Astronomers investigate highly variable polar V496 UMa

By analyzing data from ESA's XMM-Newton spacecraft and NASA's TESS telescope, German astronomers have inspected a highly variable polar known as V496 UMa. Results of the study, published May 20 on arXiv.org, deliver more hints into the properties and nature of this object.

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.

Located some 2,470 light years away from the Earth, V496 UMa (also known as MASTER OT J132104.04+560957.8) was initially identified as a CV in 2012 when it changed its brightness by about 2 mag within 30 minutes. Further observations of this source confirmed that it is a non-eclipsing polar. The orbital light curve of this system showcases a double peaked structure, with the amplitude of the second hump being strongly variable.

Now, Samet Ok and Axel Schwope of the Leibniz Institute for Astrophysics Potsdam in Germany, decided to comb through the archival data from XMM-Newton and TESS, hoping to get more insights into the properties of this highly variable polar.

"This study investigates the thermal, temporal, and geometric behavior of V496 UMa in the X-ray and optical regime," the researchers explained.

The study confirmed that V496 UMa displays a highly variable behavior of the optical light curve. The light curve shape is characterized by two alternating humps with different brightness. The system shows a periodic variability at a period of approximately 91.06 minutes, identified as the orbital period.

Furthermore, the X-ray light curve of V496 UMa is double-humped and shows a pronounced dip. It was also found that the source exhibits a multi-temperature spectrum in X-rays. It has a prominent blackbody emission at soft X-ray energy, which is typical for polars.

It was also found that V496 UMa experiences two different mass accretion states in the spectral energy distribution (SED)—an active state at different brightness levels and an inactive or low state. The data shows that the bolometric X-ray luminosity of this system in a high state corresponds to the average X-ray luminosity of the so-called intermediate polars (IPs).

Trying to determine the fundamental parameters of V496 UMa, the researchers concluded that the system most likely consists of a main-sequence donor star of spectral type M5.0 and a white dwarf with a mass of about 0.8 solar masses and effective temperature not exceeding 10,000 K.

"Our attempt to find the spectral type of the secondary was based on the empirical relationship of Knigge et al. (2011). A main-sequence star of the M5.0 spectral type, together with a 0.8 solar masses white dwarf and the temperature of 10,000 K, represented quite well the spectral energy distribution in the low accretion state," the authors of the paper concluded.

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