Pulsar Wind Nebulae
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.
Gamma-ray map of HESS J1825-137 from the HESS
telescopes. The dotted white contour shows the 95% positional confidence
contour of the unidentified EGRET source 3EG J1826-1302. The position
of the pulsar PSR J1826-1334 is marked by a white triangle. The bright
point-source to the south of HESS J1825-137 is the microquasar LS 5039.
The Galactic plane is shown as a white dashed line. http://arxiv.org/abs/astro-ph/0607548
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
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
|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).