NuSTAR Observations of Intrinsically X-ray Weak Quasar Candidates: An Obscuration-Only Scenario

Abstract

We utilize recent NuSTAR observations (co-added depth $\approx 55$-120 ks) of PG $1001+054$, PG $1254+047$, and PHL 1811 to constrain their hard X-ray ($\gtrsim 5$ 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 ${\Gamma }_{\mathrm{eff}}=1.0_{-0.6}^{+0.5}$ and ${\Gamma }_{\mathrm{eff}}=1.4_{-0.7}^{+0.8}$, respectively), while the shape is nominal for PG $1254+047$ (${\Gamma }_{\mathrm{eff}}=1.8\pm 0.3$). PG $1001+054$ and PHL 1811 are significantly weak at hard X-ray energies (by factors of $\approx 26$-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 $\approx 3$. 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 multi-epoch 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.

arXiv comment21 pages, 8 figures, accepted for publication in ApJ

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