Can Tidal Locking Occur in Binary Neutron Stars Before Merger?

[snorkack]

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?

 

[Chronos]

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].

 

[Vanadium 50]

say, 20 years, so that we could design, fund and build observatories to get a good view?

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.

 

[stefan r]

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?

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.

 

[snorkack]

Why would there be a correlation?

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?

 

[stefan r]

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?

the rotational velocity and the direction of the rotation would have an effect on that.

 

[snorkack]

the rotational velocity and the direction of the rotation would have an effect on that.

Direction of rotation would have no effect. Rotational velocity would have effect on the period of tidal stress, but not on its amplitude.

 

[stefan r]

Direction of rotation would have no effect. Rotational velocity would have effect on the period of tidal stress, but not on its amplitude.

binary period and time left to merger

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.

 

[snorkack]

If you had retrograde rotation(s) and a pro-grade orbit the tides would lower the orbit and reduce the time till merger.

Slightly.
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).

 

[alantheastronomer]

I thought this was an interesting question - tidal friction certainly does apply to neutron stars - as their orbit decays, they certainly should become tidally locked. I'm just not sure how to calculate exactly when that occurs. Sorry I couldn't be more helpful.

 

[Chronos]

Tidal locking of binary neutron stars appears to be an unlikely, albeit not impossible occurrence. This paper offers additional discussion; http://adsabs.harvard.edu/full/1992ApJ...400..175B, Tidal interactions of inspiralling compact binaries.
Neutron stars tend to be born with rapid rotation [think conservation of angular momentum] and their rotational period declines very slowly. A merger [which is also a slow process] is probable long before tidal locking could occur. Only a neutron star born with an unusually slow rotation looks like a potential candidate. That honor is currently held by IE-1613 - a ~2000 year old magnetar with a shockingly slow rotational period of 24,000 seconds [~7 hours]. This particular star, however, does not appear to have a companion - see https://arxiv.org/abs/1607.04264, for further details.