Lumpy Neutron Stars can Generate Gravitational Waves

A new simulation of neutron stars suggest they may not be as smooth as predicted. The rapidly spinning exotic bodies may have significant topological features like mountains. These “lumps” on the star’s surface may cause fluctuations in space-time as the variation of the huge gravitational field varies on each spin. This fluctuation may generate gravitational waves, propagating into the cosmos, and could be detected here on Earth…

Neutron stars are the remnants of massive stars after they have exploded as supernovae. The dense core remains behind, spinning fast and composed of only neutrons. They have immense gravitational fields and thought to have as much mass as our Sun, but measuring only 20 kilometres across. As they conserve the angular momentum of their massive sun predecessor, as they are so small, they are expected to spin hundreds of times per second.

But how can these strange objects be detected? Well, for one, they may be seen as highly radiating pulsars (or, possibly, “magnetars“), flashing a beam of radiation past the Earth as they spin like a lighthouse, beams of high energy photons emitted from the neutron star’s poles. But what about the effect they have on space-time? Can these massive bodies create gravitational waves? (Note: A gravitational wave is a totally different creature from an atmospheric “gravity wave“.)

To picture the scene: Imagine spinning a perfectly spherical ball in a swimming pool. If the ball is perfectly stationary (not bobbing up and down and not drifting), only spinning on its axis, no ripples in the pool will be seen. Therefore, any instrument measuring ripples in the pool will not detect the presence of the spinning ball. Now spin an object not spherical (like a rugby ball, or an American football) in the pool. As this object spins, the irregularities on the surface (i.e. the pointed ends) will produce a wave on each revolution of the irregular object. The ripple instrument will detect the presence of the ball in the pool.

This is the issue facing scientists trying to detect gravitational waves from neutron stars. If they are smooth objects (perhaps spherical, or slightly flattened due to the spin), they cannot produce ripples in space-time and therefore cannot be detected. If, on the other hand, they are irregularly-shaped spinning bodies, with inhomogeneities (lumps or “mountains”) on the surface, gravitational waves may be generated. The lump will sweep out a fluctuation in space-time on each rotation. This is fine, but are neutron stars lumpy?

Well, the outlook isn’t very good. The space-time “ripple” detectors set out to observe gravitational waves have so far not detected any sign of these rapidly spinning neutron stars. This could either mean that the technology we are using is not sensitive enough to detect gravitational waves or that neutron stars are naturally smooth and cannot produce gravitational waves in the first place.

Matthias Vigelius and Andrew Melatos, researchers from University of Melbourne in Australia, think they have new hope that some types of neutron star might be detected as they are naturally lumpy. Using a new computer modelling technique, the pair believes that even a small variation in the neutron star surface will produce detectable gravitational waves. But how do these lumps form? Often, stars evolve as part of a binary system (i.e. two stars orbiting a common centre of gravity), should one die as a supernova, leaving a neutron star behind, the intense gravitational field will strip its companion star of its gases. As the gas is funnelled into the neutron star, the intense magnetic field will give structural support to the incoming gas, creating an electron-proton mix of superheated plasma sitting on top of the neutron star surface. The lumps formed at the neutron star’s magnetic poles will be a long-living feature, sweeping around the star each time it rotates. Vigelius and Melatos think that detectors such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) may be able to detect this characteristic signature of an irregularly shaped neutron star…. in time.

As yet, these “lumpy” neutron stars have not been detected, but through continued observation (exposure time), it is hoped that Earth-based gravitational wave observatories may eventually receive the signal.

Source: RAS, New Scientist

14 Replies to “Lumpy Neutron Stars can Generate Gravitational Waves”

  1. Oh boy, another article filled with wisdom like “simulations suggest” and “may generate” and “might be.” Oh well, I guess somebody gets paid for writing this stuff!

  2. Advances in scientific knowledge require this type of “brainstorming”. In the classroom I have often seen the wildest brainstorm-ideas lead others to formulate very important concepts. This same process occurs among scientists.

