Discovery of EP J175257.3-351923 as a Candidate Black Hole Low-Mass X-ray Binary

Abstract

We report the discovery of a new X-ray transient, EP~J175257.3--351923 (EP250916a), by the \textit{Einstein Probe} (EP) near the Galactic plane. The outburst lasted for at least $\gtrsim 40$~days, reached a peak 2--10 keV flux of $\sim 4 \times 10^{-10}$~erg~cm$^{-2}$~s$^{-1}$, and exhibited 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 ($\sim 1.6$--$2.2$) and no evidence for a spectral state transition. Broadband spectral modeling suggests a truncated disk, a weak reflection component, and a high-energy cutoff at $\sim 217$~keV, consistent with hard-state accretion in black-hole systems. No reliable optical counterpart is detected within the Swift/XRT error circle in SVOM/VT, Swift/UVOT and GROND observations, and the inferred X-ray-to-optical flux ratio, $ξ\gtrsim 21.75$, is consistent with a low-mass companion. No pulsations or significant aperiodic variability are detected. Although the compact object cannot yet be firmly identified, the timing, spectral, and optical evidence favors EP~J175257.3--351923 as a black-hole low-mass X-ray binary candidate, highlighting EP's potential to uncover a faint, previously hidden population of X-ray binaries.