The Crab Nebula is a pretty strange crustacean. It’s a supernova remnant, from a supernova whose detonation was seen on Earth in 1054 CE. The explosion left behind a rapidly spinning neutron star and a large quantity of ejected matter that’s still expanding away from the blast site – and putting on an impressive show in optical and many other wavelengths.
Last year and earlier this year there were reports, based on detection by satellite observatories, of occasional very high-energy gamma-ray flares, cause unknown, having energies of at least 1015 eV. These are the highest energy particles detected in a discrete source, and the mechanism of their acceleration is unclear. (See here, here.)
And now very high-energy gamma-ray pulses – less energetic than the flares, but still with energies up to 400 GeV – have shown up, associated with the central pulsar itself, which spins and pulses about 30 times per second. Normally, energetic X-ray and gamma-ray photons are generated from high-speed charged particles in a strong magnetic field, by the process of synchrotron radiation (also known as “curvature radiation”). But that appears to be ruled out in this case, since the energy limit on gamma rays that could be produced by this process is about 100 GeV.
The new Science paper packages up 107 hours of Crab Nebula observations spread over the course of four years. When the data was analyzed, a clear pattern of pulses became apparent at energies above 120GeV, and the timing of the pulses lined up nicely with observations at lower energies made using the Fermi space telescope. The object there is pulsing at much higher energies than any previously detected.
The findings have a number of implications. For starters, they clearly demonstrate that the predictions of exponential decay in pulse energy, those based on observations of other pulsars, aren’t happening here. Instead, the fade-off at higher energies follows what’s called a “broken power law,” with a far more gradual tailing off. The results also allow the authors to estimate the distance from the neutron star to the source of the gamma rays: between 10 and 40 stellar radii from the surface.
If the process responsible for the gamma rays isn’t synchrotron radiation, what is it? The research paper suggests inverse Compton scattering, which involves high-energy particles transferring some of their energy to lower energy photons. But there’s as yet no evidence to confirm the nature of the process. It’s not even known whether a single process accounts for most of the gamma-ray flux, or instead different processes are responsible for the lower and higher energy portions.
Whatever is happening in the Crab Nebula, it’s of more interest than simply a curiosity of that object. There could be some connection with the still unknown mechanism in which ultra-high-energy cosmic rays are produced. There might even be new phenomena, such as a minute dependence of the speed of the highest-energy photons on their energy.