A group of astronomers from Germany led by Klaus Beuermann of the University of Göttingen have conducted photometric observations and discovered that the polar known as J1832.4-1627 (J1832 for short) is the first profoundly eclipsing stream-fed intermediate polar.
Researchers detailed their observation in a paper titled "J1832.4-1627, the first eclipsing stream-fed intermediate polar" published on November 4 in arXiv.org.
A diagram of a Cataclysmic Variable, showing the normal star, the accretion disk, and the white dwarf. The hot spot is where matter from the normal star meets the accretion disk.
What are Intermediate Polars?
According to NASA, cataclysmic variables (CV) are binary star systems that consist of a white dwarf and a normal star companion. CVs are usually small with the size of the Earth and Moon system and an orbital period of one to 10 hours.
Scientists would often refer to the white dwarf as the primary star and the normal star as the companion or secondary star, which is like the Sun in the Solar System that loses material onto the white dwarf via accretion. The gravitational potential of the white dwarf is enormous because it is highly dense and some of it is even converted into X-rays during accretion.
There are approximately more than a million cataclysmic variables in the galaxy, but scientists have only studied so far those located near Earth due to their fairly faint X-rays.
For white dwarfs that have a magnetic field above ∼10 MG, the field is strong enough to prevent an accretion disc from forming and locks the rotation rate of the white dwarf to the binary orbital period, according to a 2008 paper. Meanwhile, for those with a magnetic field strength between ∼0.1 and 10 MG, the accretion disc gets disrupted at some distance from the primary star and is now called the intermediate polar (IPs).
Astronomers hypothesize that there might be IPs that have a freely spinning white dwarf with a strong enough magnetic moment to avoid the formation of creation disk and accrete instead from a stream that feeds its two poles. But they have not found a stream-fed IP for many years, not until the recent discovery of the German scientists.
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Eclipsing IP J1832 is the First Stream-Fed Intermediate Polar
The astronomers in Germany said that eclipsing IP J1832 is a stream-fed intermediate polar and the first of its kind, Phys.org reported. Located between 4,100 and 9,300 light-years away from Earth, it was first discovered in July 2019 by Erwin Schwab using the German-Spanish Astronomical Center's Schmidt telescope.
The team said that they were able to confirm that J1832 is indeed an intermediate polar because it showcases a coherent large-amplitude 65-minute pulsation and it showed no evidence for an accretion disk in three consecutive seasons which means that this is a long-lasting feature.
Furthermore, they observed that the eclipsed light of J1832 entirely comes from two accretions spots and columns on the primary star with an indication of pole flipping. They also noted that its lack of luminous disk and observed stream flippin' in J1832 is not temporary, which supports their hypothesis that it is a deeply eclipsing stream-fed intermediate polar.
Researchers recommend further X-ray observations, optical spectroscopy, and spectropolarimetry to understand the variable accretion of J1832 and why its orbital period seems to be decreasing.
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