With its proliferation of telescopes that operate at optical and radio wavelengths, South Africa provides great opportunities for astronomers to study astronomy, taking advantage of this collection of telescopes and their capabilities. A new era of automated and robotic telescopes allows astronomers to use more than one telescope, operating at different wavelengths, to observe astronomical objects. A perfect example is a PhD research project by Zwidofhelangani Khangale, one of the South African Astronomical Observatory (SAAO) students, which capitalised on the country’s telescopes by pursuing a multi-wavelength follow-up study of the eclipsing polar. A subgroup of cataclysmic variable, known to astronomers as UZ Fornacis (in short as UZ For), observing it with the Southern African Large Telescope (SALT), SAAO 1.9-metre telescope, MeerLICHT and the MeerKAT radio array. The MeerLICHT is a robotic telescope that provides real-time optical view of the radio sky as observed by MeerKAT.

UZ For is a serendipitous X-ray source discovered in 1987 by EXOSAT then later classified as a polar based on its spectroscopic and polarimetric properties. It was the third eclipsing polar to be discovered and has an orbital period of 126.5 minutes, which means two stars orbit around a common centre of mass roughly every 2 hours. Naturally, these objects are called polars because they show a high degree of polarisation, simply meaning, they emit radiation that is linearly and circularly polarised and also variable over the orbital phase of the binary. Studying this radiation can lead to the understanding of the accretion geometry of the system among other properties. In addition, modelling of the cyclotron radiation can directly lead to the determination of the magnetic field strength of the white dwarf. 

The photometric light curve of UZ For obtained with both SAAO 1.9-m (grey points) and MeerLICHT (red, blue, green, black and yellow points) is shown in Figure 1 and the dip in brightness centred at phase zero (horizontal axis) is caused by the eclipse of the white dwarf by the red dwarf. Apart from the eclipse, the light curve is dominated by flickering -that is random non-periodic increases and decreases in magnitudes, this is expected since lights from these systems vary on time scales ranging from a few seconds to decades. Interestingly, precise measurement and analysis of the times of mid-eclipse of UZ For suggest that this binary system could be harbouring a few extra-solar planets.

The observations of UZ For were obtained with SALT operating in circular spectropolarimetry mode precisely timed to be taken around the eclipse with some exposures taken before, during and after the eclipse as shown by the vertical grey dotted lines in Figure 1. The Southern African Large Telescope (SALT) is equipped with a variety of instruments including Robert Stobie Spectrograph (RSS), SALT Imaging Camera (SALTICAM), and High-Resolution Spectrograph (HRS), each of these instruments can be operated in more than one mode depending on the science case. The cyclotron spectra of UZ For taken before the eclipse as shown in Figure 2 (middle panel) revealed negative circular polarisation which reaches minus 8% in the blue then decreases with the increasing wavelengths. This is understood since the region emitting cyclotron radiation is visible to an observer around this orbital phase. On the other hand, the polarisation observed during and after the eclipse is consistent with zero and this is expected since the region that is responsible for the emission is either eclipsed (during the eclipse) or has moved out of the site (after the eclipse).

The interesting part of the result is, before the eclipse, the cyclotron spectra showed three humps that are interpreted as cyclotron harmonics from the hot plasma. With these humps, Zwidofhelangani and his colleagues were able to determine the magnetic field strength of the white dwarf in UZ For and this was found to be ~57 million Gauss. This value is consistent with previous measurements of the field strength of the same system in the literature. While this is the first such observation to be done with SALT, similar future observations will help in further understanding of polars as well as estimation of their field strength. The uniqueness of this method lies in SALT’s capability to allow for the splitting of light beams of an object into two parts, the ordinary and the extraordinary beams, where the degree of circular polarisation and cyclotron humps can be revealed.  

In addition, UZ For was detected in radio frequencies with MeerKAT (see Figure 3) at a central frequency of 1.28 GHz and it is one of three polars to ever be detected at this frequency. According to Zwidofhelangani, “This study demonstrates that multi-wavelength observations are essential to understanding the various emission processes that are at work in magnetic cataclysmic variebles. The results presented in the research study show that UZ For is one of the most interesting polars known to date. The circular spectro-polarimetry results are consistent with those from the literature”. This shows that SALT  will be able to observe other systems like this one for detailed analysis. The radio detection of this system with MeerKAT opens a window to studying other polars at radio frequencies in the southern sky.