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Chinese astronomers have analyzed the data from NASA's Swift observatory, which extensively observed an outburst of the cataclysmic variable system known as GK Persei. Results of the study, February 20 on the arXiv preprint server, shed more light on the nature of this system.

Cataclysmic variables (CVs) are binary star systems 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.

Given that in CVs, mass transfer from the companion star often occurs through an accretion disk around the white dwarf and in some cases thermal instability in the disk triggers an outburst known as a dwarf nova (DN). These novae are CVs that undergo semi-periodic outbursts.

Polars are a subclass of cataclysmic variables distinguished from other CVs by the presence of a very strong magnetic field in their white dwarfs. In intermediate polars (IPs) the magnetic white dwarf spins asynchronously with the orbital period of the system and therefore produces a rapid oscillation with the spin period.

Located some 1,400 light years away, GK Persei (also known as A 0327+43 or Nova Persei 1901) is a cataclysmic variable consisting of a magnetized white dwarf (WD) and a K2-type subgiant star with a mass of 0.25–0.48 solar masses. It is classified as an IP and the strength of the magnetic field around the WD is estimated to be about 0.5 MG.

GK Persei underwent a classical nova explosion in 1901 and is therefore the second closest nova detected. The first DN behavior of this system was observed in 1948, and since then, numerous DN outbursts from this source have been identified, including the most recent ones in 2010, 2015 and 2018.

A team of astronomers, led by Songpeng Pei of the Liupanshui Normal University in China, focused on the outburst of GK Persei that took place 15 years ago. They analyzed Swift data covering 1.95 days after the 2010 eruption and continuing until 13.9 days before the maximum of the outburst to investigate the evolution of its X-ray light curves and spectra.

"Through X-ray and UV observations of the 2010 outburst, we made significant progress in understanding the IP nature of this system, particularly its rare, DN-like outburst behavior in a magnetic CV that also undergoes classical nova events," the researchers explained.

The study found that the X-ray spectrum of GK Persei exhibits a high level of complexity. Timing analysis performed by Pei's team suggests the presence of at least two distinct sources of X-ray emission: one producing hard X-rays (2.0–10 keV) and the other emitting in the soft X-ray range (0.3–2.0 keV).

Furthermore, the study detected a WD spin period of approximately 351.32 seconds in the 2–10 keV range during the 2010 outburst. Spin modulation in the softer energy band (0.3–2 keV) was also detected in the light curve of the second half observations (not seen in the 2015 and 2018 outbursts), but with a weaker amplitude than in the 2–10 keV range.

In addition, the astronomers found that the mass accretion rate in GK Persei varies significantly during different DN outbursts, given that the values obtained for the 2010 and 2018 outbursts turned out to be about an order of magnitude lower than those derived for the 2015 outburst.

According to the authors of the paper, the findings indicate that the soft X-ray emission in GK Persei may originate partly near the magnetic poles and partly from a wind or circumstellar material.

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