arXiv subjects

Yongquan Xue

Publications and source records attributed to Yongquan Xue.

At least 109 records · Page 6Linked to original sources

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.

astro-ph.GA↗

arXiv2018

Systematic investigation of X-ray spectral variability of TeV blazars during flares in the $RXTE$ era

Utilizing all the 16-year $RXTE$ observations, we analyze the X-ray spectra of 32 TeV blazars, and perform a systematic investigation of X-ray spectral variability for the 5 brightest sources during their major flares that lasted several days. We obtain photon spectral index ($α$), flux and synchrotron radiation peak energy ($E_{\rm{p}}$) from empirical spectral fitting, and electron spectral index ($p$) from theoretical synchrotron radiation modeling. We find that both $α$ and $p$ generally display a harder-when-brighter trend, confirming the results of many previous works. Furthermore, we confirm and strengthen the result that $p$ must vary in order to explain the observed X-ray spectral variability during flares, which would have useful implications for interpreting the associated higher-energy spectral variability. We see apparent electron spectral hysteresis in many but not all $p$-flux plots that takes a form of "loop" or oblique"8". We obtain a tight $p$-hardness ratio (HR) relation and a tighter $p$-$α$ relation using spectra of flaring periods, both of which are also applicable to stacked data of quiescent periods. We demonstrate that these two empirical relations can be used efficiently to estimate $p$ from HR or $α$ that is readily achieved. Finally, we find that, when considering TeV blazars as a whole, $α$ and X-ray luminosity are positively correlated, $E_{\rm{p}}$ is negatively correlated with $p$ and $α$, and $E_{\rm{p}}$ is positively correlated with HR; all these correlations are in line with the blazar sequence. However, after correcting for the Doppler boosting effect, $α$ and intrinsic X-ray luminosity follow an anti-correlation.

astro-ph.GA↗

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.

astro-ph.HE↗

arXiv2018

Testing photometric redshift measurements with filter definition of the Chinese Space Station Optical Survey (CSS-OS)

The Chinese Space Station Optical Survey (CSS-OS) is a major science project of the Space Application System of the China Manned Space Program. This survey is planned to perform both photometric imaging and slitless spectroscopic observations, and it will focus on different cosmological and astronomical goals. Most of these goals are tightly dependent on the accuracy of photometric redshift (photo-z) measurement, especially for the weak gravitational lensing survey as a main science driver. In this work, we assess if the current filter definition can provide accurate photo-z measurement to meet the science requirement. We use the COSMOS galaxy catalog to create a mock catalog for the CSS-OS. We compare different photo-z codes and fitting methods that using the spectral energy distribution (SED) template-fitting technique, and choose to use a modified LePhare code in photo-z fitting process. Then we investigate the CSS-OS photo-z accuracy in certain ranges of filter parameters, such as band position, width, and slope. We find that the current CSS-OS filter definition can achieve reasonably good photo-z results with sigma_z~0.02 and outlier fraction ~3%.

astro-ph.IM↗

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.

astro-ph.HE↗

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.

astro-ph.GA↗

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

Cross-correlation between X-ray and optical/near-infrared background intensity fluctuations

Angular power spectra of optical and infrared background anisotropies at wavelengths between 0.5 to 5 $μ$m are a useful probe of faint sources present during reionization, in addition to faint galaxies and diffuse signals at low redshift. The cross-correlation of these fluctuations with backgrounds at other wavelengths can be used to separate some of these signals. A previous study on the cross-correlation between X-ray and $Spitzer$ fluctuations at 3.6 $μ$m and 4.5 $μ$m has been interpreted as evidence for direct collapse blackholes (DCBHs) present at $z > 12$. Here we return to this cross-correlation and study its wavelength dependence from 0.5 to 4.5 $μ$m using $Hubble$ and $Spitzer$ data in combination with a subset of the 4 Ms $Chandra$ observations in GOODS-S/ECDFS. Our study involves five $Hubble$ bands at 0.6, 0.7, 0.85, 1.25 and 1.6 $μ$m, and two $Spitzer$-IRAC bands at 3.6 $μ$m and 4.5 $μ$m. We confirm the previously seen cross-correlation between 3.6 $μ$m (4.5 $μ$m) and X-rays with 3.7$σ$ (4.2$σ$) and 2.7$σ$ (3.7$σ$) detections in the soft [0.5-2] keV and hard [2-8] keV X-ray bands, respectively, at angular scales above 20 arcseconds. The cross-correlation of X-rays with $Hubble$ is largely anticorrelated, ranging between the levels of 1.4$-$3.5$σ$ for all the $Hubble$ and X-ray bands. This lack of correlation in the shorter optical/NIR bands implies the sources responsible for the cosmic infrared background at 3.6 and 4.5~$μ$m are at least partly dissimilar to those at 1.6 $μ$m and shorter.

