First ever, White Dwarf Pulsar Discovered!

Discussion in 'Astronomy, Exobiology, & Cosmology' started by paddoboy, Feb 7, 2017.

  1. paddoboy Valued Senior Member

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    Mysterious white dwarf pulsar discovered
    February 7, 2017

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    Credit: University of Warwick
    An exotic binary star system 380 light-years away has been identified as an elusive white dwarf pulsar – the first of its kind ever to be discovered in the universe – thanks to research by the University of Warwick.

    Professors Tom Marsh and Boris Gänsicke of the University of Warwick's Astrophysics Group, with Dr David Buckley from the South African Astronomical Observatory, have identified the star AR Scorpii (AR Sco) as the first white dwarf version of a pulsar - objects found in the 1960s and associated with very different objects called neutron stars.

    The white dwarf pulsar has eluded astronomers for over half a century.

    AR Sco contains a rapidly spinning, burnt-out stellar remnant called a white dwarf, which lashes its neighbour – a red dwarf - with powerful beams of electrical particles and radiation, causing the entire system to brighten and fade dramatically twice every two minutes.



    Read more at: https://phys.org/news/2017-02-mysterious-white-dwarf-pulsar.html#jCp
     
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  3. paddoboy Valued Senior Member

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    http://www.nature.com/articles/s41550-016-0029

    Polarimetric evidence of a white dwarf pulsar in the binary system AR Scorpii

    Abstract
    The variable star AR Scorpii (AR Sco) was recently discovered to pulse in brightness every 1.97 min from ultraviolet wavelengths into the radio regime. The system is composed of a cool, low-mass star in a tight, 3.55-hour orbit with a more massive white dwarf. Here we report new optical observations of AR Sco that show strong linear polarization (up to 40%) that varies strongly and periodically on both the spin period of the white dwarf and the beat period between the spin and orbital period, as well as low-level (up to a few per cent) circular polarization. These observations support the notion that, similar to neutron-star pulsars, the pulsed luminosity of AR Sco is powered by the spin-down of the rapidly rotating white dwarf that is highly magnetized (up to 500 MG). The morphology of the modulated linear polarization is similar to that seen in the Crab pulsar, albeit with a more complex waveform owing to the presence of two periodic signals of similar frequency. Magnetic interactions between the two component stars, coupled with synchrotron radiation from the white dwarf, power the observed polarized and non-polarized emission. AR Sco is therefore the first example of a white dwarf pulsar.

    Radio pulsars, discovered nearly 50 years ago, are fast-rotating magnetized neutron stars with spin-modulated synchrotron radio emission, powered by spin-down energy loss of fast rotating neutron stars 1 . The star AR Sco was recently discovered to be a 3.56-h close binary, containing a fast-spinning (spin period P s = 117.1 s) white dwarf, showing strong brightness variations across most of the electromagnetic spectrum (ultraviolet to radio), most strongly modulated on the P b = 118.2-s beat (synodic) period, and its harmonics 2 . The spin-down of the white dwarf ( Ṗb=3.92×10−13ss−1" role="presentation" style="display: inline; line-height: normal; letter-spacing: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">Ṗ b=3.92×10−13ss−1Ṗb=3.92×10−13ss−1 ) powers non-thermal emission, whose luminosity far exceeds (by a factor of ≥14) 2the combined luminosity of the stellar components and dominates the spectral energy distribution (SED). These observations were explained in terms of beamed synchrotron radiation from the white dwarf, some of which is reprocessed by the companion star 2 . The weak X-ray emission suggests that little accretion power is produced in AR Sco, which either implies that it is currently in a propeller mass ejection phase or there is no mass transfer at all. If the former, then it would be similar to the white dwarf in the cataclysmic variable AE Aquarii 3,4,5,6 , which has a 33-s spin period and a Ṗ=5.6×10−14ss−1" role="presentation" style="display: inline; line-height: normal; letter-spacing: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">Ṗ =5.6×10−14ss−1Ṗ=5.6×10−14ss−1 . However, the lack of flickering and broad emission lines in AR Sco, indicative of mass outflows which are seen in AE Aqr, implies no mass loss and suggests that a different mechanism is draining the rotational kinetic energy from the rapidly rotating white dwarf in AR Sco, perhaps similar to that operating in pulsars, namely dipole radiation 4 and magnetohydrodynamic (MHD) interactions.

     
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  5. paddoboy Valued Senior Member

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    So, the obvious question: What sort of paramaters of the original star, initiates a WD/Pulsar from what we know as a normal WD?
     
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