New hard polar eclipsing X-rays identified

Using NASA's XMM-Newton spacecraft from ESA and Swift, astronomers have discovered that a hard X-ray source known as 2PBCJ0658.0-1746 was a cataclysmic variable with eclipse of polar type. The discovery, presented in an article published on July 11 on arXiv.org, makes this object one of the few hard-hitting X-ray polar known to date.

Cataclysmic variables (CVs) are binary star systems composed of a white dwarf and a normal star mate. They irregularly increase the brightness of an important factor, then go back to a state of rest. The polar are a subclass of cataclysmic variables distinguished from other VCs by the presence of a very strong magnetic field in their white dwarfs.

Although more than 140 polar have been detected to date, only 33 of them have been identified as eclipsing systems. Until now, 12 of these objects have been classified in the category of hard polar eclipsing X-rays, making it a rare class. Therefore, any new addition to the still relatively short list of such objects is very important for building a larger database of these systems. This could for example offer astronomers more opportunities to study magnetic accretion in binaries

At present, a team of researchers led by Federico Bernardini of the Rome Observatory in Italy describes the latest addition to this list. By analyzing the XMM-Newton and Swift data, they discovered that 2PBCJ0658.0-1746 (J0658), an unidentified source discovered by the Swift / BAT survey, appeared to be a hard polar that eclipsed X-rays.

"The XMM-Newton observation, performed a few months later in 2018, is reported here with Swift / XRT and Swift / BAT archival curves and spectra," the researchers wrote.

According to the article, the X-ray emission shows bright and weak phases and total eclipses reproducing every 2.38 hours. In addition, it was found that the X-ray emission was modulated at the orbital period and that the intensity of the modulation was variable from one cycle to another. Such behavior is indicative of a nonstationary mass accretion rate.

Astronomers have also found that the X-ray brightness is very variable over long time scales and that the XMM-Newton probe has captured it at its lowest level ever. With regard to the X-ray spectrum, observations have shown that it is thermal and compatible with a multi-temperature structure, as observed in many magnetic systems.

In general, the results confirmed that J0658 is the 13th polar polar eclipse detected. The researchers added that its orbital period of about 2.38 hours makes it a rare discovery, since only one such pulsar with a similar orbital period (between two and three hours) has been identified to date.

The study also reported the fundamental parameters of the J0658 system. According to research, the polar is located about 681 light-years away and consists of a white dwarf with a mass of at least 0.6 solar mass and a M4 spectral-type companion about four times smaller than the sun, of a mass between 0.2 and 0.25 solar mass. It is estimated that the effective temperature of the white dwarf is between 12,000 and 22,000 K, while for the companion this value was at least 3,000 K.

© 2019 Science X Network