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An Unusual Pulsating Binary

A large part of the fascination of astronomy is the discovery of objects that don’t fit our standard definitions. KIC 8462852 — ‘Tabby’s Star’ — is deeply mysterious and high on my watchlist. But yesterday we also looked at CX330, a so-called FUor of the kind that brightens enormously over years of observation. Today we have another strange one, a system called AR Scorpii, where a white dwarf star in a binary system is releasing a blast of radiation onto a nearby red dwarf. The entire system brightens and fades every 1.97 minutes, a phenomenon that has only recently been properly understood.

“AR Scorpii was discovered over 40 years ago, but its true nature was not suspected until we started observing it in June 2015,” says Tom Marsh (University of Warwick), lead author of the paper on this work. “We realised we were seeing something extraordinary the more we progressed with our observations.”

Those observations proceeded with data from the European Southern Observatory’s Very Large Telescope (Chile) and the Isaac Newton Group of telescopes at La Palma (Canary Islands), along with other ground-based resources and the Hubble and Swift instruments in space. European amateur astronomers also played a key role in studying the star’s behavior.

AR Scorpii is about 380 light years from Earth. Its white dwarf component is roughly Earth-sized though containing 200,000 times the mass, and the associated red dwarf is about one-third the mass of the Sun. This is a tight system, with the two objects orbiting one another every 3.6 hours. What researchers have found is that the white dwarf is accelerating electrons almost to the speed of light in a beam that sweeps across the face of the M-dwarf. The brightening is dramatic, but the emissions range all the way from X-rays to radio wavelengths.

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Image: This artist’s impression shows the strange object AR Scorpii. In this unique double star a rapidly spinning white dwarf star (right) powers electrons up to almost the speed of light. These high energy particles release blasts of radiation that lash the companion red dwarf star (left) and cause the entire system to pulse dramatically every 1.97 minutes with radiation ranging from the ultraviolet to radio. Credit: M. Garlick/University of Warwick, ESA/Hubble.

AR Scorpii was classified in the 1970s as a Delta Scuti variable, a kind of star (also known as a dwarf cepheid) that shows luminosity variations due to pulsations on the star’s surface. Such variables are useful as standard candles that help astronomers calculate stellar distances. But Marsh and team discovered that AR Scorpii’s pulsations were not the result of a single variable star but a binary system in which intense brightness variations were occurring. The pulses are strong enough that the star’s optical flux can increase by a factor of four within 30 seconds.

The nature of the AR Scorpii pulses is what intrigues the researchers. From the paper:

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.

The team’s calculations show that the 1.97 minute brightness pulsations reflect the spin of a magnetic white dwarf, one that is slowing down on a timescale of 107 years.

Although the pulsations are driven by the white dwarf’s spin, they originate in large part from the cool star. AR Sco’s broad-band 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.

Synchrotron radiation involves the acceleration of charged particles in a magnetic field, but as the quote above shows, what the researchers don’t yet know is the source of the electrons. The kind of pulsations observed here have been seen before in neutron stars but AR Scorpii is the first white dwarf system to show similar behavior. The paper notes that white dwarfs and neutron stars are the only two types of object that can support a misaligned magnetic dipole and spin fast enough to match the observed pulsations. The paper goes to some length to demonstrate that the AR Scorpii pulsations are consistent only with a white dwarf and not a neutron star.

The paper is Marsh et al., “A radio pulsing white dwarf binary star,” published online in Nature 27 July 2016 (abstract).

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Comments on this entry are closed.

  • Michael July 29, 2016, 16:21

    I wonder if the stellar wind from the RD is the source of the electrons. I find it so strange that they are so close because during the RG phase the RD should have been swallowed up although it could have been further out and drag then drew it nearer.

  • Geoffrey Hillend July 30, 2016, 19:46

    @Michael I have to agree with you. The solar wind from the red dwarf star is probably being accelerated by a one million gauss magnetic field of the white dwarf. Pulsars and neutron stars have strong magnetic fields that can accelerate electrons like the Crab Nebula for example which is why it has remained luminous in the visible light for centuries. The rotating magnetic field accelerated the electrons. A close, strong magnetic field could do the same thing but the rotation of a white dwarf is slower than a neutron star. The rotation might not be needed if the white dwarfs magnetic field is strong and close enough to interact with electrons in the solar wind since a magnetic field will also accelerate electrons as long as they are in motion and perpendicular to the direction of the magnetic field like in a dynamo or electric motor.

