Implications of the measured parameters of PSR J1903+0327 for its progenitor neutron star

¹ N. Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland
e-mail: bejger@camk.edu.pl; fortin@camk.edu.pl; haensel@camk.edu.pl; jlz@camk.edu.pl
² LUTh, UMR 8102 du CNRS, Observatoire de Paris, 92195 Meudon Cedex, France

Received: 13 June 2011
Accepted: 19 October 2011

Abstract

Context. The millisecond pulsar PSR J1903+0327 rotating at 465 Hz has the second highest precisely measured mass (1.67 M_⊙) and a weak surface magnetic field (≃2 × 10⁸ G). It is located in the Galactic plane, bound in a highly eccentric (e = 0.44) orbit in a binary system with a solar-mass main-sequence star. These observational findings pose a challenge for the theory of stellar evolution.

Aims. Using the intrinsic parameters of PSR J1903+0327 evaluated from radio observations (mass M, rotation period P, and magnetic field B deduced from P and Ṗ) and a model of spin evolution during the “recycling” phase (spin-up by accretion from a low-mass companion lost afterwards) that takes into account the accretion-induced magnetic field decay, we aim to calculate the mass of its neutron star progenitor, M_i, at the onset of accretion. In addition, we derive constraints on the average accretion rate Ṁ and the pre-accretion magnetic field B_i. We also seek for the imprint of the poorly known equation of state of dense matter at supra-nuclear densities on the spin-up tracks and the progenitor neutron star.

Methods. Spin-up is modeled by accretion from a thin magnetized disk, using the magnetic-torque disk-pulsar coupling model proposed by Kluźniak and Rappaport. We adopt an observationally motivated model of the surface magnetic field dissipation caused by accretion. We consider three equations of state of dense matter, which are consistent with the existence of 2.0 M_⊙ neutron star. Orbital parameters in the accretion disk are obtained using the space-time generated by a rotating neutron star within the framework of general relativity.

Results. Constraints on the progenitor neutron star parameters and the accretion itself are obtained. The minimum average accretion rate should be higher than 2−8 × 10^-10 M_⊙ yr^-1, the highest lower bound corresponding to the stiffest equation of state. Allowed B_i-dependent values of M_i are within 1.0 − 1.4 M_⊙, much lower than the oversimplified but widely used B ≡ 0 result, where one gets M_i > 1.55 M_⊙.

Conclusions. The influence of magnetic field in the “recycling” process is crucial – it leads to a significant decrease in the spin-up rate and higher accreted masses, in comparison to the B = 0 model. The estimated initial neutron-star mass depends on the assumed dense-matter equation of state. We also show that the otherwise necessary relativistic corrections to the Newtonian model of Kluźniak and Rappaport, related to the existence of the marginally-stable circular orbit, can be neglected in the case of PSR J1903+0327.