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

Wang, Chaojun; Luo, B.; Brandt, W.N.; Alexander, D.M.; Bauer, F.E.; Gallagher, S.C.; Huang, Jian; Liu, Hezhen; Stern, D.

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


Chaojun Wang

B. Luo

W.N. Brandt

F.E. Bauer

S.C. Gallagher

Jian Huang

Hezhen Liu

D. Stern


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.


Wang, C., Luo, B., Brandt, W., Alexander, D., Bauer, F., Gallagher, S., …Stern, D. (2022). NuSTAR Observations of Intrinsically X-Ray Weak Quasar Candidates: An Obscuration-only Scenario. Astrophysical Journal, 936(2), Article 95.

Journal Article Type Article
Acceptance Date Aug 8, 2022
Online Publication Date Sep 6, 2022
Publication Date Sep 10, 2022
Deposit Date Oct 11, 2022
Publicly Available Date Oct 11, 2022
Journal Astrophysical Journal
Print ISSN 0004-637X
Electronic ISSN 1538-4357
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 936
Issue 2
Article Number 95
Public URL


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Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

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