An Extreme X-ray Variability Event of a Weak-Line Quasar. (arXiv:2001.08216v1 [astro-ph.GA])

An Extreme X-ray Variability Event of a Weak-Line Quasar. (arXiv:2001.08216v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Ni_Q/0/1/0/all/0/1">Qingling Ni</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Brandt_W/0/1/0/all/0/1">W. N. Brandt</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Yi_W/0/1/0/all/0/1">W. Yi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Luo_B/0/1/0/all/0/1">B. Luo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Timlin_J/0/1/0/all/0/1">J. D. Timlin III</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hall_P/0/1/0/all/0/1">P. B. Hall</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Liu_H/0/1/0/all/0/1">Hezhen Liu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Plotkin_R/0/1/0/all/0/1">R. M. Plotkin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shemmer_O/0/1/0/all/0/1">O. Shemmer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Vito_F/0/1/0/all/0/1">F. Vito</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wu_J/0/1/0/all/0/1">Jianfeng Wu</a>

We report the discovery of an extreme X-ray flux rise (by a factor of > 20)
of the weak-line quasar 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
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 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.

We report the discovery of an extreme X-ray flux rise (by a factor of > 20)
of the weak-line quasar 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
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 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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