astro-ph.CO↗

arXiv2016

The deepest X-ray view of high-redshift galaxies: constraints on low-rate black-hole accretion

We exploit the 7 Ms \textit{Chandra} observations in the \chandra\,Deep Field-South (\mbox{CDF-S}), the deepest X-ray survey to date, coupled with CANDELS/GOODS-S data, to measure the total X-ray emission arising from 2076 galaxies at $3.5\leq z < 6.5$. This aim is achieved by stacking the \textit{Chandra} data at the positions of optically selected galaxies, reaching effective exposure times of $\geq10^9\mathrm{s}$. We detect significant ($>3.7σ$) X-ray emission from massive galaxies at $z\approx4$. We also report the detection of massive galaxies at $z\approx5$ at a $99.7\%$ confidence level ($2.7σ$), the highest significance ever obtained for X-ray emission from galaxies at such high redshifts. No significant signal is detected from galaxies at even higher redshifts. The stacking results place constraints on the BHAD associated with the known high-redshift galaxy samples, as well as on the SFRD at high redshift, assuming a range of prescriptions for X-ray emission due to X- ray binaries. We find that the X-ray emission from our sample is likely dominated by processes related to star formation. Our results show that low-rate mass accretion onto SMBHs in individually X-ray-undetected galaxies is negligible, compared with the BHAD measured for samples of X-ray detected AGN, for cosmic SMBH mass assembly at high redshift. We also place, for the first time, constraints on the faint-end of the AGN X-ray luminosity function ($\mathrm{logL_X\sim42}$) at $z>4$, with evidence for fairly flat slopes. The implications of all of these findings are discussed in the context of the evolution of the AGN population at high redshift.

astro-ph.GA↗

arXiv2012

The X-ray Star Formation Story as Told by Lyman Break Galaxies in the 4 Ms CDF-S

We present results from deep X-ray stacking of >4000 high redshift galaxies from z~1 to 8 using the 4 Ms Chandra Deep Field South (CDF-S) data, the deepest X-ray survey of the extragalactic sky to date. The galaxy samples were selected using the Lyman break technique based primarily on recent HST ACS and WFC3 observations. Based on such high specific star formation rates (sSFRs): log SFR/M* > -8.7, we expect that the observed properties of these LBGs are dominated by young stellar populations. The X-ray emission in LBGs, eliminating individually detected X-ray sources (potential AGN), is expected to be powered by X-ray binaries and hot gas. We find, for the first time, evidence of evolution in the X-ray/SFR relation. Based on X-ray stacking analyses for z<4 LBGs (covering ~90% of the Universe's history), we find that the 2-10 keV X-ray luminosity evolves weakly with redshift (z) and SFR as log LX = 0.93 log (1+z) + 0.65 log SFR + 39.80. By comparing our observations with sophisticated X-ray binary population synthesis models, we interpret that the redshift evolution of LX/SFR is driven by metallicity evolution in HMXBs, likely the dominant population in these high sSFR galaxies. We also compare these models with our observations of X-ray luminosity density (total 2-10 keV luminosity per Mpc^3) and find excellent agreement. While there are no significant stacked detections at z>5, we use our upper limits from 5<z<8 LBGs to constrain the SMBH accretion history of the Universe around the epoch of reionization.

astro-ph.CO↗

arXiv2012

An X-ray Detected Group of Quiescent Early-type Galaxies at z=1.6 in the Chandra Deep Field South

(Abridged) We report the discovery of an X-ray group of galaxies located at a high redshift of z=1.61 in the Chandra Deep Field South. The group is first identified as an extended X-ray source. We use a wealth of deep multi-wavelength data to identify the optical counterpart -- our red sequence finder detects a significant over-density of galaxies at z~1.6 and the brightest group galaxy is spectroscopically confirmed at z=1.61. We measure an X-ray luminosity of L_{0.1-2.4 keV}= 1.8\pm0.6 \times 10^{43} erg/s, which then translates into a group mass of 3.2\pm0.8 \times 10^{13} M_sun. This is the lowest mass group ever confirmed at z>1.5. The deep optical-nearIR images from CANDELS reveal that the group exhibits a surprisingly prominent red sequence. A detailed analysis of the spectral energy distributions of the group member candidates confirms that most of them are indeed passive galaxies. Furthermore, their structural parameters measured from the near-IR CANDELS images show that they are morphologically early-type. The newly identified group at z=1.61 is dominated by quiescent early-type galaxies and the group appears similar to those in the local Universe. One possible difference is the high fraction of AGN (38^{+23}_{-20}%), which might indicate a role for AGN in quenching. But, a statistical sample of high-z groups is needed to draw a general picture of groups at this redshift. Such a sample will hopefully be available in near future surveys.

