Probing multi-band variability and mode switching in the candidate transitional millisecond pulsar 3FGL J1544.6–1125

¹ INAF–Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate (LC), Italy
² INAF-Osservatorio Astronomico di Roma, Via Frascati 33, I-00078 Monte Porzio Catone (RM), Italy
³ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain
⁴ Institut d’Estudis Espacials de Catalunya (IEEC), E-08860 Castelldefels (Barcelona), Spain
⁵ ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
⁶ INAF–Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, I-80131 Naples, Italy
⁷ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via U. La Malfa 153, I-90146 Palermo, Italy
⁸ Institució Catalana de Recerca i Estudis Avan cats (ICREA), E-08010 Barcelona, Spain
⁹ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
¹⁰ Department of Astronomy, University of Geneva, chemin d’Écogia, 16, 1290 Versoix, Switzerland
¹¹ Fundación Galileo Galilei – INAF, Rambla J.A.Fernández P. 7, 38712 B.Baja, (S.C.Tenerife), Spain
¹² Dipartimento di Fisica e Astronomia, Sezione Astrofisica, Università di Catania, Via S. Sofia 78, I-95123 Catania, Italy
¹³ INAF–Osservatorio Astrofisico di Catania, Via S. Sofia 78, I-95123 Catania, Italy

^⋆ Corresponding author: giulia.illiano@inaf.it

Received: 10 March 2025
Accepted: 23 June 2025

Abstract

We present the most extensive high-time resolution multi-band campaign to date on the candidate transitional millisecond pulsar (tMSP) 3FGL J1544.6−1125 in the enigmatic sub-luminous disk state with coordinated observations from the radio to the X-ray band. While XMM-Newton and NuSTAR X-ray light curves exhibit the characteristic high and low-mode bimodality, the source’s faintness prevents firm evidence from being obtained for similar bimodality in the ultraviolet and near-infrared light curves, which are presented here for the first time. A re-analysis of archival XMM-Newton/OM data revealed an optical flare without an X-ray counterpart, likely originating from the outer accretion disk or the companion star. During our observations, no radio emission was detected, with a 3σ flux density upper limit of ∼8 μJy at 6 GHz. While past works already reported radio variability in the source, this limit is a factor of ≳3.5 below the average value measured in 2019 under similar conditions, underscoring significant radio variability despite the relatively stable X-ray flux. Simultaneous optical light curves in five filters obtained with GTC/HiPERCAM revealed flickering and dipping activities that resemble the observed X-ray variability along with a reddening trend at lower fluxes. The latter is consistent with discrete mass ejections that disrupt the inner flow and reduce both X-ray and optical fluxes, thereby driving the high-to-low-mode switches. The observed reddening suggests a common origin for most optical and X-ray emission at the boundary region between the pulsar wind and the inner disk, as also supported by our modeling of the spectral energy distribution in the high mode. Overall, our findings reinforce the mini-pulsar nebula picture for tMSPs in the sub-luminous state and demonstrate how coordinated high-time resolution multi-wavelength campaigns are essential to understanding the physical processes governing rapid mode switches in these systems.