Letter to the Editor

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

¹ INAF–Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate (LC), Italy
² Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain
³ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels (Barcelona), Spain
⁴ Department of Physics, University of Alberta, CCIS 4-181 Edmonton, AB T6G 2E1, Canada
⁵ Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁶ INAF-Osservatorio Astronomico di Roma, Via Frascati 33, I-00076 Monte Porzio Catone (RM), Italy
⁷ INAF–Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, I-80131 Naples, Italy
⁸ Center for Astrophysics and Space Science, New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE
⁹ Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys 23, E-08010 Barcelona, Spain
¹⁰ INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, I-00133 Rome, Italy
¹¹ Como Lake centre for AstroPhysics (CLAP), DiSAT, Universitá dell’Insubria, Via Valleggio 11, 22100 Como, Italy
¹² ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, I-00133 Rome, Italy
¹³ Dipartimento di Fisica, Universitá degli Studi di Milano, Via Celoria 16, I-20133 Milan, Italy

^⋆ Corresponding author; cristina.baglio@inaf.it

Received: 16 December 2024
Accepted: 31 January 2025

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 polarisation drops from ≃1.4% to ≃0.5%, indicating the emergence of an unpolarised 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 ionisation 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 polarisation 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 accretion-ejection coupling.