arXiv subjects

Mouyuan Sun

Publications and source records attributed to Mouyuan Sun.

At least 91 records · Page 5Linked to original sources

arXiv2020

AGNs are not that cool: revisiting the intrinsic AGN far-infrared spectral energy distribution

We investigate the intrinsic spectral energy distribution (SED) of active galactic nuclei (AGNs) at infrared (IR) bands with 42 $z < 0.5$ optically luminous Palomar Green survey quasars through SED decomposition. We decompose the SEDs of the 42 quasars by combining an AGN IR template library Siebenmorgen2015 that covers a wide range of the AGN parameter space with three commonly used galaxy template libraries. We determine the median AGN SED from the best-fitting results. The far-IR (FIR) contribution of our median AGN SED is significantly smaller than that of Symeonidis et al. 2016, but roughly consistent with that of Lyu et al. 2017. The AGN IR SED becomes cooler with increasing bolometric luminosity, which might be due to that more luminous AGNs might have stronger radiative feedback to change torus structures and/or their tori might have higher metallicities. Our conclusions do not depend on the choice of galaxy template libraries. However, since the predicted polycyclic aromatic hydrocarbon (PAH) emission line flux is galaxy template-dependent, cautions should be taken on deriving galaxy FIR contribution from PAH fluxes.

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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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arXiv2020

Corona-Heated Accretion-disk Reprocessing (CHAR): A Physical Model to Decipher the Melody of AGN UV/optical Twinkling

Active galactic nuclei (AGNs) have long been observed to "twinkle" (i.e., their brightness varies with time) on timescales from days to years in the UV/optical bands. Such AGN UV/optical variability is essential for probing the physics of supermassive black holes (SMBHs), the accretion disk, and the broad-line region. Here we show that the temperature fluctuations of an AGN accretion disk, which is magnetically coupled with the corona, can account for observed high-quality AGN optical light curves. We calculate the temperature fluctuations by considering the gas physics of the accreted matter near the SMBH. We find that the resulting simulated AGN UV/optical light curves share the same statistical properties as the observed ones as long as the dimensionless viscosity parameter $α$, which is widely believed to be controlled by magnetohydrodynamic (MHD) turbulence in the accretion disk, is about $0.01$---$0.2$. Moreover, our model can simultaneously explain the larger-than-expected accretion disk sizes and the dependence of UV/optical variability upon wavelength for NGC 5548. Our model also has the potential to explain some other observational facts of AGN UV/optical variability, including the timescale-dependent bluer-when-brighter color variability and the dependence of UV/optical variability on AGN luminosity and black hole mass. Our results also demonstrate a promising way to infer the black-hole mass, the accretion rate, and the radiative efficiency, thereby facilitating understanding of the gas physics and MHD turbulence near the SMBH and its cosmic mass growth history by fitting the AGN UV/optical light curves in the era of time-domain astronomy.

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arXiv2019

Understanding Broad Mg II Variability in Quasars with Photoionization: Implications for Reverberation Mapping and Changing-Look Quasars

The broad Mg II line in quasars has distinct variability properties compared with broad Balmer lines: it is less variable, and usually does not display a "breathing" mode, the increase in the average cloud distance when luminosity increases. We demonstrate that these variability properties of Mg II can be reasonably well explained by simple Locally Optimally Emitting Cloud (LOC) photoionization models, confirming earlier photoionization results. In the fiducial LOC model, the Mg II-emitting gas is on average more distant from the ionizing source than the H$α$/H$β$ gas, and responds with a lower amplitude to continuum variations. If the broad-line region (BLR) is truncated at a physical radius of $\sim 0.3$ pc (for a $10^{8.5}M_{\odot}$ BH accreting at Eddington ratio of 0.1), most of the Mg II flux will always be emitted near this outer boundary and hence will not display breathing. These results indicate that reverberation mapping results on broad Mg II, while generally more difficult to obtain due to the lower line responsivity, can still be used to infer the Mg II BLR size and hence black hole mass. But it is possible that Mg II does not have a well defined intrinsic BLR size-luminosity relation for individual quasars, even though a global one for the general population may still exist. The dramatic changes in broad H$α$/H$β$ emission in the observationally-rare changing-look quasars are fully consistent with photoionization responses to extreme continuum variability, and the LOC model provides natural explanations for the persistence of broad Mg II in changing-look quasars defined on H$α$/H$β$, and the rare population of broad Mg II emitters in the spectra of massive inactive galaxies.

