Constraining the magnetic field geometry of the millisecond pulsar PSR J0030+0451 from joint radio, thermal X-ray, and γ-ray emission

¹ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
e-mail: jerome.petri@astro.unistra.fr
² IRAP, CNRS, 9 avenue du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
³ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace, Université d’Orléans/CNRS, 45071 Orléans Cedex 02, France
⁴ Observatoire Radioastronomique de Nançay, Observatoire de Paris, Université PSL, Université d’Orléans, CNRS, 18330 Nançay, France
⁵ LUTH, Observatoire de Paris, Université PSL, Université Paris Cité, CNRS, 92195 Meudon, France
⁶ E.A. Milne Centre for Astrophysics, University of Hull, Cottingham Road, Kingston-upon-Hull HU6 7RX, UK
⁷ Centre of Excellence for Data Science, Artificial Intelligence and Modelling (DAIM), University of Hull, Cottingham Road, Kingston-upon-Hull HU6 7RX, UK
⁸ LESIA, Observatoire de Paris, Université, PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
⁹ Institute of Radio Astronomy of NAS of Ukraine, 4 Mystetstv St., 61002 Kharkiv, Ukraine
¹⁰ Onsala Space Observatory, Chalmers University of Technology, Onsala, Sweden
¹¹ SKA Observatory, Jodrell Bank, Lower Withington, Macclesfield SK11 9FT, UK
¹² Department of Physics and Astronomy, University of the Western Cape, Bellville, Cape Town 7535, South Africa

Received: 16 May 2023
Accepted: 5 September 2023

Abstract

Context. With the advent of multi-wavelength electromagnetic observations of neutron stars – spanning many decades in photon energies – from radio wavelengths up to X-rays and γ-rays, it has become possible to significantly constrain the geometry and the location of the associated emission regions.

Aims. In this work, we use results from the modelling of thermal X-ray observations of PSR J0030+0451 from the Neutron Star Interior Composition Explorer (NICER) mission and phase-aligned radio and γ-ray pulse profiles to constrain the geometry of an off-centred dipole that is able to reproduce the light curves in these respective bands simultaneously.

Methods. To this aim, we deduced a configuration with a simple dipole off-centred from the location of the centre of the thermal X-ray hot spots. We show that the geometry is compatible with independent constraints from radio and γ-ray pulsations only, leading to a fixed magnetic obliquity of α ≈ 75° and a line-of-sight inclination angle of ζ ≈ 54°.

Results. We demonstrate that an off-centred dipole cannot be rejected by accounting for the thermal X-ray pulse profiles. Moreover, the crescent shape of one spot is interpreted as the consequence of a small-scale surface dipole on top of the large-scale off-centred dipole.