Journey to the Center of a Neutron Star
by Christopher Wanjek
Astronomers are using whatever means possible to probe the ultradense interiors of neutron stars.
A neutron star is not a place most would want to visit. This dense remnant of a collapsed star has a magnetic field billions of times stronger than Earth's, enough to shuffle your body's molecules long before you even land. The featureless surface is no fun either. Crushing gravity ensures that the star is a near perfect sphere, compressing all matter so that a sand-grain-sized scoop of neutron star material would weigh as much as a battleship on Earth.
At least black holes offer the promise of funky singularity, time warps, and the Odyssean temptation to venture beyond a point of no return. What's a journey to a neutron star good for, one might ask? Well, for starters, it offers the possibility of confirming a theorized state of matter called quark-gluon plasma, which likely existed for a moment after the Big Bang and now might only exist in the superdense interiors of neutron stars.
Beneath the neutron star crust, a kilometer-thick plate of crystalline matter, lies the great unknown. The popular theory is that the neutron star interior is made up of a neutron superfluid ? a fluid without friction. With the help of two NASA satellites ? the Rossi X-Ray Timing Explorer and the Chandra X-Ray Observatory ? scientists are journeying to the center of a neutron star. Matter might be so compressed there that it breaks down into quarks, the building blocks of protons and neutrons, and gluons, the carrier of the strong nuclear force.
To dig inside a neutron star, no simple drill bit will do. Scientists gain insight into the interior through events called glitches, a sudden change in the neutron star's precise spin rate. "Glitches are one of the few ways we have to study the neutron star interior," says Frank Marshall of NASA's Goddard Space Flight Center, who has used the Rossi Explorer to follow the escapades of the glitchiest of all neutron stars, dubbed the Big Glitcher and known scientifically as PSR J0537-6910.
Glitches speed up neutron star spins. Marshall said that PSR J0537-6910's frequent glitches, about three per year, strongly support the neutron superfluid theory. The neutron star's magnetic field slows the crust's spin, yet the superfluid interior spins down at a slower rate. As the difference of the two speeds widens, the superfluid reaches a critical threshold and, like a flywheel, suddenly transfers its built-up angular momentum to the crust, causing the "exterior" of the neutron star to spin faster. The glitch's size that is, how much it quickens the spin ? is a reflection of the mass, density, and other characteristics of the superfluid.
"Given enough pressure, the neutron superfluid will dissolve into quarks," perhaps in the core, said Bennett Link of Montana State University. Quarks normally exist in pairs or triplets and are bound to each other by gluons. Theory has it that quarks cannot exist in a free state, but that theory may fall by the wayside in a neutron star's inner core. No one knows for sure. Glitches can only provide information about the amount of superfluid, not quarks.
Some neutron stars might accrete enough gas from a companion star to add enough pressure on the surface to squeeze the neutron superfluid interior into free quarks. Or, as the neutron star's spin slows down over millions of years, the centrifugal force from within could ease enough to allow the crust to collapse, liberating quarks. In either case, the diameter of the neutron star would shrink, quickening the spin. The neutron star would become a theorized quark star. If it collapsed any further, it would become a black hole.
To understand the nature of the neutron star's interior, astronomers need to determine the ratio between the star's mass and radius. Masses are well known, particularly for neutron stars in binary systems in which the orbital period is dictated by mass and distance. No one has accurately measured a neutron star radius, however, because these objects are so tiny.
Frederick Walter of SUNY at Stony Brook used Chandra to determine the radius of an isolated neutron star called RX J185635-3754. This radius, about 4.5 kilometers, is on the cusp between that theorized for quark stars and neutron stars. Unfortunately, Walter's Hubble Space Telescope data don't quite match up, so he is reanalyzing his observation.
Many in the field believe we are only a few years from nailing down radii in significant numbers to determine a neutron star's interior. Particle physicists hope to see fleeting free quarks and gluons with the Brookhaven National Lab's Relativistic Heavy Iron Collider. Using the universe as a laboratory, astronomers just might beat them, for neutron star diners might serve up a tasty bowl of quark soup 24/7, no reservations required.
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