Browsing by Author "Liu, Hezhen"

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    An Extreme X-Ray Variability Event of a Weak-line Quasar
    (2020) Ni, Q.; Brandt, W. N.; Yi, W.; Luo, B.; Timlin, J. D., III; Hall, P. B.; Liu, Hezhen; Plotkin, R. M.; Shemmer, O.; Vito, F.; Wu, Jianfeng
    We report the discovery of an extreme X-ray flux rise (by a factor of greater than or similar to 20) of the weak-line quasar Sloan Digital Sky Survey (SDSS) J153913.47+395423.4 (hereafter SDSS J1539+3954) at z = 1.935. SDSS J1539+3954 is the most-luminous object among radio-quiet type 1 active galactic nuclei (AGNs) where such dramatic X-ray variability has been observed. Before the X-ray flux rise, SDSS J1539+3954 appeared X-ray weak compared with the expectation from its ultraviolet (UV) flux; after the rise, the ratio of its X-ray flux and UV flux is consistent with the majority of the AGN population. We also present a contemporaneous HET spectrum of SDSS J1539+3954, which demonstrates that its UV continuum level remains generally unchanged despite the dramatic increase in the X-ray flux, and its C IV emission line remains weak. The dramatic change only observed in the X-ray flux is consistent with a shielding model, where a thick inner accretion disk can block our line of sight to the central X-ray source. This thick inner accretion disk can also block the nuclear ionizing photons from reaching the high-ionization broad emission-line region, so that weak high-ionization emission lines are observed. Under this scenario, the extreme X-ray variability event may be caused by slight variations in the thickness of the disk. This event might also be explained by gravitational light-bending effects in a reflection model.
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    NuSTAR Observations of Intrinsically X-Ray Weak Quasar Candidates: An Obscuration-only Scenario
    (2022) Wang, Chaojun; Luo, B.; Brandt, W. N.; Alexander, D. M.; Bauer, F. E.; Gallagher, S. C.; Huang, Jian; Liu, Hezhen; Stern, D.
    We utilize recent NuSTAR observations (co-added depth approximate to 55-120 ks) of PG 1001+054, PG 1254+047, and PHL 1811 to constrain their hard X-ray (greater than or similar to 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(eff )= 1.0(-0.6)(+0.5) and Gamma(eff )= 1.4(-0.7)(+0.8) respectively), while the shape is nominal for PG 1254+047 ( Gamma(eff) = 1.8 +/- 0.3). PG 1001+054 and PHL 1811 are significantly weak at hard X-ray energies (by factors of approximate to 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 approximate to 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.