[Paper Review] The double pulsar -- A new testbed for relativistic gravity

M. Krämer, A. G. Lyne+10 more|arXiv (Cornell University)|May 10, 2004

Pulsars and Gravitational Waves Research4 citations

TL;DR

This paper presents the discovery of the first double pulsar system, PSR J0737−3039, which provides an unprecedented laboratory for testing relativistic gravity in the strong-field regime. By measuring multiple post-Keplerian parameters—such as periastron advance, gravitational redshift, Shapiro delay, and mass ratio—the system over-constrains general relativity, yielding a precise mass determination and confirming predictions with a 0.22% uncertainty on the Shapiro parameter ratio.

ABSTRACT

The first ever double pulsar, discovered by our team a few months ago, consists of two pulsars, one with period of 22 ms and the other with a period of 2.7 s. This binary system with a period of only 2.4-hr provides a truly unique laboratory for relativistic gravitational physics. In this contribution we summarize the published results and look at the prospects of future observations.

Motivation & Objective

• To investigate the potential of the double pulsar system PSR J0737−3039 as a new testbed for relativistic gravity in the strong-field regime.
• To measure multiple post-Keplerian parameters (e.g., periastron advance, Shapiro delay, gravitational redshift) to test general relativity beyond the weak-field limit.
• To determine the masses of both neutron stars with high precision using timing measurements and orbital constraints.
• To explore the possibility of measuring neutron star spin-orbit coupling and moment of inertia through relativistic precession effects.
• To assess the system’s potential for future high-precision tests of gravity, including geodetic precession and light propagation corrections.

Proposed method

• Utilized high-precision timing of radio pulses from both pulsars using the Parkes and Lovell radio telescopes to derive Keplerian and post-Keplerian orbital parameters.
• Measured the Shapiro delay in pulse arrival times of pulsar A due to the gravitational field of pulsar B, enabling a precise determination of orbital inclination and the range parameter r.
• Applied the mass-mass diagram method, where general relativity predicts that multiple post-Keplerian parameters intersect at a single point for the true masses.
• Used the mass ratio R = mA/mB derived from projected semi-major axes as a theory-independent constraint that must intersect with post-Keplerian constraints.
• Modeled higher-order relativistic effects such as geodetic precession, spin-orbit coupling, and light-bending corrections to assess their detectability.
• Projected that future measurements of the orbital period derivative and parallax via VLBI will further refine tests of gravity and account for kinematic effects.

Experimental results

Research questions

• RQ1 Can the double pulsar system PSR J0737−3039 provide a more stringent test of general relativity in the strong-field regime than previous binary pulsar systems?
• RQ2 To what extent do the measured post-Keplerian parameters (e.g., periastron advance, Shapiro delay, gravitational redshift) agree with general relativity predictions?
• RQ3 Can the mass ratio R = mA/mB serve as a theory-independent constraint that validates or rules out alternative gravity theories?
• RQ4 Is the relativistic spin-orbit coupling effect measurable in this system, and if so, can it be used to determine the neutron star moment of inertia?
• RQ5 What is the expected detectability of higher-order effects such as light-bending and geodetic precession, and how do they affect timing models?

Key findings

• The system PSR J0737−3039 is the first known double pulsar, with pulsar A having a period of 22.7 ms and pulsar B of 2.7 s, orbiting every 2.4 hours.
• Four post-Keplerian parameters were measured: periastron advance (16.90(1) deg/yr), gravitational redshift (0.38(5) ms), Shapiro delay parameter s = 0.9995(+4/-32), and range parameter r = 5.6(−12/+18) μs.
• The mass ratio R = mA/mB = 1.069(6) was derived from the projected semi-major axes, providing a theory-independent constraint on the mass plane.
• The intersection of the four post-Keplerian constraints and the mass ratio line in the mass-mass diagram confirms general relativity with a consistency check of s_obs/s_exp = 1.0001 ± 0.00220.
• The system's high relativistic effects (e.g., spin-orbit coupling) suggest that the moment of inertia of a neutron star could be measured for the first time with future high-precision timing.
• Simulations indicate that a few years of high-precision timing data should be sufficient to detect the spin-orbit coupling effect, which is expected to be about an order of magnitude larger than in PSR B1913+16.