[Paper Review] Gamma Rays From Rotation-Powered Pulsars

A. K. Harding|arXiv (Cornell University)|Aug 22, 2002

Pulsars and Gravitational Waves Research3 citations

TL;DR

This paper investigates the origin of pulsed gamma-ray emission in rotation-powered pulsars, comparing polar cap and outer gap models to explain observed gamma-ray and radio emission characteristics. It predicts that the upcoming GLAST mission will detect many more radio-quiet gamma-ray pulsars, especially under outer gap models, resolving long-standing uncertainties about emission site and particle acceleration mechanisms in pulsars.

ABSTRACT

The seven known gamma-ray pulsars represent a very small fraction of the more than 1000 presently known radio pulsars, yet they can give us valuable information about pulsar particle acceleration and energetics. Although the theory of acceleration and high-energy emission in pulsars has been studied for over 25 years, the origin of the pulsed gamma rays is a question that remains unanswered. Characteristics of the pulsars detected by the Compton Gamma-Ray Observatory could not clearly distinguish between an emission site at the magnetic poles (polar cap models) and emission from the outer magnetosphere (outer gap models). There are also a number of theoretical issues in both type of model which have yet to be resolved. The two types of models make contrasting predictions for the numbers of radio-loud and radio-quiet gamma-ray pulsars and of their spectral characteristics. GLAST will probably detect at least 50 radio-selected pulsars and possibly many more radio-quiet pulsars. With this large sample, it will be possible to fully test the model predictions and finally resolve this longstanding question.

Motivation & Objective

• To determine the emission site of pulsed gamma rays in rotation-powered pulsars—whether at the magnetic poles (polar cap models) or in the outer magnetosphere (outer gap models).
• To resolve longstanding uncertainties in pulsar particle acceleration and high-energy emission mechanisms despite over 25 years of theoretical study.
• To predict the expected numbers of radio-loud and radio-quiet gamma-ray pulsars under different emission models, using observational constraints from the Compton Gamma-Ray Observatory.
• To evaluate the potential of the upcoming GLAST mission to detect a large sample of radio-selected and radio-quiet pulsars, enabling definitive testing of competing emission models.

Proposed method

• Analyzes observational data from the Compton Gamma-Ray Observatory (CGRO), including pulse profiles and spectral characteristics of seven known gamma-ray pulsars.
• Compares predictions of polar cap and outer gap models for the ratio of radio-loud to radio-quiet gamma-ray pulsars, based on emission geometry and beaming patterns.
• Uses simulations of radio and gamma-ray pulsar populations to test model predictions, incorporating variations in model assumptions and detection thresholds.
• Evaluates the sensitivity of future missions—GLAST, AGILE, and Integral—especially GLAST’s capability for blind period searches and detection of pulsed gamma-ray emission.
• Considers off-beam gamma-ray emission in polar cap models, where curvature radiation from high-altitude particles may be detectable even when the radio beam is not.
• Assesses the potential of ground-based Cherenkov telescopes to detect pulsed emission at TeV and GeV energies, providing additional constraints on emission models.

Experimental results

Research questions

• RQ1 What is the dominant emission site for pulsed gamma rays in rotation-powered pulsars: the magnetic poles (polar cap models) or the outer magnetosphere (outer gap models)?
• RQ2 Why are only seven gamma-ray pulsars detected among over 1,000 known radio pulsars, and what does this imply about the emission mechanisms and beaming patterns?
• RQ3 How do the predicted numbers of radio-loud and radio-quiet gamma-ray pulsars differ between polar cap and outer gap models, and what observational tests can distinguish them?
• RQ4 To what extent can the GLAST mission detect radio-quiet gamma-ray pulsars, and how will this help resolve the long-standing ambiguity in pulsar emission models?
• RQ5 Can off-beam gamma-ray emission from polar cap models account for some of the EGRET unidentified sources, particularly those associated with the young Gould Belt?

Key findings

• The seven known gamma-ray pulsars are all radio-loud, with the possible exception of Geminga, and all have young ages (less than 10^6 years), except for the millisecond pulsar J0218+4232.
• Outer gap models predict a significantly higher ratio of radio-quiet to radio-loud gamma-ray pulsars than polar cap models, with GLAST potentially detecting 13 times more radio-quiet than radio-loud pulsars.
• Polar cap models predict that GLAST will detect slightly more radio-quiet than radio-loud pulsars, but only about 10% of radio-quiet sources will show detectable pulsed gamma-ray emission due to sensitivity limits.
• The observed correlation between gamma-ray and radio pulse phases—where radio pulses often lead gamma-ray pulses—favors emission from a single magnetic pole with a hollow cone or wide fan beam geometry.
• Simulations suggest that off-beam gamma-ray emission from polar cap models, though fainter and broader, could explain some EGRET unidentified sources, particularly those in the nearby Gould Belt.
• GLAST is expected to detect at least 50 radio-selected pulsars and possibly many more radio-quiet pulsars, enabling a definitive test of emission model predictions and resolving the origin of high-energy emission in pulsars.