Astronomers accidentally discovered a new magnetic cataclysmic event

(Samet Ok et al. / arXiv, 2023 https://bit.ly/3L6K7Gl)

The eROSITA telescope of the Spektr-RG X-ray space observatory has discovered a new eclipsing polar, which is an X-ray binary system of a white dwarf and an ordinary star.

It is noteworthy that the discovery was completely accidental, the telescope explored an open star cluster not associated with the polar.

Polars are a kind of cataclysmic variable systems.

This is a close binary where the white dwarf is accreting material from an ordinary star filling its Roche lobe.

At the same time, the dwarf has a strong magnetic field.

This is why the system either does not have an accretion disk at all (classical polar), or only has its outer part (intermediate polar)

Also, the star matter flows into the polar zones of the dwarf, moving along the lines of its magnetic field.

Another feature of the polars, which arises from the presence of a magnetic field, is the synchronization of the dwarf's rotation periods with the orbital period of the binary system.

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THE DISCOVERY
Now a group of astronomers from the Potsdam Astrophysical Institute in Germany announced the discovery of a new cataclysmic variable SRGE J075818-612027.

The team led by Samet Ok used the eROSITA telescope of the Spektr-RG X-ray space observatory.

In addition, the scientists used photometric observations from the TESS space telescope and spectroscopic data from the SALT telescope.

The discovery was accidental and occurred during observations of the open cluster NGC 2516, which was previously observed by the ROSAT and Swift telescopes.

Curious enough, those telescopes did not notice this object: it is likely that during these observations the variable was in a phase of low accretion rate.

SRGE J075818-612027 was in the field of view of eROSITA near the cluster and was visible as a bright X-ray source.

However, the system is not associated with the cluster and is located at a distance of 1225-4147 parsecs from Earth, the distance to NGC 2516 is estimated at 400 parsecs.

The system really belongs to the class of magnetized cataclysmic variables, but so far scientists cannot determine whether it is a polar or an intermediate polar.

SRGE J075818-612027 demonstrates long-term emission variability in both optical and X-ray wavelengths.

The rotation period of the white dwarf is estimated at 106 minutes; and the bolometric X-ray luminosity of SRGE J075818-612027 is 8×1032 erg per second.

The dwarf's companion can be a low-mass star.

The dips in the X-ray light curve are explained by the absorption of radiation in the accretion flow and the self-eclipse of the region where accretion occurs.

Sources:

• Astronomy & Astrophysics: https://www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/202345966
• Phys.org: https://phys.org/news/2023-03-magnetic-cataclysmic-variable.html

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