XMM-Newton and NuSTAR Observations of an Optically Quiescent Quasar

Optically quiescent quasars (OQQs) represent a recently systematised class of infrared-luminous active galactic nuclei (AGN) which have galaxy-like optical continua. They may represent an interesting, brief phase in the AGN life cycle, e.g. either cocooned within high-covering-factor media or indicative of recent triggering, though their nature remains unclear. Here, we present the first targeted simultaneous X-ray observations of an OQQ, our previously identified prototype, SDSS J075139.06+402811.2 at $z$=0.587. The source is significantly detected over 0.5-16 keV with XMM-Newton and NuSTAR, unambiguously confirming the presence of current accretion activity. Spectral modelling yields an intrinsic luminosity $L_{2-10\mathrm{k}\mathrm{e}\mathrm{V}}$ $\approx$4.4$\times$10${}^{43}$ erg s${}^{-1}$, well within the AGN regime, but underluminous relative to its infrared power. It is lightly obscured, with log $N_{\mathrm{H}}$ [cm${}^{-2}$] $\approx$22.