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I Binary pulsar tidal locking

  1. Jan 13, 2018 #1
    How many neutron stars are binaries of other neutron stars?

    Hulse-Taylor binary, discovered in 1974, has orbital period of 7,75 hours. And the pulsar component rotates at 59 milliseconds. The orbit is also eccentric, from 1,1 to 4,8 solar radii. Expected to merge in 300 million years.

    PSR J0737-3039, discovered in 2003, has orbital period of 2,4 hours. The periods of the pulsars are 22 milliseconds and 2,7 seconds. Merger due in 85 million years.

    The one merger detected so far was noticed in gravity waves about 100 seconds ahead.

    What would be the present orbital period of a binary neutron star due to merge in, say, 20 years, so that we could design, fund and build observatories to get a good view?

    Does tidal friction operate on pulsars?

    About how long before merger, in which range of orbital periods, would tidal friction cause the rotational period of a pulsar to become equal to the orbital period of the neutron star binary?
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  3. Jan 14, 2018 #2


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    The inspiral process of binary neutron star systems is painfully slow. For example, the predicted time before merger for the Hulse Taylor binary [of nobel fame] is roughly 300 million years, despite the fact the orbital period is under 8 housr and they are only separated by around a half million miles. For a binary system on the brink of merger [<100 years] the orbital period would be very much shorter than 8 hours and the two would be separated by much less than half a million miles. The extremely low luminosity of neutron stars would make EM observation of sny such system extremely challenging. Gravitational wave emissions are also very weak prior to merger The short gamma ray burst associated GO event GW170817 was not detected until more than 10 hours later by SWIFT, which probably has as much to do with the low luminosity of initial EM emissions as any time delay in light speed vs gravity wave differences. The long and short of it is EM detection of neutron stars is very difficult and binary neutron star systems are quite rare, and assuming the usual result of merger is a short gamma burst, the odds stronglyy disfavor observing such a system within our own galaxy at present [GW170817A is the nearest short GRB on record at z=.010, oi about 130 million light years distant].
  4. Jan 14, 2018 #3

    Vanadium 50

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    Not going to happen.

    If Taylor-Hulse has a remaining lifetime of hundreds of millions of years, that means there needs to be tens of millions of (observed) such objects to find one with only decades left. There are 223.
  5. Jan 15, 2018 at 12:35 AM #4

    stefan r

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    Why would there be a correlation? Two neutron stars could merge without locking. If they are spinning retrograde to the orbit tidal forces could help spiral them in.
  6. Jan 15, 2018 at 3:14 AM #5
    Because there would be tidal forces and tidal friction.

    Pulsars are known to sustain elastic stresses and undergo brittle failures (pulsar glitches).
    When a pulsar rotates really close to another pulsar or a black hole, the rotation should cause periodic elastic stresses, right?

    At which distances, which binary period and time left to merger does the tidal stress from the second component become an important component for brittle failures?
  7. Jan 15, 2018 at 5:57 AM #6

    stefan r

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    the rotational velocity and the direction of the rotation would have an effect on that.
  8. Jan 15, 2018 at 8:44 AM #7
    Direction of rotation would have no effect. Rotational velocity would have effect on the period of tidal stress, but not on its amplitude.
  9. Jan 16, 2018 at 3:59 PM #8

    stefan r

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    Tides on earth push the moon into a higher orbit. The earth is rotating pro-grade and the moon is orbiting pro-grade. If you had retrograde rotation(s) and a pro-grade orbit the tides would lower the orbit and reduce the time till merger.
  10. Jan 17, 2018 at 5:32 AM #9
    Tides result in exchange of angular momentum between rotation and revolution. Only gravitational radiation removes angular momentum from the system for good.
    And the bulk of the angular momentum of the binary is in revolution (for reasons of lever arm).
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