Neutron Star Mystery Confounds Existing Science


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Amsterdam astronomers have discovered a neutron star that confounds existing models for thermonuclear explosions in such extreme objects. In the case of the accreting pulsar IGR J17480-2446, it seems to be a strong magnetic field that causes some parts of the star to burn more brightly than the rest.

The specific neutron star is part of the X-ray binary IGR J17480-2446 (hereafter J17480). X-ray binaries consist of a neutron star and a companion star in orbit around each other. Neutron stars, which are about 1.5 times as massive as the Sun, with a diameter of about 25 km, have a strong gravitational field that can pull gas from the companion star. This gas can build up on the neutron star surface and explode in a fast, high-energy thermonuclear reaction.

Normally, the entire surface of the star explodes uniformly. However, in about 10 percent of cases, some parts of the star become much brighter than the rest. Why this occurs is not understood.

In recent years a number of theoretical models have been developed to explain this phenomenon. According to one model, the rapid rotation of the neutron star prevents the burning material from spreading, just as the rotation of the Earth contributes to the formation of hurricanes via the Coriolis force. Another idea is that the explosion generates global-scale waves in the surface ‘ocean’ layers of the star. The ocean on one side of the star cools and dims as it rises up, while the other stays warmer and brighter.

The new study of J17480 excludes both of these models. Like other stars, J17480 develops unusually bright surface patches during thermonuclear explosions. However the star rotates much more slowly than other neutron stars that exhibit this behavior — only 10 times per second (the next slowest rotates 245 times per second). At this speed, the Coriolis force is not strong enough to affect the flame front, preventing the formation of thermonuclear hurricanes. The development of large-scale ocean waves can also be ruled out.

Instead, the astronomers think that the magnetic field of the star might explain the uneven burning. The exploding gas expands, moving upwards and outwards. This churns up the magnetic field, which acts like an elastic band to prevent the burning bubble from spreading any further.

“More theoretical work is needed to confirm this, but in the case of J17480 it is a very plausible explanation for our observations”, says lead author Yuri Cavecchi (University of Amsterdam, the Netherlands).

Co-author Anna Watts (University of Amsterdam) stressed that their new model may not necessarily explain non-uniform burning for all stars. “The new mechanism may only work in stars like this one, with magnetic fields that are strong enough to stop the flame front from spreading. For other stars with this odd burning behavior, the old models might still apply.”

Astronomer's at the Harvard Chandra X-Ray Telescope took the image at the top of the page that shows the 2,000 year-old-remnant of such a cosmic explosion, known as RCW 103, which occurred about 10,000 light years from Earth.

In Chandra's image (above), the colors of red, green, and blue are mapped to low, medium, and high-energy X-rays. At the center, the bright blue dot is likely the neutron star that astronomers believe formed when the star exploded. For several years astronomers have struggled to understand the behavior of the this object, which exhibits unusually large variations in its X-ray emission over a period of years.

Neutron stars are extremely dense remnants of exploded stars about the size of Manhattan consisting of tightly packed neutrons. When stars are more massive than about 8 times the Sun, they end their lives in a spectacular explosion called a supernova. The outer layers of the star are hurtled out into space at thousands of miles an hour, leaving a debris field of gas and dust. Where the star once was located, a small, collapsed, incredibly dense object, a neutron star, is often found. While only 10 miles or so across, the tightly packed neutrons in such a star contain more mass than the entire Sun.

The result of the final implosion is an unimaginably compacted core: atoms would be crushed together with their electrons squeezed into the nucleus, forming neutrons and a neutron star, with a core so dense that a single spoonful would weigh 200 billion pounds.

Most neutron stars house incredibly large magnetic fields. If they are spinning rapidly they make fabulous clocks, cosmic radio beacons we call pulsars. Pulsars can keep time to an accuracy better that one microsecond per year. Some pulsars generate more than 1000 pulses per second, which means, as Lawrence Krauss wrote in The Physics of Star Trek, that an object with the mass of the Sun packed into an object 10 to 20 kilometers across is rotating over 1000 times per second, or more that half the speed of light!

Oddly, though new evidence from Chandra implies that the neutron star near the center of RCW 103 is rotating only once every 6.7 hours, confirming recent work from the XMM-Newton Space Telescope. This is much slower than a neutron star of its age should be spinning.

One possible solution to this mystery is that the massive progenitor star to RCW 103 may not have exploded in isolation. Rather, a low-mass star that is too dim to see directly may be orbiting around the neutron star. Gas flowing from this unseen neighbor onto the neutron star might be powering its X-ray emission, and the interaction of the magnetic field of the two stars could have caused the neutron star to slow its rotation.

The Daily Galaxy via and University of Amsterdam


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