The first continuous optical monitoring of the transitional millisecond pulsar PSR J1023+0038 with Kepler

TL;DR

This study presents the first continuous 80-day optical monitoring of the transitional millisecond pulsar PSR J1023+0038 with Kepler, revealing frequent flares, orbital modulation, and variability likely linked to accretion disk dynamics and companion star reprocessing.

Contribution

It provides the first detailed optical light curve of PSR J1023+0038 during an accretion disk state, highlighting new variability patterns and their possible origins.

Findings

Frequent flares lasting minutes to 14 hours observed in 15.6% of the time.

Orbital modulation with ~16% amplitude, varying over days.

Optical luminosity changes likely due to accretion disk and system geometry variations.

Abstract

We report on the first continuous, 80 day optical monitoring of the transitional millisecond pulsar PSR J1023+0038 carried out in mid-2017 with Kepler in the K2 configuration, when an X-ray subluminous accretion disk was present in the binary. Flares lasting from minutes to 14 hr were observed for 15.6% of the time, which is a larger fraction than previously reported on the basis of X-ray and past optical observations, and more frequently when the companion was at the superior conjunction of the orbit. A sinusoidal modulation at the binary orbital period was also present with an amplitude of ~16%, which varied by a few percent over timescales of days, and with a maximum that took place 890 +/- 85 s earlier than the superior conjunction of the donor. We interpret these phenomena in terms of reprocessing of the X-ray emission by an asymmetrically heated companion star surface and/or a non-axisymmetric outflow possibly launched close to the inner Lagrangian point. Furthermore, the non-flaring average emission varied by up to ~ 40% over a time scale of days in the absence of correspondingly large variations of the irradiating X-ray flux. The latter suggests that the observed changes in the average optical luminosity might be due to variations of the geometry, size, and/or mass accretion rate in the outer regions of the accretion disk.