As I know, neutron stars are almost perfect sphere and no net moments, does it mean it is not possible to tidally lock it?
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$\begingroup$ How much time and money do you have? :-) Now, seriously... of course it is theoretically possible to tidal lock a neutron star, gravity itself will cause the necessary asymmetry, like in any other object, the problem is the incredibly large angular momentum that you are trying to transfer... to what... exactly? Another neutron star or a black hole sounds like the best alternative. $\endgroup$– CuriousOneCommented Mar 10, 2016 at 9:44
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$\begingroup$ For a more technical answer, see e.g. "The coupled effect of tides and stellar winds on the evolution of compact binaries", Serena Repetto, Gijs Nelemans. $\endgroup$– CuriousOneCommented Mar 10, 2016 at 9:51
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$\begingroup$ The coupled effect of tides and stellar winds on the evolution of compact binaries $\endgroup$– rob ♦Commented Mar 14, 2016 at 18:47
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This article may solve the question that you asked, https://ui.adsabs.harvard.edu/abs/1992ApJ...400..175B/abstract
Title: Tidal Interactions of Inspiraling Compact Binaries
Authors: Bildsten, Lars; Cutler, Curt
Abstract: We discuss the tidal interaction in neutron star-neutron star and neutron star-black hole binaries and argue that they will not be tidally locked during the gravitational inspiral. More specifically, we show that, for inspiraling neutron stars of mass greater than about 1.2 solar mass, the shortest possible tidal synchronization time exceeds the gravitational decay time, so that the neutron star cannot be tidally locked prior to tidal disruption, regardless of its internal viscosity. For smaller mass neutron stars, an implausibly large kinematic viscosity - nearly the speed of light times the stellar radius - is required for tidal locking. We also argue that the mass transfer which occurs when the neutron star reaches the tidal radius will be unstable in neutron star-black hole binaries, and the instability will destroy the neutron star in a few orbital periods. The implications of our work for the detection of these sources by LIGO and other gravitational wave observatories and for the gamma-ray burst scenarios of Paczynski (1986, 1991) are discussed.
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If a neutron star were a perfect sphere with a monopole moment and zero quadrupole, etc., moments, it would be impossible to observe its rotation. For example, observe the rotation of Earth's roundest object (whose rotational symmetry is broken by its crystalline structure).
We are able to observe that neutron stars rotate. Therefore neutron stars have nonzero moments of inertia and are subject to tidal locking. However it might take a very long time. A commenter links to a paper on the subject.
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1$\begingroup$ I think its important to note that even a neutron star which is on its own spherical would not be spherical in the presence of another mass, which is necessary anyway for there to be something to tidally lock to. $\endgroup$– AsherCommented Mar 21, 2016 at 19:19
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$\begingroup$ @Asher Also true; my point was that that the asker's premise (that neutron stars don't have any non-spherical moments) is flawed. $\endgroup$– rob ♦Commented Mar 21, 2016 at 19:53
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$\begingroup$ Ah, yes. I read the title and paid little attention to the actual question, so I hadn't realized the root of the question. $\endgroup$– AsherCommented Mar 21, 2016 at 20:00