Using NASA's Rossi X-ray Timing Explorer (RXTE), astronomers have discovered the first fast X-ray pulsar to be eclipsed by its companion star.

Pulsars are rapidly spinning neutron stars – the dense cores of massive stars that exploded as dramatic supernovae events at the end of their lives. They pack Sun-sized masses into a core that spans just 16-24 kilometres across, and spin hundreds of times per second.

RXTE detected an X-ray outburst from an object known as Swift J1749, which was originally detected by the Swift satellite in 2006 and found to be part of a binary system, feeding off gas from its stellar companion. The two stars orbit around each other every 8.8 hours and J1749 is spinning at a dizzy 518 times a second.

“Like many accreting binary systems, J1749 undergoes outbursts when instabilities in the accretion disc allow some of the gas to crash onto the neutron star,” says RXTE project scientist Tod Strohmayer. This is facilitated by the pulsar's intense magnetic fields that direct the infalling gas onto the star's magnetic poles.

During a week-long outburst in April of this year, RXTE recorded three periods where J1749’s X-ray emission appeared to disappear, corresponding to a 36 minute-long eclipse when the neutron star passes behind its companion. “This is the first time we’ve detected X-ray eclipses from a fast pulsar that is also accreting gas,” says Craig Markwardt at NASA’s Goddard Space Flight Center. “Using this information, we now know the size and mass of the companion star with unprecedented accuracy.”

The team determined that the normal star weighs around 70 percent of the Sun's mass, but that it is puffed up to some 20 percent larger than it should be for its mass and 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,” says Markwardt. “This additional heating probably also makes the star’s surface especially disturbed and stormy.”

The pulsar's mass lies between 1.4 and 2.2 times the Sun's mass, but the team need one more piece of information to constrain this value. “We need to detect the normal star in the system with optical or infrared telescopes,” Strohmayer said. “Then we can measure its motion and extract the same information about the pulsar that the pulsar’s motion told us about the star.”

However, high-precision measurements of the X-ray pulses just before and after an eclipse – well within the capability of RXTE – may give the team the answer they are looking for. One consequence of relativity is that a signal (such as an X-ray pulse) experiences a slight timing delay when it passes very close to a massive object. Known as the Shapiro delay after its proposer Irwin Shaprio, this value is predicted as 21 microseconds for J1749. Although RXTE’s impressive timing resolution allows it to record changes seven times faster, with only three eclipses observed during the 2010 outburst it did not capture enough data to reveal a large delay. The data did set an upper limit on the star's mass however, for if the mass was greater than 2.2 times the Sun’s, RXTE would have seen the delay.

“We believe this is the first time anyone has set realistic limits for this effect at X-ray wavelengths outside of our Solar System,” says Markwardt. “The next time J1749 has an outburst, RXTE absolutely could measure its Shapiro delay.”

The results are reported in the 10 July issue of The Astrophysical Journal Letters.