Letter to the Editor

Discovery of EP J175257.3–351923 as a candidate black hole low-mass X-ray binary

¹ Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, PR China
² University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, China
³ Université Paris Cité, CNRS, Astroparticule et Cosmologie, F-75013 Paris, France
⁴ Max Planck Institute for Extraterrestrial Physics, Garching 85748, Germany
⁵ School of Astronomy and Space Science, Nanjing University, Nanjing 210023, China
⁶ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210023, China
⁷ INAF-Osservatorio Astrofisico di Torino, Strada Osservatorio 20, I-10025 Pino Torinese, Italy
⁸ Sydney Institute for Astronomy, School of Physics A28, The University of Sydney, NSW 2006, Australia
⁹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n E-08193 Barcelona, Spain
¹⁰ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels (Barcelona), Spain
¹¹ IRAP, Université de Toulouse/OMP, CNRS, CNES, 9 avenue du Colonel Roche, BP 44346 F-31028 Toulouse Cedex 4, France
¹² National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
¹³ CEA Paris-Saclay, Institut de Recherche sur les lois Fondamentales de l’Univers, 9111 Gif-sur-Yvette, France
¹⁴ School of Science, Qingdao University of Technology, Qingdao 266525, China
¹⁵ Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS, 11 rue de l’Université F-67000 Strasbourg, France
¹⁶ School of Physics and Astronomy, University of Southampton, Highfield Southampton SO17 1BJ, UK
¹⁷ Yunnan Observatory, Chinese Academy of Sciences, Kunming 650011, China

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 April 2026
Accepted: 20 June 2026

Abstract

We report the discovery of a new X-ray transient, EP J175257.3–351923 (EP250916a), by the Einstein Probe (EP) near the galactic plane. The outburst lasted for at least 250 days and reached a peak 2–10 keV flux of ∼4 × 10⁻¹⁰ erg cm⁻² s⁻¹, exhibiting a fast-rise exponential-decay (FRED) profile typical of X-ray binary outbursts. The source remained in the hard state throughout the outburst, with only modest variations in the photon index (∼1.6 − 2.2) and no evidence of a spectral state transition. Broadband spectral modeling suggests a truncated disk, a weak reflection component, and a high-energy cutoff at ∼217 keV, consistent with hard-state accretion in black-hole systems. No reliable optical or radio counterpart has been detected within the Swift/XRT error circle. The inferred X-ray-to-optical and X-ray-to-radio flux ratios are consistent with a low-mass X-ray binary (LMXB) classification. Neither pulsations nor significant aperiodic variability were detected. Although the compact object cannot yet be firmly identified, the timing, spectral, and multiwavelength evidence favors the classification of EP J175257.3–351923 as a black-hole (BH) LMXB candidate, highlighting EP’s potential to uncover a faint, previously hidden population of X-ray binaries.