arXiv subjects

Jun-Xian Wang

Publications and source records attributed to Jun-Xian Wang.

At least 55 records · Page 3Linked to original sources

arXiv2022

The Relation between X-ray and Ultraviolet Variability of Quasars

The relation between X-ray and UV/optical variability in AGNs has been explored in many individual sources, however a large sample study is yet absent. Through matching the XMM-Newton serendipitous X-ray and UV source catalogs with SDSS quasars, we build a sample of 802 epoch-pairs of 525 quasars showing clear variability in logFx-logFuv space. After correcting for the effect of photometric noise, we find 35.6\pm2.1% of the epoch-pairs show asynchronous variability between X-ray and UV (brightening in one band but dimming in the other). This indicates only in 28.8\pm4.2% of the epoch-pairs the X-ray and UV variability are intrinsically coordinated. The variability synchronicity exhibits no dependence on physical parameters of quasars or the time lag of the epoch-pairs, except for stronger variability tends to have stronger synchronicity. Switches between synchronous and asynchronous variability are also seen in individual sources. The poor coordination clearly contradicts both the X-ray reprocessing model and the accretion rate variation model for AGN variability. The ratios of the observed X-ray variability amplitude to that in UV span a broad range and peak at ~ 2. The dominant fraction of the ratios appear too small to be attributed to X-ray reprocessing, and too large for accretion rate variation. The inhomogeneous disk model which incorporates both X-ray and UV/optical variability in AGNs is favored by the observed stochastic relation between X-ray and UV variations.

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arXiv2022

The X-ray coronae in NuSTAR bright active galactic nuclei

We present systematic and uniform analysis of NuSTAR data with 10-78 keV S/N > 50, of a sample of 60 SWIFT BAT selected AGNs, 10 of which are radio-loud. We measure their high energy cutoff Ecut or coronal temperature Te using three different spectral models to fit their NuSTAR spectra, and show a threshold in NuSTAR spectral S/N is essential for such measurements. High energy spectral breaks are detected in the majority of the sample, and for the rest strong constraints to Ecut or Te are obtained. Strikingly, we find extraordinarily large Ecut lower limits ( > 400 keV, up to > 800 keV) in 10 radio-quiet sources, whereas none in the radio-loud sample. Consequently and surprisingly, we find significantly larger mean Ecut/Te of radio-quiet sources compared with radio-loud ones. The reliability of these measurements are carefully inspected and verified with simulations. We find a strong positive correlation between Ecut and photon index Γ, which can not be attributed to the parameter degeneracy. The strong dependence of Ecut on Γ, which could fully account for the discrepancy of Ecut distribution between radio-loud and radio-quiet sources, indicates the X-ray coronae in AGNs with steeper hard X-ray spectra have on average higher temperature and thus smaller opacity. However, no prominent correlation is found between Ecut and λedd. In the l-Θ diagram, we find a considerable fraction of sources lie beyond the boundaries of forbidden regions due to runaway pair production, posing (stronger) challenges to various (flat) coronal geometries.

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arXiv2022

Strong [OIII]λ5007 emission line compact galaxies in LAMOST DR9: Blueberries, Green Peas and Purple Grapes

Green Pea and Blueberry galaxies are well-known for their compact size, low mass, strong emission lines and analogs to high-z Lyα emitting galaxies. In this study, 1547 strong [OIII]λ5007 emission line compact galaxies with 1694 spectra are selected from LAMOST DR9 at the redshift range from 0.0 to 0.59. According to the redshift distribution, these samples can be separated into three groups: Blueberries, Green Peas and Purple Grapes. Optical [MgII]λ2800 line feature, BPT diagram, multi-wavelength SED fitting, MIR color, and MIR variability are deployed to identify 23 AGN candidates from these samples, which are excluded for the following SFR discussions. We perform the multi-wavelength SED fitting with GALEX UV and WISE MIR data. Color excess from Balmer decrement shows these strong [OIII]λ5007 emission line compact galaxies are not highly reddened. The stellar mass of the galaxies is obtained by fitting LAMOST calibrated spectra with the emission lines masked. We find that the SFR is increasing with the increase of redshift, while for the sources within the same redshift bin, the SFR increases with mass with a similar slope as the SFMS. These samples have a median metallicity of 12+log(O/H) of 8.10. The metallicity increases with mass, and all the sources are below the mass-metallicity relation. The direct-derived Te-based metallicity from the [OIII]λ4363 line agrees with the empirical N2-based empirical gas-phase metallicity. Moreover, these compact strong [OIII]λ5007 are mostly in a less dense environment.

