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Abstract Long-period radio transients (LPTs) are coherent bursts of polarized radio emission that repeat periodically on timescales of minutes to hours. Little is known about the physical origins of these systems. Astronomers have proposed magnetars that rotate slowly and white dwarfs that rapidly orbit with a companion star as potential explanations. While several recent examples appear to support the latter hypothesis, the mechanism generating these bright radio pulses remains poorly understood. Here we report our discovery and classification of the LPT ASKAP J174508.9-505149 as an accreting white dwarf binary. This object has an ~1.3 h spectroscopic orbital period and exhibits orbitally modulated X-ray emission and radio bursts. These elliptically polarized radio bursts drift in emission frequency, potentially due to a longer beat period, and turn off for several hours at a time. Some LPTs have been associated with non-interacting white dwarf binaries. We have spectroscopically confirmed this system as an accreting cataclysmic variable, identified through characteristic optical emission lines and an ongoing X-ray outburst. Our results strengthen the link between at least some LPTs and white dwarf binaries.
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This content will become publicly available on June 1, 2027
The X-ray emission of the long-period transient and accreting cataclysmic variable ASKAP J174508.9-505149
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 byXMM-NewtonandEinstein Probebetween 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 ofP = 4868(22) s, consistent with radio and optical periods. We also observe the same periodicity in the hardness ratio extracted from theXMM-Newtonobservation, 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 (∼0.1 keV), a collisionally ionised plasma (∼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.
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- PAR ID:
- 10697963
- Publisher / Repository:
- ESO
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 710
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- L27
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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