Einstein Probe discovery of the short-period intermediate polar EP J115415.8−501810

The X-ray transient source EP240309a/EP J115415.8−501810 was first detected by the Wide-field X-ray Telescope on board the Einstein Probe (EP) instrument during the commissioning phase. Subsequent optical observations confirmed it as a cataclysmic variable of the intermediate polar type with a 238.2 s spinning white dwarf on a ∼3.76 h orbit. We report the source discovery and follow-up studies carried out with the EP’s Follow-up X-ray Telescope. A periodic variation of 231 s was detected in the 0.3−2 keV band, while no obvious pulsation appeared in the 2−10 keV band. The spectral analysis showed that the X-ray emission could be described by an absorbed bremsstrahlung model with kT > 11 keV. The partial covering absorption, with a hydrogen column density N H = 2.0 × 10 22 cm −2 and covering fraction of around 0.9, is much higher than the interstellar absorption along the line of sight. According to the distance of d = 309.5 pc obtained from the Gaia parallax, we estimated that the luminosity of this source in the 0.3−10 keV range is ∼2 × 10 32 erg s −1 . In addition, a phase-resolved spectral analysis revealed that the detected periodic variation is mainly caused by the change in the absorption column density. In this scenario, the spin modulation arises due to absorption from the pre-shock accretion flow of the X-ray emitting pole, while the optical radiation is modulated at the orbital side band ( ω spin − Ω orbit ) due to the reprocessing in regions within the binary system. Due to this unusual transient behaviour for an intermediate polar, we also searched for radio signals similar to those observed in the new class of long period transients. We derived upper limits with ASKAP of 200–300 μJy beam −1 in the range 800–1500 MHz and with MWA of 40–90 mJy beam −1 in the range 80–300 MHz.

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Journal
Astronomy and Astrophysics
Published
2025-07-18
DOI
https://doi.org/10.1051/0004-6361/202554876
Primary Topic
Astrophysical Phenomena and Observations
Type
article
Field-Weighted Citation Impact
0.00

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article

Einstein Probe discovery of the short-period intermediate polar EP J115415.8−501810

N. Rea, N. Hurley-Walker, Huahuai Sun, Yihong Xiao et al.
Astronomy and Astrophysics
Astrophysical Phenomena and Observations
article

Einstein Probe discovery of the short-period intermediate polar EP J115415.8−501810

N. Rea, N. Hurley-Walker, Huahuai Sun, Yihong Xiao, Mingyu Ge, Chichuan Jin, X. T. Hou, E. Kuulkers, Lian Tao, Siju Zhang, Domitilla de Martino, Kaya Mori, S. J. McSweeney, F. Coti Zelati, F. J. Lu, A. Marino, B. Zhang, W. Yuan, D. A. H. Buckley, S. Scaringi, Yunfei Xu, Hua Feng, Zehao Yu, Ji Wu
article en

Abstract

The X-ray transient source EP240309a/EP J115415.8−501810 was first detected by the Wide-field X-ray Telescope on board the Einstein Probe (EP) instrument during the commissioning phase. Subsequent optical observations confirmed it as a cataclysmic variable of the intermediate polar type with a 238.2 s spinning white dwarf on a ∼3.76 h orbit. We report the source discovery and follow-up studies carried out with the EP’s Follow-up X-ray Telescope. A periodic variation of 231 s was detected in the 0.3−2 keV band, while no obvious pulsation appeared in the 2−10 keV band. The spectral analysis showed that the X-ray emission could be described by an absorbed bremsstrahlung model with kT > 11 keV. The partial covering absorption, with a hydrogen column density N H = 2.0 × 10 22 cm −2 and covering fraction of around 0.9, is much higher than the interstellar absorption along the line of sight. According to the distance of d = 309.5 pc obtained from the Gaia parallax, we estimated that the luminosity of this source in the 0.3−10 keV range is ∼2 × 10 32 erg s −1 . In addition, a phase-resolved spectral analysis revealed that the detected periodic variation is mainly caused by the change in the absorption column density. In this scenario, the spin modulation arises due to absorption from the pre-shock accretion flow of the X-ray emitting pole, while the optical radiation is modulated at the orbital side band ( ω spin − Ω orbit ) due to the reprocessing in regions within the binary system. Due to this unusual transient behaviour for an intermediate polar, we also searched for radio signals similar to those observed in the new class of long period transients. We derived upper limits with ASKAP of 200–300 μJy beam −1 in the range 800–1500 MHz and with MWA of 40–90 mJy beam −1 in the range 80–300 MHz.

Astronomy and AstrophysicsVol. 701
University of Nevada, Las Vegas (US), University of Cape Town (ZA), Durham University (GB), Xiamen University (CN), Chinese Academy of Sciences (CN), Beijing Normal University (CN), University of the Free State (ZA), Yunnan Observatories (CN), Astronomical Observatory of Capodimonte (IT), Institut d'Estudis Espacials de Catalunya (ES), Qilu Normal University (CN), Institute of Space Sciences (ES), National Space Science Center (CN), South African Radio Astronomy Observatory (ZA), Institute of High Energy Physics (CN), International Centre for Radio Astronomy Research (AU), National Astronomical Observatories (CN), University of Chinese Academy of Sciences (CN), European Space Research and Technology Centre (NL), Columbia University (US)
National Aeronautics and Space Administration, Science and Industry Endowment Fund, Astronomy Australia Limited, Australian Government, Commonwealth Scientific and Industrial Research Organisation, Curtin University of Technology, National Natural Science Foundation of China, Chinese Academy of Sciences, Centre National d’Etudes Spatiales, Youth Innovation Promotion Association of the Chinese Academy of Sciences, Government of Western Australia, National Key Research and Development Program of China, Youth Innovation Promotion Association
Openalex Percentile: Top 86%
Astrophysical Phenomena and Observations
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