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Binary star system 1RXS J165424.6-433758 revealed to be polar, new observations find

An international team of astronomers has performed X-ray, ultraviolet, and optical observations of an X-ray source known as 1RXS J165424.6-433758. Results of the observational campaign, published June 8 on the pre-print server arXiv, shed more light on the nature of this source, providing evidence that it is a polar.

Binary System 1RXS J165424.6-433758 revealed to be polar, new observations find

Cataclysmic variables (CVs) are binary star systems comprising a white dwarf and a normal star companion. They irregularly increase in brightness by a large factor, then drop back down to a quiescent state. Polars are a subclass of cataclysmic variables distinguished from other CVs by the presence of a very strong magnetic field in their white dwarfs.

1RXS J165424.6-433758 (or AX J165420-4337) was first identified as an X-ray source by the ROSAT satellite more than three decades ago. However, although the source is known to astronomers for such a long time, its true nature remains a mystery.

Now, a new study published by a group of astronomers led by Brendan O'Connor of the George Washington University in Washington, DC, claims that the nature of 1RXS J165424.6-433758 has been finally determined, thanks to multiwavelength observations using various spacecraft and ground-based facilities.

"Here, we present DGPS [Deep Galactic Plane Survey] observations and follow-up of the source with Swift, NuSTAR, the Southern African Large Telescope, and the South African Astronomical Observatory 1-m telescope, as well as archival XMM-Newton observations. Our data show that 1RXS J165424.6-43375 (hereafter J1654) is a typical polar mCV [magnetic cataclysmic variable] at a distance of ∼ 460 pc," the researchers wrote in the paper.

The observations found that J1654 showcases a hard X-ray spectrum in the high-state characterized by thermal bremsstrahlung radiation with temperature of 10.1 keV and luminosity at a level of about 65 nonillion erg/s. Further investigation led to the finding that J1654 is a magnetic CV with an orbital period of 2.87 hours and a magnetic field of 3.5 MG.

The astronomers noted that J1654 lies within the so-called CV period gap (between two and three hours). This is a common property of the population of known polar mCVs, which do not exhibit a prominent period gap. Moreover, the overall X-ray, ultraviolet, optical, and infrared properties of J1654 also confirm the polar scenario for this system.

The results suggest that the donor star in the J1654 system is a main-sequence star of a late spectra type, with a radius of about 0.38 solar radii and an effective temperature of 4,300 K. The white dwarf is estimated to have a mass of some 0.58 solar masses.

The accretion rate for J1654 was calculated to be about 10^-11 solar masses per year. According to the authors of the paper, this result is comparable to the expected values for systems whose evolution is driven purely by gravitational wave radiation.