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The setup is very simple: you have a regular ($1.35$ to $2$ solar masses) evolved neutron star, and you shine plane electromagnetic waves on it with given $\lambda$. Very roughly, what shall be the total flux of absorbed/scattered EM radiation?

Shall the result change if the neutron star is young and not evolved?

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4 Answers 4

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A neutron star will have a thin layer of normal matter at the surface, and of course this reflects light just like any other normal matter.

But I guess you're really asking if neutronium reflects light, and that's a very good question that a quick Google failed to answer. EM radiation generally interacts with dipoles or scatters off electrons, so I'd guess matter made of neutrons should be transparent.

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    $\begingroup$ AFAIK, neutron star $\neq$ neutronium, neutronium $\implies$ " $n$ revolving around $\bar{n}$ atoms". Neutron star\implies "pure $n$ with atomic matter shell. $\endgroup$
    – Manishearth
    Mar 23, 2012 at 12:44
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    $\begingroup$ Scratch that, neutronium=pure neutrons. Wierd--the rest of the particles become particle-antiparticle pairs when appended with -onium. $\endgroup$
    – Manishearth
    Mar 23, 2012 at 12:46
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    $\begingroup$ Neutron stars don't just consist of neutrons even in their interiors. I believe this makes your guess wrong. There are plenty of (free) electrons inside neutron stars. $\endgroup$
    – ProfRob
    Jan 14, 2015 at 18:04
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    $\begingroup$ Why would neutron stars be reflective? They are usually assumed to be near-perfect black bodies and therefore absorb almost all light incident upon them. If the surface was reflective, neutron stars would be unable to cool through radiative processes. $\endgroup$
    – ProfRob
    Jan 14, 2015 at 18:27
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    $\begingroup$ The question does not ask about neutronium, which does not exist inside neutron stars. Your answer is vague about the reflective properties of the surface - everything bar a black hole has a reflectivity somewhere between 0 and 100%. $\endgroup$
    – ProfRob
    Jan 15, 2015 at 7:38
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Neutron stars are usually assumed to have intrinsic spectra that approximate closely to black bodies - though the details are many and need a full consideration of the strong magnetic fields and electrodynamics near the surface. They have a thin layer (few cm) of non-degenerate, gaseous material at their surfaces, rich in ionised or partially ionised iron-peak elements. However, accretion from the ISM possibly means that most neutron stars have a skin of hydrogen or helium present.

Modelling of neutron star atmospheres is well established (I found this review by Potehkin (2014) extremely useful). Isolated, non-magnetic neutron stars have spectra that are close, but not identical to black bodies.There can be absorption features if there are significant densities of iron-peak elements in the atmosphere. H/He atmosphere have spectra shifted towards higher energies. Strong magnetic fields introduce more complications, including cyclotron absorption lines. In these models the neutron star is unlikely to be very reflective because by definition, something that approximates a blackbody absorbs most radiation incident upon it. Some isolated neutron stars have now been observed at X-ray wavelengths and their spectra are close to blackbodies with some evidence for broad absorption features that might be due to cyclotron lines (e.g. Haberl 2005).

However, there is an idea that in some neutron stars, the outer atmosphere may be cool enough and/or the magnetic field strong enough to cause a phase transition to a solid state just below a very thin hydrogen atmosphere (Turolla et al. 2004). In these circumstances the surface can become (partially) reflective. Potekhin et al.(2012) considered a condensed iron surface in strong magnetic fields ($10^{12}-10^{14}\ G$) covered by a very thin atmosphere. They find certain photon energy ranges where the reflectivity can reach of order 50 percent.

Older neutron stars will have lower temperatures and probably lower magnetic fields. The ionisation state of the atmosphere will change and the effects of magnetic fields would diminish.

