Astronomers study the nature of a rapidly rotating intermediate polar

Using various space X-ray observatories, astronomers at Columbia University in New York and elsewhere studied CTCV J2056-3014, a polar intermediate containing one of the fastest rotating white dwarfs. Study results, published September 26 on the preprint server arXivshed more light on the nature of this object.

Cataclysmic variables (CVs) are binary star systems consisting of white dwarf accretion material from a normal stellar companion. Their brightness increases irregularly and considerably, then falls back to a resting state. Polars are a subclass of cataclysmic variables that are distinguished from other CVs by the presence of a very strong magnetic field in their white dwarfs.

In some CVs, accretion occurs via a truncated accretion disk when the white dwarf is moderately magnetic. These systems are called intermediate polar (IP). Observations showed that in IPs the magnetic white dwarf rotates asynchronously with the orbital period of the system and therefore produces a rapid oscillation with the rotation period. Therefore, determining a precise rotation period and precise oscillation ephemeris could be the key to revealing the IP nature of certain CVs.

Located approximately 853 light years away, CTCV J2056-3014 is a nearby IP detected in 2010. Its white dwarf has a rotation period of 29.6 seconds, making the system one of the few rotating CVs known fast (FSCV). The orbital period of CTCV J2056-3014 was measured to be approximately 1.76 hours.

Previous observations of CTCV J2056-3014 have shown that the system is weak in the X-ray band and also exhibits dwarf novae (DNe) as well as superexplosions in the optical band, thus properties indicative of WZ Sge-type CVs. In general, WZ Sge CVs exhibit occasional explosions, indicating more variable mass accretion, including dwarf novae explosions and much rarer superexplosions.

To obtain more information about the true nature of CTCV J2056-3014, a team of astronomers led by Ciro Salcedo of Columbia University decided to use ESA’s XMM-Newton satellite, the NASA’s NuSTAR (Nuclear Spectroscopic Telescope Array) spacecraft and the neutron star. Interior Composition Explorer (NICER) aboard the International Space Station (ISS), to perform in-depth radiographic and optical observations of this system.

Salcedo’s team observed CTCV J2056-3014 between 2019 and 2022. It was found that the pulsed profile of this system is characterized by a single broad peak with a modulation of around 25%. Additionally, a four-fold increase in the flux of unabsorbed X-rays was detected, coinciding with an optical flare occurring in November 2022.

According to the paper, properties of CTCV J2056-3014, such as low plasma temperatures and lack of significant absorption of low-energy X-rays, are generally similar to those of other known FSCVs.

However, the researchers noted that accretion-fueled emission in CTCV J2056-3014 and evidence of an accretion column, combined with the non-magnetic CV properties of the system and the faint iron line identified in its state ” low”, make its classification uncertain. Therefore, the team plans to publish another research paper in which the optical properties of CTCV J2056-3014 will also be discussed, which could help determine the true classification of this CV.

The authors of the study added that, based on the collected X-ray spectral data, they were able to estimate the mass of the white dwarf in CTCV J2056-3014, which turned out to be 0.7 to 1 .0 solar mass. This is a typical value for the population of known CV systems.

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