Modelling annual and orbital variations in the scintillation of the relativistic binary PSR J1141$-$6545

Abstract

We have observed the relativistic binary pulsar PSR J1141$-$6545 over a period of $\sim$6 years using the Parkes 64 m radio telescope, with a focus on modelling the diffractive intensity scintillations to improve the accuracy of the astrometric timing model. The long-term scintillation, which shows orbital and annual variations, allows us to measure parameters that are difficult to measure with pulsar timing alone. These include: the orbital inclination $i$; the longitude of the ascending node $\hat{I}\text{c◯}$; and the pulsar system transverse velocity. We use the annual variations to resolve the previous ambiguity in the sense of the inclination angle. Using the correct sense, and a prior probability distribution given by a constraint from pulsar timing ($i=73\pm 3^{\circ }$), we find $\hat{I}\text{c◯}=24.8\pm 1.8^{\circ }$ and we estimate the pulsar distance to be $D=10_{-3}^{+4}$ kpc. This then gives us an estimate of this pulsar's proper motion of $\hat{I}{¼}_{\hat{I}}\pm \cos \hat{I}´=2.9\pm 1.0$ mas yr${}^{-1}$ in right ascension and $\hat{I}{¼}_{\hat{I}}´=1.8\pm 0.6$ mas yr${}^{-1}$ in declination. Finally, we obtain measurements of the spatial structure of the interstellar electron density fluctuations, including: the spatial scale and anisotropy of the diffraction pattern; the distribution of scattering material along the line of sight; and spatial variation in the strength of turbulence from epoch to epoch. We find that the scattering is dominated by a thin screen at a distance of $(0.724\pm 0.008)D$, with an anisotropy axial ratio $A_{\mathrm{r}}=2.14\pm 0.11$.

17 pages, 8 figures, 2 tables. Accepted for publication in MNRAS