| 29th of July 2016 |
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| A radio-pulsing white dwarf binary star |
| by Phil Edwards (CASS) |
| 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. Marsh et al. report in this week's Nature the discovery of a white dwarf/cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii, was classified in the early 1970s as a delta-Scuti star, a common variety of periodic variable star. New 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 were also detectable with the Australia Telescope Compact Array (ATCA) at 5.5 and 9.0 GHz, albeit at a lower level than at higher
frequencies. The pulses reflect the spin of a magnetic white dwarf, which is
slowing down on a 107-year timescale. The figure above shows amplitude spectra for the multi-wavelength data (the ATCA 5.5 and 9.0 GHz data are displayed in the bottom panel). All bands show signals with a fundamental period of about 1.97 min (8.46 mHz) and its second harmonic. The signals have two components, clearest in the harmonic, which we identify as the spin frequency vS and beat frequency vB = vS - vO, where vO is the orbital frequency. The pairs of grey dashed lines mark the positions of the beat (left) and spin (right) frequencies and their second harmonics. The beat component is the stronger of the two and defines the dominant 1.97 min pulsation period; the spin period is 1.95 min. Reference: Marsh et al. 2016, Nature, in press |