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 X ≲ 10 36 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.

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Publication Details

Journal
The Astrophysical Journal Letters
Published
2025-09-25
DOI
https://doi.org/10.3847/2041-8213/adf104
Primary Topic
Astrophysical Phenomena and Observations
Type
article
Field-Weighted Citation Impact
0.00

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article

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

Francesco Carotenuto, N. Rea, Weimin Yuan, A. K. H. Kong et al.
The Astrophysical Journal Letters
Astrophysical Phenomena and Observations
article

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

Francesco Carotenuto, N. Rea, Weimin Yuan, A. K. H. Kong, M. Y. Ge, Ge Jin, S. M. Jia, B. F. Liu, Xiaofang Sun, Xinlei Chen, F. Coti Zelati, A. Marino, A. P. Beardmore, S. X. Wen, Roberto Soria, Chandreyee Maitra, Yuan-Pei Yang, M. Zhang, Chichuan Jin, Erlin Qiao, Yong Chen, Y. H. Zhang, W. J. Zhang, Dongyue Li, Cheng-Kui Li, P. G. Jonker, He‐Yang Liu, E. Kuulkers, Shuang‐Nan Zhang, J. P. U. Fynbo, Lian Tao, Yunfei Xu, Shao-Yu Fu, Chenzhou Cui, Song Wang, Samaporn Tinyanont, Haiwu Pan, Shanshan Weng, Peng Zhang, Liang Zhang, Yongqing Yan, Qingchang Zhao, Huaqing Cheng, Junjie Jin, Joe Bright, A. Rau, Zhen Yan, Yun Wang, Jinzhong Liu, Y. L. Qiu, D. Xu, Zhiqiang Lu, Z. X. Ling, Jianfeng Wu, Long Ji, Ming‐Hsi Huang, J. Y. Hu, Yunfei Chen, Hua Feng, Yuanqi Liu, Chunhua Zhang, Yanan Wang, Bing Zhang, Xiaowei Liu, Weidong Zhang, Zhou Fan, Chenxu Liu, Ailing Wang, Hua-Li Li, Qingzhou Zhao, Guobao Zhang, W. W. Cui, Hong-Na Mu, Z. -M. Cai, Z. K. Lin, H. B. Hu, Haiqing Liu, C.‐Y. Ng, S. L. Sun
article en

Abstract

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 X ≲ 10 36 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.

The Astrophysical Journal LettersVol. 991(2)
National Aeronautics and Space Administration, California Institute of Technology, Australian Government, University of Leicester, European Commission, Commonwealth Scientific and Industrial Research Organisation, National Research Foundation, Deutsche Forschungsgemeinschaft, Danmarks Grundforskningsfond, National Natural Science Foundation of China, Chinese Academy of Sciences, Centre National d’Etudes Spatiales, China Postdoctoral Science Foundation, Jet Propulsion Laboratory, Leibniz-Gemeinschaft, National Key Research and Development Program of China, Precursory Research for Embryonic Science and Technology
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Astrophysical Phenomena and Observations
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