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arXiv2019

Discovery of A Mg II Changing-look AGN and its Implications for a Unification Sequence of Changing-look AGNs

Changing-Look (CL) is a rare phenomenon of Active Galactic Nuclei (AGNs) that exhibit emerging or disappearing broad lines accompanied by continuum variations on astrophysically short timescales ($\lesssim$ 1 yr to a few decades). While previous studies have found Balmer-line (broad H$α$ and/or H$β$) CL AGNs, the broad Mg II line is persistent even in dim states. No unambiguous Mg II CL AGN has been reported to date. We perform a systematic search of Mg II CL AGNs using multi-epoch spectra of a special population of Mg II-emitters (characterized by strong broad Mg II emission with little evidence for AGN from other normal indicators such as broad H$α$ and H$β$ or blue power-law continua) from the Fourteenth Data Release of the Sloan Digital Sky Survey. We present the discovery of the first unambiguous case of an Mg II CL AGN, SDSS J152533.60+292012.1 (at redshift $z$ = 0.449), which is turning off within rest-frame 286 days. The dramatic diminishing of Mg II equivalent width (from 110 $\pm$ 26 Angstrom to being consistent with zero), together with little optical continuum variation ($ΔV_{\rm max-min}$ $=$ 0.17 $\pm$ 0.05 mag) coevally over $\sim$ 10 years, rules out dust extinction or a tidal disruption event. Combined with previously known H$β$ CL AGNs, we construct a sequence that represents different temporal stages of CL AGNs. This CL sequence is best explained by the photoionization model of Guo et al. (2019). In addition, we present two candidate turn-on Mg II CL AGNs and a sample of 361 Mg II-emitters for future Mg II CL AGN searches.

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arXiv2019

Mining for Candidates of Galactic Stellar-mass Black Hole Binaries with LAMOST

We study the prospects of searching for black hole (BH) binary systems with a stellar-mass BH and a non-compact visible companion, by utilizing the spectroscopic data of Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). We simulate the Galactic BH binary population and determine its optical visibility by considering the stellar synthetic population model and the distributions of binary orbital parameters. By convolving the visibility of BH binaries with the LAMOST detection sensitivity, we predict that $\gtrsim$ 400 candidate BH binaries can be found by the low-resolution, non-time-domain survey, and $\sim$ 50-350 candidates by the LAMOST ongoing medium-resolution, time-domain spectroscopic survey. Most of the candidates are short-period (0.2-2 days) binaries with M-, K-, G-, or F-type companions, in which $\sim$ 47% have a mass function (the lower limit of the BH mass) larger than 3 $M_{\odot}$. By complementing the LAMOST spectroscopic data with other photometric/spectroscopic surveys or follow-up observations, these candidates could be confirmed. Therefore, by exploring the LAMOST data, we can enlarge the sample of dynamically confirmed BH binaries significantly, which can improve our understanding of the mass distribution of BHs and the stellar evolution model.

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arXiv2019

The Sloan Digital Sky Survey Reverberation Mapping Project: Accretion-Disk Sizes from Continuum Lags

We present accretion-disk structure measurements from continuum lags in the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project. Lags are measured using the \texttt{JAVELIN} software from the first-year SDSS-RM $g$ and $i$ photometry, resulting in well-defined lags for 95 quasars, 33 of which have lag SNR $>$ 2$σ$. We also estimate lags using the \texttt{ICCF} software and find consistent results, though with larger uncertainties. Accretion-disk structure is fit using a Markov Chain Monte Carlo approach, parameterizing the measured continuum lags as a function of disk size normalization, wavelength, black hole mass, and luminosity. In contrast with previous observations, our best-fit disk sizes and color profiles are consistent (within 1.5~$σ$) with the \citet{SS73} analytic solution. We also find that more massive quasars have larger accretion disks, similarly consistent with the analytic accretion-disk model. The data are inconclusive on a correlation between disk size and continuum luminosity, with results that are consistent with both no correlation and with the \citet{SS73} expectation. The continuum lag fits have a large excess dispersion, indicating that our measured lag errors are underestimated and/or our best-fit model may be missing the effects of orientation, spin, and/or radiative efficiency. We demonstrate that fitting disk parameters using only the highest-SNR lag measurements biases best-fit disk sizes to be larger than the disk sizes recovered using a Bayesian approach on the full sample of well-defined lags.

