Crushed star puts Einstein to the test

Artist's impression of the J1749 system

Artist's impression of the J1749 system, which comprises a superdense pulsar (a spinning neutron star) and a normal star closely orbiting each other.

  • Stellar pair includes pulsar and a normal star
  • X-rays from the pulsar picked up by space telescope
  • Measurements can test an aspect of Einstein’s theories

A space telescope that “sees” X-rays could be used to test a key prediction of Einstein’s relativity theory.

Scientists using NASA’s Rossi X-ray Timing Explorer (RXTE) have studied a pair of stars that orbit each other so closely that one of them moves in front of the other and causes regular eclipses.

The astronomers can use these eclipses, along with standard physics laws, to estimate the size and mass of one of the stars.

Known collectively as Swift J1749.4-2807—or J1749 for short—one of the objects is a super-dense body called a pulsar, while the other is a normal star. The system is 22,000 light-years from Earth.

Pulsars are spinning neutron stars, the remnant cores left over after a giant star explodes at the end of its life. The matter in a neutron star is so heavily squashed that electrons have been forced into their atoms’ cores and combine with protons to form neutrons, leaving just a huge mass of neutrons.

Neutron stars pack more than the Sun’s mass into a ball just 20 to 25 kilometres across. In fact, their matter is so densely compressed that just one teaspoonful would have a staggering mass of 4,500 million tonnes.

Pulsar is eating its neighbour

Pulsars emit lots of radiation in tight beams, and as they spin they can appear to pulse or flash on and off like lighthouses.

Artist's impression of a pulsar

Artist's impression of a pulsar dragging gas from its companion star.

Astronomers can learn a lot about a pulsar from those flashes, such as how fast it is spinning. The J1749 pulsar spins at 518 times per second!

With J1749, the RXTE satellite spotted three eclipses as well as three pulses of X-rays as the pulsar experienced a series of outbursts.

The X-rays came from hotspots on the pulsar, where gas—sucked (or accreted) from the outer atmosphere of the companion star—had spiralled down and crashed onto the pulsar’s surface. The pulsar is slowly eating its neighbour.

It was these bright X-ray flashes that drew the astronomers’ attention to the J1749 system.

Small variations in the flashes arise from the pulsar’s orbital motion with the companion star, and indicate that the pulsar whizzes around its companion in just 8.8 hours.

The duration of the eclipses have enabled the astronomers to calculate that the companion is about 70% as massive as our Sun, but about 20% bigger than it would normally be for a star of this type—this is because the energy emitted by the pulsar is heating the companion’s outer layers, making them puff out further into space.

“This is the first time we’ve detected X-ray eclipses from a fast pulsar that is also accreting gas,” said Craig Markwardt of NASA’s Goddard Space Flight Centre. “Using this information, we now know the size and mass of the companion star with unprecedented accuracy.”

Artist's impression of RXTE

Artist's impression of the Rossi X-ray Timing Explorer space telescope, which made the observations.

Einstein to the rescue

What the astronomers don’t yet have is an accurate measure of the mass of the pulsar. The standard way to get it would be to use other telescopes to make optical and infrared observations of the companion star’s motion, from which they could work backward mathematically and deduce the pulsar’s mass.

But there is another way. Einstein’s relativity says that massive bodies distort space and slow down time. So what the astronomers hope to do is measure delays in the pulsar flashes as they travel past the companion star, something that RXTE is easily capable of doing.

This will be a good test of Einstein’s theory under extreme stellar conditions.

“High-precision measurements of the X-ray pulses just before and after an eclipse would give us a detailed picture of the entire system,” said Tod Strohmayer, RXTE’s project scientist at Goddard.

But for this, they’ll have to wait for RXTE to spot more X-ray outbursts…so you can be sure they’ll be keeping a close eye on this dynamic stellar duo in the months and years to come.

Story by Jonathan Nally, editor,

Images courtesy NASA / GSFC.

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