A black widow population dissection through HiPERCAM multi-band light curve modelling

TL;DR

This study uses multi-band photometry from HIPERCAM on the GTC telescope to model and analyze six black widow pulsar systems, revealing details about their companion stars and proposing new correlations in their properties.

Contribution

First multi-band light curve modeling of six black widow systems with HIPERCAM, providing detailed companion star parameters and new insights into their irradiation and orbital characteristics.

Findings

Identified some of the hottest and densest companion stars known.

Confirmed correlation between companion density and orbital period.

Proposed a new correlation between orbital inclination and irradiation temperature.

Abstract

Black widows are extreme millisecond pulsar binaries where the pulsar wind ablates their low-mass companion stars. Their optical light curves vary periodically due to the high irradiation and tidal distortion of the companion, which allows us to infer the binary parameters. We present simultaneous multi-band observations obtained with the HIPERCAM instrument at the 10.4-m GTC telescope for six of these systems. The combination of this five-band fast photometer with the world's largest optical telescope enables us to inspect the light curve range near minima. We present the first light curve for PSR J1641+8049, as well as attain a significant increase in signal-to-noise and cadence compared with previous publications for the remaining 5 targets: PSR J0023+0923, PSR J0251+2606, PSR J0636+5129, PSR J0952-0607 and PSR J1544+4937. We report on the results of the light curve modelling with the Icarus code for all six systems, which reveals some of the hottest and densest companion stars known. We compare the parameters derived with the limited but steadily growing black widow population for which optical modelling is available. We find some expected correlations, such as that between the companion star mean density and the orbital period of the system, but also a puzzling positive correlation between the orbital inclination and the irradiation temperature of the companion. We propose such a correlation would arise if pulsars with magnetic axis orthogonal to their spin axis are capable of irradiating their companions to a higher degree.