PSR J1903 + 0327 , a millisecond pulsar in an eccentric ( e = 0.44 ) 95-day orbit with a ( \sim 1 M _ { \odot } ) companion poses a challenge to our understanding of stellar evolution in binary and multiple-star systems .
Here we describe optical and radio observations which rule out most of the scenarios proposed to explain formation of this system .
Radio timing measurements of three post-Keplerian effects yield the most precise measurement of the mass of a millisecond pulsar to date : 1.667 \pm 0.021 solar masses ( 99.7 % confidence limit ) .
This rules out some equations of state for super-dense matter , furthermore it is consistent with spin-up of the pulsar by mass accretion , as suggested by its short spin period and low magnetic field .
Optical spectroscopy of a proposed main sequence counterpart show that its orbital motion mirrors the pulsar ’ s 95-day orbit ; being therefore its binary companion .
This finding rules out a previously suggested scenario which proposes that the system is presently a hierarchical triple .
Conventional binary evolution scenarios predict that , after recycling a neutron star into a millisecond pulsar , the binary companion should become a white dwarf and its orbit should be nearly circular .
This suggests that if PSR J1903 + 0327 was recycled , its present companion was not responsible for it .
The optical detection also provides a measurement of the systemic radial velocity of the binary ; this and the proper motion measured from pulsar timing allow the determination of the systemic 3-D velocity in the Galaxy .
We find that the system is always within 270 pc of the plane of the Galaxy , but always more than 3 kpc away from the Galactic centre .
Thus an exchange interaction in a dense stellar environment ( like a globular cluster or the Galactic centre ) is not likely to be the origin of this system .
We suggest that after the supernova that formed it , the neutron star was in a tight orbit with a main-sequence star , the present companion was a tertiary farther out .
The neutron star then accreted matter from its evolving inner MS companion , forming a millisecond pulsar .
The former donor star then disappears , either due to a chaotic 3-body interaction with the outer star ( caused by the expansion of the inner orbit that necessarily results from mass transfer ) , or in the case of a very compact inner system , due to ablation/accretion by the newly formed millisecond pulsar .
We discuss in detail the possible evolution of such a system before the supernova .