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Main Authors:	Cheng, Huaqing, Zhao, Qingchang, Tao, L., Feng, H., Zelati, F. Coti, Pan, H. W., Wang, A. L., Wang, Y. N., Ge, M. Y., Rau, A., Marino, A., Zhang, L., Zhang, W. J., Carotenuto, F., Ji, L., Jin, C. C., Li, D. Y., Liu, B. F., Liu, Y., Qiao, E. L., Rea, N., Soria, R., Wang, S., Yan, Z., Yuan, W., Zhang, B., Zhang, G. B., Zhang, S. N., Zhang, W. D., Beardmore, A., Bright, J. S., Chen, X. L., Fan, Z., Fu, S. Y., Fynbo, J. P. U., Hu, J. W., Jin, J. J., Jonker, P. G., Kong, A. K. H., Kuulkers, E., Li, C. K., Li, H. L., Lin, Z. K., Liu, C. X., Liu, H. -Y., Liu, J. Z., Liu, X. W., Lu, Z., Maitra, C., Mu, H. Y., Ng, C. -Y., Qiu, Y. L., Tinyanont, S., Wang, Y., Wen, S. X., Weng, S. S., Wu, Jianfeng, Xu, D., Yan, Y. K., Yang, Y. -P., Zhang, P., Zhang, S., Zhao, Q., Cai, Z. M., Chen, Y., Chen, Y. F., Cui, C. Z., Cui, W. W., Hu, H. B., Huang, M. H., Jia, S. M., Jin, G., Ling, Z. X., Liu, H. Q., Sun, S. L., Sun, X. J., Xu, Y. F., Zhang, C., Zhang, M., Zhang, Y. H.
Format:	Preprint
Published:	2025
Subjects:	High Energy Astrophysical Phenomena
Online Access:	https://arxiv.org/abs/2507.12876
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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 $L_{\rm X}\lesssim10^{36}~{\rm erg~s^{-1}}$, 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 multi-wavelength 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 non-thermal and consistent with a power-law model with a nearly constant photon index of $Γ\sim2$ throughout the outburst. A long-lasting millihertz quasi-periodic oscillation (QPO) signal was detected in its X-ray light curve, centered around a frequency of $\sim0.04$ 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 $α\sim0.45$. The X-ray spectral and temporal characteristics, along with the multi-wavelength 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.

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