The spectacular gravity of neutron stars offers great opportunities for thought experiments. For example, if you dropped an object from a height of 1 meter above a neutron star’s surface, it would hit the surface within a millionth of a second having been accelerated to over 7 million kilometers an hour.
But these days you should first be clear what kind of neutron star you are talking about. With ever more x-ray sensitive equipment scanning the skies, notably the ten year old Chandra space telescope, a surprising diversity of neutron star types are emerging.
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Neutron stars are the compressed remnant of a star left behind after it went supernova. They retain much of that stars angular momentum, but within a highly compressed object only 10 to 20 kilometers in diameter. So, like ice skaters when they pull their arms in – neutron stars spin pretty fast.
Furthermore, compressing a star's magnetic field into the smaller volume of the neutron star, increases the strength of that magnetic field substantially. However, these strong magnetic fields create drag against the stars' own stellar wind of charged particles, meaning that all neutron stars are in the process of 'spinning down'.
This spin down correlates with an increase in luminosity, albeit much of it is in x-ray wavelengths. This is presumably because a fast spin expands the star outwards, while a slower spin lets stellar material compress inwards – so like a bicycle pump it heats up. Hence the name rotation powered pulsar (RRP) for your ‘standard’ neutron stars, where that beam of energy flashing at you once every rotation is a result of the braking action of the magnetic field on the star's spin.
It’s been suggested that magnetars may just be a higher order of this same RRP effect. Victoria Kaspi has suggested it may be time to consider a ‘grand unified theory' of neutron stars where all the various species might be explained by their initial conditions, particularly their initial magnetic field strength, as well as their age.
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Other neutron stars might begin life in less dramatic fashion, as the much more common and just averagely luminous RRPs, which spin down at a more leisurely rate – never achieving the extraordinary luminosities that magnetars are capable of, but managing to remain luminous for longer time periods.
The relatively quiet Central Compact Objects, which don’t seem to even pulse in radio anymore, could represent the end stage in the neutron star life cycle, beyond which the stars hit the dead line, where a highly degraded magnetic field is no longer able to apply the brakes to the stars' spin. This removes the main cause of their characteristic luminosity and pulsar behaviour – so they just fade quietly away.
For now, this grand unification scheme remains a compelling idea – perhaps awaiting another ten years of Chandra observations to confirm or modify it further.
Victoria M. Kaspi (McGill University) wrote:The last decade has shown us that the observational properties of neutron stars are remarkably diverse. From magnetars to rotating radio transients, from radio pulsars to `isolated neutron stars,' from central compact objects to millisecond pulsars, observational manifestations of neutron stars are surprisingly varied, with most properties totally unpredicted. The challenge is to establish an overarching physical theory of neutron stars and their birth properties that can explain this great diversity. Here I survey the disparate neutron stars classes, describe their properties, and highlight results made possible by the Chandra X-ray Observatory, in celebration of its tenth anniversary. Finally, I describe the current status of efforts at physical `grand unification' of this wealth of observational phenomena, and comment on possibilities for Chandra's next decade in this field.
The spectacular gravity of neutron stars offers great opportunities for thought experiments. For example, if you dropped an object from a height of 1 meter above a neutron star’s surface, it would hit the surface within a millionth of a second having been accelerated to over 7 million kilometers an hour.
Moral: People who live on neutron stars shouldn't drop stones.
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Just what do neutron stars quietly fade away into?
Billion pound dust bunnies?
Traffic hazzards for UFO's ?
Mediocre dispersion of leftover gas?
Or perhaps it manages to shrink enough to become too heavy to stay in this realm and it falls through to another realm alongside of this realm?
Or perhaps i am just killing time to avoid putting the mower deck on my lawn tractor and mowing the lawn for the first time this year?
Questions - Questions - everywhere and not an ambitious me in sight
The relatively quiet Central Compact Objects, which don’t seem to even pulse in radio anymore, could represent the end stage in the neutron star life cycle, beyond which the stars hit the dead line, where a highly degraded magnetic field is no longer able to apply the brakes to the stars' spin. This removes the main cause of their characteristic luminosity and pulsar behaviour – so they just fade quietly away.>>
Just what do neutron stars quietly fade away into?
Billion pound dust bunnies?
Traffic hazzards for UFO's ?
Mediocre dispersion of leftover gas?
Or perhaps it manages to shrink enough to become too heavy to stay in this realm and it falls through to another realm alongside of this realm?
Or perhaps i am just killing time to avoid putting the mower deck on my lawn tractor and mowing the lawn for the first time this year?
Questions - Questions - everywhere and not an ambitious me in sight
Think the Cassiopeia A Neutron Star sans remnant nebula:
Craig Heinke, University of Alberta
Wynn Ho (University of Southampton)
<<The nature of the central compact objects in supernova remnants, showing thermal spectra but no radio pulsations, is a major mystery of the Chandra era. The youngest known CCO in Cassiopeia A is perhaps the most mysterious, as spectral fits with single-component models have given emitting areas too small for neutron star radii, while a hot spot should produce pulsations that have not yet been detected. We have produced a variety of light-element atmosphere models for neutron stars, and found that only unmagnetized carbon atmospheres provide both acceptable fits to the Cas A CCO spectrum (T~1.5e6 K, R~12 km) and radius constraints consistent with modern NS models. Our result has ramifications for the evolution of NS surfaces, NS thermal evolution, and constraints on NS interior structure.>>
Central Compact Objects in Supernova Remnants as Anti-magnetars
Gotthelf, Eric V.; Halpern, J. P.
