A non-thermal pulsed X-ray emission of AR~Scorpii

TL;DR

This paper reports non-thermal pulsed X-ray emission from AR Scorpii, suggesting relativistic electron acceleration and synchrotron radiation, with emission originating from the M-type star surface rather than accretion columns.

Contribution

It provides evidence for non-thermal X-ray emission and proposes a model involving magnetic dissipation and relativistic electrons in AR Scorpii.

Findings

X-ray/UV emission varies with orbital phase.

X-ray emission modulates with white dwarf spin, showing a power-law spectrum.

Emission is dominated by thermal plasma with evidence of non-thermal components.

Abstract

We report the analysis result of UV/X-ray emission from AR~Scorpii, which is an intermediate polar (IP) composed of a magnetic white dwarf and a M-type star, with the XMM-Newton data. The X-ray/UV emission clearly shows a large variation over the orbit, and their intensity maximum (or minimum) is located at the superior conjunction (or inferior conjunction) of the M-type star orbit. The hardness ratio of the X-ray emission shows a small variation over the orbital phase, and shows no indication of the absorption by an accretion column. These properties are naturally explained by the emission from the M-type star surface rather than from the accretion column on the WD's star similar to the usual IPs. Beside, the observed X-ray emission also modulates with WD's spin with a pulse fraction of $\sim 14\%$. The peak position is aligned in the optical/UV/X-ray band. This supports the hypothesis that the electrons in AR~Scorpii are accelerated to a relativistic speed, and emit non-thermal photons via the synchrotron radiation. In the X-ray bands, the evidence of the power-law spectrum is found in the pulsed component, although the observed emission is dominated by the optically thin thermal plasma emissions with several different temperatures. It is considered that the magnetic dissipation/reconnection process on the M-type star surface heats up the plasma to a temperature of several keV, and also accelerates the electrons to the relativistic speed. The relativistic electrons are trapped in the WD's closed magnetic field lines by the magnetic mirror effect. In this model, the observed pulsed component is explained by the emissions from the first magnetic mirror point.