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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.

Fast X-ray pulses

On 10 April the system known as Swift J1749.4-2807 - or J1749 for short - erupted with an X-ray outburst. During the event, NASA's Rossi X-ray Timing Explorer (RXTE) observed three eclipses, detected X-ray pulses that identified the neutron star as a pulsar, and even recorded pulse variations indicating the neutron star's orbital period. J1749 was discovered in June 2006, when a smaller eruption brought it to the attention of NASA's Swift satellite. Observations by Swift, RXTE and other telescopes revealed the star to be part of a binary system located 250 thousand billion kilometres from the earth in the constellation Sagittarius. They also revealed that the neutron star is actively capturing gas from its stellar partner, which has gathered into an accretion disk around the neutron star.

Like many accreting binary systems, J1749 undergoes outbursts when gas from the disk crashes onto the neutron star. The pulsar's powerful magnetic field directs infalling gas onto the star's magnetic poles. This results in high-energy hot spots that rotate with the neutron star, producing fast X-ray pulses. J1749 spins on its axis at a rate of 518 times a second (the earth takes 24 hours to complete just one rotation). In addition, the pulsar's orbital motion causes small but regular changes in the frequency of the X-ray pulses. These changes make it possible to establish that it takes the stars just under nine hours to revolve around each other. Compare that with the 365 days it takes the earth to complete a single revolution around the sun.

Neutron star slowly sucking pulsar away

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).

Shapiro time delay

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.