Deepest View of AGN X-Ray Variability with the 7 Ms Chandra Deep Field-South Survey

Zheng, Xuechen; Xue, Yongquan; Brandt, W. N.; Li, J. Y.; Paolillo, M.; Yang, Guang; Zhu, Shifu; Luo, B.; Sun, Mouyuan; Hughes, T. M.; Bauer, F. E.; Vito, F.; Wang, Junxian; Liu, Teng; Vignali, C.; Shu, Xinwen

doi:10.3847/1538-4357/aa9378

Cited by 38 publications

(41 citation statements)

References 73 publications

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“…Since the detection limit of the 7 Ms CDF-S is a function of off-axis angle, we divide the central r = 8 FOV into a series of narrow concentric annuli with a width of ∆r to determine the minimum 0.5-7 keV counts of an X-ray source required for a detection in each annulus, i.e., the detection limit in each annular region. Following the original transient-searching procedure 14 and considering that the background of the 7 Ms CDF-S is stable 17 , we assume a background region 10 times that of the source region and estimate the expected background counts for the source in a 70 ks observation (i.e., the exposure of ObsID 16453) using the mean background count rate of the 7 Ms CDF-S 17 . The fluctuation (σ) of the background counts is given by the Poisson distribution.…”

Section: Methodsmentioning

confidence: 99%

“…South survey (CDF-S), which consists of 102 individual Chandra/Advanced CCD Imaging Spectrometer imaging array (ACIS-I) observations spanning 16.4 yrs 17,18 , we performed a search for X-ray transient events and discovered two notable fast outbursts 14 , with one dubbed CDF-S XT1 and reported elsewhere 13 and CDF-S XT2 being the focus here. The We present the binned Chandra 0.5-7 keV light curves and spectra of CDF-S XT2 in Figure 1 for viewing purposes, and fit the unbinned light curves and spectra for physical constraints (see Methods).…”

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confidence: 99%

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A magnetar-powered X-ray transient as the aftermath of a binary neutron-star merger

Xue

Zheng

et al. 2019

Nature

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Neutron star-neutron star mergers are known to be associated with short gammaray bursts 1-4 . If the neutron star equation of state is sufficiently stiff, at least some of such mergers will leave behind a supramassive or even a stable neutron star that spins rapidly with a strong magnetic field (i.e., a magnetar) 5-8 . Such a magnetar signature may have been observed as the X-ray plateau following a good fraction (up to 50%) of short gamma-ray bursts 9, 10 , and it has been expected that one may observe short gamma-ray burst-less X-ray transients powered by double neutron star mergers 11, 12 . A fast X-ray transient (CDF-S XT1) was recently found to be associated with a faint host galaxy whose redshift is unknown 13 . Its X-ray and host-galaxy properties allow several possible explanations including a short gamma-ray burst seen off axis, a low-luminosity gamma-ray burst at high redshift, or a tidal disruption event involving an intermediate mass black hole and a white dwarf 13 . Here we report a second X-ray transient, CDF-S XT2, that is associated with a galaxy at redshift z = 0.738 14 .The light curve is fully consistent with being powered by a millisecond magnetar. More intriguingly, CDF-S XT2 lies in the outskirts of its star-forming host galaxy with a moderate offset from the galaxy center, as short bursts often do 15,16 . The estimated event rate density of similar X-ray transients, when corrected to the local value, is consistent with the double neutron star merger rate density inferred from the detection of GW170817 1 .Upon the completion of the deepest X-ray survey to date, the 7 Ms Chandra Deep Field-

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Section: Methodsmentioning

confidence: 99%

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confidence: 99%

A magnetar-powered X-ray transient as the aftermath of a binary neutron-star merger

Xue

Zheng

et al. 2019

Nature

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“…The results are listed in Table 9. To check whether our calculation of σ 2 nxv is affected by the limited counts (e.g., Zheng et al 2017; hereafter Z17), we perform Spearman rank correlation tests and find no significant correlation between σ 2 nxv,F , σ 2 nxv,N , σ 2 nxv,L and counts with Spearman's ρ = 0.07, p-value = 0.70; ρ = −0.11, p-value = 0.56 and ρ = 0.32, p-value = 0.08, respectively. We also calculate the fractional root-mean-square (frms) variability amplitude, which is defined as σ 2 nxv for sources having σ 2 nxv > 0 and can be treated as the percentage of the variability amplitude.…”