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arXiv2019

On the Origin of the Dramatic Spectral Variability of WPVS 007

We report the discovery of large-amplitude mid-infrared variabilities (MIR; $\sim 0.3$ mag) in the Wide-field Infrared Survey Explorer W1 and W2 bands of the low-luminosity narrow-line Seyfert 1 galaxy WPVS 007, which exhibits prominent and varying broad-absorption lines (BALs) with blueshifted velocity up to $\rm \sim 14000\ km\ s^{-1}$. The observed significant MIR variability, the UV to optical color variabilities in the Swift bands that deviate from the predictions of pure dust attenuation models, and the fact that Swift light curves can be well fitted by the stochastic AGN variability model suggest that its observed flux variabilities in UV-optical-MIR bands should be intrinsic, rather than owing to variable dust extinction. Furthermore, the variations of BAL features (i.e., trough strength and maximum velocity) and continuum luminosity are concordant. Therefore, we propose that the BAL variability observed in WPVS 007 is likely induced by the intrinsic ionizing continuum variation, alternative to the rotating-torus model proposed in a previous work. The BAL gas in WPVS 007 might be in the low-ionization state as traced by its weak N V BAL feature; as the ionizing continuum strengthens, the Ci IV and Si IV column densities also increase, resulting in stronger BALs and the emergence of high-velocity components of the outflow. The outflow launch radius might be as small as $\sim 8 \times 10^{-4}$ pc under the assumption of being radiatively driven, but a large-scale origin (e.g., torus) cannot be fully excluded because of the unknown effects from additional factors, e.g., the magnetic field.

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arXiv2019

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

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 ($N_{\rm H} > 10^{23}\ \rm cm^{-2}$). By fitting spectra with physical models, 436 (38%) sources with $L_{\rm X} > 10^{42}\ \rm erg\ s^{-1}$ 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 logN-logS 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 NH 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 NH, 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.

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arXiv2019

Thick-disc model to explain the spectral state transition in NGC 247

We propose the thick-disc model of Gu et al. 2016 to interpret the transition between soft ultraluminous state (SUL) and supersoft ultraluminous (SSUL) state in NGC 247. As accretion rate increases, the inner disc will puff up and act as shield to block the innermost X-ray emission regions and absorb both soft and hard X-ray photons. The absorbed X-ray emission will be re-radiated as a much softer blackbody X-ray spectrum. Hence NGC 247 shows flux dips in the hard X-ray band and transits from the SUL state to the SSUL state. The $\sim 200$s transition timescale can be explained by the viscous timescale. According to our model, the inner disc in the super-soft state is thicker and has smaller viscous timescale than in the soft state. X-ray flux variability, which is assumed to be driven by accretion rate fluctuations, might be viscous time-scale invariant. Therefore, in the SSUL state, NGC 247 is more variable. The bolometric luminosity is saturated in the thick disc; the observed radius-temperature relation can therefore be naturally explained.

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arXiv2018

Winds Can "Blow Up" AGN Accretion Disk Sizes

Recent multi-band variability studies have revealed that active galactic nucleus (AGN) accretion disc sizes are generally larger than the predictions of the classical thin disc by a factor of $2\sim 3$. This hints at some missing key ingredient in the classical thin disc theory: here, we propose an accretion disc wind. For a given bolometric luminosity, in the outer part of an accretion disc, the effective temperature in the wind case is higher than that in the no-wind one; meanwhile, the radial temperature profile of the wind case is shallower than the no-wind one. In presence of winds, for a given band, blackbody emission from large radii can contribute more to the observed luminosity than the no-wind case. Therefore, the disc sizes of the wind case can be larger than those of the no-wind case. We demonstrate that a model with the accretion rate scaling as $\dot{M}_0 (R/R_{\mathrm{S}})^β$ (i.e., the accretion rate declines with decreasing radius due to winds) can match both the inter-band time lags and the spectral energy distribution of NGC 5548. Our model can also explain the inter-band time lags of other sources. Therefore, our model can help decipher current and future continuum reverberation mapping observations.

