Very High Energy Pulsed Gamma Ray Emission from Crab Pulsar
Institute for Cosmic Ray Research
A research group led by Professor Masahiro Teshima (Institute for Cosmic Ray Research, the University of Tokyo, and the Max Planck Institute for Physics) using the MAGIC (Major Atmospheric Gamma-Ray Imaging Cherenkov) telescopes, located on the Canary Island of La Palma, has observed gamma rays from the Crab pulsar in the range 25 GeV to 400 GeV (10 billion to 200 billion times the energy of visible light), the highest energy pulsed emission recorded to date. These measurements show that the emissions of this swiftly rotating neutron star reach extremely high energies, 50 times more energetic than current theoretical predictions of pulsar emissions.
This finding indicates that there is an unknown process at work which is not part of current pulsar theories. The results were published on March 30, 2012 in the journal Astronomy & Astrophysics. The Crab pulsar is a neutron star that rotates rapidly around its axis 30 times per second, and which is highly magnetized (100 million Tesla, more than 1000 billion times the magnetic field of the Earth). It powers the famous Crab Nebula, located around 6,000 light-years from Earth in the Taurus constellation. Both the pulsar and the nebula are remnants of a supernova explosion that occurred in 1054 A.D. Neutron stars as the Crab pulsar are very compressed stars with a mass similar to that of the Sun, but only about 20 km in diameter.
MAGIC previously announced in a 2008 paper published in Science the detection of gamma rays of 25 GeV coming from the Crab pulsar, which result led to the conclusion that the radiation had to be produced far (more than 60 km) from the neutron star surface. This is because the star’s magnetic field shields high-energy photons very effectively, so a production process close to the neutron star would not have allowed for detection at such high energies at all. Now, the latest MAGIC measurements have yielded another surprise: with data taken from 73 hours of observation over a two-year research period, pulsed emissions exceeding all expectations were detected up to at least 400 GeV as extremely short pulses of about one millisecond duration. This finding casts doubt on the previous understanding of the mechanism of pulsars, whose hard energy limit seemed a settled issue in the field.
MAGIC Collaboration, J. Aleksi?, J. Kushida, R. Orito, K. Saito, T. Saito, H. Takami, M. Teshima, et al.,
“Phase-resolved energy spectra of the Crab pulsar in the range of 50-400 GeV measured with the MAGIC Telescopes”,
Astronomy and Astrophysics, 540, A69 (2012) doi: 10.1051/0004-6361/201118166.