[Paper Review] Optical studies of companions to millisecond pulsars

M. H. van Kerkwijk, C. Bassa+2 more|arXiv (Cornell University)|May 14, 2004

Pulsars and Gravitational Waves Research11 citations

TL;DR

This paper investigates the optical properties of white dwarf companions to millisecond pulsars to constrain binary evolution and pulsar ages. By combining optical spectroscopy and cooling models, it finds that the cooling age of PSR B0820+02 exceeds its characteristic spin-down age, implying a braking index below 3, and uses cooling ages to test formation scenarios for PSR J1911−5958A in NGC 6752, suggesting a young origin via dynamical exchange.

ABSTRACT

Optical observations of the companions of pulsars can help determine the properties of the binaries, as well as those of their components, and give clues to the preceding evolution. In this review, we first describe the different classes of binary pulsars, and present a table with a summary what is known about their optical counterparts. Next, we focus on the class of pulsars that have low-mass, helium-core white dwarf companions. We discuss attempts to determine the masses of both components using optical spectroscopy, and compare the pulsar spin-down ages with cooling ages of the white dwarfs. We confirm that for a given age, the lowest-mass white dwarfs are much hotter than the more massive ones, consistent with recent evolutionary models, although with one glaring exception. We discuss the case of PSR B0820+02, where the cooling age indicates a braking index less than 3, and conclude by describing how cooling ages can be used to test formation scenarios for PSR J1911-5958A, a pulsar binary in the outskirts of NGC 6752.

Motivation & Objective

• To determine the masses and evolutionary histories of low-mass white dwarf companions to millisecond pulsars using optical spectroscopy.
• To compare pulsar spin-down ages with white dwarf cooling ages to test the consistency of braking laws and evolutionary models.
• To use cooling age estimates to distinguish between formation scenarios for PSR J1911−5958A in the globular cluster NGC 6752.
• To assess the reliability of white dwarf cooling models by comparing predicted and observed temperatures and masses.

Proposed method

• Optical spectroscopy of white dwarf companions to measure effective temperature and surface gravity.
• Use of white dwarf cooling models to derive cooling ages from observed temperatures and masses.
• Comparison of pulsar spin-down age (τ_c = P / (2Ṗ)) with cooling age to infer braking index n.
• Application of mass-radius relations and flux-temperature-distance measurements to constrain white dwarf mass independently.
• Use of archival HST and ESO imaging to identify and characterize the optical counterpart of PSR J1911−5958A.
• Statistical analysis of orbital parameters and companion masses to test evolutionary models of binary pulsars.

Experimental results

Research questions

• RQ1 Is the braking index of PSR B0820+02 consistent with magnetic dipole braking, given its cooling age exceeds its spin-down age?
• RQ2 Can the discrepancy between the pulsar’s characteristic age and the white dwarf’s cooling age be resolved by adjusting the braking index?
• RQ3 What does the young cooling age of PSR J1911−5958A’s white dwarf imply about its formation mechanism in NGC 6752?
• RQ4 How do observed white dwarf temperatures and masses compare with predictions from evolutionary models for low-mass binary pulsars?
• RQ5 Can cooling ages be used to distinguish between primordial binary formation and dynamical exchange events in globular clusters?

Key findings

• The cooling age of PSR B0820+02’s white dwarf companion is 221 ± 11 Myr, significantly longer than its characteristic spin-down age of 130 Myr.
• The discrepancy implies a braking index of approximately 2.2, suggesting deviations from standard magnetic dipole braking even in older pulsars.
• The white dwarf companion of PSR J1911−5958A has a temperature of ~11,000 K and a mass of ~0.2 M☉, indicating a young age of ~1 Gyr.
• The young cooling age of PSR J1911−5958A’s white dwarf supports a formation via a recent dynamical exchange interaction in NGC 6752.
• The absence of hydrogen in the white dwarf atmosphere of PSR B0820+02 suggests a helium-rich or CO-core composition, consistent with low-mass white dwarf evolution.
• Cooling models are in good agreement with observations, enabling reliable age estimates for white dwarfs in binary pulsar systems.