astro-ph.CO↗

arXiv2010

Variable O VI and N V Emission from the X-ray Binary LMC X-3: Heating of the Black Hole Companion

Based on high-resolution ultraviolet spectroscopy obtained with FUSE and COS, we present new detections of O VI and N V emission from the black-hole X-ray binary (XRB) system LMC X-3. We also update the ephemeris of the XRB using recent radial velocity measurements obtained with the echelle spectrograph on the Magellan-Clay telescope. We observe significant velocity variability of the UV emission, and we find that the O VI and N V emission velocities follow the optical velocity curve of the XRB. Moreover, the O VI and N V intensities regularly decrease between binary phase = 0.5 and 1.0, which suggests that the source of the UV emission is increasingly occulted as the B star in the XRB moves from superior to inferior conjunction. These trends suggest that illumination of the B-star atmosphere by the intense X-ray emission from the accreting black hole creates a hot spot on one side of the B star, and this hot spot is the origin of the O VI and N V emission. However, the velocity semiamplitude of the ultraviolet emission, K_{UV}~180 km/s, is lower than the optical semiamplitude; this difference could be due to rotation of the B star. If our hypothesis about the origin of the highly ionized emission is correct, then careful analysis of the emission occultation could, in principle, constrain the inclination of the XRB and the mass of the black hole.

astro-ph.HE↗

arXiv2007

Modeling the Broadband Spectral Energy Distribution of the Microquasars XTE J1550-564 and H 1743-322

We report results from a systematic study of the spectral energy distribution (SED) and spectral evolution of XTE J1550--564 and H 1743--322 in outburst. The jets of both sources have been directly imaged at both radio and X-ray frequencies, which makes it possible to constrain the spectrum of the radiating electrons in the jets. We modelled the observed SEDs of the jet `blobs' with synchrotron emission alone and with synchrotron emission plus inverse Compton scattering. The results favor a pure synchrotron origin of the observed jet emission. Moreover, we found evidence that the shape of the electron spectral distribution is similar for all jet `blobs' seen. Assuming that this is the case for the jet as a whole, we then applied the synchrotron model to the radio spectrum of the total emission and extrapolated the results to higher frequencies. In spite of significant degeneracy in the fits, it seems clear that, while the synchrotron radiation from the jets can account for nearly 100% of the measured radio fluxes, it contributes little to the observed X-ray emission, when the source is relatively bright. In this case, the X-ray emission is most likely dominated by emission from the accretion flows. When the source becomes fainter, however, the jet emission becomes more important, even dominant, at X-ray energies. We also examined the spectral properties of the sources during outbursts and the correlation between the observed radio and X-ray variabilities. The implication of the results is discussed.

astro-ph↗

arXiv2007

On the Radio/X-ray Correlation in Microquasars

[Context] The broadband spectral energy distribution of microquasars has proven to be a valuable tool for assessing the roles of jets and accretion flows in microquasars, as well as the coupling between the two. The coupling might manifest itself observationally in the correlated radio and X-ray variabilities of a source. Such a radio/X-ray correlation has indeed been seen in several microquasars during the low-hard state and subsequently been claimed to be universal for all. If proven, the universal correlation would have profound implications on theoretical models. However, there is already observational evidence that suggests otherwise. [Aims] In this paper, we critically examine the radio/X-ray correlation in a sample of microquasars, in the low-hard state as well as during state transitions, with a goal of testing the claimed universality of the correlation on observational grounds. [Methods] We have assembled a comprehensive data set from the simultaneous/contemporaneous radio and X-ray observations of representative microquasars. The data have allowed us to quantify the radio/X-ray correlation on a source by source basis. [Results] We find that the radio/X-ray correlation of microquasars exhibits diverse behaviors, both in the low-hard and transitional states, ranging from being very week (or none at all) to very strong. There is even a hint of spectral dependence of the correlation in some cases. [Conclusions] Our results rule out the claimed universality of the radio/X-ray correlation of microquasars, even for the low-hard state. On the other hand, we do find that the radio and X-ray variabilities are, to varying degrees, correlated in most cases.

