NASA satellites find high-energy surprises in ò??constantò?? Crab Nebula

The combined data from several NASA satellites has astonished astronomers by revealing unexpected changes from the Crab Nebula (M1), once thought to be the steadiest high-energy source in the sky.

ò??For 40 years, most astronomers regarded the Crab as a standard candle,ò?? said Colleen Wilson-Hodge, an astrophysicist at NASAò??s Marshall Space Flight Center in Huntsville, Alabama, who presented the findings Wednesday at the American Astronomical Society meeting in Seattle, Washington. ò??Now, for the first time, weò??re clearly seeing how much our candle flickers.ò??

The Crab Nebula is the wreckage of an exploded star whose light reached Earth in 1054. It is one of the most studied objects in the sky. At the heart of an expanding gas cloud lies whatò??s left of the original starò??s core, a super-dense neutron star that spins 30 times each second. All of the Crabò??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 Crabò??s X-ray emissions as so stable that theyò??ve used it to calibrate space-borne instruments. They also customarily describe the emissions of other high-energy sources in ò??millicrabs,ò?? a unit derived from the nebulaò??s output.

ò??The Crab Nebula is a cornerstone of high-energy astrophysics,ò?? said team member Mike Cherry at Louisiana State University in Baton Rouge, ò??and this study shows us that our foundation is slightly askew.ò?? The story unfolded when Cherry and Gary Case, also at Louisiana State, first noticed the Crabò??s dimming in observations by the Gamma-ray Burst Monitor (GBM) aboard NASAò??s Fermi Gamma-ray Space Telescope.

The team then analyzed GBM observations of the object from August 2008 to July 2010 and found an unexpected but steady decline of several percent at four different ò??hardò?? X-ray energies, from 12,000 to 500,000 electron volts (eV). For comparison, visible light has energies between 2 and 3 eV.

With the Crabò??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. ò??If only one satellite instrument had reported this, no one would have believed it,ò?? Wilson-Hodge said.

ÒšSo the team amassed data from the fleet of sensitive X-ray observatories now in orbit: NASAò??s Rossi X-Ray Timing Explorer (RXTE) and Swift satellites and the European Space Agencyò??s International Gamma-Ray Astrophysics Laboratory (INTEGRAL). The results confirm a real intensity decline of about 7 percent at energies between 15,000 to 50,000 eV over 2 years. 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.

Fermiò??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 nebulaò??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 nebulaò??s central light-year, but observations with future telescopes will be needed to know for sure.

This region is dominated by four high-energy structures: an X-ray-emitting jet; an outflow of particles moving near the speed of light, called a ò??pulsar windò??; a disk of accumulating particles where the wind terminates; and a shock front where the wind abruptly slows.

Òšò??This environment is dominated by the pulsarò??s magnetic field, which we suspect is organized precariously,ò?? said Roger Blandford, who directs the Kavli Institute for Particle Astrophysics and Cosmology, jointly located at the Department of Energyò??s SLAC National Accelerator Laboratory and Stanford University. ò??The X-ray changes may involve some rearrangement of the magnetic field, but just where this happens is a mystery.ò??

The Crab Nebula is a supernova remnant located 6,500 light-years away in the constellation Taurus.

The combined data from several NASA satellites has astonished astronomers by revealing unexpected changes from the Crab Nebula (M1), once thought to be the steadiest high-energy source in the sky.

ò??For 40 years, most astronomers regarded the Crab as a standard candle,ò?? said Colleen Wilson-Hodge, an astrophysicist at NASAò??s Marshall Space Flight Center in Huntsville, Alabama, who presented the findings Wednesday at the American Astronomical Society meeting in Seattle, Washington. ò??Now, for the first time, weò??re clearly seeing how much our candle flickers.ò??

The Crab Nebula is the wreckage of an exploded star whose light reached Earth in 1054. It is one of the most studied objects in the sky. At the heart of an expanding gas cloud lies whatò??s left of the original starò??s core, a super-dense neutron star that spins 30 times each second. All of the Crabò??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 Crabò??s X-ray emissions as so stable that theyò??ve used it to calibrate space-borne instruments. They also customarily describe the emissions of other high-energy sources in ò??millicrabs,ò?? a unit derived from the nebulaò??s output.

ò??The Crab Nebula is a cornerstone of high-energy astrophysics,ò?? said team member Mike Cherry at Louisiana State University in Baton Rouge, ò??and this study shows us that our foundation is slightly askew.ò?? The story unfolded when Cherry and Gary Case, also at Louisiana State, first noticed the Crabò??s dimming in observations by the Gamma-ray Burst Monitor (GBM) aboard NASAò??s Fermi Gamma-ray Space Telescope.

The team then analyzed GBM observations of the object from August 2008 to July 2010 and found an unexpected but steady decline of several percent at four different ò??hardò?? X-ray energies, from 12,000 to 500,000 electron volts (eV). For comparison, visible light has energies between 2 and 3 eV.

With the Crabò??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. ò??If only one satellite instrument had reported this, no one would have believed it,ò?? Wilson-Hodge said.

ÒšSo the team amassed data from the fleet of sensitive X-ray observatories now in orbit: NASAò??s Rossi X-Ray Timing Explorer (RXTE) and Swift satellites and the European Space Agencyò??s International Gamma-Ray Astrophysics Laboratory (INTEGRAL). The results confirm a real intensity decline of about 7 percent at energies between 15,000 to 50,000 eV over 2 years. 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.

Fermiò??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 nebulaò??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 nebulaò??s central light-year, but observations with future telescopes will be needed to know for sure.

This region is dominated by four high-energy structures: an X-ray-emitting jet; an outflow of particles moving near the speed of light, called a ò??pulsar windò??; a disk of accumulating particles where the wind terminates; and a shock front where the wind abruptly slows.

Òšò??This environment is dominated by the pulsarò??s magnetic field, which we suspect is organized precariously,ò?? said Roger Blandford, who directs the Kavli Institute for Particle Astrophysics and Cosmology, jointly located at the Department of Energyò??s SLAC National Accelerator Laboratory and Stanford University. ò??The X-ray changes may involve some rearrangement of the magnetic field, but just where this happens is a mystery.ò??

The Crab Nebula is a supernova remnant located 6,500 light-years away in the constellation Taurus.