Description
Transitional millisecond pulsars (tMSPs) represent a dynamic class of systems linking low-mass
X-ray binaries to rotation-powered millisecond radio pulsars, providing key laboratories to study
accretion–ejection physics and neutron star environments. These sources undergo transitions between
accretion- and rotation-powered states, and in their active phase, they display rapid switches
between distinct high and low luminosity modes, occasionally accompanied by flares.
In this talk, I will present high time-resolution optical spectroscopic observations of the prototypical
tMSP J1023+0038 in its sub-luminous disc state. The dataset probes variability on minute
timescales, revealing significant changes in colour as well as in the strength and profile of the main
emission lines. These variations offer insight into the structure and dynamics of the accretion flow
and its interaction with the neutron star during its disc state.
I will emphasise how combining high-time resolution optical spectroscopy with fast optical photometry,
as enabled by instruments such as SiFAP2, can provide a more complete view of these
systems. In particular, the synergy between fast photometry and spectroscopy, combined with
X-ray observations, can be used to trace correlations between high/low mode transitions across
different energy bands while simultaneously probing the spectral response of the accretion disc.
This approach opens the possibility of linking rapid flux variability to changes in emission line
properties and disc structure.