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Quasi-periodic oscillations detected in a long-period eclipsing dwarf nova

Quasi-periodic oscillations detected in a long-period eclipsing dwarf nova
Light curves of HS 2325+8205. Credit: Sun et al, 2023

Using NASA's Transiting Exoplanet Survey Satellite (TESS), Chinese astronomers have detected quasi-periodic oscillations (QPOs) in the light curve of HS 2325+8205—a long-period eclipsing dwarf nova. The discovery was detailed in the January 2023 issue of Monthly Notices of the Royal Astronomical Society.

Cataclysmic variables (CVs) are consisting of a white dwarf primary accreting matter from a normal star companion. They irregularly increase in brightness by a large factor, then drop back down to a quiescent state. These binaries have been found in many environments, such as the center of the Milky Way galaxy, the solar neighborhood, and within open and globular clusters.

Some CVs, which experience recurrent outbursts of 2–10 magnitude caused by accretion disk instabilities, are called dwarf novae (DNe). Based on the different characteristics of outbursts, DNe can be divided into three main subtypes: Z Cam, SU UMa, and U Gem. Z Cams are characterized by standstills at an intermediate brightness level below the outburst maximum and above the quiescence state.

Observations show that orbital cyclic variation exists in a large number of CVs. In particular, DNe in general showcase three types of rapid oscillations: DNe oscillations (DNOs), long-period DNe oscillations (lp-DNOs), and quasiperiodic oscillations (QPOs). QPOs usually have periods longer than DNOs and lp-DNOs, ranging from hundreds of seconds to thousands of seconds.

Now, a team of astronomers led by Qi-Bin Sun of Yunnan Observatories in Kunming, China, reports the detection of QPOs in a Z Cam-type eclipsing DN with a relatively long orbital period of approximately 0.194 days, known as HS 2325+8205.

First of all, by analyzing the TESS data, the team found that HS 2325+8205 appeared to exhibit four different types of outbursts; however, two of them may not be actual. The outburst period was measured to be about 13.83 days. In addition, the results indicate that there are double humps in the quiescence period, which may be caused by ellipsoidal modulation.

Furthermore, the astronomers detected quasi-periodic oscillations with a period of about 2,160 seconds in the long of HS 2325+8205. It turned out that the oscillation intensity of these QPOs is related to the orbital phase as the intensity of QPOs in orbital phases 0.5–0.9 is stronger than in orbital phases 0.1–0.5.

"Therefore, the relationship between the intensity of QPOs and the orbital phase may become a research window for the origin of QPOs," the researchers concluded.

The authors of the paper assume that when the accretion flow from the secondary star in HS 2325+8205 collides with one side of the accretion disk, forming hot spots, these may have influence on the intensity of QPOs. However, more observations are required in order to confirm this hypothesis.

More information: Qi-Bin Sun et al, First discovery of quasi-periodic oscillations in the dwarf nova HS 2325+8205 based on TESS photometry, Monthly Notices of the Royal Astronomical Society (2023). DOI: 10.1093/mnras/stac3272. On arXiv: DOI: 10.48550/arxiv.2302.05887

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