NuSTAR catches the unveiling nucleus of NGC 1068

We present a NuSTAR and XMM-Newton monitoring campaign in 2014/2015 of the Compton-thick Seyfert 2 galaxy, NGC 1068. During the 2014 August observation, we detect with NuSTAR a flux excess above 20 keV (32 ± 6 per cent) with respect to the 2012 December observation and to a later observation performed in 2015 February. We do not detect any spectral variation below 10 keV in the XMM-Newton data. The transient excess can be explained by a temporary decrease of the column density of the obscuring material along the line of sight (from N_H ≃ 10²⁵ cm^-2 to N_H = 6.7 ± 1.0 × 10²⁴ cm^-2), which allows us for the first time to unveil the direct nuclear radiation of the buried active galactic nucleus in NGC 1068 and to infer an intrinsic 2-10 keV luminosity L_X=7^{+7}_{-4} × 10^{43} erg s^-1.