The otherwise steadfast Crab Nebula has stunned scientists by emitting a huge burst of gamma rays, the source of which remains a mystery. According to NASA’s Fermi Gamma Ray Space Telescope (formerly the Gamma Ray Large Area Telescope, or GLAST), the burst emitted was up to 30 times the nebula’s normal output of gamma rays, and five times more powerful.
The Crab Nebula is located in the constellation Taurus, approximately 6,500 light years from Earth. It is comprised primarily of the remnants of a supernova – the explosion of a dying star. The light from the supernova which formed the Crab Nebula reached Earth and was recorded by astronomers at the time in the year 1054 AD.
What’s left of the core of the star, located in the center of the iconic gas cloud, is a spinning neutron star, or pulsar. Each of the neutron star’s 30-second rotations blasts huge beams of radiation outward. Referred to as a “constant candle”, the Crab Nebula has long since been considered a steady emitter of light, radio, and high-energy radiation, making the recent bursts of gamma rays particularly singular.
Of all the waves in the electromagnetic spectrum, gamma rays have the smallest wavelengths and the highest energy. Gamma ray bursts release significantly more energy than traditional stars.
“An individual gamma ray burst can release in a matter of seconds the same amount of energy that our Sun will radiate over its 10-billion-year lifetime,” stated NASA scientist Neil Gehrels.
While waves of visible light or radio emissions permeate through our atmosphere, gamma rays are absorbed before they reach the Earth’s surface. This is beneficial for organisms on Earth, as gamma rays can be fatal for living cells.
However, this also makes gamma rays more difficult to study, requiring highly sensitive equipment, like the Fermi Telescope, often placed in the upper reaches of or above the Earth’s atmosphere.
Speaking with the BBC, scientist Rolf Buehler explained part of the mystery surrounding the Crab Nebula’s sudden gamma ray bursts. “If you look in optical light, the Crab is very steady,” he stated. “In radio emission, it’s very steady; in very, very high-energy gamma rays it’s very steady. Only in this part between do we see it varying.”
The Fermi Telescope, which through its GLAST Burst Monitor (GBM) specifically studies gamma ray bursts, was able to record the first burst on April 12th, 2011. The GBM has the capacity to detect as many as 200 gamma rays bursts annually.
“It’s amazing that gamma-ray bursts are so powerful that a small detector you could hold in one hand can observe them from distances of billions of light-years,” stated GBM Principal Investigator Charles Meegan of NASA’s Marshall Space Flight Center.
The information was discussed at the Third Fermi Symposium in Rome, which ended yesterday. Fermi’s observations were corroborated by similar findings on the AGILE, the Italian Space Agency’s satellite.
Speaking on the gamma ray flares, NASA’s Goddard Space Flight Center’s Alice Harding stated, “These superflares are the most intense outbursts we’ve seen to date, and they are all extremely puzzling events.”
Theories do exist as to the source and nature of the gamma-ray bursts. Scientists with NASA believe that the flares are a result of restructuring of the pulsar’s magnetic field, which can accelerate particles to very near the speed of light.
Gamma ray bursts are thought to arise from the consequent interaction of these high-speed particles, like electrons, with the pulsar’s magnetic field. While scientists estimate that the emitting region must be within a third of a light year from the pulsar and approximately as large as our solar system, the exact source of the bursts remain unclear.
Photo credit: NASA/DOE/Fermi LAT/R. Buehler, ESA, J. Hester, A. Loll (ASU) nasa.gov/multimedia/imagegallery/image_feature_1604.html