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arXiv2021

More variable quasars have stronger emission lines

The UV/optical variation, likely driven by accretion disc turbulence, is a defining characteristic of type 1 active galactic nuclei (AGNs) and quasars. In this work we investigate an interesting consequence of such turbulence using quasars in SDSS Stripe 82 for which the measurements of the UV/optical variability amplitude are available from $\sim$ 10 years long light curves. We discover positive correlations between UV/optical variability amplitude $σ_{rms}$ and equivalent widths of CIV, Mg II and [OIII]5007 emission lines. Such correlations remain statistically robust through partial correlation analyses, i.e., after controlling the effects of other variables including bolometric luminosity, central supermassive black hole mass, Eddington ratio and redshift. This, for the first time, indicates a causal link between disc turbulence and emission line production. We propose two potential underlying mechanisms both of which may be involved: 1) quasars with stronger disc turbulence have on average bluer/harder broadband SED, an expected effect of the disc thermal fluctuation model; 2) stronger disc turbulence could lead to launch of emission line regions with larger covering factors.

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arXiv2021

Distinct High Energy Cutoff Variation Patterns in Two Seyfert Galaxies

Investigating how the cutoff energy $E_{\rm cut}$ varies with X-ray flux and photon index $Γ$ in individual AGNs opens a new window to probe the yet unclear coronal physics. So far $E_{\rm cut}$ variations have only been detected in several AGNs but different patterns have been reported. Here we report new detections of $E_{\rm cut}$ variations in two Seyfert galaxies with multiple NuSTAR exposures. While in NGC 3227 $E_{\rm cut}$ monotonically increases with $Γ$, the $E_{\rm cut}$-$Γ$ relation exhibits a $Λ$ shape in SWIFT J2127.4+5654 ($E_{\rm cut}$ increasing with $Γ$ at $Γ$ $\lesssim$ 2.05, but reversely decreasing at $Γ$ $\gtrsim$ 2.05), indicating more than a single underlying mechanism is involved. Meanwhile both galaxies show softer spectra while they brighten in X-ray, a common phenomenon in Seyfert galaxies. Plotting all 7 AGNs with $E_{\rm cut}$ variations ever reported with NuSTAR observations in the $E_{\rm cut}$-$Γ$ diagram, we find they could be unified with the $Λ$ pattern. Although the sample is small and SWIFT J2127.4+5654 is the only source with $Γ$ varying across the break point thus the only one exhibiting the complete $Λ$ pattern in a single source, the discoveries shed new light on the coronal physics in AGNs. Possible underlying physical mechanisms are discussed.

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arXiv2020

The Feasibility and Flexibility of Selecting Quasars by Variability Using Ensemble Machine Learning Algorithms

In this work we train three decision-tree based ensemble machine learning algorithms (Random Forest Classifier, Adaptive Boosting and Gradient Boosting Decision Tree respectively) to study quasar selection in the variable source catalog in SDSS Stripe 82. We build training and test samples (both containing 1:1 of quasars and stars) using the spectroscopic confirmed sources in SDSS DR14 (including 8330 quasars and 3966 stars). We find that, trained with variation parameters alone, all three models can select quasars with similarly and remarkably high precision and completeness ($\sim$ 98.5% and 97.5%), even better than trained with SDSS colors alone ($\sim$ 97.2% and 96.5%), consistent with previous studies. Through applying the trained models on the variable sources without spectroscopic identifications, we estimate the spectroscopically confirmed quasar sample in Stripe 82 variable source catalog is $\sim$ 93% complete (95% for $m_i<19.0$). Using the Random Forest Classifier we derive the relative importance of the observational features utilized for classifications. We further show that even using one- or two-year time domain observations, variability-based quasar selection could still be highly efficient.

