Large panel: time-averaged spectral energy distribution of J1109 from the radio to the gamma-ray band. Vertical bars represent 1σ statistical uncertainties, whereas arrowheads indicate 95% upper limits. The blue dashed line represents the best-fitting power-law model for the average X-ray emission detected with XMM-Newton and NuSTAR, and its extrapolation to the optical and nIR bands. The X-ray spectrum has not been corrected for absorption. Inset: SEDs of J1109, PSR J1023+0038 (taken from Bogdanov et al. 2015), XSS J12270−4859 (from de Martino et al. 2015) and 1RXS J154439.4−112820 (from Bogdanov et al. 2018 for the optical, UV and X-ray data) plotted together to highlight their similarities among these systems in the sub-luminous accretion disk state (we note in particular the virtually identical slope of the power law model in the X-ray band).