The discovery of gamma-ray flares in the Crab Nebula was rated by Astronomy Magazine as the number two space story of 2011. Now KIPAC scientists report on another, larger, flaring episode, and are beginning to crack the mystery of why this source can be so variable.
Gamma-ray flux from the Crab Nebula as measured by the Fermi-LAT for 14 days in April, 2011.
The Crab nebula is the remnant of a supernova explosion which was observed by naked-eye star gazers in 1054. The explosion left behind a rapidly rotating neutron star, known as the Crab pulsar, which sends beams of particles and radiation streaming into space as it spins. As the pulsar's rotation gradually slows down, it releases energy to the surrounding space in the form of a magnetized particle flux, the so called 'pulsar wind'. Because this wind interacts with the ejecta moving out from the original supernova explosion of the star, it glows bright across the electromagnetic spectrum in a shell surrounding the pulsar.
Due to its proximity and brightness, the Crab nebula is one of the most studied objects in the sky and serves as a laboratory for relativistic plasma physics. Furthermore, it is widely used as a reference 'standard candle' to cross-calibrate instruments and test their stability over time. It therefore came as a surprise when the Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope detected strong gamma-ray flares from the nebula earlier this year, in which the flux increased by a factor of several from its average value. A team of astronomers - led by KIPAC postdoc Rolf Buehler, and including KIPAC director Roger Blandford, professors Stefan Funk and Roger Romani, and staff scientist James Chiang, along with colleagues from several other institutions - has now reported and analyzed the most dramatic Crab flare yet, which happened over nine days in April.
After observing three previous flares, astronomers were prepared as they awaited further flaring activity. As soon as the gamma-ray flux observed by Fermi started to increase again, the Chandra X-ray satellite swung to point at the source. Surprisingly, these observations revealed no significant increase in the X-ray emission anywhere in the nebula. Furthermore, optical light observations with the Keck telescope in Hawaii during the flare revealed no flux increases. The lack of any correlation between gamma-rays and these other wavebands of light implies that the energy spectrum of the electrons responsible for the gamma-ray emission has to be steeply rising with increasing energy. It is an interesting question which acceleration mechanism is able to accelerate electrons into such a spectrum of energies. The standard explanations for the acceleration of high energy particles in cosmic environments, that originated with the same Enrico Fermi for whom the satellite is named, do not work here and new explanations have been proposed and are being investigated, according to KIPAC director Blandford.
During the April 2011 Flare, the Fermi satellite interrupted its usual all-sky monitoring for one week to target the Crab nebula. Together with the exceptional brightness of the flare, this allowed Buehler and Funk to explore the gamma-ray flares in greater detail than ever before, allowing the flux behavior to be resolved into ten-minute time intervals by the LAT. It was seen that the flux doubled on time scales less than eight hours. This rapid variability implies that the region emitting the gamma-rays must be very small, with a length smaller than a few light days. A source this compact and variable is expected in be located in the inner region of the nebula, within half of a light year from the pulsar.
Another surprise from the Fermi-LAT observations of the April flare is that gamma-ray emission was detected up to energies beyond 1 GeV. The presence of such high frequency emission, and the brightness of the flare, are most easily explained if the emission region is moving towards us at extremely fast speeds. This fact is also indicated by the spectral evolution of the gamma-ray emission during the outburst, which suggests that changes in this motion likely induce the observed flux variations. The interplay of astronomy with a cosmic relativistic plasma physics laboratory is one of the reasons that the Crab flares were selected by the popular press as a top space science story.
This highlight is based on work submitted for publication to the Astrophysical Journal and available from astro-ph at arXiv:1112.1979.
Tidbit Authors: Rolf Buehler and Jack Singal