PSR J1903+0327 : A Unique Milli-Second Pulsar with a Main-Sequence Companion Star

TL;DR

PSR J1903+0327 is a unique millisecond pulsar with a main-sequence companion in a highly eccentric orbit, challenging existing models of binary evolution and providing new observational constraints.

Contribution

This study presents new optical spectroscopic data confirming the high eccentricity and characterizing the companion star, offering insights into the system's mass ratio, spectral type, and metallicity.

Findings

Confirmed high eccentricity of the orbit

Estimated mass ratio as 1.56±0.15

Constrained spectral type between F5 V and G0 V

Abstract

PSR J1903+0327 is a mili-second pulsar with a mass of 1.67 M_{\odot} in a highly eccentric orbit (e=0.44) around a main-sequence star. This unique system cannot be reconciled with current observations where milli-second pulsars are generally seen to orbit white dwarfs in almost exactly circular orbits. Current theoretical models of binary and stellar formation and evolution cannot explain the high eccentricity of this system either. In this work, we present three new epochs of optical spectroscopy for the companion to PSR J1903+0327, obtained to confirm the association of the main-sequence star with the pulsar. These 3 new epochs together with the 2 previous ones obtained by Freire et al. (2011) firmly establish the high eccentricity of the companion's orbit as predicted by pulsar timing. Using all five epochs of optical data, we have provided an independent estimate of the mass-ratio, R=1.56\pm0.15 as well as the systemic radial velocity of the binary, \gamma= 42.1\pm2.5 km/s. We constrain the spectral type of the pulsar companion to lie between F5 V -- G0 V (a slightly earlier type than suggested previously) by measuring the equivalent widths of two of the three Ca-triplet lines (8498 \AA, 8542 \AA) and the Paschen line at 8598 \AA (P14); we also broadly constrain the metallicity of the companion. Additionally, we have placed a somewhat better limit on the rotational velocity of the pulsar companion of v_{rot}\sin{i} \leq 66 km/s which is still not sufficient to allow a test of general relativity using this system.