PSR J1012+5307: A millisecond pulsar with an extremely low-mass white dwarf companion

Abstract

Binaries harbouring millisecond pulsars (MSPs) enable a unique path to determine neutron star (NS) masses: radio pulsations reveal the motion of the NS, while that of the companion can be characterized through studies in the optical range. PSR J1012+5307 is an MSP in a 14.5-h orbit with a helium-core white dwarf (WD) companion. In this work we present the analysis of an optical spectroscopic campaign, where the companion star absorption features reveal one of the lightest known WDs. We determine a WD radial velocity semi-amplitude of K2 = 218.9 ± 2.2km s-1, which combined with that of the pulsar derived from the precise radio timing, yields amass ratio of q=10.44±0.11.We also attempt to infer theWDmass from observational constraints using new binary evolution models for extremely low-mass (ELM) WDs, but find that they cannot reproduce all observed parameters simultaneously. In particular, we cannot reconcile the radius predicted from binary evolution with the measurement from the photometric analysis (RWD = 0.047+0.003-0.002 R⊙). Our limited understanding of ELM WD evolution, which results from binary interaction, therefore comes as the main factor limiting the precision with which we can measure the mass of the WD in this system. Our conservative WDmass estimate of MWD = 0.165 ± 0.015M⊙, along with the mass ratio enables us to infer a pulsar mass of MNS = 1.72 ± 0.16M⊙. This value is clearly above the canonical ∼ 1.4M⊙, therefore adding PSR J1012+5307 to the growing list of massive MSPs.