All in a Spin: The Physics of Pulsars
The magnetic fields of pulsars are known to act as efficient cosmic
accelerators, yet there is no final model for this acceleration
mechanism, a process which involves electrodynamics in very high
magnetic fields as well as the effects of general relativity. Pulsed
gamma-ray emission is the key to this study, as its detection allows the
separation of processes occurring in the magnetosphere from the
emission of the surrounding nebula.
observations of the Crab pulsar have already demonstrated that pulsed
emission at tens of GeV can be detected with Cherenkov telescopes.
results point to models in which gamma-ray emission occurs far out in
the magnetosphere (i.e. in the ‘outer gaps’). In these models,
exponential cut-offs in the spectral energy distribution are expected at
a few GeV, and these have already been found in several pulsars
detected with Fermi.
Pulsed emission from the Crab Pulsar measured in different energy
bands.(A) Emission at E > 60 GeV measured by MAGIC. (B) Emission at E
> 25 GeV, also measured by MAGIC. (C) Emission > 1 GeV, measured
by EGRET. (D) Emission > 100 MeV, measured by EGRET. (E) Optical
emission measured by MAGIC with the central pixel of the camera. http://arxiv.org/abs/0809.2998
To make progress in understanding the emission mechanisms of
pulsars we need to study their radiation at extreme energies. In
particular, the characteristics of pulsar emission in the GeV domain
(currently best examined by Fermi LAT) and at very high energies will
tell us more about the electrodynamics within their magnetospheres.
Between ~10 GeV and ~50 GeV (where Fermi performance is limited), using a
special low-energy trigger for pulsed sources, CTA will offer a closer
look at unidentified Fermi sources and deeper analysis of Fermi pulsar
candidates. Above 50 GeV, CTA will explore the most extreme energetic
processes in millisecond pulsars. The VHE domain will be particularly
important for the study of millisecond pulsars, very much like the HE
domain for classical pulsars.
Magnetars, pulsars with extremely high magnetic fields, are much less
well-studied and the high-energy emission mechanism from magnetars is
essentially unknown. Due to the large magnetic field, all high-energy
photons would be absorbed if emitted close to the magnetar, so we expect
emission much like the outer gap emission in the classical pulsars.
Magentars are known to produce large X-ray flares, which may be
accompanied by short timescale gamma-ray emission. The high sensitivity
of CTA is required to study the GeV-TeV emission related to such rapid
pulsar phenomena, which is beyond the current reach of working
CTA can also observe possible high-energy phenomena related to timing
noise, in which the pulse phase and/or frequency of radio pulses drifts
stochastically, or sudden increases in the pulse frequency (glitches)
produced by apparent changes in the momentum of inertia of neutron stars
are observed. Unlike satellite-based instruments, which require long
integration times, CTA’s large effective area means that it will be able
to make meaningful measurements on short timescales.
Caliandro et al., On the High Energy Pulsar Population Detected by Fermi; http://arxiv.org/abs/0912.3857
|An artist's impression of the gamma-ray pulsar in the supernova
remnant CTA 1. Clouds of charged particles move along the pulsar's
magnetic field lines (blue) and create a lighthouse-like beam of gamma
rays (purple). Image credit: NASA/Fermi.