Pulsar Physics

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.

MAGIC observations of the Crab pulsar have already demonstrated that pulsed emission at tens of GeV can be detected with Cherenkov telescopes. Current Fermi 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

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 instruments.

Short-Timescale Variability

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.

 

Further Reading

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.