The featureless and non-variable optical spectral energy distribution of AXP 4U 0142+61

We present GTC-10.4 m spectroscopy and multiband photometry of the faint (r similar to 26) optical counterpart of the anomalous X-ray pulsar 4U 0142+61. The 5000-9000 angstrom spectrum - the first obtained for a magnetar -is featureless, allowing us to set an equivalent width upper limit EW < 25 angstrom to the presence of emission lines in the H alpha region. Multiband photometry in the g, r, i, z Sloan Digital Sky Survey (SDSS) bands obtained at different epochs over 12 yr shows no significant variability from minutes-to-years time-scales. The photometry has been calibrated, for the first time, against the SDSS itself, resulting in solid upper limits to variability ranging from similar to 0.2 mag in i (over 12 yr) to 0.05 mag in z (over 1.5 yr). The shape of the optical + near-infrared (literature values) spectral energy distribution is not well constrained due to the high extinction along the line of sight. Using a Markov Chain Monte Carlo analysis, we find that it can be described by a power law with a spectral index beta = -0.7 +/- 0.5 and E(B - V) = 1.5 +/- 0.4. We also discuss on the implications of adding hard X-ray flux values from literature to the spectral fitting.