Einstein Probe Discovery of EP J182730.0−095633: A New Black Hole X-Ray Binary Candidate in Faint Outburst?
Year: 2025
Type: article
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 ≲ 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.
Authors: H. Q. Cheng, Q. C. 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, Z. 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
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $2,400
Funders: 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, H2020 European Research Council
Awards: G1284474790, G1976769578, G2073246600, G3074516354, G3132630735, G3577539154, G3746673049, G4069961460, Cosmic Dawn Center (DAWN), The impact of highly magnetic neutron stars in the explosive and transient Universe, G4461113280, G465191129, G4701920126, G5050199812, G5366591201, G5703186329, G646913773, G6484117879, G7502779462, G8004848151, G8669088157
OpenAlex ID: W4414516538