Pulsar Wind Nebulae
|Combined X-ray and optical image of the Crab Nebula. Image credits: NASA/CXC/ASU/J. Hester et al. (X-ray); NASA/HST/ASU/J. Hester et al. (optical).|
The largest single class of identified Galactic VHE gamma-ray sources is that of the pulsar wind nebulae (PWNe).
The Crab Nebula is the best-known member of this class; indeed, it is generally regarded as the ‘standard candle’ for high energy astrophysics. Its emission over more than 15 decades in energy shows it to be an effective particle accelerator. However, the gamma-ray luminosity of the Crab Nebula is actually much less than the considerable spin-down power of the pulsar.
This surprising fact is explained by the high magnetic field of this young pulsar. This has the effect of suppressing the gamma ray emission, which is produced from high-energy electrons via the inverse Compton mechanism. Paradoxically, a less powerful pulsar with a weaker magnetic field would result in a higher gamma-ray production efficiency.
An example of one of these efficient, low-magnetic field pulsar wind nebulae is HESS J1825-137. This has a similar gamma-ray luminosity to the Crab Nebula, but the pulsar’s spin-down power is two orders of magnitude smaller than the Crab’s and its magnetic field looks to be in the range of a few microGauss instead of hundreds. The gamma-ray spectrum of the whole emission region from HESS J1825-137 is measured over more than two orders of magnitude, from 270 GeV to 35 TeV, and shows signs of a cut-off at high energies that CTA, with its excellent spectra coverage, could confirm.
The gamma-ray spectra in the different regions of the PWN HESS J1825-137 show a softening with increasing distance from the pulsar and therefore an energy-dependent morphology. If this emission is due to the inverse Compton effect, the pulsar power is not sufficient to generate the gamma-ray luminosity, suggesting that the pulsar was able to inject more high-energy particles into its nebula in the past. Is this common for other PWNe and what can that tell us about the evolution of pulsar winds? CTA will have the angular resolution and sensitivity necessary to perform more detailed morphological studies of objects like HESS J1825-137.
Another PWN, Vela X, shows a curvature in its VHE gamma ray spectrum that suggests the ‘Compton peak’ in the gamma ray spectrum has been found. Such a spectrum would be generated by electrons (more generally, leptons) It has been suggested that the feature could in fact be due to protons – hadrons – but it’s not clear how much of a contribution hadrons could make to the pulsar wind. If such a contribution could be confirmed, we would know that ions are being torn from the pulsar’s surface, which would contribute significantly to the understanding of the magnetohydrodynamic flow in PWNe.
Finally, only the sensitivity and angular resolution as achieved with CTA will allow detailed multiwavelength studies of large/close PWNe, and an understanding of particle propagation, the magnetic field profile in the nebula, and interstellar medium feedback.
Gaensler and Slane, The Evolution and Structure of Pulsar Wind Nebulae, Annual Review of Astronomy & Astrophysics (2006), 44, 1, p.17-47; http://arxiv.org/abs/astro-ph/0601081
de Jager and Djannati-Atai, Springer Lecture notes 'Neutron Stars and Pulsars: 40 years after their discovery', ed. W. Becker; http://arxiv.org/abs/0803.0116