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It seems that this question has not really been explored in the literature. Do isolated neutron stars (which do not accrete material) emit stellar wind? If yes, what composition would it have? If yes, what will be the rate of mass loss for the star?

One might also think up that the process of Hawking radiation might possibly be applicable to neutron stars, where negative energy particles get trapped in the nuclear star atmosphere (instead of crossing the horizon in case of black holes), which would also lead to some sort of evaporation and corresponding wind. However, in this question I am more interested in 'classical' winds.

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  • $\begingroup$ Neutron stars are really complex objects that have a lot of dynamics to their constituent parts, and very large magnetic fields. I expect that the answer to your question is going to be a "probably, but it depends on a lot of things." I would also expect there to be no quantum Hawking-style radiation, as the magnitude of that effect is governed by the black hole horizon's area, which is zero for a neutron star. $\endgroup$
    – Zo the Relativist
    Commented Jan 22, 2013 at 18:35
  • $\begingroup$ I suspect the answer is probably "no" for a naked neutron star. If the star retains an envelope from earlier in its evolution it might still be able to fuel a wind. With just the star, though, I think the essentially solid nature of the stellar surface would prevent a sustained wind. Unfortunately I can't find any references to back up my intuition... $\endgroup$
    – Kyle Oman
    Commented Jan 22, 2013 at 19:02
  • $\begingroup$ Dear @JerrySchirmer, concerning neutron stars, they surely exhibit great diversity, they also evolve (cool down), may be in accreting binary, have an envelope of remnants, etc. Still, it would be interesting to know the magnitude of the most typical mass loss rates for most typical isolated neutron stars. Concerning Hawking radiation, the key thing for it is not an event horizon, but a "membrane" surface, which lets everything go one way only. As neutron stars are very opaque, I could imagine easily particles getting stuck in their atmosphere, as if behind a membrane. $\endgroup$
    – Alexey Bobrick
    Commented Jan 22, 2013 at 22:14
  • $\begingroup$ Dear @Kyle, the word solid might be misleading. Surely, the densities of matter are extreme. However, the structure of the atmosphere is not crystalline, as is the case for the solids we are used to. Instead, it is dense gas-like matter, and even though the scale height of an atmosphere is very small, the transition between the star and the vacuum is smooth, and the density (and pressure) drops to zero continuously. This resembles atmospheres of real stars, except for that here they are tremendously compressed and have rather exotic state of matter. $\endgroup$
    – Alexey Bobrick
    Commented Jan 22, 2013 at 22:29
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    $\begingroup$ @Kyle The bulk material is generally described as a "degenerate gas", but that should not be confused with a diffuse gas of the sorts we encounter on Earth. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Jan 23, 2013 at 3:32

2 Answers 2

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I don't claim to be an expert on this topic, but I found this recent review paper: arXiv:1211.0852. Here's a short summary covering the questions above.

Isolated neutron stars can emit a wind powered by the rotational energy of the star. As it slows, it loses energy at a rate $\dot{E} = 4\pi^2I\dot{P}/P^3$ where $E$ is the rotational kinetic energy of the star, $P$ is its rotational period, $I$ is its moment of inertia and a dot denotes a time derivative. This energy loss can drive a relativistic wind of electrons and positrons, which is visible via synchrotron radiation from the interaction of the wind particles with the (often very strong!) local magnetic field. Whether all neutron stars DO emit a wind is as yet unknown.

87 out of 103 galactic objects (at least tentatively) identified as neutron stars have an associated detection showing or suggesting a wind. These are primarily stars not in binary systems, but instead discovered in supernova remnants.

As to Hawking radiation, my understanding is that the process requires an event horizon to occur, but maybe someone with a better understanding of that process can weight in.

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  • $\begingroup$ Thank you very much for the reference! Though the article is non-refereed, it has a number of references therein. My impression is that they talk mainly about newly born and/or highly magnetised pulsars, focusing on the effect of those on the interstellar medium. Young pulsars apriori emit a lot (especially, neutrinos), as they are very hot. For example, two references (the text, section about winds) Pacini & Salvati (1973), and Rees & Gunn (1974) discuss Crab pulsar (very young one). $\endgroup$
    – Alexey Bobrick
    Commented Jan 23, 2013 at 21:38
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    $\begingroup$ Further, Pacini (1967)‡, Gold (1968) suggest two other mechanisms for the neutron stars to emit energy: 1) Dipole emission, 2) Strong magnetic field drags the ambient plasma to corotate with the neutron star, which will accelerate it to ultrarelativistic speeds. The latter possibility is not a wind from the neutron star precisely speaking, but looks very interesting. In the end the article leaves it as a question, if winds from neutron stars can power pulsar wind nebulae, found around the most magnetised pulsars. Yet, one can see from Fig 1, nuetron stars in the article are not very typical. $\endgroup$
    – Alexey Bobrick
    Commented Jan 23, 2013 at 21:46
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Young neutron stars and the winds they energize, lay cause to some of the most extreme physical environments in the universe. The exact plasma and wind production mechanism are not well understood, but the basic picture is as follows.

At the stellar surface, the pulsar’s huge magnetic fields and rapid rotation induce enormous electric fields within the magnetosphere, these consequently tear particles from the stellar surface and accelerate them to high energies. Plasma then fills the magnetosphere and the extreme magnetic field present is sufficient to cause the plasma to rigidly co-rotate. However, this co-rotation must cease somewhere near the light cylinder, and the particles flow along the opened magnetic field lines, carrying away energy in the form of an ultrarelativistic magnetized wind.

In those cases, where the conditions are conducive to the formation of a rapidly rotating neutron star, a pulsar wind, driven by the pulsar spin-down power is likely to always be formed.

If yes, what composition would it have? If yes, what will be the rate of mass loss for the star?

The first of the above questions depends on the angle that the axis of the stars magnetic field makes with its rotational axis. In reality, pulsars will almost always be oblique rotators (with $\mathbf{B}$ miss-aligned to the axis of rotation). In this case the wind from the star will take the form of a striped wind. However, most mathematical models of pulsars assume and aligned rotator and instead of modelling this stripped wind explicitly they do this implicitly - this avoids 3D models and a massive increase in complexity.

The rate of mass loss due to such a wind is something that we do not know for sure. However, it is likely to be directly ascociated with the spin-down power of the star - but again, depending on the 'obliqeness' of the rotation, this will vary.

One might also think up that the process of Hawking radiation might possibly be applicable to neutron stars...

I know of no reason to suggest that Hawking Radiation would act in the way you have suggested. This mechanism is purely associated with event horizons. However, one mechanism associated predominantly with black-holes and that may have some bearing for pulsars is the Blandford–Znajek process. This is a mechanism for the extraction of energy from a rotating black hole. It is one of the best explanations for the way quasars are powered. It requires an accretion disc with a strong polar magnetic field around a spinning black hole. The magnetic field extracts spin energy and the power can be estimated as the energy density at the speed of light cylinder. I have never seen suggestion of this process being applicable to neutron stars, but I have not looked - to me it would be a far more likely mechanism...

I hope this helps.

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