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arXiv2006

Modeling the hard states of XTE J1550--564 during its 2000 outburst

We study hard states of the black-hole binary XTE J1550--564 during its 2000 outburst. In order to explain those states at their highest luminosities, $L\sim 10%$ of the Eddington luminosity, $L_{\rm E}$, we propose a specific hot accretion flow model. We point out that the highest values of the hard-state $L$ are substantially above the $L$ an advection-dominated accretion flow (ADAF) can produce, $\sim 0.4α^2 L_{\rm E}$, which is only $\sim (3$--$4)%L_{\rm E}$ even for $α$ as high as 0.3. On the other hand, we successfully explain the hard states with $L\sim (4$--$10)%$ using the luminous hot accretion flow (LHAF) model. As $10%L_{\rm E}$ is also roughly the highest luminosity an LHAF can produce, such an agreement between the predicted and observed highest luminosities provides by itself strong support for this model. Then, we study multi-waveband spectral variability during the 2000 outburst. In addition to the primary maxima in the optical light curves, secondary maxima were detected after the transition from the very high state to the hard state. We show that the secondary maxima are well modeled by synchrotron emission from a jet formed during the state transition. We argue that the absence of the corresponding secondary peak in the X-ray light curve indicates that the X-ray jet emission, regardless of its radiative process, synchrotron or its Comptonization, is not important in the hard state compared to the emission from the accretion flow.

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arXiv2006

Accretion Disk Spectra of the Brightest Ultra-luminous X-ray Source in M82

Emission spectra of hot accretion disks characteristic of advection dominated accretion flow (ADAF) models are investigated for comparison with the brightest ultra-luminous source, X-1, in the galaxy M82. If the spectral state of the source is similar to the low luminosity hard state of stellar mass black holes in our Galaxy, a fit to the {\it Chandra} X-ray spectrum and constraints from the radio and infrared upper limits, require a black hole mass in the range of $9 \times 10^4 - 5 \times 10^5 \msun$. Lower black hole masses ($\la 10^4 \msun$) are possible if M82 X-1 corresponds to the high luminosity hard state of Galactic black hole X-ray binary sources. Both of these spectrally degenerate hot accretion disk solutions lead to an intermediate mass black hole interpretation for M82 X-1. Since these solutions have different spectral variability with X-ray luminosity and predict different infrared emission, they can be distinguished by future off axis {\it Chandra} observations or simultaneous sensitive infrared detections.

astro-ph↗

arXiv2006

X-ray Spectral Variability of TeV Blazars during Rapid Flares

The spectral energy distribution (SED) of TeV blazars peaks both at keV and TeV energies. The X-ray emission is generally believed to originate in the synchrotron emission from relativistic electrons (and positrons) in the jet of these sources, while the origin of the gamma-ray emission is still being debated. We report results from a systematic study of X-ray spectral variability of Mrk 421 and Mrk 501 during individual flares that last for several days, making use of some of the high-quality data that have recently become available. The X-ray spectra of the two sources fall on the opposite sides of the synchrotron peak of their respective SEDs, so they together may offer additional insights into the physical origin of X-ray variability. We modeled each of the time-resolved X-ray spectra over a {\em single} flare by adopting a homogeneous spatial distribution and an instantaneous power-law spectral distribution for the emitting particles. We focused on the variation of four key parameters: particle spectral index, maximum Lorentz factor, energy density, and magnetic field. While there is considerable degeneracy in the fits, we show that, in order to account for the X-ray spectral variability observed in Mrk 421, at least three of the parameters are required to vary in most cases, with the spectral index being one of them. The observations of Mrk 501 support the conclusion, although the quality of the data is not as good. We discuss the implications of the results.

astro-ph↗

arXiv2005

Hot One-Temperature Accretion Flows Revisited

The effectiveness of the thermal coupling of ions and electrons in the context of optically thin, hot accretion flows is investigated. In the limit of complete coupling, we focus on the one-temperature accretion flows. Based on a global analysis, the results are compared with two-temperature accretion flow models and with the observations of black hole sources. Many features are quite similar. That is, hot one-temperature solutions are found to exist for mass flow rates less than a critical value; i.e., $\dot{M}\la 10α^2\dot{M}_{\rm Edd}$, where $\dot{M}_{\rm Edd}= L_{\rm Edd}/c^2$ is the Eddington accretion rate. At low mass flow rates, $\dot{M}\la 10^{-3}α^2 \dot{M}_{\rm Edd}$, the solution is in the advection-dominated accretion flow (ADAF) regime. But at higher rates, radiative cooling is effective and is mainly balanced by advective {\em heating}, placing the solution in the regime of luminous hot accretion flow (LHAF). To test the viability of the one-temperature models, we have fitted the spectra of the two black hole sources, Sgr A* and XTE J1118+480, which have been examined successfully with two-temperature models. It is found that the one-temperature models do not provide acceptable fits to the multi-wavelength spectra of Sgr A* nor to XTE J1118+480 as a result of the higher temperatures characteristic of the one-temperature models. It is concluded that the thermal coupling of ions and electrons cannot be fully effective and that a two-temperature description is required in hot accretion flow solutions.

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