Speeding neutron star caught with 'radar gun'
Posted: April 23, 2002

NASA scientists have observed a rare thermonuclear explosion on a neutron star that brightened it for so long that they could detect its motion as it moved towards and away from us on its orbit around a companion star. This enabled them to measure the star's orbital velocity using the Doppler effect in the same way a state trooper nabs speeding motorists.

This three-hour "superburst" also revealed the neutron star's spin frequency, confirming two key theories about neutron stars: that they can evolve into faster-spinning pulsars, and that the flickering of X rays frequently seen in short bursts, called burst oscillations, are a direct measurement of spin frequency.

Drs. Tod Strohmayer and Craig Markwardt of NASA's Goddard Space Flight Center in Greenbelt, Md., discussed the observation, made with NASA's Rossi X-ray Timing Explorer, at the joint meeting of the American Physical Society and the High Energy Astrophysics Division of the American Astronomical Society in Albuquerque, New Mexico.

"This burst allowed us to do some interstellar police work," said Markwardt of this neutron star, named 4U 1636-53. "Because the burst was so powerful -- a trillion times greater than the total U.S. energy consumption last year -- we could study the neutron star for much longer than usual. Its fast spin, well beyond the speed limit of most neutron stars, is strong evidence that these objects evolve into faster-spinning pulsars."

A pulsar is a neutron star that emits steady pulses of radiation with each spin. A neutron star is the skeletal remains of a massive star that exhausted its nuclear fuel and subsequently ejected its outer shell in a supernova explosion. The remaining core, still possessing about a sun's worth of mass, collapses to a sphere no larger than Albuquerque, about 7 miles in diameter.

Neutron stars in "low mass" binary star systems such as the one observed here (where the companion has less mass than the Sun) have been suspected as the sites where slowly spinning neutron stars are spun-up to millisecond spin periods. A neutron star has a powerful gravitational field, and it can accrete gas from its companion. Matter spirals toward the neutron star in the form of an accretion disk, a journey visible in X-ray radiation. In doing so, it transfers its orbital energy to the neutron star, making it spin faster and faster. 4U 1636-53 is now spinning 582 times per second. The theory is that such a neutron star may eventually cannibalize its companion, becoming an isolated millisecond pulsar.

"By seeing X rays produced from both accretion and the rapidly spinning neutron star, we have almost an open and shut case that millisecond pulsars are born in binary systems like 4U 1636-53," Strohmayer said.

Also, Strohmayer and Markwardt, a researcher with the University of Maryland, College Park, could determine conclusively that burst oscillations -- X-ray pulsations seen in many shorter, normal bursts -- are directly related to the spin of the neutron star.

"We've seen many normal bursts from 4U 1636-53 with 582 Hz burst oscillations, but they usually only last about 10 seconds," Strohmayer said. "The superburst let us see the pulses for much longer, but since both types of bursts have the same pulsation frequency, we know they must be caused by the spinning neutron star. We can now confidently assume that burst oscillations seen in other stars are indeed a reflection of the spin frequency."

By taking advantage of the Doppler effect, Strohmayer and Markwardt measured the neutron star's orbital velocity, a "bonus" measurement which enabled them to also determine an overall mass for the system. Future observations may determine the mass of the companion star, which would then lead to an estimate of the neutron star's mass and radius, two hard-sought properties of neutron stars that would reveal their structure and evolution.

The Rossi Explorer, launched in 1995, is operated by NASA's Goddard Space Flight Center.