Modelling the broadband emission from the white dwarf binary system AR Scorpii

AR Scorpii is a compact binary system which consists of a magnetic white dwarf and an M-type main sequence cool star. This binary system was discovered as a source of pulsating radiation in radio, infrared, optical, ultraviolet and X-ray wavebands. In this work, we have analyzed the γ-ray data in the energy range 100 MeV to 500 GeV from the Fermi-Large Area Telescope (LAT) observations for the period August 4, 2008 to March 31, 2019. The γ-ray emission from AR Scorpii over the last decade is not statistically significant and therefore 2σ upper limit on the integral flux above 100 MeV has been estimated. We reproduce the non-thermal broadband spectral energy distribution of AR Scorpii using an emission model having two synchrotron components due to the relativistic electrons in very high magnetic fields. The first component (Synchrotron-1) broadly describes the emissions at radio to high energy X-rays through the synchrotron radiation originating from a spherical region of radius ~ 1.8 × 1010 cm and a magnetic field strength of ~ 103 Gauss. The second component (Synchrotron-2) which reproduces the X-ray emission at lower energies and predicts the γ-ray emission, originates from another spherical region with radius ~ 1.4 × 1010 cm and a magnetic field strength of ~ 106 Gauss. The relativistic electron populations in both the emission regions are described by a smooth broken power law energy distribution. The γ-ray emission predicted by the Synchrotron-2 model is below the broadband sensitivity of the Fermi-LAT and is also consistent with the 95% confidence level upper limit on the integral flux above 100 MeV derived from more than 10 years of observations. According to our model, the binary system AR Scorpii could be a γ-ray source, although its emission level must be below the current detection limit of the Fermi-LAT.