Irradiated but not eclipsed, the case of PSR J0610-2100

TL;DR

This study presents 16-year radio and optical observations of the black widow pulsar J0610-2100, revealing no eclipses, low irradiation, and providing insights into its binary system properties and potential for gravitational wave detection.

Contribution

It provides the longest radio timing baseline for PSR J0610-2100, characterizes its optical companion, and discusses the implications for pulsar timing arrays and black widow pulsar properties.

Findings

No significant orbital variations detected.

Optical light curves confirm companion irradiation.

Companion likely not filling its Roche lobe.

Abstract

We report on radio timing observations of the black widow binary pulsar J0610$-$2100 and optical observations of its binary companion. The radio timing observations extend the timing baseline to 16yr and reveal a marginal detection of the orbital period derivative, but they show no significant evidence of orbital variations such as those seen in other black widow pulsars. Furthermore, no eclipses are seen in the observations at observing frequencies ranging from 310 to 2700MHz. The optical VRI light curves were modulated with the orbital period, reaching maximum brightness of $V=26.8$, $R=25.4$, and $I=23.8$ at superior conjunction of the companion, confirming irradiation of the companion by the pulsar. Modelling the light curves indicates that the companion is likely not filling its Roche lobe, while having a moderate inclination ($i > 54\degr$). We find an unusually low temperature and a low irradiation for the irradiated hemisphere of the companion. We investigate the absence of radio eclipses in PSR J0610-2100 and in other black widow systems in relation to their binary, pulsar, and companion properties. We also discuss the suitability of PSR J0610-2100 for pulsar timing array observations aimed at detecting nano-Hertz gravitational waves.