# The binary millisecond pulsar PSR J1023+0038 -- II. Optical spectroscopy

## TL;DR

This study uses optical spectroscopy to analyze the binary millisecond pulsar PSR J1023+0038, revealing how its companion star's spectral type and radial velocity vary across different states, shedding light on heating effects and system dynamics.

## Contribution

It provides detailed observational evidence of spectral and radial velocity changes in PSR J1023+0038 across pulsar and accretion states, improving understanding of heating and accretion effects in redback systems.

## Key findings

- Companion star heats during disc state, spectral type changes from G5 to F6.
- Radial velocity semi-amplitude K2 determined as 276.3 km/s.
- Observed semi-amplitude variations explained by heating and accretion disc presence.

## Abstract

We present time-resolved optical spectroscopy of the `redback' binary millisecond pulsar system PSR J1023+0038 during both its radio pulsar (2009) and accretion disc states (2014 and 2016). We provide observational evidence for the companion star being heated during the disc-state. We observe a spectral type change along the orbit, from G5 to F6 at the secondary star's superior and inferior conjunction, respectively, and find that the corresponding irradiating luminosity can be powered by the high energy accretion luminosity or the spin-down luminosity of the neutron star. We determine the secondary star's radial velocity semi-amplitude from the metallic (primarily Fe and Ca) and Halpha absorption lines during these different states. The metallic and Halpha radial velocity semi-amplitude determined from the 2009 pulsar-state observations allows us to constrain the secondary star's true radial velocity K_2=276.3+/-5.6 km/s and the binary mass ratio q=0.137+/-0.003. By comparing the observed metallic and Halpha absorption-line radial velocity semi-amplitudes with model predictions, we can explain the observed semi-amplitude changes during the pulsar-state and during the pulsar/disc-state transition as being due to different amounts of heating and the presence of an accretion disc, respectively.