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arXiv2020

More than softer-when-brighter: the X-Ray powerlaw spectral variability in NGC 4051

The powerlaw X-ray spectra of active galactic nuclei at moderate to high accretion rates normally appear softer when they brighten, for which the underlying mechanisms are yet unclear. Utilizing XMM-Newton observations and excluding photons $<$ 2 keV to avoid contamination from the soft excess, in this work we scrutinize the powerlaw spectral variability of NCG 4051 from two new aspects. We first find that a best-fit "softer-when-brighter" relation is statistically insufficient to explain the observed spectral variabilities, and intervals deviated from the empirical relation are clearly visible in the light curve of 2 -- 4 keV/4 -- 10 keV count rate ratio. The deviations are seen not only between but also within individual XMM-Newton exposures, consistent with random variations of the corona geometry or inner structure (with timescales as short as $\sim$ 1 ks), in addition to those behind the smooth "softer-when-brighter" trend. We further find the "softer-when-brighter" trend gradually weakens with the decreasing timescale (from $\sim$ 100 ks down to 0.5 ks). These findings indicate that the powerlaw spectral slope is not solely determined by its brightness. We propose a two-tier geometry, including flares/nano-flares on top of the inner disc and an embedding extended corona (heated by the flares, in analogy to solar corona) to explain the observations together with other observational clues in literature. Rapid spectral variabilities could be due to individual flares/nano-flares, while slow ones are driven by the variations in the global activity of inner disc region (akin to the variation of solar activity, but not the accretion rate) accompanied with heating/cooling and inflation/contraction of the extended corona.

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arXiv2020

EUCLIA. II. On the puzzling large UV to X-ray lags in Seyfert galaxies

Recent intense X-ray and UV monitoring campaigns with Swift have detected clear UV lags behind X-ray in several local active galactic nuclei (AGNs). The UV to X-ray lags are often larger (by a factor up to ~ 20) than expected if the UV variation is simply due to the X-ray reprocessing. We previously developed a model in which the UV/optical variations are attributed to disk turbulences, and the effect of large-scale turbulence is considered. Our model, which overcomes many severe challenges to the reprocessing scheme, can well explain the observed variations in NGC 5548, particularly the correlations and lags among the UV/optical bands. In this work, assuming the corona heating is associated with turbulences in the inner accretion disk, we extend our study to model the correlations and lags between the X-ray and UV/optical bands. We find that our model, without the need of light echoing, can well reproduce the observed UV to X-ray lags and the optical to UV lags simultaneously in four local Seyfert galaxies, including NGC 4151, NGC 4395, NGC 4593, and NGC 5548. In our scenario, relatively larger UV to X-ray lag is expected for AGN with smaller innermost disk radius and thus more compact corona. Interestingly, for these Seyfert galaxies studied in this work, sources with relatively larger UV to X-ray lags do have broader Fe Ka lines, indicative of relativistic broadening due to more compact corona and smaller innermost disk radius. If confirmed with more X-ray and UV monitoring campaigns, this interesting discovery would provide a new probe to the inner disk/corona.

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arXiv2018

New Clues to Jet Launching: The inner disks in radio loud quasars may be more stable

Jet launching in radio loud (RL) quasars is one of the fundamental problems in astrophysics. Exploring the differences in the inner accretion disk properties between RL and radio quiet (RQ) quasars might yield helpful clues to this puzzle. We previously discovered that the shorter term UV/optical variations of quasars are bluer than the longer term ones, i.e., the so-called timescale-dependent color variation. This is consistent with the scheme that the faster variations come from the inner and hotter disk regions, thus providing a useful tool to map the accretion disk which is otherwise unresolvable. In this work we compare the UV/optical variations of RL quasars in SDSS Stripe 82 to those of several RQ samples, including those matched in redshift-luminosity-black hole mass and/or color-magnitude. We find that while both RL and RQ populations appear bluer when they brighten, RL quasars potentially show a weaker/flatter dependence on timescale in their color variation. We further find that while both RL and RQ populations on average show similar variation amplitudes at long timescales, fast variations of RL sources appear weaker/smaller (at timescales of ~ 25 -- 300 days in the observer's frame), and the difference is more prominent in the g-band than in the r-band. Inhomogeneous disk simulations can qualitatively reproduce these observed differences if the inner accretion disk of RL quasars fluctuates less based on simple toy models. Though the implications are likely model dependent, the discovery points to an interesting diagram that magnetic fields in RL quasars may be prospectively stronger and play a key role in both jet launching and the stabilization of the inner accretion disk.

