Observers examine the polar BS that is eclipsing. Tri

 by Tomasz Nowakowski, phys.org

Jan 06 2023

Examples of BS Tri spectra from the observations in 2011 and 2012. The upper panel displays spectra that were collected during phases that were near to their brightest. The spectra that were collected close to the object's lowest brightness are displayed in the bottom panel. Source: Kolbin et al.

Spectroscopic and photometric studies of the odd eclipsing polar known as BS Tri have been made by Russian astronomers. The findings of this observational effort, which were detailed in a publication published on arXiv.org on December 23, shed new light on BS Tri's characteristics, particularly on the accretion process going on in this system.

CVs are near binary star systems made up of a low mass main-sequence star and a white dwarf (most often a red dwarf). They erratically get significantly brighter before returning to a quiet condition. Polars are a subclass of cataclysmic variables that are distinguishable from other CVs by having white dwarfs with extremely powerful magnetic fields.

The ROSAT space observatory was the first to notice BS Tri as a brilliant X-ray source. Later studies of this source revealed profound eclipses in its light curve as well as the cataclysmic variable-typical Balmer series hydrogen and neutral helium emission lines in its spectra. Later, it was determined that BS Tri was magnetic, which was essential for classifying it as a polar.

A white dwarf with a mass of 0.75 solar masses and a companion red dwarf with an estimated mass of roughly 0.16 solar masses make up BS Tri. The orbital period of the system is around 96.3 minutes. Although this polar has been the subject of several investigations, many of its properties are still unknown.

For this reason, a team of astronomers from the Special Astrophysical Observatory (SAO) in Russia, under the direction of Alexander I. Kolbin, chose to examine spectroscopic and photometric observations of BS Tri. They looked at a set of BS Tri spectra collected by the 6-m BTA telescope of SAO as well as photometric data from the ZTE 125-cm and Zeiss-600 60-cm telescopes, all of which are housed at the South Astronomical Station of Moscow State University.

The photometric data reveals changes in the BS Tri light curves' form that are brought on by the accretion stream's various contributions to the system's integral radiation. However, throughout the whole time that BS Tri was being observed, there were no noticeable fluctuations in average brightness.

The scientists were able to adjust several system parameters based on the radial velocity curves of the red dwarf's irradiated hemisphere. For instance, they discovered that the primary and secondary stars of BS Tri, respectively, had solar masses of 0.6 and 0.12, which are both less massive than previously thought. The system's orbital inclination was found to be 85 degrees.

In addition, the research discovered that the spectra of BS Tri exhibit wide cyclotron harmonics that move around when the white dwarf spins. The researchers estimated the magnetic field strength in the accretion spot to be at 22.7 MG and the average spot temperature to be at a level of 10 keV by modelling the cyclotron spectra.

The researchers also discovered that the hydrogen-alpha line Zeeman components in the spectra of BS Tri emerge concurrently with cyclotron harmonics and are generated at a magnetic field intensity of 21.5 MG.

The presence of a chilly halo surrounding the accretion area, which signals are observed in certain other polars, is a potential explanation for this phenomena, according to the paper's authors.