High-temporal-resolution optical spectroscopic observations of the transitional millisecond pulsar PSR J1023+0038

TL;DR

This study presents the first high-time-resolution optical spectroscopic observations of the transitional millisecond pulsar PSR J1023+0038, revealing rapid, uncorrelated variability in emission lines and continuum in its sub-luminous disc state.

Contribution

It provides the first short-timescale optical spectroscopic data of a tMSP, uncovering rapid emission line variability and its lack of correlation with continuum changes.

Findings

Detected up to twofold variability in emission line properties over minutes.

Observed erratic optical continuum variability without clear mode or orbital modulation.

Found uncorrelated variability between emission lines and continuum, indicating complex emission processes.

Abstract

Transitional millisecond pulsars (tMSPs) represent a dynamic category of celestial sources that establish a crucial connection between low-mass X-ray binaries and millisecond radio pulsars. These systems exhibit transitions from rotation-powered states to accretion-powered ones and vice versa, highlighting the tight evolutionary link expected by the so-called recycling scenario. In their active phase, these sources manifest two distinct emission modes named high and low, occasionally punctuated by sporadic flares. Here, we present high-time-resolution spectroscopic observations of the binary tMSP J1023+0038, in the sub-luminous disc state. This is the first short-timescale (~ 1 min) optical spectroscopic campaign ever conducted on a tMSP. The campaign was carried out over the night of June 10, 2021 using the Gran Telescopio Canarias. The optical continuum shows erratic variability, without clear evidence of high and low modes or of orbital modulation. Besides, the analysis of these high-temporal-cadence spectroscopic observations reveals, for the first time, evidence for a significant (up to a factor of ~ 2) variability in the emission line properties (equivalent width and full width half maximum) over a timescale of minutes. Intriguingly, the variability episodes observed in the optical continuum and in the emission line properties seem uncorrelated, making their origin unclear.