Crab nebula outbursts shock astronomers

Spacecraft detect powerful gamma-ray flares from a supernova remnant known for its steady glow

HEIDELBERG, Germany — Astronomers consider the Crab nebula one of the steadiest sources of high-energy radiation in the universe. Radiation from the supernova remnant is believed to be so constant that astronomers use it as a standard candle with which to measure the energetic radiation of other astronomical sources.

This composite X-ray, visible-light and infrared view of the Crab nebula shows an array of complex knots or wisps. One such wisp may be the site of a recently detected gamma-ray outburst in the Crab. X-ray: J. Hester/ASU, CXC. Optical: J. Hester and A. Loll/ASU, NASA, ESA. Infrared: R. Gehrz/U of Minn., JPL/NASA

That’s why researchers are astounded that two spacecraft recently recorded giant gamma-ray hiccups from the Crab, the remnants of a stellar explosion 6,500 light-years from Earth that was observed by humans in 1054. The intensity of the Crab’s gamma-ray radiation suddenly became two to three times stronger for three days beginning September 19, scientists with the Italian Space Agency’s AGILE telescope reported in a September 22 Astronomical Telegram, an e-mail communication. Researchers with Fermi’s Gamma-ray Space Telescope found an even larger increase over roughly the same time period, they reported in a telegram on the following day. Both teams also announced they had found evidence of previous flares — the AGILE telescope recorded an outburst in the fall of 2007 while the Fermi team spotted one in February 2009.

The suspected source of the energetic flares, along with steadier radiation emanating from the nebula, is blizzards of electrons spat out by the Crab’s pulsar — the rapidly rotating, exploded cinder of a star that lies at the very center of the Crab nebula. But figuring out exactly how the electrons got revved up to energies of at least 1015 electron volts — the most energetic charged particles ever associated with a distinct astrophysical object — for so short a time has astronomers at the biannual Texas Symposium on Relativistic Astrophysics, held this year in Heidelberg, Germany, scratching their heads and searching for new models.

Finding the flares “was a shock,” said AGILE team member Marco Tavani of the INAF-IASF in Rome and the University of Rome Tor Vergata, who spoke about the findings at the meeting on December 6 and 7. In fact, when his team first noticed a sudden, short-lived rise in gamma-ray emissions from the Crab in the fall of 2007, soon after AGILE was launched, the researchers didn’t believe it. Only when the craft recorded the 2010 outburst was the team convinced enough to go public with both findings. “If you say a steady source like the Crab is variable and it’s not true, you burn yourself for life,” Tavani said at the meeting.

In a paper posted online at on November 17 (, the Fermi team noted that the findings “pose special challenges to particle acceleration theory.”

Fermi researcher Rolf Buehler of the SLAC National Accelerator Laboratory in Menlo Park, Calif., joined Tavani in a hastily convened session on December 6, not part of the scheduled program, to discuss variable sources of energetic radiation in the Milky Way. Tavani and Buehler declined to talk to reporters because both of their teams have submitted their findings to Science.

In a widely accepted model, the stage is set for any kind of gamma-ray emissions — steady or short-lived — when electrons hurled from the Crab’s central pulsar encounter strong magnetic fields in the surrounding debris. The electrons gyrate around the magnetic fields and get revved up to energies high enough to emit gammas.

But the Crab’s recently detected outbursts would seem to pose problems for that acceleration model. The brevity of the flares indicates that the electrons couldn’t have gyrated long enough to produce the energetic radiation, Buehler noted. Another problem: Because electrons accelerated to very high energies lose that energy quickly, the nebula’s magnetic field might have to be three to 10 times stronger — 3 to 10 milliGauss — than is commonly assumed. (By comparison, Earth’s surface magnetic field is about 500 milliGauss.)

The short duration suggests the gamma rays originate in a relatively small part of the inner nebula. Buehler suggested that the pulsar’s own electric field helped accelerate the electrons in the inner part of the nebula to energies high enough to emit the gammas.

Wlodek Bednarek and a colleague from the University of Lodz in Poland offered another explanation. In a paper posted at on November 19 (, they suggest that the pulsar’s wind of charged particles rams into and compresses the magnetic field in the nebula. As the disrupted field snaps like a rubber band and reconfigures itself, it unleashes an enormous amount of energy that accelerates the electrons, the researchers propose.

As researchers puzzle over the details, astronomers are also trying to pinpoint the exact region from which the September outburst originated. As revealed in visible light and X- ray images, the nebula contains a complex array of wisps and jets. A series of portraits taken by the Chandra X-ray Observatory beginning a few weeks after the September flare shows that the base of one of the jets has brightened. This might be where the gamma-ray flare originated, says Tavani.

Figuring out the riddle presented by the Crab nebula is likely to shed new light on the nature of its pulsar, noted Jonathan Arons of the University of California, Berkeley. “All these particles come screaming out [of the pulsar] and get stopped in the nebula,” which acts like the pulsar’s catch basin, Arons said. “Studying what’s going on in the inner nebula is as close as we can get to a laboratory experiment” to probe the pulsar, he added.

It may also help elucidate the physics of a host of other astronomical systems that feature a central compact object, Arons said. These include black holes whose jets of charged particles slam into surrounding interstellar space or collisions between clumps of material within such jets that are thought to create the most energetic explosions in the universe, events called gamma-ray bursts.

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