The X-ray emission of the long-period transient and accreting cataclysmic variable ASKAP J174508.9-505149

Abstract

Long-period transients (LPTs) challenge our knowledge of the mechanism producing radio periodic pulsations in compact objects. Some LPTs have been associated with systems hosting a white dwarf and a low-mass star in a detached binary. Recently, a new LPT (ASKAP\,J174508.9-505149) has been classified as an accreting cataclysmic variable (CV). In the present letter, we report on the detailed study of the X-ray variability of ASKAP\,J174508.9-505149 as observed by \emph{XMM-Newton} and \emph{Einstein Probe} between September 2025 and May 2026. Simultaneous optical and radio observations are also presented. We studied the timing variability of the source, and estimated an X-ray periodicity of $P=4868(22)$\,s, consistent with radio and optical periods. We also observe the same periodicity in the hardness ratio extracted from the \emph{XMM-Newton} observation, peaking at the minimum of the modulation. A long-term modulation is also present in the X-rays and in the B-band photometry, but it is poorly constrained by the current dataset. Spectral X-ray analysis shows the presence of a black-body component ($\sim$0.1\,keV), a collisionally ionised plasma ($\sim$15\,keV), and an absorption feature at 0.77 keV (possibly due to Oxygen-VII). This is the third LPT detected in the X-ray band, the second with a detected X-ray periodicity and variable X-ray emission, and the first conclusively recognised as an accreting magnetic CV.

Accepted for publication in A&A Letters on 27/05/2026. 7 pages (4 main text+ 3 appendices)

References in corpus (16)

• Magnetic White Dwarfs
• Australian Square Kilometre Array Pathfinder: I. System Description
• A radio pulsing white dwarf binary star
• A radio transient with unusually slow periodic emission
• A long-period radio transient active for three decades
• Polarimetric evidence of a white dwarf pulsar in the binary system AR Scorpii
• An emission state switching radio transient with a 54 minute period
• Long-period Pulsars as Possible Outcomes of Supernova Fallback Accretion
• Sporadic radio pulses from a white dwarf binary at the orbital period
• Spectroscopic Detection of a 2.9-hour Orbit in a Long Period Radio Transient
• Evolutionary implications of a magnetar interpretation for GLEAM-X J162759.5-523504.3
• Detection of X-ray Emission from a Bright Long-Period Radio Transient
• Unveiling the white dwarf in J191213.72-441045.1 through ultraviolet observations
• DW Cnc: a micronova with a negative superhump and a flickering spin
• In-flight Characteristics and Modelling of the Instrumental Background of EP/FXT
• A broadband X-ray study of the asynchronous polar: CD Ind