UT: Making Sense Of The Neutron Zoo
Posted: Sat May 15, 2010 7:41 pm
Astronomy Without a Telescope – Making Sense Of The Neutron Zoo
Universe Today - 15 May 2010
Grand Unification in Neutron Stars
Universe Today - 15 May 2010
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.
Grand Unification in Neutron Stars
- arXiv.org > astro-ph > arXiv:1005.0876 > 06 May 2010
<-- PreviousVictoria 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.