A radio-pulsing white dwarf binary star

Abstract

White dwarfs are compact stars, similar in size to Earth but approximately 200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf/cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a δ-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56-hour period close binary, pulsing in brightness on a period of 1.97 minutes. The pulses are so intense that AR Sco's optical flux can increase by a factor of four within 30 seconds, and they are also detectable at radio frequencies. They reflect the spin of a magnetic white dwarf, which we find to be slowing down on a 10-year timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf's spin, they mainly originate from the cool star. AR Sco's broadband spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf's magnetosphere.

A Model of White Dwarf Pulsar Ar Scorpii

“…Interpreting the high pulsating frequency as the "beat" frequency of the system, the inferred rotation period of the WD is 1.95 minutes. The spectral energy distribution of the pulsed emission supports a synchrotron origin for the radiation (Marsh et al 2016). These peculiar observational properties make AR Sco a unique system.…”

“…Observationally, the spectrum of AR Sco shows a peak flux density 1.6 × 10 −24 erg cm −2 s −1 Hz −1 (Marsh et al 2016). We therefore obtain n e = 3.5 × 10 8 F ν,peak 1.6 × 10 −24 erg cm −2 s −1 Hz…”

“…A white dwarf (WD) -M dwarf (MD) binary, AR Scorpii (henceforth AR Sco), was recently reported to emit pulsed broad-band (radio, IR, optical and UV) emission (Marsh et al 2016). The brightness of the system varies in long time scales with the orbital period of 3.56 hours, and shows pulsation in short time scales with a period of 1.97 minutes.…”

AR Scorpii and possible gravitational wave radiation from pulsar white dwarfs

“…For all stars considered in this work, we use a distance d ∼116 pc (1pc = 3.08×10 13 km) which is the distance of AR Sco from Earth, see Marsh et al (2016). Although we are fixing d, note that the closer the star, the larger the amplitude h 0 .…”

“…This, together with the fact that white dwarfs are found closer to earth, is one of the reasons why white dwarfs are much more understood than neutron stars. The main motivation of this work is provided by the recent binary system discovered by Marsh et al (2016). This system is composed of a main sequence star and a fast spinning and magnetized white dwarf with a mass that lies in the range 0.81M ⊙ < M wd ≤ 1.29M ⊙ .…”