Multi-band study of the flaring mode emission in the transitional millisecond pulsar PSR J1023+0038

Abstract

We present a comprehensive study of the flaring mode of the transitional millisecond pulsar (tMSP) PSR J1023+0038 during its X-ray sub-luminous state, using strictly simultaneous X-ray, UV, optical, and radio observations. The X-ray flares exhibit UV and optical counterparts and coincide with the brightest radio flare observed in the past decade, reaching 1.2 mJy at 6 GHz and lasting ~1 hour. During the flare, the optical polarization drops from ~1.4% to ~0.5%, indicating the emergence of an unpolarized component. We propose that the thickening of the disc, which enlarges the shock region between the pulsar wind and the accretion flow and may drive the X-ray flaring observed in tMSPs, enhances the ionization level of the disc, thereby generating an increased number of free electrons. These electrons could then be channelled by magnetic field lines into the jet. This increased jet mass-loading could drive the associated radio and optical variability. The radio spectral evolution during flares is consistent with synchrotron self-absorption in jet ejecta or internal shocks within the compact jet. We infer radio polarization upper limits (<8.7%, <2.3%, and <8.2%, before, during, and after the radio flare) that further support a compact jet origin but do not rule out discrete ejections. Our findings suggest that tMSPs could serve as essential laboratories for investigating jet-launching mechanisms, mainly because they operate under very low mass accretion rates. This accretion regime has not been explored before in the context of the accretion-ejection coupling.