Washington: NASA's Rossi X-ray Timing Explorer (RXTE) has found the first fast X-ray pulsar to be eclipsed by its companion star.
The study will help scientists understand some of the most compressed matter in the universe and test a key prediction of Einstein’s relativity theory.
The pulsar is a rapidly spinning neutron star -- the crushed core of a massive star that long ago exploded as a supernova.
Neutron stars are between 10 and 15 miles across and compress more than the Sun’s mass into a ball nearly 60,000 times smaller.
The system will help the team narrow down the precise masses for neutron stars.
Known as Swift J1749.4-2807 - J1749 for short, its source was a binary system located 22,000 light-years away in the constellation Sagittarius and that the neutron star was actively capturing, or accreting, gas from its stellar partner. This gas gathers into a disk around the neutron star.
The pulsar’s powerful magnetic field directs infalling gas onto the star’s magnetic poles. This means that the energy release occurs in hot spots that rotate with the neutron star, producing fast X-ray pulses - of the order of 518 times a second.
"This is the first time we’ve detected X-ray eclipses from a fast pulsar that is also accreting gas," Markwardt said.
"Using this information, we now know the size and mass of the companion star with unprecedented accuracy," he added.
But the eclipses indicate that the star is 20 percent larger than it should be for its mass and apparent age.
"We believe that the star’s surface is ''puffed up'' by radiation from the pulsar, which is only about a million miles away from it," Markwardt explained.
One consequence of relativity is that a signal -- such as a radio wave or an X-ray pulse -- experiences a slight timing delay when it passes very close to a massive object.
"High-precision measurements of the X-ray pulses just before and after an eclipse would give us a detailed picture of the entire system," Tod Strohmayer said.
"We believe this is the first time anyone has set realistic limits for this effect at X-ray wavelengths outside of our solar system," Markwardt noted.
Their findings are published in the July 10 issue of The Astrophysical
First Published: Wednesday, August 18, 2010, 18:44