X-ray and optical observations of the millisecond pulsar binary PSR J1431–4715

¹ INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, I-80131 Naples, Italy
² Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
³ Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK
⁴ Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
⁵ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain
⁶ Institut d’Estudis Espacials de Catalunya (IEEC), Carrer de Gran Capitá 2-4, E-08034 Barcelona, Spain
⁷ Institució Catalana de Recerca i Estudis Avançats (ICREA), E-08010 Barcelona, Spain
⁸ INAF-Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate, LC, Italy
⁹ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
¹⁰ ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Rome, Italy
¹¹ INAF-Osservatorio Astronomico di Roma, Via Frascati 33, I-00040 Monte Porzio Catone, (RM), Italy

^⋆ Corresponding author; domitilla.demartino@inaf.it

Received: 31 May 2024
Accepted: 2 September 2024

Abstract

We present the first X-ray observation of the energetic millisecond pulsar binary PSR J1431−4715, performed with XMM-Newton and complemented with fast optical multi-band photometry acquired with the ULTRACAM instrument at ESO-NTT. It is found as a faint X-ray source without a significant orbital modulation. This contrasts with the majority of systems that instead display substantial X-ray orbital variability. The X-ray spectrum is dominated by non-thermal emission and, due to the lack of orbital modulation, does not favour an origin in an intrabinary shock between the pulsar and companion star wind. While thermal emission from the neutron star polar cap cannot be excluded in the soft X-rays, the dominance of synchrotron emission favours an origin in the pulsar magnetosphere that we describe at both X-ray and gamma-ray energies with a synchro-curvature model. The optical multi-colour light curve folded at the 10.8 h orbital period is double-humped and dominated by ellipsoidal effects, but also affected by irradiation. The ULTRACAM light curves are fit with several models encompassing direct heating and a cold spot, or heat redistribution after irradiation either through convection or convection plus diffusion. Despite the inability to constrain the best irradiation models, the fits provide consistent system parameters, giving an orbital inclination of 59 ± 6° and a distance of 3.1 ± 0.3 kpc. The companion is found to be an F-type star, underfilling its Roche lobe (f_RL = 73 ± 4%) with a mass of 0.20 ± 0.04 M_⊙, confirming the redback status, but hotter than the majority of redbacks. The stellar dayside and nightside temperatures of 7500 K and 7400 K, respectively, indicate a weak irradiation effect on the companion, likely due to its high intrinsic luminosity. Although the pulsar mass cannot be precisely derived, a heavy (1.8−2.2 M_⊙) neutron star is favoured.