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  • $\begingroup$ Hm, generally, one assumes a general statement first and then argues for a particular one. For example, one would generally expect even optically thick media to be reflective, and indeed why would it not scatter some of the incident light back? $\endgroup$
    – Alexey Bobrick
    Jan 14, 2015 at 18:51
  • $\begingroup$ @AlexeyBobrick Yes, I expect there will be small, wavelength dependent departures from black body behaviour, just as there are for other stellar atmospheres. I suppose it depends what you mean by "some". What ionised gases are highly reflective? $\endgroup$
    – ProfRob
    Jan 14, 2015 at 19:13
  • $\begingroup$ Highly - none, but reflection coefficient of 0.1-0.2 is not that small either. Consider clouds - they are not ionised, granted, but are opaque and rather reflective. Why would dense neutron star atmosphere not be similarly reflective? $\endgroup$
    – Alexey Bobrick
    Jan 15, 2015 at 12:18
  • $\begingroup$ @AlexeyBobrick So you are saying that if I shine a laser (plane EM wave as per the question) at a "cloud" I will receive 10-20% of the radiation back?? No. And neither can a cloud be considered a black body, whereas a neutron star can (to a good approximation). $\endgroup$
    – ProfRob
    Jan 15, 2015 at 12:25
  • $\begingroup$ I was having in mind something like this: en.wikipedia.org/wiki/Cloud_albedo . The laser probably would be better reflected if it were tuned at some of the absorption lines. And yes, it is not a black body. But black body approximation is used for getting emission and due to the fact that neutron star's atmosphere is optically thick at the wavelengths it is emitting. $\endgroup$
    – Alexey Bobrick
    Jan 15, 2015 at 12:36
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A neutron star is mostly neutrons, but it contains protons to a certain extent allowed by the gravity, since a pure neutron state is unstable to beta decay. The protons collect on the surface due to their electrostatic repulsion, and form a fermi-gas like state there.

The fermi-gas of protons will reflect long-wavelength light very much like an ordinary metal-- the surface will, if you scrape off the ordinary matter, be shiny like a mirror.

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  • $\begingroup$ Neutron star remains neutral due to charge conservation, so macroscopic motion of protons due to electrostatic forces will not happen. Another point to mention is that neutron stars have to have an atmosphere, as the surface pressure contiuously drops to zero (otherwise the surface would expand until the outer pressure gets to zero). Hence close to the surface in a thin layer there will be all sorts of physical states of matter. One definite conclusion from all this and from what you say, indirectly, is that neutron stars are not transparent. $\endgroup$
    – Alexey Bobrick
    Sep 9, 2012 at 15:30
  • $\begingroup$ @AlexeyBobrick:yes, I agree, the electron gas neutralizing the proton is also like a metal. So you have two charged fluids reflecting light, it's still shiny once you scrape the ordinary matter off. $\endgroup$
    – Ron Maimon
    Sep 9, 2012 at 15:47
  • $\begingroup$ This is right. Though the atmosphere could make it look lika a metal in a thin hydrogen cloud, with some wierd layers in between (strong magnetic field, high gradients of species distribution, etc.) $\endgroup$
    – Alexey Bobrick
    Sep 10, 2012 at 17:58
  • $\begingroup$ The problem here is that free protons don't gather on the surface. They recombine with electrons to form neutrons. The surfaces of neutron stars probably consist of non-degenerate, partially ionised, iron-peak nuclei and free electrons. $\endgroup$
    – ProfRob
    Jan 14, 2015 at 18:33
  • $\begingroup$ @RobJeffries: Their is an equilibrium density of protons among the neutrons, because neutron purification by gravitational instability is never perfect. The extra protons collect on the surface. The ordinary matter is a separate thing, forming what is called an "atmosphere" over the neutron star surface. The nuclear ball contains a proton fraction in thermodynamic equilibrium through beta decay. $\endgroup$
    – Ron Maimon
    Jan 14, 2015 at 21:15
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Martin, the infalling light is blue-shifted, and red-shifted on reflection. No overall change, I think. However, a suitably mechanically strong light-source on the surface of the neutron star (!) will be seen to emit light that is redder than usual.

If the neutron star had its normal matter scraped off (left as an exercise for the student) then I don't see how light would interact at all with it. Hard gamma rays would be absorbed, but anything else? Nah.

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    $\begingroup$ Neutrons have a dipole moment, and there is a proton gas density in neutron stars, which should reflect light like a shiny metal. $\endgroup$
    – Ron Maimon
    Aug 23, 2012 at 6:08
  • $\begingroup$ Same comment as for the accepted answer. Neutron stars contain plenty of free protons and electrons. They would be utterly opaque to light. Whether they would be reflective is a different matter. You certainly can't scrape the normal matter off, because more normal matter would form - like a tarnish perhaps. $\endgroup$
    – ProfRob
    Jan 14, 2015 at 18:07
  • $\begingroup$ Would there also be a layer of white dwarf matter between the neutron matter and the normal matter? $\endgroup$
    – ikrase
    Sep 9, 2019 at 6:56

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