The parallax, mass and age of the PSR J2145-0750 binary system

Abstract

We present results of high-precision timing measurements of the binary millisecond pulsar PSR J2145-0750. Combining 10 yrs of radio timing data obtained with the Effelsberg 100-m radio telescope and the Lovell 76-m radio telescope we measure a significant timing parallax of 2.0(6) mas placing the system at 500 pc distance to the solar system. The detected secular change of the projected semi-major axis of the orbit ̇x = 1.8(6) × 10 -14 lt-s s-1, where x = (ap sin i)/c, is caused by the proper motion of the system. With this measurement we can constrain the orbital inclination angle to i < 61°, with a median likelihood value of 46° which is consistent with results from polarimetric studies of the pulsar magnetosphere. This constraint together with the non-detection of Shapiro delay rules out certain combinations of the companion mass, m2, and the inclination, i. For typical neutron star masses and using optical observations of the carbon/oxygen-core white dwarf we derive a mass range for the companion of 0,7 M· ≤ m2 ≤ 1.0 M·. We apply evolutionary white dwarf cooling models to revisit the cooling age of the companion. Our analysis reveals that the companion has an effective temperature of Teff = 5750 ± 600 K and a cooling age of τCOOl = 3.6(2) Gyr, which is roughly a factor of three lower than the pulsar's characteristic age of 10.4 Gyr. The cooling age implies an initial spin period of P0 = 13.0(5) ms, which is very close to the current period.