The Crab Nebula Is Not So Steady

May 5, 2015

The Crab Nebula is a system with a pulsar and a surrounding ball of material that emits light all across the electromagnetic spectrum. For many years it was thought to be a constant steady source and was used as a calibration reference for telescopes. Now, KIPAC scientists using the Fermi Space telescope have shown that the emission from the Crab in gamma rays varies with time.

Image of the Crab Nebula in optical light (red - from the Hubble Space Telescope) and X-rays (purple - from the Chandra X-ray observatory)

The Crab Nebula is the remnant of a historical supernova explosion that was recorded in 1054 AD. The explosion left behind a rapidly rotating neutron star or pulsar, spinning on its axis thirty times a second, as well as a surrounding 'nebula' of material which was ejected from the star. The pulsar creates a wind of high energy particles and strong electromagnetic fields which interact with the surrounding material that leads to bright emission seen from radio to gamma-ray frequencies.
 
Today the Crab Nebula is one of the most studied objects in the sky. Its bright and apparently steady emission has been used as a reference source to calibrate telescopes in several wavebands, in particular at high energies. It has also become one of the main astronomical laboratories to test theories of particle acceleration and plasma physics under extreme conditions.
 
The Fermi Large Area Telescope has been used to make the surprising discovery that the emission from the Nebula is highly variable at soft gamma-ray energies (photons ranging from 0.1-10 GeV). In particular, two extreme outbursts were detected in February 2009 and December 2010, with flux increases of a factor of around six and a duration of only a few days. The discovery of these unexpected events was led by KIPAC Scientists Rolf Buehler and Stefan Funk.
 
The brevity and strength of the flares imply that the soft gamma-ray emission must come from a relatively small region, likely located less than a light year from the neutron star. Even more interestingly, it shows that the emission is radiated by electrons of energies higher than 1 billion GeV. These are the most energetic particles associated with any discrete astronomical source.
 
Understanding the mechanism for accelerating particles rapidly to these energies is a big puzzle, and as argued by KIPAC director Roger Blandford, many existing proposals are inadequate for explaining the presence of such high energy gamma rays. It is possible that the central pulsar, which can act like a giant generator creating voltages of 30 billion GigaVolts, is somehow responsible. The next step is to use optical and X-ray telescopes to pinpoint the site of the Crab Nebula flares and thereby further understand their origin.
 
This highlight is based on work submitted for publication and available from astro-ph at arXiv:1011.3855.
 
Science Contact:
Rolf Buehler
KIPAC
buehler@slac.stanford.edu
 
Tidbit Authors: Rolf Buehler, Roger Blandford, and Jack Singal