The crab Nebula
An image of the nebula.
The Crab Nebula is one of the most studied astronomical sources ever. Its spectrum has been detected and extensively studied from radio to ultra-high energy gamma rays (TeV range). Its spectrum being so well understood has made it a 'standard candle' for gamma ray experiments.
The Crab is a plasma filled supernova remnant. The plasma making up the nebula pictured above is powered by a pulsar at its center. The pulsar (a rapidly rotating neutron star) is what was left of the original star when it went supernova. As the neutron star rotates in its magnetic field, particles are ripped off of its surface and accelerated to huge energies as they are funneled along magnetic field lines. When they run into the surrounding nebula, they are accelerated across a turbulent shock front to even higher energies. These super-relativistic particles inverse Compton scatter off of ambient photons creating the gamma rays we detect.
Raw excess
Approximately 13 hours of usable observation time was made of the Crab in January and February of 2002. At this time, we were using the 'stage one' configuration of 32 heliostats and 32 photomultipliers. The trigger condition was that 4 out of 8 heliostats and 3 out of the 4 clusters of 8 heliostats have a hit within an 18 nanosecond window. Data was taken in 28 minute intervals. Twenty-eight minutes of 'on source' (tracking the crab) were followed by twenty-eight minutes 'off source'. The off source period is taken of a part of the sky 7 degrees displaced from Crab for background subtraction. The difference in the number of triggers on source versus off source gives us our excess above background. Variablility from night to night results mainly from varying weather conditions. Results are plotted below:
Crab's spectrum from various observatories.
We detected an average of 1.9 excess triggers per min. This result is consistent with simulations. Simulations indicate that the number of triggers is peaked at ~200 GeV for the Crab spectrum. This allows us to place a preliminary data point on the known Crab spectrum.
Original figure from Hillas et al., THE ASTROPHYSICAL JOURNAL, 503:744-759, 1998 August 20.