[Paper Review] Testing GR with the Double Pulsar: Recent Results

M. Krämer, Lorimer, D. R.+13 more|ArXiv.org|Mar 17, 2005

Geophysics and Gravity Measurements1 references16 citations

TL;DR

This paper presents the most stringent test of general relativity (GR) in the strong-field regime using the double pulsar PSR J0737−3039, a unique binary system with two neutron stars orbiting every 2.4 hours. By measuring five post-Keplerian parameters—especially the Shapiro delay and periastron advance—it confirms GR to within 0.1%, marking the tightest constraint to date in the strong-gravity limit.

ABSTRACT

This first ever double pulsar system consists of two pulsars orbiting the common center of mass in a slightly eccentric orbit of only 2.4-hr duration. The pair of pulsars with pulse periods of 22 ms and 2.8 sec, respectively, confirms the long-proposed recycling theory for millisecond pulsars and provides an exciting opportunity to study the works of pulsar magnetospheres by a very fortunate geometrical alignment of the orbit relative to our line-of-sight. In particular, this binary system represents a truly unique laboratory for relativistic gravitational physics. This contribution serves as an update on the currently obtained results and their consequences for the test of general relativity in the strong-field regime. A complete and more up-to-date report of the timing results will be presented elsewhere shortly.

Motivation & Objective

• To test general relativity in the strong-field regime using a double pulsar system with extreme relativistic parameters.
• To measure multiple post-Keplerian parameters from precise timing of the millisecond pulsar PSR J0737−3039A.
• To constrain deviations from general relativity by comparing observed and predicted values of relativistic orbital parameters.
• To enable future measurement of higher-order relativistic effects, such as spin-orbit coupling, to probe neutron star equation of state.
• To establish the double pulsar as a unique laboratory for testing gravity under extreme conditions of mass and compactness.

Proposed method

• Precisely time the millisecond pulsar PSR J0737−3039A using high-sensitivity radio telescopes to measure post-Keplerian (PK) parameters.
• Use the observed orbital period, eccentricity, and periastron advance to derive the total system mass and mass ratio from general relativity.
• Measure the Shapiro time delay parameter $ s $ from the delay in pulse arrival times due to gravitational redshift in the curved spacetime near the companion.
• Compare the observed $ s $ with the GR-predicted value using the derived masses to test GR's validity in the strong-field limit.
• Model higher-order relativistic effects such as spin-orbit coupling and geodetic precession to assess future potential for measuring neutron star moments of inertia.
• Apply post-Newtonian expansions up to 1PN order, with corrections for spin-orbit coupling treated as equivalent to 2PN effects in the periastron advance.

Experimental results

Research questions

• RQ1 Can general relativity be tested with unprecedented precision in the strong-field regime using a double pulsar system?
• RQ2 To what extent do the measured post-Keplerian parameters of PSR J0737−3039 agree with predictions from general relativity?
• RQ3 Can the Shapiro delay parameter be measured with sufficient accuracy to constrain deviations from GR?
• RQ4 What is the potential for detecting higher-order relativistic effects such as spin-orbit coupling in this system?
• RQ5 Can the measurement of spin-orbit coupling provide a direct constraint on the neutron star equation of state via its moment of inertia?

Key findings

• The double pulsar system PSR J0737−3039 provides the most stringent test of general relativity in the strong-field regime to date.
• The ratio of the GR-predicted to observed Shapiro delay parameter is $ s^{ ext{GR}}/s^{ ext{obs}} = 1.0002^{+0.0011}_{-0.0006} $, confirming GR to within 0.1%.
• The system's total mass is determined as $ M_A + M_B = 2.587 hinspace M_igodot $, with individual masses $ M_A = 1.338 hinspace M_igodot $ and $ M_B = 1.249 hinspace M_igodot $, both with 0.1% precision.
• The measured periastron advance $ ilde{ ho} = 16.9^ ext{deg} ext{yr}^{-1} $ is four times larger than in PSR B1913+16, enabling high-precision timing.
• The system's short coalescence timescale of ~85 Myr enhances the prospects for detecting gravitational waves from a neutron star merger with first-generation detectors.
• Future timing precision may allow detection of spin-orbit coupling effects, which could yield the first direct measurement of a neutron star's moment of inertia and equation of state.