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arXiv2018

An intrinsic link between long-term UV/optical variations and X-ray loudness in quasars

Observations have shown that UV/optical variation amplitude of quasars depend on several physi- cal parameters including luminosity, Eddington ratio, and likely also black hole mass. Identifying new factors which correlate with the variation is essential to probe the underlying physical processes. Combining ~ten years long quasar light curves from SDSS stripe 82 and X-ray data from Stripe 82X, we build a sample of X-ray detected quasars to investigate the relation between UV/optical variation amplitude ($σ_{rms}$) and X-ray loudness. We find that quasars with more intense X-ray radiation (com- pared to bolometric luminosity) are more variable in UV/optical. Such correlation remains highly significant after excluding the effect of other parameters including luminosity, black hole mass, Ed- dington ratio, redshift, rest-frame wavelength (i.e., through partial correlation analyses). We further find the intrinsic link between X-ray loudness and UV/optical variation is gradually more prominent on longer timescales (up to 10 years in the observed frame), but tends to disappear at timescales < 100 days. This suggests a slow and long-term underlying physical process. The X-ray reprocessing paradigm, in which UV/optical variation is produced by a variable central X-ray emission illuminating the accretion disk, is thus disfavored. The discovery points to an interesting scheme that both the X-ray corona heating and UV/optical variation is quasars are closely associated with magnetic disc turbulence, and the innermost disc turbulence (where corona heating occurs) correlates with the slow turbulence at larger radii (where UV/optical emission is produced).

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arXiv2018

Probing AGN Inner Structure with X-ray Obscured Type 1 AGN

Using the X-ray-selected active galactic nuclei (AGN) from the XMM-XXL north survey and the SDSS Baryon Oscillation Spectroscopic Survey (BOSS) spectroscopic follow-up of them, we compare the properties of X-ray unobscured and obscured broad-line AGN (BLAGN1 and BLAGN2; $N_\textrm{H}$below and above $10^{21.5}$ cm$^{-2}$), including their X-ray luminosity $L_X$, black hole mass, Eddington ratio $λ_{\textrm{Edd}}$, optical continuum and line features. We find that BLAGN2 have systematically larger broad line widths and hence apparently higher (lower) $M_{\textrm{BH}}$ ($λ_{\textrm{Edd}}$) than BLAGN1. We also find that the X-ray obscuration in BLAGN tends to coincide with optical dust extinction, which is optically thinner than that in narrow-line AGN (NLAGN) and likely partial-covering to the broad line region. All the results can be explained in the framework of a multi-component, clumpy torus model by interpreting BLAGN2 as an intermediate type between BLAGN1 and NLAGN in terms of an intermediate inclination angle.

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arXiv2018

On Measuring the Variation of High Energy Cut-off in Active Galactic Nuclei

The variation in the high energy cut-off E_c in active galactic nuclei uniquely probes the corona physics. In this work we show that the ratio of two NuSTAR spectra (in analogy to difference imaging technique widely used in astronomy) is uniquely useful in studying E_c variations. The spectra ratio could directly illustrate potential E_c variation between two spectra. By comparing with the ratio of two spectral fitting models, it also examines the reliability of the spectral-fitting measured E_c variation. Assisted with this technique, we revisit the 5 AGNs in literature (MCG -5-23-16, 3C 382, NGC 4593, NGC 5548 and Mrk 335) for which E_c (kT_e) variations have been claimed with NuSTAR observations. We show the claimed E_c variations appear inconsistent with the spectra ratios in three of them, thus need to be revised, demonstrating the striking usefulness of spectra ratio. We present thereby improved spectral fitting results and E_c variations. We also report a new source with E_c variations based on NuSTAR observations (radio galaxy 4C +74.26). We find the corona tends to be hotter when it brightens (hotter-when-brighter) in 3C 382, NGC 5548, Mrk 335 and 4C +74.27, but MCG -5-23-16 and NGC 4593 show no evidence of significant E_c variations. Meanwhile all 6 sources in this small sample appear softer-when-brighter. Changes in corona geometry are required to explain the observed hotter-when-brighter trends.