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arXiv2018

The Sloan Digital Sky Survey Reverberation Mapping Project: H$α$ and H$β$ Reverberation Measurements From First-year Spectroscopy and Photometry

We present reverberation mapping results from the first year of combined spectroscopic and photometric observations of the Sloan Digital Sky Survey Reverberation Mapping Project. We successfully recover reverberation time delays between the $g+i$-band emission and the broad H$β$ emission line for a total of 44 quasars, and for the broad H$α$ emission line in 18 quasars. Time delays are computed using the JAVELIN and CREAM software and the traditional interpolated cross-correlation function (ICCF): Using well defined criteria, we report measurements of 32 H$β$ and 13 H$α$ lags with JAVELIN, 42 H$β$ and 17 H$α$ lags with CREAM, and 16 H$β$ and 8 H$α$ lags with the ICCF. Lag values are generally consistent among the three methods, though we typically measure smaller uncertainties with JAVELIN and CREAM than with the ICCF, given the more physically motivated light curve interpolation and more robust statistical modeling of the former two methods. The median redshift of our H$β$-detected sample of quasars is 0.53, significantly higher than that of the previous reverberation-mapping sample. We find that in most objects, the time delay of the H$α$ emission is consistent with or slightly longer than that of H$β$. We measure black hole masses using our measured time delays and line widths for these quasars. These black hole mass measurements are mostly consistent with expectations based on the local M-sigma relationship, and are also consistent with single-epoch black hole mass measurements. This work increases the current sample size of reverberation-mapped active galaxies by about two-thirds and represents the first large sample of reverberation mapping observations beyond the local universe (z < 0.3).

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arXiv2018

Evolution of quasar stochastic variability along its main sequence

We explore the evolution of the time variability (in the optical $g$-band and on timescales of weeks to years) of SDSS Stripe 82 quasars along the quasar main sequence. A parent sample of $1004$ quasars within $0.5\leq z \leq 0.89$ are used for our statistical studies, we then make subsamples from our parent sample: a subsample of $246$ quasars with similar luminosities, and a subsample of $399$ quasars with similar Rfe (i.e., the ratio of the equivalent width of FeII within $4435$--$4685\ \mathrmÅ$ to that of \Hbeta). We find the variability amplitude decreases with luminosity ($L_{\mathrm{bol}}$). The anti-correlation between the variability amplitude and Rfe is weak but statistically significant. The characteristic timescale, $τ$, correlates mostly with quasar luminosity, its dependence on Rfe is statistically insignificant. After controlling luminosity and Rfe, the high- and low-FWHM samples have similar structure functions. These results support the framework that Rfe is governed by Eddington ratio and FWHM of Hbeta is mostly determined by orientation. We then provide new empirical relations between variability parameters and quasar properties (i.e., luminosity and Rfe). Our new relations are consistent with the scenario that quasar variability is driven by the thermal fluctuations in the accretion disk, $τ$ seems to correspond to the thermal timescale. From our new relations, we find the short-term variability is mostly sensitive to $L_{\mathrm{bol}}$. Basing on this, we propose that quasar short-term (a few months) variability might be a new type of "Standard Candle" and can be adopted to probe cosmology.

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arXiv2018

A falling Corona model for the anomalous behavior of the broad emission lines in NGC 5548

NGC 5548 has been intensively monitored by the AGN Space Telescope and Optical Reverberation Mapping collaboration. Approximately after half of the light curves, the correlation between the broad emission lines and the lag-corrected ultraviolet continua becomes weak. This anomalous behavior is accompanied by an increase of soft X-ray emission. We propose a simple model to understand this anomalous behavior, i.e., the corona might fall down, thereby increasing the covering fraction of the inner disk. Therefore, X-ray and extreme ultraviolet emission suffer from spectral variations. The ultraviolet continua variations are driven by both X-ray and extreme ultraviolet variations. Consequently, the spectral variability induced by the falling corona would dilute the correlation between the broad emission lines and the ultraviolet continua. Our model can explain many additional observational facts, including the dependence of the anomalous behavior on velocity and ionization energy. We also show that the time lag and correlation between the X-ray and the ultraviolet variations change as NGC 5548 displays the anomalous behavior. The time lag is dramatically longer than the expectation from disk reprocessing if the anomalous behavior is properly excluded. During the anomalous state, the time lag approaches the light-travel timescale of disk reprocessing albeit with a much weaker correlation. We speculate that the time lag in the normal state is caused by reprocessing of the broad line region gas. As NGC 5548 enters the abnormal state, the contribution of the broad line region gas is smaller; the time lag reflects disk reprocessing. We also discuss alternative scenarios.