  • spaceman July 31, 2016, 12:58

    Interesting object! I am always amazed by the power and beauty of astrophysical objects such as this binary system.

    The amount of energy released by stars boggles the mind and gets me to imagine what kinds of projects could be enabled if even a modest fraction of this energy could be tapped, as energy–which is currently a real limitation to human endeavors–would essentially no longer be the limiting factor to, say, the construction and maintenance of a huge space-based particle accelerator or perhaps a solar powered antimatter factory (like C. Pelligrino’s fictional factory based on the surface of Mercury) that could produce tons of antihydrogen per year for various uses. Other uses would be beamed propulsion via huge solar powered lasers.

    In terms of the beauty, the artist’s impression included with this piece is striking. I often wonder what these objects would look like to the human eye from close-up such as from the vantage point of a hypothetical close orbiting world or starship visiting from another system. Space artist’s impressions, image processing and enhancement, and the increasingly detailed simulations are great, but has anybody attempted to depict, to the extent possible, how various astrophysical objects might appear (e.g the ‘true color’ view) to the unaided eye from close proximity?

  • Geoffrey Hillend August 3, 2016, 18:14

    I am not an expert in quantum physics but the acceleration of electrons from the area of the white dwarf is a little more complicated than what I wrote. A magnetic field does not accelerate electrons and shrink their quantum wavelength so it gets shorter and particle size gets smaller like in a particle accelerator. It does curve particles and electrons so they move in circles and produce synchrotron radiation.
    It takes an electric field to accelerate a particle which probably comes from the compression of the white dwarfs magnetosphere by the solar wind. The Red dwarf is quite close to the white dwarf since they have a rotation period of 3.6 hours? This could make a strong compression of the solar wind and an electric field which is what happens on Jupiter where the rotation and solar wind combine to produce and effect like a particle accelerator. There is “magneto disk or magnetic equator of directional electric current called the current sheet around Jupiter. Electrons are accelerated close to the speed of light around Jupiter’ equator and they make the journey all the way to the vicinity of the Earth. Barbato and Ayer, P. 186, and 215, Atmospheres.
    I don’t think that the electrons are from the white dwarf since they don’t emit electrons which have to overcome to electron degeneracy and gravity to escape, so they have to come from the Red dwarf or come from some other source which is then captured by the gravity of the white dwarf into a plasma of electrons around it.

  • Geoffrey Hillend August 3, 2016, 18:18

    The magnetosphere of Jupiter is compressed by the solar wind of the Sun which creates an electric field so the white dwarfs magnetic field might be compressed by the solar wind of the red dwarfs to create an electric field?

  • Geoffrey Hillend August 5, 2016, 18:36

    I forgot to write that electrons are ejected from Jupiter’s magneto disk at close to the speed of light. This same process be happening from the white dwarf.

  • Harry R Ray September 1, 2016, 10:00

    CAUTION: This may be just a WEE OT, but it does involve pulsating binaries, so here goes: Merger of a White Dwarf-Neutron Star”. by Ben Margalit, Brian D Metzger. It looks like the puzzle of the formation of the PSR 1257+12 planetary system has FINALLY BEEN SOLVED! What intrigues me is whether this process can be RAMPED UP to ALSO explain PSR J1719-1438b, which has ALWAYS been attributed to a WD being CONVERTED to a diamond “planet” via a “Black Widow” pulsar, and thus, not REALLY a planet at all. I have ALWAYS had issues with this explanation because the DENSITY of the surface material of the WD should make it HIGHLY RESISTANT to weathering, EVEN BY A PULSAR BEAM!

  • Harry R Ray September 1, 2016, 10:00

    This paper is up on Arxiv RIGHT NOW!

  • Harry R Ray September 1, 2016, 10:13

    The FULL TITLE of the paper is: Merger of a White Dwarf-Neutron Star binary to 10 to the twenty ninth power Carat Diamonds: Origin of the Pulsar Planets.