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Crab’s candle starts to flicker

  • Crab Nebula is 6,500 light-years from Earth
  • It is the remains of an exploded star (a supernova)
  • Now shown to unexpectedly vary its energy output

DATA FROM SEVERAL NASA satellites has astonished astronomers by revealing unexpected changes in X-ray emission from the Crab Nebula, once thought to be the steadiest high-energy source in the sky.

“For 40 years, most astronomers regarded the Crab as a standard candle,” said Colleen Wilson-Hodge, an astrophysicist at NASA’s Marshall Space Flight Centre, who presented the findings recently at the American Astronomical Society meeting in Seattle.

“Now, for the first time, we’re clearly seeing how much our candle flickers.”

The Crab Nebula is the wreckage of an exploded star whose light reached Earth in 1054. Located 6,500 light-years away, it is one of the most studied objects in the sky.

At the heart of the expanding gas cloud lies what’s left of the original star’s core, a superdense neutron star that spins 30 times a second. All of the Crab’s high-energy emissions are thought to be the result of physical processes that tap into this rapid spin.

For decades, astronomers have regarded the Crab’s X-ray emissions as so stable that they’ve used it to calibrate space-borne instruments. They also customarily describe the emissions of other high-energy sources in “millicrabs,” a unit derived from the nebula’s output.

Crab Nebula

This view of the Crab Nebula comes from the Hubble Space Telescope and spans 12 light-years. The supernova remnant, located 6,500 light-years away, is among the best-studied objects in the sky. Image courtesy NASA / ESA / ASU / J. Hester.

“The Crab Nebula is a cornerstone of high-energy astrophysics,” said team member Mike Cherry at Louisiana State University (LSU), “and this study shows us that our foundation is slightly askew.”

Satellite tag teams

The story unfolded when Cherry and Gary Case, also at LSU, first noticed the Crab’s dimming in observations by the Gamma-ray Burst Monitor (GBM) aboard NASA’s Fermi Gamma-ray Space Telescope.

The team then analysed GBM observations of the object from August 2008 to July 2010 and found an unexpected but steady decline of several percent at four different “hard” X-ray energies.

With the Crab’s apparent constancy well established, the scientists needed to prove that the fadeout was real and was not an instrumental problem associated with the GBM.

“If only one satellite instrument had reported this, no one would have believed it,” Wilson-Hodge said.

Graph showing multi-wavelength observations of the Crab Nebula

Data from four satellites show that the Crab Nebula's energy output has varied. Powerful gamma-ray flares (pink vertical lines) have been detected as well. Graph courtesy NASA Goddard Space Flight Centre.

So the team amassed data from the fleet of sensitive X-ray observatories now in orbit—NASA’s Rossi X-Ray Timing Explorer (RXTE) and Swift satellites and the European Space Agency’s International Gamma-Ray Astrophysics Laboratory (INTEGRAL).

The results confirm a real intensity decline of about 7 percent at certain energy ranges. They also show that the Crab has brightened and faded by as much as 3.5 percent a year since 1999.

The scientists say that astronomers will need to find new ways to calibrate instruments in flight and to explore the possible effects of the inconstant Crab on past findings.

Showing some flare

Fermi’s other instrument, the Large Area Telescope, has detected unprecedented gamma-ray flares from the Crab, showing that it is also surprisingly variable at much higher energies.

The nebula’s power comes from the central neutron star, which is also a pulsar that emits fast, regular radio and X-ray pulses. This pulsed emission exhibits no changes associated with the decline, so it cannot be the source.

Instead, researchers suspect that the long-term changes probably occur in the nebula’s central light-year, but observations with future telescopes will be needed to know for sure.

Adapted from information issued by NASA MSFC.

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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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