The Nearest Millisecond Pulsar Revisited with XMM-Newton: Improved Mass-Radius Constraints for PSR J0437-4715

TL;DR

This study uses deep X-ray observations of the nearest millisecond pulsar to refine neutron star mass-radius constraints, revealing complex thermal emission components and a displaced magnetic dipole, with implications for neutron star physics and pulsar populations.

Contribution

It provides the first phase-resolved X-ray spectroscopy accounting for pulsar geometry, constrains neutron star radius, and suggests a displaced magnetic dipole in PSR J0437-4715.

Findings

Detection of a secondary X-ray pulse offset by ~0.55 in phase.

Identification of at least three thermal emission components.

Neutron star radius constrained to R>11.1 km, favoring stiff equations of state.

Abstract

I present an analysis of the deepest X-ray exposure of a radio millisecond pulsar (MSP) to date, an X-ray Multi Mirror-Newton European Photon Imaging Camera spectroscopic and timing observation of the nearest known MSP, PSR J0437--4715. The timing data clearly reveal a secondary broad X-ray pulse offset from the main pulse by $\sim$0.55 in rotational phase. In the context of a model of surface thermal emission from the hot polar caps of the neutron star, this can be plausibly explained by a magnetic dipole field that is significantly displaced from the stellar center. Such an offset, if commonplace in MSPs, has important implications for studies of the pulsar population, high energy pulsed emission, and the pulsar contribution to cosmic ray positrons. The continuum emission shows evidence for at least three thermal components, with the hottest radiation most likely originating from the hot magnetic polar caps and the cooler emission from the bulk of the surface. I present pulse phase-resolved X-ray spectroscopy of PSR J0437--4715, which for the first time, properly accounts for the system geometry of a radio pulsar. Such an approach is essential for unbiased measurements of the temperatures and emission areas of polar cap radiation from pulsars. Detailed modelling of the thermal pulses, including relativistic and atmospheric effects, provides a constraint on the redshift-corrected neutron star radius of R>11.1 km (at 3 sigma conf.) for the current radio timing mass measurement of 1.76 M_sun. This limit favors "stiff'" equations of state.