The discovery will not only shed new light on the size and internal structure of such elusive neutron stars, but will also make it possible to test one of the fundamental predictions of Einstein's relativity theory.
A team of astronomers from the University of Amsterdam (UvA) and NASA have discovered the first millisecond X-ray pulsar to be eclipsed by its companion star. Their discovery will not only shed new light on the size and internal structure of such elusive neutron stars, but will also make it possible to test one of the fundamental predictions of Einstein's relativity theory. The discovery was recently published in the scientific journal Astrophysical Journal Letters.
A pulsar is a rapidly spinning neutron star; the crushed core of a massive star that exploded long ago as a supernova. Despite packing one and a half times the mass of the sun, neutron stars measure no more than 15 to 20 kilometres across.
During the week-long outburst, RXTE observed three periods when J1749's X-ray emission briefly disappeared. These eclipses, each lasting 36 minutes, occurred whenever the neutron star passed behind the normal star in the system.
‘This is an amazing discovery', states the article's main author, Diego Altamirano (UvA). ‘We know of only 13 neutron stars that rotate at speeds higher than 100 times a second. This is the first one to also eclipse. We can now establish the size and mass of the companion star with unprecedented accuracy.' The astronomers determined that the star has 60-80% of the sun's mass. ‘But', says Altamirano, ‘because its mass is slowly being sucked away by the extreme gravitational field of the neutron star, it's getting lighter and lighter'.
The pulsar's mass can be estimated to be between 1.4 and 2.2 times that of the sun. But in order to be able to nail down its precise size and mass, the astronomers will need to study the companion star with optical or infrared telescopes (to obtain the same information about the pulsar that the pulsar's orbital motion revealed about the star).
High-precision measurements of the X-ray pulses just before and after an eclipse will also make it possible to perform one of the tests of Einstein's General Theory of Relativity known as the Shapiro time delay. One consequence of relativity is that a signal experiences a slight timing delay when it passes very close to a massive object. For J1749, the predicted Shapiro delay is 21 microseconds, or 10,000 times faster than the blink of an eye. With only three eclipses observed during the 2010 outburst, RXTE was not able to capture enough data to reveal a large delay. However, the measurements did set a limit on how massive the normal star can be. If the star's mass had been greater than 2.2 times the sun's, RXTE would have seen the delay.