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.