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

TL;DR

This study models the broadband emission from AR Scorpii, a white dwarf binary, using two synchrotron components and finds its gamma-ray emission likely falls below current detection limits, consistent with Fermi-LAT observations.

Contribution

The paper presents a two-component synchrotron emission model for AR Scorpii, explaining its broadband spectrum and predicting gamma-ray flux below current detection thresholds.

Findings

Gamma-ray emission from AR Scorpii is not statistically significant.

The model predicts gamma-ray flux below Fermi-LAT sensitivity.

AR Scorpii could be a gamma-ray source below detection limits.

Abstract

In this work, we have analyzed the $\gamma$-ray data in the energy range 100 MeV to 500 GeV from the \emph{Fermi}-Large Area Telescope (LAT) observations for the period August 4, 2008 to March 31, 2019. The $\gamma$-ray emission from AR Scorpii over the last decade is not statistically significant and therefore 2$\sigma$ 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 $\sim$ 1.8$\times$10$^{10}$ cm and a magnetic field strength of $\sim$ 10$^3$ Gauss. The second component (Synchrotron-2) which reproduces the X-ray emission at lower energies and predicts the $\gamma$-ray emission, originates from another spherical region with radius $\sim$ 1.4$\times$10$^{10}$ cm and a magnetic field strength of $\sim$ 10$^6$ Gauss. The relativistic electron populations in both the emission regions are described by a smooth broken power law energy distribution. The $\gamma$-ray emission predicted by the Synchrotron-2 model is below the broadband sensitivity of the \emph{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 $\gamma$-ray source, although its emission level must be below the current detection limit of the \emph{Fermi}-LAT.