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arXiv2018

On the UV/optical variation in NGC 5548: new evidence against the reprocessing diagram

The reprocessing scenario is widely adopted in literature to explain the observed tight inter-band correlation and short lags in the UV/optical variations of active galactic nuclei (AGNs). In this work we look into the color variability of the famous Seyfert galaxy NGC 5548 with high quality Swift multi-band UV/optical light curves. We find the color variation of NGC 5548 is clearly timescale-dependent, in a way that it is more prominent at shorter timescales. This is similar to that previously detected in quasar samples, but for the first time in an individual AGN. We show that while a reprocessing model with strict assumptions on the driving source and the disk size can apparently match the observed light curves and inter-band lags, it fails to reproduce the observed timescale dependency in the color variation. Such discrepancy raises a severe challenge to, and can hardly be reconciled under the widely accepted reprocessing diagram. It also demonstrates that the timescale dependency of the color variation is uniquely powerful in probing the physics behind AGN UV/optical variations.

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arXiv2018

Recurrent outbursts revealed in 3XMM J031820.8-663034

3XMM J031820.8-663034, first detected by ROSAT in NGC 1313, is one of a few known transient ultraluminous X-ray sources (ULXs). In this paper, we present decades of X-ray data of this source from ROSAT, XMM-Newton, Chandra and the Neil Gehrels Swift Observatory. We find that its X-ray emission experienced four outbursts since 1992, with a typical recurrent time $\sim$ 1800 days, an outburst duration $\sim 240-300$ days, and a nearly constant peak X-ray luminosity $\sim 1.5 \times 10^{39}$ erg/s. The upper limit of X-ray luminosity at the quiescent state is $\sim 5.6 \times 10^{36}$ erg/s, and the total energy radiated during one outburst is $\sim 10^{46}$ erg. The spectra at the high luminosity states can be described with an absorbed disk black-body, and the disk temperature increases with the X-ray luminosity. We compare its outburst properties with other known transient ULXs including ESO 243-49 HLX-1. As its peak luminosity only marginally puts it in the category of ULXs, we also compare it with normal transient black hole binaries. Our results suggest that the source is powered by an accreting massive stellar-mass black hole, and the outbursts are triggered by the thermal-viscous instability.

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arXiv2018

EUCLIA - Exploring the UV/optical continuum lag in active galactic nuclei. I. a model without light echoing

The tight inter-band correlation and the lag-wavelength relation among UV/optical continua of active galactic nuclei have been firmly established. They are usually understood within the widespread reprocessing scenario, however, the implied inter-band lags are generally too small. Furthermore, it is challenged by new evidences, such as the X-ray reprocessing yields too much high frequency UV/optical variations as well as it fails to reproduce the observed timescale-dependent color variations among {\it Swift} lightcurves of NGC 5548. In a different manner, we demonstrate that an upgraded inhomogeneous accretion disk model, whose local {\it independent} temperature fluctuations are subject to a speculated {\it common} large-scale temperature fluctuation, can intrinsically generate the tight inter-band correlation and lag across UV/optical, and be in nice agreement with several observational properties of NGC 5548, including the timescale-dependent color variation. The emergent lag is a result of the {\it differential regression capability} of local temperature fluctuations when responding to the large-scale fluctuation. An average speed of propagations as large as $\gtrsim 15\%$ of the speed of light may be required by this common fluctuation. Several potential physical mechanisms for such propagations are discussed. Our interesting phenomenological scenario may shed new light on comprehending the UV/optical continuum variations of active galactic nuclei.

