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Register a new userAn Extreme X-ray Variability Event of a Weak-Line Quasar
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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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We report the discovery of extreme X-ray variability in a type 1 quasar: SDSS J$075101.42+291419.1$. It has a black hole mass of $1.6times 10^7~rm M_odot$ measured from reverberation mapping (RM), and the black hole is accreting with a super-Eddington accretion rate. Its XMM-Newton observation in 2015 May reveals a flux drop by a factor of $sim 22$ with respect to the Swift observation in 2013 May when it showed a typical level of X-ray emission relative to its UV/optical emission. The lack of correlated UV variability results in a steep X-ray-to-optical power-law slope ($alpha_{rm OX}$) of -1.97 in the low X-ray flux state, corresponding to an X-ray weakness factor of 36.2 at rest-frame 2 keV relative to its UV/optical luminosity. The mild UV/optical continuum and emission-line variability also suggest that the accretion rate did not change significantly. A single power-law model modified by Galactic absorption describes well the $0.3-10$ keV spectra of the X-ray observations in general. The spectral fitting reveals steep spectral shapes with $Gammaapprox3$. We search for active galactic nuclei (AGNs) with such extreme X-ray variability in the literature and find that most of them are narrow-line Seyfert 1 galaxies and quasars with high accretion rates. The fraction of extremely X-ray variable objects among super-Eddington accreting AGNs is estimated to be $approx 15-24%$. We discuss two possible scenarios, disk reflection and partial covering absorption, to explain the extreme X-ray variability of SDSS J$075101.42+291419.1$. We propose a possible origin for the partial covering absorber, which is the thick inner accretion disk and its associated outflow in AGNs with high accretion rates.
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We show that the optical flare event discovered by Graham et al. (2020) towards the active galactic nucleus J1249+3449 is fully consistent with being a quasar microlensing event due to a $simeq 0.1 M_{odot}$ star, although other explanations, such as that, mainly supported by Graham et al. (2020), of being the electromagnetic counterpart associated to a binary black hole merger, cannot be completely excluded at present.
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