XMM-Newton and NuSTAR Observations of an Optically Quiescent Quasar

TL;DR

This study presents the first simultaneous X-ray observations of an optically quiescent quasar, confirming active accretion despite its optical quiescence, and provides insights into its obscuration and luminosity characteristics.

Contribution

It provides the first targeted X-ray data of an optically quiescent quasar, revealing active accretion and spectral properties that inform understanding of this class.

Findings

Confirmed active accretion in an optically quiescent quasar

Measured intrinsic X-ray luminosity consistent with AGN activity

Found the source to be lightly obscured with specific column density

Abstract

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 $L_{\rm 2-10 keV}$ $\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 $N_{\rm H}$ [cm$^{-2}$] $\approx$22.