Long term optical variations in Swift J1858.6-0814: evidence for ablation and comparisons to radio properties

TL;DR

This study monitors optical and radio variability in the neutron star binary Swift J1858.6-0814, revealing accretion-driven long-term changes, jet propagation effects, and evidence of ablation influencing its evolution into millisecond pulsars.

Contribution

It provides the first detailed optical variability analysis during outburst and quiescence, linking accretion changes to jet activity and demonstrating ablation effects in neutron star binaries.

Findings

Optical variability persists throughout outburst with steady average flux.

Optical spectral energy distribution is predominantly blue, indicating accretion disc emission.

Radio and optical variability amplitudes are comparable, suggesting linked accretion and jet processes.

Abstract

We present optical monitoring of the neutron star low-mass X-ray binary Swift J1858.6-0814 during its 2018-2020 outburst and subsequent quiescence. We find that there was strong optical variability present throughout the entire outburst period covered by our monitoring, while the average flux remained steady. The optical spectral energy distribution is blue on most dates, consistent with emission from an accretion disc, interspersed by occasional red flares, likely due to optically thin synchrotron emission. We find that the fractional rms variability has comparable amplitudes in the radio and optical bands. This implies that the long-term variability is likely to be due to accretion changes, seen at optical wavelengths, that propagate into the jet, seen at radio frequencies. We find that the optical flux varies asymmetrically about the orbital period peaking at phase ~0.7, with a modulation amplitude that is the same across all optical wavebands suggesting that reprocessing off of the disc, companion star and ablated material is driving the phase dependence. The evidence of ablation found in X-ray binaries is vital in understanding the long term evolution of neutron star X-ray binaries and how they evolve into (potentially isolated) millisecond pulsars.