Connecting the X-ray properties of weak-line and typical quasars: testing for a geometrically thick accretion disk

Ni, Quan Xing; Brandt, W. N.; Shen, Yue; Anderson, Scott F.; Plotkin, Richard M.; Richards, Gordon T.; Schneider, Donald P.; Shemmer, Ohad; Wu, Jianfeng

doi:10.1093/mnras/sty1989

Cited by 41 publications

(104 citation statements)

References 78 publications

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“…However, about half of the WLQs have notably lower X-ray luminosities compared to the expectation from the α ox -L 2500Å relation (e.g., Luo et al 2015;Ni et al 2018). For this half of the WLQ population, high apparent levels of intrinsic X-ray absorption, Compton reflection, and/or scattering have been suggested through X-ray stacking and spectral analyses (e.g., Wu et al 2011Wu et al , 2012Luo et al 2015;Ni et al 2018). Also, the notably steep X-ray spectra of the other half of the WLQ population that is not X-ray weak indicate accretion at high Eddington ratios (e.g., Luo et al 2015;Marlar et al 2018).…”

Section: Introductionmentioning

confidence: 89%

An Extreme X-Ray Variability Event of a Weak-line Quasar

Brandt

et al. 2020

ApJL

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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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Section: Introductionmentioning

confidence: 89%

An Extreme X-Ray Variability Event of a Weak-line Quasar

Brandt

et al. 2020

ApJL

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“…Along with the increased sample size, our Full quasar sample also spans a wide range in luminosity. Figure 2 depicts the absolute i−band magnitude of our Full quasar sample compared to Sample B, all SDSS quasars (through DR14), and the sample of WLQs from Ni et al (2018). Again we measure absolute magnitudes that have been K-corrected to z = 2 using the K-corrections in Richards et al (2006).…”

Section: Finding Chandra Counterpartsmentioning

confidence: 99%

“…Another reason that C iv line emission has been associated with quasar winds is the large blueshift that it can exhibit with respect to the systemic redshift of the quasar Figure 5, however the WLQs (brown points; brown arrows depict quasars with X-ray upper limits) from Ni et al (2018) are included. If the WLQs behaved in the same manner as the typical quasars in this space, we would expect a comparable amount of dispersion around the best-fit trend (black line).…”

Section: IV Blueshift Relationship With α Ox and ∆α Oxmentioning

confidence: 99%

“…As an additional test of kinematics, we investigated α ox (and ∆α ox ) as a function of the full-width at half-maximum (FWHM) of the C iv emission line, but found no significant correlation in this space. We additionally depict the C iv blueshift values of the WLQs in Figure 8 which tend to be much larger than those of the rest of the quasar population (see also Ni et al 2018 and references therein). Also apparent in Figure 8 is that the 3σ confidence interval on Equation 7 greatly expands which we attribute to the large scatter in ∆α ox .…”

Section: IV Blueshift Relationship With α Ox and ∆α Oxmentioning

confidence: 99%

“…In this analysis, we assemble a large, unbiased sample of typical quasars, akin to Sample B, in order to constrain better correlations between the X-ray and optical/UV broad emission-line properties, and to compare the relationships derived for typical quasars with measurements for the large WLQ sample in Ni et al (2018). Since 2008, the SDSS has discovered and published nearly seven times the number of spectroscopically-confirmed quasars than are in SDSS DR5.…”

Section: Introductionmentioning

confidence: 99%

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The correlations between optical/UV broad lines and X-ray emission for a large sample of quasars

Timlin

Brandt

et al. 2019

Monthly Notices of the Royal Astronomical Society

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We present Chandra observations of 2106 radio-quiet quasars in the redshift range 1.7 ≤ z ≤ 2.7 from the Sloan Digital Sky Survey (SDSS), through data release fourteen (DR14), that do not contain broad absorption lines (BAL) in their rest-frame UV spectra. This sample adds over a decade worth of SDSS and Chandra observations to our previously published sample of 139 quasars from SDSS DR5 which is still used to correlate X-ray and optical/UV emission in typical quasars. We fit the SDSS spectra for 753 of the quasars in our sample that have high-quality (exposure time ≥ 10 ks and off-axis observation angle ≤ 10 ) X-ray observations, and analyze their X-ray-to-optical SED properties ( α ox and ∆α ox ) with respect to the measured CIV and MgII emission-line rest-frame equivalent width (EW) and the CIV emission-line blueshift. We find significant correlations (at the ≥ 99.99% level) between α ox and these emission-line parameters, as well as between ∆α ox and C iv EW. Slight correlations are found between ∆α ox and C iv blueshift, Mg ii EW, and the C iv EW to Mg ii EW ratio. The best-fit trend in each parameter space is used to compare the X-ray weakness ( ∆α ox ) and optical/UV emission properties of typical quasars and weak-line quasars (WLQs). The WLQs typically exhibit weaker X-ray emission than predicted by the typical quasar relationships. The best-fit relationships for our typical quasars are consistent with predictions from the disk-wind quasar model. The behavior of the WLQs compared to our typical quasars can be explained by an X-ray "shielding" model.

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Stochastic Modeling Handbook for Optical AGN Variability

Moreno

Vogeley

Richards

et al. 2019

PASP

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This work develops application techniques for stochastic modeling of Active Galactic Nuclei (AGNs) variability as a probe of accretion disk physics. Stochastic models, specifically Continuous Auto-Regressive Moving Average (CARMA) models, characterize light curves with a perturbation spectrum and an Impulse-Response function, which crucially provides an interpretation for variability timescales. CARMA timescales are not physical but rather, they describe correlation structure and ordered information in stochastic processes. We begin this tutorial by reviewing discrete auto-regressive and moving-average processes, we bridge these components to their continuous analogs, and lastly we investigate the significance of CARMA timescales, obtained by modeling a light curve in the time domain, in relation to the shape of the power spectrum (PSD) and structure function. We determine that higher order CARMA models, for example the Damped Harmonic Oscillator (DHO or CARMA(2, 1)) are more sensitive to deviations from a single-slope power-law description of AGN variability; unlike Damped Random Walks (DRW or CAR(1)) where the PSD slope is fixed, the DHO slope is not. Higher complexity stochastic models than the DRW capture additional covariance in data and output additional characteristic timescales that probe the driving mechanisms of variability. We provide code using Kali software to generate simulations of diverse complexity stochastic light curves. We also provide a heuristic discussion of aliasing effects in ground-based cadences and the importance of light curve length in regards to uncertainty and limitations in timescale estimation.

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Connecting the X-ray properties of weak-line and typical quasars: testing for a geometrically thick accretion disk

Cited by 41 publications

References 78 publications

An Extreme X-Ray Variability Event of a Weak-line Quasar

An Extreme X-Ray Variability Event of a Weak-line Quasar

The correlations between optical/UV broad lines and X-ray emission for a large sample of quasars

Stochastic Modeling Handbook for Optical AGN Variability

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