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. Image: A composite photograph of the Crab Nebula showing X-ray light (light blue), visible light (green and dark blue) and infrared light (red). Credit: NASA, ESA, CXC, JPL-Caltech, J. Hester and A. Loll (Arizona State Univ.), R. Gehrz (Univ. Minn.), and STScI
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 Sept. 19, scientists with the Italian Space Agency’s AGILE telescope reported in a Sept. 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.
In a paper posted online at www.arXiv.org on Nov. 17, the Fermi team noted that the findings “pose special challenges to particle-acceleration theory.”
Fermi esearcher Rolf Buehler of the SLAC National Accelerator Laboratory in Menlo Park, California, joined Tavani in a hastily convened session on Dec. 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.
Wlodek Bednarek and a colleague from the University of Lodz in Poland offered another explanation. In a paper posted at www.arXiv.org on Nov. 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.
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.