NuSTAR Observations of Intrinsically X-Ray Weak Quasar Candidates: An Obscuration-only Scenario

Abstract

We utilize recent NuSTAR observations (co-added depth ≈55–120 ks) of PG 1001+054, PG 1254+047, and PHL 1811 to constrain their hard X-ray (≳5 keV) weakness and spectral shapes and thus to investigate the nature of their extreme X-ray weakness. These quasars showed very weak soft X-ray emission, and they were proposed to be intrinsically X-ray weak, with the X-ray coronae producing weak continuum emission relative to their optical/UV emission. However, the new observations suggest an alternative explanation. The NuSTAR 3–24 keV spectral shapes for PG 1001+054 and PHL 1811 are likely flat (effective power-law photon indices ${{\rm{\Gamma }}}_{\mathrm{eff}}={1.0}_{-0.6}^{+0.5}$ and ${{\rm{\Gamma }}}_{\mathrm{eff}}={1.4}_{-0.7}^{+0.8}$, respectively), while the shape is nominal for PG 1254+047 (Γeff = 1.8 ± 0.3). PG 1001+054 and PHL 1811 are significantly weak at hard X-ray energies (by factors of ≈26–74 at rest-frame 8 keV) compared to the expectations from their optical/UV emission, while PG 1254+047 is only hard X-ray weak by a factor of ≈3. We suggest that X-ray obscuration is present in all three quasars. We propose that, as an alternative to the intrinsic X-ray weakness + X-ray obscuration scenario, the soft and hard X-ray weakness of these quasars can be uniformly explained under an obscuration-only scenario. This model provides adequate descriptions of the multiepoch soft and hard X-ray data of these quasars, with variable column density and leaked fraction of the partial covering absorber. We suggest that the absorber is the clumpy dust-free wind launched from the accretion disk. These quasars probably have super-Eddington accretion rates that drive powerful and high-density winds.