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arXiv2017

SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES): Faint-End Counts at 450 um

The SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES) is a three-year JCMT Large Program aiming at reaching the 450 $μ$m confusion limit in the COSMOS-CANDELS region, to study a representative sample of the high-redshift far-infrared galaxy population that gives rise to the bulk of the far-infrared background. We present the first-year data from STUDIES. We have reached a 450 $μ$m noise level of 0.91~mJy for point sources at the map center, covered an area of 151 arcmin$^2$, and detected 98 and 141 sources at 4.0 and 3.5 $σ$, respectively. Our derived counts are best constrained in the 3.5-25 mJy regime using directly detected sources. Below the detection limits, our fluctuation analysis further constrains the slope of the counts down to 1 mJy. The resulting counts at 1-25 mJy are consistent with a power law having a slope of $-2.59$ ($\pm0.10$ for 3.5-25 mJy, and $^{+0.4}_{-0.7}$ for 1-3.5 mJy). There is no evidence of a faint-end termination or turn-over of the counts in this flux density range. Our counts are also consistent with previous SCUBA-2 blank-field and lensing cluster surveys. The integrated surface brightness from our counts down to 1 mJy is $90.0\pm17.2$ Jy deg$^{-2}$, which can account for up to $83^{+15}_{-16}\%$ of the COBE 450 $μ$m background. We show that Herschel counts at 350 and 500 $μ$m are significantly higher than our 450 $μ$m counts, likely caused by its large beam and source clustering. High-angular resolution instruments like SCUBA-2 at 450 $μ$m are therefore highly beneficial for measuring the luminosity and spatial density of high-redshift dusty galaxies.

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arXiv2017

X-ray spectral analyses of AGNs from the 7Ms Chandra Deep Field-South survey: the distribution, variability, and evolution of AGN's obscuration

We present a detailed spectral analysis of the brightest Active Galactic Nuclei (AGN) identified in the 7Ms Chandra Deep Field South (CDF-S) survey over a time span of 16 years. Using a model of an intrinsically absorbed power-law plus reflection, with possible soft excess and narrow Fe K$α$ line, we perform a systematic X-ray spectral analysis, both on the total 7Ms exposure and in four different periods with lengths of 2-21 months. With this approach, we not only present the power-law slopes, column densities $N_H$, observed fluxes, and absorption-corrected 2-10~keV luminosities $L_X$ for our sample of AGNs, but also identify significant spectral variabilities among them on time scales of years. We find that the $N_H$ variabilities can be ascribed to two different types of mechanisms, either flux-driven or flux-independent. We also find that the correlation between the narrow Fe line EW and $N_H$ can be well explained by the continuum suppression with increasing $N_H$. Accounting for the sample incompleteness and bias, we measure the intrinsic distribution of $N_H$ for the CDF-S AGN population and present re-selected subsamples which are complete with respect to $N_H$. The $N_H$-complete subsamples enable us to decouple the dependences of $N_H$ on $L_X$ and on redshift. Combining our data with that from C-COSMOS, we confirm the anti-correlation between the average $N_H$ and $L_X$ of AGN, and find a significant increase of the AGN obscured fraction with redshift at any luminosity. The obscured fraction can be described as $f_{obscured}\thickapprox 0.42\ (1+z)^{0.60}$.

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arXiv2016

The Timescale-Dependent Color Variability of Quasars Viewed with GALEX

In recent work done by Sun et. al., the color variation of quasars, namely the bluer-when-brighter trend, was found to be timescale-dependent using SDSS $g/r$ band light curves in the Stripe 82. Such timescale dependence, i.e., bluer variation at shorter timescales, supports the thermal fluctuation origin of the UV/optical variation in quasars, and can be well modeled with the inhomogeneous accretion disk model. In this paper, we extend the study to much shorter wavelengths in the rest frame (down to extreme UV), using GALaxy Evolution eXplorer (GALEX) photometric data of quasars collected in two ultraviolet bands (near-UV and far-UV). We develop Monte-Carlo simulations to correct possible biases due to the considerably larger photometric uncertainties in GALEX light curves (particularly in far-UV, comparing with SDSS $g/r$ bands), which otherwise could produce artificial results. We securely confirm the previously discovered timescale dependence of the color variability with independent datasets and at shorter wavelengths. We further find the slope of the correlation between the amplitude of color variation and timescale however appears even steeper than that predicted by the inhomogeneous disk model, which assumes that disk fluctuations follow damped random walk process. In line with the much flatter structure function observed in far-UV comparing with that at longer wavelengths, this implies deviation from DRW process in the inner disk where rest frame extreme UV radiation is produced.

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