arXiv · 2507.12876
Einstein Probe Discovery of EP J182730.0-095633: A New Black Hole X-ray Binary Candidate in Faint Outburst?
Abstract
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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Huaqing Cheng, Qingchang Zhao, L. Tao, H. Feng, F. Coti Zelati, H. W. Pan, A. L. Wang, Y. N. Wang, M. Y. Ge, A. Rau, A. Marino, L. Zhang, W. J. Zhang, F. Carotenuto, L. Ji, C. C. Jin, D. Y. Li, B. F. Liu, Y. Liu, E. L. Qiao, N. Rea, R. Soria, S. Wang, Z. Yan, W. Yuan, B. Zhang, G. B. Zhang, S. N. Zhang, W. D. Zhang, A. Beardmore, J. S. Bright, X. L. Chen, Z. Fan, S. Y. Fu, J. P. U. Fynbo, J. W. Hu, J. J. Jin, P. G. Jonker, A. K. H. Kong, E. Kuulkers, C. K. Li, H. L. Li, Z. K. Lin, C. X. Liu, H. -Y. Liu, J. Z. Liu, X. W. Liu, Z. Lu, C. Maitra, H. Y. Mu, C. -Y. Ng, Y. L. Qiu, S. Tinyanont, Y. Wang, S. X. Wen, S. S. Weng, Jianfeng Wu, D. Xu, Y. K. Yan, Y. -P. Yang, P. Zhang, S. Zhang, Q. Zhao, Z. M. Cai, Y. Chen, Y. F. Chen, C. Z. Cui, W. W. Cui, H. B. Hu, M. H. Huang, S. M. Jia, G. Jin, Z. X. Ling, H. Q. Liu, S. L. Sun, X. J. Sun, Y. F. Xu, C. Zhang, M. Zhang, Y. H. Zhang. 2025-07-17. Einstein Probe Discovery of EP J182730.0-095633: A New Black Hole X-ray Binary Candidate in Faint Outburst?. https://arxiv.org/abs/2507.12876
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