[Paper Review] Radio Pulsars: The Neutron Star Population Fundamental Physics

V. M. Kaspi, M. Krämer|arXiv (Cornell University)|Feb 24, 2016

Pulsars and Gravitational Waves Research5 references19 citations

TL;DR

This comprehensive review synthesizes the role of radio pulsars as natural laboratories for fundamental physics, emphasizing their use in testing gravity through precise timing in binary systems. It examines the neutron star population—including pulsars, magnetars, and XDINS—highlighting their formation, evolution, and observational biases, while also discussing implications for gravitational theory and emerging phenomena like fast radio bursts.

ABSTRACT

Radio pulsars are unique laboratories for a wide range of physics and astrophysics. Understanding how they are created, how they evolve and where we find them in the Galaxy, with or without binary companions, is highly constraining of theories of stellar and binary evolution. Pulsars' relationship with a recently discovered variety of apparently different classes of neutron stars is an interesting modern astrophysical puzzle which we consider in Part I of this review. Radio pulsars are also famous for allowing us to probe the laws of nature at a fundamental level. They act as precise cosmic clocks and, when in a binary system with a companion star, provide indispensable venues for precision tests of gravity. The different applications of radio pulsars for fundamental physics will be discussed in Part II. We finish by making mention of the newly discovered class of astrophysical objects, the Fast Radio Bursts, which may or may not be related to radio pulsars or neutron stars, but which were discovered in observations of the latter.

Motivation & Objective

• To review the diversity of neutron star populations, including radio pulsars, magnetars, and quiescent neutron stars (XDINS), and their astrophysical implications.
• To examine the observational selection effects and biases that shape the known radio pulsar population, particularly in the Galactic plane.
• To explore how radio pulsars serve as precise cosmic clocks for testing gravitational theories, especially in strong-field regimes.
• To assess the role of binary pulsars in constraining alternative theories of gravity and probing neutron star equation of state.
• To discuss the potential connection between fast radio bursts and neutron stars, including pulsars and magnetars.

Proposed method

• Analysis of the spatial distribution and kinematics of over 2300 known radio pulsars using data from surveys and the Galactic electron distribution.
• Application of spin-down timing measurements (P and ˙P) to infer magnetic field strengths and age estimates via the dipole braking model.
• Use of radio pulsar timing in binary systems to test general relativity and alternative gravity theories through relativistic orbital parameters.
• Comparison of pulsar-based constraints on gravity with solar system and other astrophysical tests, emphasizing strong-field regimes.
• Review of theoretical models for radio emission mechanisms and their implications for neutron star magnetospheres.
• Integration of multi-wavelength data (radio, X-ray, gamma-ray) to classify neutron star types and understand their evolutionary pathways.

Experimental results

Research questions

• RQ1 How do observational selection effects influence the observed distribution and properties of radio pulsars in the Galaxy?
• RQ2 What constraints do binary pulsar systems place on alternative theories of gravity, particularly in strong-field regimes?
• RQ3 How do the magnetic field strengths and spin-down rates of radio pulsars inform our understanding of neutron star evolution and emission mechanisms?
• RQ4 What is the relationship between radio pulsars, magnetars, and other neutron star classes such as XDINS?
• RQ5 Could fast radio bursts be related to neutron star phenomena, and what evidence supports or challenges this hypothesis?

Key findings

• The known radio pulsar population of over 2300 sources is strongly biased toward the Galactic plane due to dispersion, scattering, and survey sensitivity, with a scale height increasing with age.
• Radio pulsars exhibit a wide range of spin periods (from 1.4 ms to ~8 s) and spin-down rates, enabling precise measurements of magnetic fields and ages via the dipole braking model.
• Binary pulsars provide the most stringent tests of general relativity in strong gravitational fields, with constraints on post-Newtonian parameters at the level of 10−4 or better.
• The discovery of fast radio bursts (FRBs) has opened new avenues for probing neutron star populations, though their connection to pulsars remains uncertain.
• Magnetars and XDINS represent distinct classes of neutron stars with high magnetic fields or low surface temperatures, challenging standard pulsar evolution models.
• Pulsar timing arrays and high-precision measurements in binary systems have placed tight limits on deviations from general relativity, particularly in the context of scalar-tensor theories.