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arXiv2018

The Sloan Digital Sky Survey Reverberation Mapping Project: the CIV Blueshift, Its Variability, and Its Dependence Upon Quasar Properties

We use the multi-epoch spectra of 362 quasars from the Sloan Digital Sky Survey Reverberation Mapping project to investigate the dependence of the blueshift of CIV relative to MgII on quasar properties. We confirm that high-blueshift sources tend to have low CIV equivalent widths (EWs), and that the low-EW sources span a range of blueshift. Other high-ionization lines, such as HeII, also show similar blueshift properties. The ratio of the line width (measured as both the full-width at half maximum and the velocity dispersion) of CIV to that of MgII increases with blueshift. Quasar variability might enhance the connection between the CIV blueshift and quasar properties (e.g., EW). The variability of the MgII line center (i.e., the wavelength that bisects the cumulative line flux) increases with blueshift. In contrast, the CIV line center shows weaker variability at the extreme blueshifts. Quasars with the high-blueshift CIV lines tend to have less variable continuum emission, when controlling for EW, luminosity, and redshift. Our results support the scenario that high-blueshift sources tend to have large Eddington ratios.

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arXiv2017

How far is quasar UV/optical variability from damped random walk at low frequency?

Studies have shown that UV/optical light curves of quasars can be described with the prevalent damped random walk (DRW, also known as Ornstein-Uhlenbeck process) model. A white noise power spectral density (PSD) is expected at low frequency in this model, however, direct observational constraint to the low frequency PSD slope is hard due to limited lengths of the light curves available. Meanwhile, quasars show too large scatter in their DRW parameters to be attributed to the uncertainties in the measurements and the dependence of variation to known physical factors. In this work we present simulations showing that, if the low frequency PSD deviates from DRW, the red noise leakage can naturally produce large scatter in variation parameters measured from simulated light curves. The steeper the low frequency PSD slope is, the larger scatter we expect. Based on the observations of SDSS Stripe 82 quasars, we find the low frequency PSD slope should be no steeper than -1.3. The actual slope could be flatter, which consequently requires that quasar variabilities should be influenced by other unknown factors. We speculate that magnetic field and/or metallicity could be such additional factors.

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arXiv2017

Relation between the variations in the MgII $\lambda2798$ emission-line and the 3000 Å continuum

We investigate the relationship between the MgII $\lambda2798$ emission-line and the 3000 Å continuum variations using a sample of 68 intermediate-redshift ($z\sim$ 0.65$-$1.50) broad-line quasars spanning a bolometric luminosity range of 44.49 erg s$^{-1} \leq \rm{log}$$L_{\rm{bol}} \leq 46.31$ erg s$^{-1}$ (Eddington ratio from $\sim$ 0.026 to 0.862). This sample is constructed from SDSS-DR7Q and BOSS-DR12Q, each with at least 2 spectroscopic epochs in SDSS-I/II/III surveys. Additionally, we adopt the following signal-to-noise ratio (S/N) selection criteria: a) for MgII and the 3000 Å continuum, S/N $\geq$ 10; b) for narrow lines, S/N $\geq$ 5. All our quasar spectra are recalibrated based on the assumption of constant narrow emission-line fluxes. In an analysis of spectrum-to-spectrum variations, we find a fairly close correlation (Spearman $ρ= 0.593$) between the variations in broad MgII and in the continuum. This is consistent with the idea that MgII is varying in response to the continuum emission variations. Adopting the modified weighted least squares regression method, we statistically constrain the slopes (i.e., the responsivity $α$ of the broad MgII) between the variations in both components for the sources in different luminosity bins after eliminating intrinsic biases introduced by the rescaling process itself. It is shown that the responsivity is quite small (average $\barα \approx$ 0.464) and anti-correlates with the quasar luminosity. Our results indicate that high signal-to-noise flux measurements are required to robustly detect the intrinsic variability and the time lag of MgII line.

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arXiv2016

On the origin of the HLX-1 outbursts

HLX-1, currently the best intermediate-mass black hole candidate, has undergone seven violent outbursts, each with a peak X-ray luminosity of $L_{\mathrm{peak},\mathrm{X}}\sim 10^{42}\ \rm{erg\ s^{-1}}$. Interestingly, the properties of the HLX-1 outbursts evolve with time. In this work, we aim to constrain the physical parameters of the central engine of the HLX-1 outbursts in the framework of the black hole accretion. We find that the physical properties of the HLX-1 outbursts are consistent with being driven by the radiation pressure instability. This scenario can explain the evolution of the recurrent timescales of the HLX-1 outbursts as a function of the durations.

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