Observations of the binary pulsar system PSR B1718-19 -- The Role of Tidal Circularisation

TL;DR

This study uses optical and infrared observations to analyze the companion star of the binary pulsar PSR B1718-19, focusing on how tidal forces have circularized its orbit and affected its evolution.

Contribution

It provides new constraints on the companion's physical parameters and explores the role of tidal circularisation in the system's orbital evolution.

Findings

Companion's radius is between that of a main sequence star and its Roche limit.

The orbit has been circularized from an initially eccentric state.

Energy balance models are consistent with the initial eccentricity but challenge current tidal dissipation theories.

Abstract

We present optical and infrared observations taken with the Very Large Telescope of the eclipsing binary pulsar system PSR B1718-19. The candidate companion of the pulsar, identified earlier in Hubble Space Telescope observations, has been detected in all three bands, R, I, and J. These detections allowed us to derive constraints on temperature, radius, and mass, pointing to a companion that has expanded to a radius between one of a main sequence star and one at the Roche-limit. We focus on the role of tidal circularisation in the system, which will have transformed the initially eccentric orbit expected from formation scenarios into the nearly circular orbit presently observed. Based on simple energy balance arguments, we are able to draw a picture of the companion's evolution resulting from the energy deposition in the star due to circularisation. In this picture, our measurement of the companion's parameters is consistent with the expected initial eccentricity. However, with the present understanding of tidal dissipation it remains difficult to account for the short time in which the system was circularised.