New magnetic cataclysmic variable discovered

X-ray image of NGC 2516 obtained by eROSITA. The white dashed circle marks the position of SRGE J075818-612027. False colors in the image were made using photon energies between 0.2 – 1.1 keV, 1.1 – 2.3 keV and 2.3 – 5 keV for the red, green and blue channels respectively. The image shows an area of ​​1×1 degree. Credit: Ok et al, 2023

By analyzing the data from the Spektr-RG (SRG) spacecraft and the Transiting Exoplanet Survey Satellite (TESS), astronomers have discovered a new magnetic cataclysmic variable. The new object, designated SRGE J075818-612027, is most likely of the polar subtype. The finding was reported in a paper published February 26 on the pre-print server arXiv.

Catalysmic variables (CVs) are binary star systems consisting of a white dwarf accreting material from a normal companion star. They irregularly increase in brightness by a large factor, then fall 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.

Recently, a team of astronomers led by Samet Ok from the Leibniz Institute for Astrophysics Potsdam (AIP) in Germany analyzed observational data from the field of open cluster NGC 2516 using Spektr-RG’s eROSITA X-ray telescope and TESS. As a result, they serendipitously discovered a new resume.

“We report the discovery of SRGE J075818-612027, a deep stream-eclipsing magnetic cataclysmic variable found serendipitously in SRG/eROSITA CalPV observations of the open cluster NGC 2516 as an unrelated X-ray source,” the researchers wrote in the paper.

SRGE J075818-612027 was identified as one of the brightest X-ray objects in the field during the eROSITA Calibration and Performance Verification (CalPV) phase of observations. It was found to be an accretion-driven background object at a distance of between 4,000 and 13,500 light years.

Further observations of SRGE J075818-612027 revealed its variability with a period of about 106 minutes. The low-resolution identification spectrum of this variable shows hydrogen Balmer emission superimposed on a flat or light blue continuum. These results confirmed SRGE J075818-612027 as a magnetic cataclysmic variable.

According to the study, the X-ray spectrum of SRGE J075818-612027 is compatible with thermal plasma emission at a single temperature of around 10 keV, which is typical for polars. Furthermore, the object exhibits large size differences on long time scales at both optical and X-ray wavelengths. Such behavior could be interpreted as high and low states, further supporting its classification as a polar.

Summarizing the results, the authors of the paper noted that the discovery of SRGE J075818-612027 underlines the great sensitivity of the eROSITA telescope. This, together with eROSITA’s optimized scanning strategy, allowed them to detect SRGE J075818-612027 despite its great distance and the transition between high and low states. Therefore, the researchers hope to uncover many more magnetic CVs using this instrument.

“It can be expected that eROSITA will reveal many more magnetic CVs through systematic follow-up observations of all point-like X-ray sources found in eRASS (eROSITA All-Sky Survey). In particular, there is a high chance of finding those that escaped its detection in ROSAT all-cloud survey while it was in a low state at the time, because polar duty can be as short as 50 percent,” the astronomers concluded.

More information:
Samet Ok et al, Serendipitous discovery of the magnetic cataclysmic variable SRGE J075818-612027, arXiv (2023). DOI: 10.48550/arxiv.2302.13315

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