Einstein Probe Discovery of EP J182730.0−095633: A New Black Hole X-Ray Binary Candidate in Faint Outburst?

Black hole X-ray binaries (candidates) currently identified in our Galaxy are mainly transient sources, with the majority discovered through the detection of their X-ray outbursts. Among these, only four were found during faint outbursts exhibiting peak X-ray luminosities LX ≲ 1036 erg s−1, likely due to the previous lack of sensitive, wide-field monitoring instruments in the X-ray band. In this Letter, we present the discovery of an intriguing X-ray transient, EP J182730.0−095633, via the Einstein Probe (EP) and subsequent multiwavelength follow-up studies. This transient, located on the Galactic plane, experienced a faint and brief X-ray outburst lasting about 20 days. Its X-ray spectrum is nonthermal and consistent with a power-law model with a nearly constant photon index of Γ ∼ 2 throughout the outburst. A long-lasting mHz quasiperiodic oscillation signal was detected in its X-ray light curve, centered around a frequency of ∼0.04 Hz. A transient near-infrared source was identified as its counterpart, although no optical emission was detectable, likely due to significant extinction. A radio counterpart was also observed, displaying an inverted radio spectrum with α ∼ 0.45. The X-ray spectral and temporal characteristics, along with the multiwavelength properties, indicate that the source is a faint low-mass X-ray binary, with the compact object likely being a black hole. This work demonstrates the potential of the EP in discovering new X-ray binaries by capturing faint-level X-ray outbursts.