  3. I think you meant “pulsar” where you wrote “quasar”. Quasars are active galactic nuclei.

  4. Hi Ian many thanks for this enlightening article, again.
    And again I felt like a MIT student.
    All is so easy.
    And it is so thrilling. Nothing is certain, proven and boring.
    All is in flux.

    But for the next article I kindly ask you to bring in some more scary things like dark protons or submarine black holes or midnightly death rays.

    I like these things so much.
    Please do it again.

    Your sincere fan bellinda

  5. Judging from numerous comments about the various Universe Today articles, many subscribers are looking for entertainment and/or a forum for emotional venting about non-science subjects, not actual science. Maybe Youtube wouild be a better place for them to hang out.

  6. “YourBrain Says:
    April 2nd, 2008 at 7:53 am

    Oh boy, another article filled with wisdom like “simulations suggest” and “may generate” and “might be.” ”

    Your point being? This is actually how science moves forward – deductions based upon the best available knowledge and theoretical considerations, and then you go out and see if the evidence backs it up. If it does – great. If it doesn’t – great. No matter what happens, you have learned something new and you are necessarily closer to understanding some aspect of nature.

    If you want something more unquestioningly definitive, science isn’t for you.

  7. this is an interesting possibility. if gravitational waves do indeed exist and have the predicted properties, then this should be a very powerful source.

  8. Yeal indeed a good point.

    So you think universe today is a SCIENCE place ?
    It is not.
    It is a sciencebabble place one time and again.

    And it is NOT the ‘subscribers’ to be blamed, but the authors and the editors.

    Be aware that this site is to generate money not wisdom.

    So anyone creating a site hit increasing the count is welcome.

  9. According to (perhaps old) magnetar theory, the common flashes that are the hallmark of SGRs are
    caused by “starquakes” in the outer rigid crust of the magnetar. As a magnetar’s colossal magnetic field shifts, it strains and may break the crust When the crust snaps, it vibrates with seismic waves like in an earthquake and emits a flash of soft gamma-rays. Assuming that the crustal vibrations are unlikely to be spherically symmetrical: perhaps behaving more like the first few components of spherical harmonics: they would produce propagating lumps in the spinning neutron stars outer shell. This brief period of asymmetry should also generate gravity waves, should it not?

  10. From the ancestral time the world space research established on blind theory. We although demand that we have discovered lots of mysteries viz. milk ways, galaxies, nebulas, white drafts, black holes etcs., in real they are creation of space mirror. SPACE MIRROR is the truth and hidden mystery of the space. Since we are unknown about space mirror, our research has diverted from original truth and we have spent lots of time and money behind the false truth.
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  11. Ah Cathbad, how I’ve missed you! How have you been?

    I always enjoy your valuable and insightful input to stories on the Universe Today.

    There’s never any substance to your arguments, there’s never a real name behind your pseudonym, there’s never any science to back up your point of view. There is never any challenge to what I write, just a lot of hot air and nonsense.

    As we’ve said to you before, give us some convincing scientific debate, and back that up by coming out into the open and providing your name or your own blog. Even write an article or two to show us that the “sciencebabble” we report on can be written better.

    As yet we’ve had no response about this offer, so I will let you continue to rant and rave if that makes you happy.

    All the best, Ian

  12. The question I have is that binary neutron stars will generate much stronger gravitational waves and we know they are out there and yet we can not detect them. Recently, I’ve read that binary neutron stars or stars going super nova are 6 times more likey than previously thought and now this suggestion that neutron stars are lumpier than previously thought. It all sounds like propaganda to raise the funding for projects like the next generation of LIGO. Things are getting desperate in the science funding world as is shown by NASA refusing the funds to finance the final analysis of the GPB data, despite the fact the funds are only a fraction of the money that already been spent over the last 40 years on that project.

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