Section: Variability Amplitude Estimationmentioning

confidence: 99%

Piercing through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields. I. X-Ray Spectral and Long-term Variability Analyses

Xue

Sun

et al. 2019

ApJ

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We present a detailed X-ray spectral analysis of 1152 AGNs selected in the Chandra Deep Fields (CDFs), in order to identify highly obscured AGNs (N H > 10 23 cm −2 ). By fitting spectra with physical models, 436 (38%) sources with L X > 10 42 erg s −1 are confirmed to be highly obscured, including 102 Compton-thick (CT) candidates. We propose a new hardness-ratio measure of the obscuration level which can be used to select highly obscured AGN candidates. The completeness and accuracy of applying this method to our AGNs are 88% and 80%, respectively. The observed log N − log S relation favors cosmic X-ray background models that predict moderate (i.e., between optimistic and pessimistic) CT number counts. 19% (6/31) of our highly obscured AGNs that have optical classifications are labeled as broad-line AGNs, suggesting that, at least for part of the AGN population, the heavy X-ray obscuration is largely a line-of-sight effect, i.e., some high-column-density clouds on various scales (but not necessarily a dust-enshrouded torus) along our sightline may obscure the compact X-ray emitter. After correcting for several observational biases, we obtain the intrinsic N H distribution and its evolution. The CT-to-highly-obscured fraction is roughly 52% and is consistent with no evident redshift evolution. We also perform long-term (≈ 17 years in the observed frame) variability analyses for 31 sources with the largest number of counts available. Among them, 17 sources show flux variabilities: 31% (5/17) are caused by the change of N H , 53% (9/17) are caused by the intrinsic luminosity variability, 6% (1/17) are driven by both effects, and 2 are not classified due to large spectral fitting errors. arXiv:1904.03827v1 [astro-ph.GA] 8 Apr 2019Li, Xue, Sun et al.

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“…When the observed α OX value of an AGN is lower than that expected from the α OX -L 2500 Å relation (∆α OX = α OX − α OX,exp < 0, where more negative values indicate more extreme X-ray weakness relative to the 2500 Å monochromatic luminosity), it is X-ray weak by a linear factor of f weak = 10 −∆αOX/0.384 . X-ray luminosity variability is a characteristic property of AGNs, which is generally related to instabilities of the corona or fluctuations of the accretion flow (e.g., Nandra 2001;McHardy et al 2006;MacLeod et al 2010;Yang et al 2016;Zheng et al 2017). The intrinsic X-ray variability of AGNs contributes partially to the dispersion of the α OX -L 2500 Å relation (e.g., Vagnetti et al 2010;Gibson & Brandt 2012;Vagnetti et al 2013;Chiaraluce et al 2018).…”

Section: Introductionmentioning

confidence: 99%

SDSS J075101.42+291419.1: A Super-Eddington Accreting Quasar with Extreme X-Ray Variability

Liu

Brandt

Brotherton

et al. 2019

ApJ

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We report the discovery of extreme X-ray variability in a type 1 quasar: SDSS J075101.42+291419.1. It has a black hole mass of 1.6 × 10 7 M ⊙ 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 ∼ 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 (α 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 Γ ≈ 3. 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 ≈ 15-24%. We discuss two possible scenarios, disk reflection and partial covering absorption, to explain the extreme X-ray variability of SDSS J075101.42 + 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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Deepest View of AGN X-Ray Variability with the 7 Ms Chandra Deep Field-South Survey

Cited by 38 publications

References 73 publications

A magnetar-powered X-ray transient as the aftermath of a binary neutron-star merger

A magnetar-powered X-ray transient as the aftermath of a binary neutron-star merger

Piercing through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields. I. X-Ray Spectral and Long-term Variability Analyses

SDSS J075101.42+291419.1: A Super-Eddington Accreting Quasar with Extreme X-Ray Variability

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