Submillimeter observations of the white dwarf pulsar AR Sco

TL;DR

This study presents the first submillimeter observations of AR Scorpii, revealing a spectral break, detecting the white dwarf's spin modulation at radio frequencies, and suggesting a weaker magnetic field than previously thought.

Contribution

It provides new submillimeter data, detects WD spin modulation at radio frequencies, and revises the magnetic field strength estimate of the white dwarf in AR Scorpii.

Findings

Detected a spectral break at ~200 GHz in synchrotron emission.

First direct radio detection of the WD spin period at 58.26 s.

Revised the WD magnetic field estimate to ~15 MG.

Abstract

AR Scorpii, the so called white dwarf pulsar, contains a rapidly rotating magnetic white dwarf (WD; Pspin = 117.0564 s) interacting with a cool, red dwarf (RD) companion in a 3.56 hour orbit. It is a strong radio source with an inverted spectral index between 1-200 GHz that is indicative of synchrotron emission. This paper presents the first submillimeter observations of AR Scorpii using the Submillimeter Array, helping to fill a critical gap in the spectral energy distribution between 10-600 GHz. The average flux densities at 220 and 345 GHz are 124 and 86 mJy, respectively. The lower than expected flux density at 345 GHz suggests a break in the synchrotron emission at about 200 GHz. A periodogram analysis of the 220 GHz observations shows a modulation with an amplitude of ~6% at a period of 58.26 s or at twice the spin frequency of the white dwarf. This modulation is the first direct detection of the WD spin period at radio frequencies and implies that the synchrotron emission arises near the WD and and not from an interaction with the photosphere of the RD. A fit to the spectral energy distribution shows that the synchrotron emission arises from a small, low density region with a magnetic field of 43 MG at a distance of 0.6 orbital radii from the WD. This result implies that AR Scorpii contains a weakly magnetic WD (~15 MG) and not a strongly magnetic WD (~500 MG) as previously asserted.