The Parallax, Mass and Age of the PSR J2145-0750 binary system

Abstract

We present results of timing measurements of the binary millisecond pulsar PSR J2145-0750. Combining timing data obtained with the Effelsberg and Lovell radio telescopes 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 $\dot{x}=1.8(6)\times 10^{-14}$ lt-s s${}^{-1}$, where $x=(a_{\mathrm{p}}\sin i)/c$, is caused by the proper motion of the system. With this measurement we can constrain the orbital inclination angle to $i<61\degr$, with a median likelihood value of $46\degr$ 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, $m_2$, 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 $0.7M_{\odot }\le m_2\le 1.0M_{\odot }$. 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 $T_{\mathrm{eff}}=5750\pm 600$ K and a cooling age of $\ddot{I}{„}_{\mathrm{cool}}=3.6(2)$ 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 $P_0=13.0(5)$ ms, which is very close to the current period.

11 pages, 5 figures, accepted for publication in A&A