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

We present the first X-ray observation of the energetic millisecond pulsar binary PSR\,J1431$-$4715, performed with and complemented with fast optical multi-band photometry acquired with the instrument at ESO-NTT. It is found as a faint X-ray source without a significant orbital modulation. This contrasts with the majority of systems that instead display substantial X-ray orbital variability. The X-ray spectrum is dominated by non-thermal emission and, due to the lack of orbital modulation, does not favour an origin in an intrabinary shock between the pulsar and companion star wind. While thermal emission from the neutron star polar cap cannot be excluded in the soft X-rays, the dominance of synchrotron emission favours an origin in the pulsar magnetosphere that we describe at both X-ray and gamma-ray energies with a synchro-curvature model. The optical multi-colour light curve folded at the 10.8\,h orbital period is double-humped and dominated by ellipsoidal effects, but also affected by irradiation. The light curves are fit with several models encompassing direct heating and a cold spot, or heat redistribution after irradiation either through convection or convection plus diffusion. Despite the inability to constrain the best irradiation models, the fits provide consistent system parameters, giving an orbital inclination of 59$ circ $ and a distance of 3.1pm 0.3\,kpc. The companion is found to be an F-type star, underfilling its Roche lobe RL with a mass of 0.20$ odot $, 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\ odot $) neutron star is favoured.