Peeking Between the Pulses: The Far-UV Spectrum of the Previously Unseen White Dwarf in AR Scorpii

Abstract

The compact object in the interacting binary AR Sco has widely been presumed to be a rapidly rotating, magnetized white dwarf (WD), but it has never been detected directly. Isolating its spectrum has proven difficult because the spin-down of the WD generates pulsed synchrotron radiation that far outshines the WD's photosphere. As a result, a previous study of AR Sco was unable to detect the WD in the averaged far-ultraviolet spectrum from a Hubble Space Telescope (HST) observation. In an effort to unveil the WD's spectrum, we reanalyze these HST observations by calculating the average spectrum in the troughs between synchrotron pulses. We identify weak spectral features from the previously unseen WD and estimate its surface temperature to be 11500$\pm$500K. Additionally, during the synchrotron pulses, we detect broad Lyman-$α$ absorption consistent with hot WD spectral models. We infer the presence of a pair of hotspots, with temperatures between 23000K and 28000K, near the magnetic poles of the WD. As the WD is not expected to be accreting from its companion, we describe two possible mechanisms for heating the magnetic poles. The Lyman-$α$ absorption of the hotspots appears relatively undistorted by Zeeman splitting, constraining the WD's field strength to be 100 MG, but the data are insufficient to search for the subtle Zeeman splits expected at lower field strengths.