An intriguing new model of the emission surrounding the Vela Pulsar may explain a famous mystery in high energy astrophysics.

The left panel shows a VLA radio map of the Vela pulsar extended radio region with Fermi-LAT gamma-ray contours overlaid. The right panel shows the ROSAT satellite X-ray map with H.E.S.S. TeV gamma ray contours overlaid.

The Vela pulsar is in many ways the archetypal pulsar, one of nature's lighthouses where a rapidly spinning, highly magnetized neutron star sends out beams of particles and radiation sweeping through space like a beacon. Vela's proximity to Earth has allowed it to be studied in detail across the electromagnetic spectrum, including with radio waves, X-rays, and the highest energy "TeV" gamma rays. The latest contributions to our wealth of data on this phenomenon are the observations from the Large Area Telescope (LAT) of the Fermi Gamma-ray Space Telescope, the orbiting observatory in which KIPAC is the primary scientific institution. These observations have revealed an extended gamma-ray emitting structure surrounding the pulsar and coincident with a similarly extended area of radio emission, known as the "extended radio nebula," where electrons ejected from near the pulsar emit light through interactions with the ambient magnetic field.
 
In spite of, or perhaps because of, the wealth of observations of Vela, questions have persisted on the mechanisms of its emission on different scales. The extended radio region of Vela seems quite different than other so-called Pulsar Wind Nebulae, with a spectrum of gamma-rays as measured by Fermi that seems to indicate a lack of high energy electrons. Recently KIPAC professor Stefan Funk, on a team led by Jim Hinton of the University of Leicester and two colleagues from the University of Leeds in the UK, has proposed a new model of emission from Vela which explains the observed features and gives an additional bonus.
 
The model accounts for the seeming lack of high energy electrons in the extended region, as as well as the spectrum of TeV gamma rays seen closer to the pulsar. It assumes that the higher energy electrons in the extended region can be tossed out completely by interactions with the outflowing matter that is left over from the supernova explosion that gave birth to the pulsar around 11,000 years ago. Funk and colleagues have performed calculations to estimate the expected emission given this scenario and realistic estimates of parameters describing quantities such as the energy of the injected particles and the magnetic field.
 
While the team's results aid our understanding of the environment around Vela and young pulsar wind nebulae more generally, they also may shed light on a contemporary enigma in high energy astrophysics. As both the PAMELA satellite and Fermi-LAT observations have confirmed, there is an unexpected rise in the fraction of cosmic rays that are positrons at high energies. Higher energy particles escaping from pulsar wind nebulae are a potential source of cosmic ray positrons, and if what has been modeled in regard to Vela is typical of such systems early in their evolution, then these pulsar environments in our Galaxy may collectively be the source of the anomalous cosmic ray positrons.  A competing hypothesis fingers annihilating dark matter as the ultimate source of the excess positrons, and of course one, both, or neither of these options may fully explain the data.  Future observations of the cosmic-ray electron and positron spectra, such as with the Cherenkov Telescope Array under development at KIPAC and other institutions, will allow further investigation of these high energy enigmas and possible solutions.
 
This work is described in a paper to appear in Astrophysical Journal Letters and available from astro-ph at arXiv:1111.2036.
 
Science Contact:
Stefan Funk
KIPAC
email: funk@slac.stanford.edu