Recycling Pulsars – The Millisecond Matters…

Article written: 4 Feb , 2012
Updated: 24 Dec , 2015


It’s a millisecond pulsar… a rapidly rotating neutron star and it’s about to reach the end of its mass gathering phase. For ages the vampire of this binary system has been sucking matter from a donor star. It has been busy, spinning at incredibly high rotational speeds of about 1 to 10 milliseconds and shooting off X-rays. Now, something is about to happen. It is going to lose a whole lot of energy and age very quickly.

Astrophysicist Thomas Tauris of Argelander-Institut für Astronomie and Max-Planck-Institut für Radioastronomie has published a paper in the February 3 issue of Science where he has shown through numerical equations the root of stellar evolution and accretion torques. In this model, millisecond pulsars are shown to dissipate approximately half of their rotational energy during the last phase of the mass-transfer process and just before it turns into a radio source. Dr. Tauris’ findings are consistent with current observations and his conclusions also explain why a radio millisecond pulsar appears age-advanced over their companion stars. This may be the answer as to why sub-millisecond pulsars don’t exist at all!

“Millisecond pulsars are old neutron stars that have been spun up to high rotational frequencies via accretion of mass from a binary companion star.” says Dr. Tauris. “An important issue for understanding the physics of the early spin evolution of millisecond pulsars is the impact of the expanding magnetosphere during the terminal stages of the mass-transfer process.”

By drawing mass and angular momentum from a host star in a binary system, a millisecond pulsar lives its life as a highly magnetized, old neutron star with an extreme rotational frequency. While we might assume they are common, there are only about 200 of these pulsar types known to exist in galactic disk and globular clusters. The first of these millisecond pulsars was discovered in 1982. What counts are those that have spin rates between 1.4 to 10 milliseconds, but the mystery lay in why they have such rapid spin rates, their strong magnetic fields and their strangely appearing ages. For example, when do they switch off? What happens to the spin rate when the donor star quits donating?

“We have now, for the first time, combined detailed numerical stellar evolution models with calculations of the braking torque acting on the spinning pulsar”, says Thomas Tauris, the author of the present study. “The result is that the millisecond pulsars lose about half of their rotational energy in the so-called Roche-lobe decoupling phase. This phase is describing the termination of the mass transfer in the binary system. Hence, radio-emitting millisecond pulsars should spin slightly slower than their progenitors, X-ray emitting millisecond pulsars which are still accreting material from their donor star. This is exactly what the observational data seem to suggest. Furthermore, these new findings can help explain why some millisecond pulsars appear to have characteristic ages exceeding the age of the Universe and perhaps why no sub-millisecond radio pulsars exist.”

Thanks to this new study we’re now able to see how a spinning pulsar could possibly brake out of an equilibrium spin. At this age, the mass-transfer rate slows down and affects the magnetospheric radius of the pulsar. This in turn expands and forces the incoming matter to act as a propeller. The action then causes the pulsar to slow down its rotation and – in turn – slow its spin rate.

“Actually, without a solution to the “turn-off” problem we would expect the pulsars to even slow down to spin periods of 50-100 milliseconds during the Roche-lobe decoupling phase”, concludes Thomas Tauris. “That would be in clear contradiction with observational evidence for the existence of millisecond pulsars.”

Original Story Source: Max-Planck-Institut für Radioastronomie News Release>. For Further Reading: Spin-Down of Radio Millisecond Pulsars at Genesis.

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11 Responses

  1. Christopher says

    LOSE, not LOOSE. I can’t believe I am seeing that spelling error on UT!

    • Member
      Tammy Plotner says

      maybe i’m just a bit touchy today, but i get so tired of being afraid to publish anything for fear of what grammatical error i might make that folks just can’t wait to pounce on. you know, it really takes a lot of the joy and excitement out of what i do. i appreciate constructive criticism… and i’m always willing to correct errors and thank those who spot them. but, come on… i don’t look at people’s postings and cut them down when they’ve got something to say. there’s a real person behind this keyboard, and quite frankly it hurts to know that you’d rather correct my article than read it.

      • morphics says

        I don’t think it was a personal remark (at least I hope it wasn’t), people just get touchy about grammar sometimes. I read the article and it was fascinating; every time I read about pulsars I get my mind blown, it’s hard to imagine something the size of a city spinning several hundred times per second. Thanks for the post, and keep it up!

      • Anonymous says

        It depends on how good news reporting is expected here. If you were doing this without monetary gain such grammatical errors would quite acceptable. Is the senior editor here reviewing all work being released on-line? Isn’t it ultimately, it is their responsibility

        Yet, I do notice that you also did corrected it, so Christopher’s point must have been well taken. Also I see nowhere here you actually acknowledging the correction with a retraction, as is normally expected in any form of the print or electronic media. Again, that depends on how good news reporting is expected here.

        Just saying…

      • Torbjörn Larsson says

        That gets into the commercial vs non-commercial side.

        If we abstract the discussion to readability and science we can simplify.

        The first issue would imply that too much deviation in spelling and grammar from the average should be avoided.

        Then too complex or simplistic styles are out, as well as “formal” grammar, for the benefit of the evolving language. Especially beneficiary on new and competitive media like the web.

        The second issue would imply that only spelling that affects science (names and terms) would benefit from a correction.

        So to correct happenstances of too loose spelling is to set oneself up to lose.

        Add back commercialism and you would need some measure of quality. Having a discussion of spelling and grammar on a science thread is IMHO lowering quality of science vs noise and, with it, user experience.

        I wish such comments could be banned under a site policy. An invisible channel could be mail to the site and/or author.

      • Anonymous says

        It happens to us all. I am a stickler for spelling and grammar but I recently noticed that I had recommended someone on LinkedIn and used “incite” instead of “insight” agh! It’s out there, in public, the shame!

      • Anonymous says

        I completely agree with you.
        It is killing off any fun to write an article.

        It is nothing wrong that someone helps you in correcting the spelling, but it is very wrong when someone with a huge ego shoots you down publicly for something that is just a mistake. It is like these people have no life.

        I don’t care it the spelling is wrong, I care what the article is explaining.

  2. Anonymous says

    Well, it was a quotation so I hope they made it in the original statement [sic]. But I agree, I read it as loo-sss and it’s just SO wrong!
    Wondering what were the conditions which made the pulsars seem older than the universe.

    • Member
      Tammy Plotner says

      thank you, peristroika. the error was in their quote and i missed it when i edited.

    • Torbjörn Larsson says

      The condition should be the spin down rate after decoupling from the feed star, I think. If there is no apparent initial braking mechanism and you extrapolate backward the system would look older.

      The article is confusing, because it seems the flip side is that earlier mechanisms proposed too much braking!?

  3. Anonymous says

    The end of mass transfer from a companion star ceases to adjust the angular momentum upwards. Further the magnetic field of the pulsar induces torques of material exterior to the star which causes the spin-down.

    A neutron star with around 10km radius spinning with a 1 millisecond rate has its equatorial region moving at a tangential velocity 3.14×10^4km/sec, which is about .1c (10% the speed of light). That is pretty fast and considering the mass being moved that is considerable. The motion is of course circular, which means the frame of the neutron star is an accelerated frame. If we focused on a local region the frame is then in a Rindler wedge spacetime and two local regions will then have different particle horizons. This means that after some period of time regions of the neutron star will becomes causally disconnected, which physically would be similar to hanging a rope into a black hole. The forces would become enormous and start to shear the material apart. I wonder if this is one reason neutron stars are not able to rotate faster than 1 millisecond.


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