Optical field of SRGA J213151.5+491400. Credit: arXiv (2024). DOI: 10.48550/arxiv.2401.06001
Astronomers from Turkey and Russia have made optical observations of a magnetic cataclysmic variable known as SRGA J213151.5+491400. Results of the observation campaign, presented on January 11 on the pre-print server arXivprovide important information about the properties of this particular system.
Cataclysmic variables (CVs) are binary star systems consisting of a primary white dwarf that accumulates material from a normal companion star. Their brightness increases irregularly and considerably, then falls back to a resting state. These binaries have been found in many environments, such as the center of the Milky Way, the solar neighborhood, and within open and globular clusters.
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. One of them is SRGA J213151.5+491400, which was first identified in 2020 as an X-ray source, when it experienced a significant flare.
A team of astronomers led by Solen Balman of Istanbul University, Turkey, began observing SRGA J213151.5+491400 in optical wavelengths, shortly after its flaring activity began. For this purpose, they used the TÜBITAK National Observatory in Turkey and the Special Astrophysical Observatory (SAO) in Russia.
“In order to identify and study the source, we used observations with the optical telescopes of TUG (TÜBITAK National Observatory, Turkey) – mainly the 1.5m RTT150 telescope and SAO RAS (Special Astrophysics Observatory , Russian Academy of Sciences) – mainly the 6 m telescope (BTA),” the researchers wrote.
Observations by Balman’s team show that SRGA J213151.5+491400 moved from a low state in early 2021 to a high state in 2020 with a 3.0 mag decrease in brightness. The spin pulse profile of the source was found to be single-peaked (mostly sinusoidal) in the high state, while in the low state a double-peaked profile was observed, indicating an accretor at two poles.
Astronomers have pointed out that the change in accretion geometry and the shift from one pole to two poles within or between states are detected in several polar magnetic cataclysmic variables.
Based on the collected data, the rotation period of the white dwarf in SRGA J213151.5+491400 was measured to be approximately 85.98 minutes. This is one of the shortest spin periods obtained for polar-type systems.
Low-resolution spectroscopy performed by the researchers revealed prominent Balmer lines and a helium line. It was found that Doppler tomography using these spectral lines confirmed the polar character of this system.
The paper’s authors added that additional observations of SRGA J213151.5+491400 in X-rays, using the Neutron Star Interior Composition ExploreR (NICER), identified the soft X-ray component in the state down. This is therefore the first detection of a soft component in the low state of a polar.
More information:
Ş. Balman et al, Optical identification and follow-up observations of SRGA J213151.5+491400 — a new magnetic cataclysmic variable discovered with the SRG observatory, arXiv (2024). DOI: 10.48550/arxiv.2401.06001
Journal information:
arXiv
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