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

¹ Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100049, China
² Institute of Space Sciences (ICE-CSIC), Campus UAB, C/ de Can Magrans s/n, Cerdanyola del Vallès, (Barcelona), 08193, Spain
³ Institut d’Estudis Espacials de Catalunya (IEEC), Esteve Terradas 1, RDIT Building, Of. 212 Mediterranean Technology Park (PMT), 08860 Castelldefels, Spain
⁴ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiarello 16, 80131 Napoli, Italy
⁵ ESA/ESTEC, Noordwijk, 2201 AZ, The Netherlands
⁶ National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100101, PR China
⁷ School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, Beijing, 100049, PR China
⁸ Department of Astronomy, Xiamen University, Xiamen, Fujian, 361005, China
⁹ International Centre for Radio Astronomy Research, Curtin University, Kent Street, Bentley, WA, 6102, Australia
¹⁰ South African Astronomical Observatory, PO Box 9 Observatory, 7935, South Africa
¹¹ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch, 7701, South Africa
¹² Department of Physics, University of the Free State, PO Box 339 Bloemfontein, 9300, South Africa
¹³ The Nevada Center for Astrophysics, University of Nevada, Las Vegas, 89154 NV, USA
¹⁴ Department of Physics and Astronomy, University of Nevada, Las Vegas, 89154 NV, USA
¹⁵ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK
¹⁶ Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
¹⁷ College of Physics and Electronic Engineering, Qilu Normal University, 250200 Jinan, PR China
¹⁸ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, PR China
¹⁹ Institute for Frontier in Astronomy and Astrophysics, Beijing Normal University, Beijing, 102206, PR China

^⋆ Corresponding authors: xiaoyunxiang@ihep.ac.cn, gemy@ihep.ac.cn, rea@ice.csic.es

Received: 31 March 2025
Accepted: 4 July 2025

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 × 10²² 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 × 10³² 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.