American Astronomical Society, HEAD meeting #11, #23.05
<<Central compact objects (CCOs) in supernova remnants (SNRs) are apparently isolated neutron stars, with steady flux, predominantly thermal X-ray emission, lacking an optical or radio counterpart, and without evidence of a pulsar wind nebula. Three are found to be pulsars, with periods of 0.105, 0.112, and 0.424 s. Until now, no spin-down was detected from a CCO, which we interpret as indicating a weak dipole magnetic field. In two cases, the upper limits on the magnetic field inferred from spin period measurements are 3.3x1011 G and 9.8x1011 G. Most of the properties of the CCOs can thus be explained by an "anti-magnetar" model, including the possibility that their weak magnetic fields are causally related to their slow rotation periods at birth through the turbulent dynamo that generates the magnetic field. While CCOs are inconspicuous relative to ordinary young pulsars and magnetars, the fact that they are found in SNRs in comparable numbers to other classes of neutron stars implies that they must represent a significant fraction of neutron star births.>>
biddie67 wrote:Can these CCOs be a source of -or- a concentration of dark energy/dark matter?
If they contain a significant concentration of dark matter then they would have to be smaller than the expected ~25 km in diameter.
If they contain a significant concentration of dark energy then they would have to be larger than the expected ~25 km in diameter.
The upper crust of a neutron star is thought to be composed of crystallized iron, may have centimeter high mountains and experiences occasional ‘star quakes’ which may proceed what is technically known as a glitch. These glitches and the subsequent post-glitch recovery period may offer some insight into the nature and behavior of the superfluid core of neutron stars.
The events leading up to a neutron star quake go something like this. All neutron stars tend to ‘spin down’ during their life cycle, as their magnetic field applies the brakes to the star’s spin. Magnetars, having particularly powerful magnetic fields, experience more powerful braking.
During this dynamic process, two conflicting forces operate on the geometry of the star. The very rapid spin tends to push out the star’s equator, making it an oblate spheroid. However, the star’s powerful gravity is also working to make the star conform to hydrostatic equilibrium (i.e. a sphere).
Thus, as the star spins down, its crust – which is reportedly 10 billion times the strength of steel – tends to buckle but not break. There may be a process like a tectonic shifting of crustal plates – which create 'mountains' only centimeters high, although from a base extending for several kilometres over the star's surface. This buckling may relieve some of stresses the crust is experiencing – but, as the process continues, the tension builds up and up until it ‘gives’ suddenly.
The sudden collapse of a 10 centimeter high mountain on the surface of a neutron star is considered to be a possible candidate event for the generation of detectable gravitational waves – although this is yet to be detected. But, even more dramatically, the quake event may be either coupled with – or perhaps even triggered by – a readjustment in the neutron’s stars magnetic field.
It may be that the tectonic shifting of crustal segments works to ‘wind ‘up’ the magnetic lines of force sticking out past the neutron star’s surface. Then, in a star quake event, there is a sudden and powerful energy release - which may be a result of the star’s magnetic field dropping to a lower energy level, as the star’s geometry readjusts itself. This energy release involves a huge flash of x and gamma rays.
In the case of a magnetar-type neutron star, this flash can outshine most other x-ray sources in the universe. Magnetar flashes also pump out substantial gamma rays – although these are referred to as soft gamma ray (SGR) emissions to distinguish them from more energetic gamma ray bursts (GRB) resulting from a range of other phenomena in the universe.
However, 'soft' is a bit of a misnomer as either burst type will kill you just as effectively if you are close enough. The magnetar SGR 1806-20 had one of largest (SGR) events on record in December 2004.
Along with the quake and the radiation burst, neutron stars may also experience a glitch – which is a sudden and temporary increase in the neutron’s star spin. This is partly a result of conservation of angular momentum as the star’s equator sucks itself in a bit (the old 'skater pulls arms in' analogy), but mathematical modeling suggests that this may not be sufficient to fully account for the temporary ‘spin up’ associated with a neutron star glitch.
González-Romero and Blázquez-Salcedo have proposed that an internal readjustment in the thermodynamics of the superfluid core may also play a role here, where the initial glitch heats the core and the post-glitch period involves the core and the crust achieving a new thermal equilibrium – at least until the next glitch.
Core-crust transition pressure evolution in post-glitch epoch for a Vela-type pulsar
We propose that the post-glitch epoch in a Vela-type pulsar corresponds to a transition time in which the broken crust is readjusting its temperature and core-crust transition pressure after the changes produced by the glitch. To describe the evolution of the pulsar in this epoch we use a sequence of stationary and axisymmetric relativistic models of slowly rotating neutron stars with a surface layer crust. Previously, we have formulated the matching conditions on the surface of the star in terms of physical properties (total mass of the star, core-crust transition pressure,...).
