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Zhu Liu

Publications and source records attributed to Zhu Liu.

At least 109 records · Page 6Linked to original sources
arXiv2020

Following up the afterglow: strategy for X-ray observation triggered by gravitational wave events

The multi-messenger observation of compact binary coalescence promises great scientific treasure. However, a synthetic observation from both gravitational wave and electromagnetic channels remains challenging. Relying on the day-to-week long macronova emission, GW170817 remains the only event with successful electromagnetic followup. In this manuscript, we explore the possibility of using the early stage X-ray afterglow to search for the electromagnetic counterpart of gravitational wave events. Two algorithms, the sequential observation and the local optimization are considered and applied to three simulated events. We consider the proposed Einstein probe as a candidate X-ray telescope. Benefiting from the large field of view and high sensitivity, we find that the sequential observation algorithm not only is easy to implement, but also promises a good chance of actual detection.

astro-ph.HE↗
arXiv2020

A Tidal Disruption Event Candidate Discovered in the Active Galactic Nucleus SDSS J022700.77-042020.6

We report the discovery of a Tidal Disruption Event (TDE) candidate occurring in the Active Galactic Nucleus SDSS J022700.77-042020.6. A sudden increase in flux of J0227-0420 during the second half of 2009 is clearly shown in the long-term optical, UV, and NIR light curves. A plateau phase, following an initial decline, is seen in the NUV and optical u, g, r, i light curves. Moreover, we find possible evidence that the plateau phase in the NUV band may lag behind the optical ones by approximately 70-80 days with also a much shorter duration, i.e. $\sim$7-15 days against $\sim$40-50 days. The long-term NUV/optical (after the plateau phase), NIR and MIR light curves can be well characterized with a form of $L(t)\propto t^{-β}$, consistent with the expectation of a TDE. The plateaus can be explained if the stellar streams collide with the pre-existing AGN disk at different radii. Though the overall fallback rate decreases, the material in the outer disk gradually drifts inward and increases the local accretion rate at the inner region, producing the optical and UV plateaus. The possible lag between the optical and NUV plateaus can then be attributed to viscosity delay. The index $β$ of the NIR $J, H, K_s$ bands ($\sim1.4-3.3$) is steeper than that of the UV/optical ($\sim0.7-1.3$) and MIR bands ($\sim0.9-1.8$), which may suggest that a certain fraction of the dust in the inner region of the dusty torus may be sublimated during the TDE phase. Our results indicate that, due to collisions between stellar debris and pre-existing disk, the light curves of TDEs occurring in AGN may show distinctive features, which may shed new light on the accretion process.

astro-ph.GA↗
arXiv2020

A detailed study on the reflection component for the Black Hole Candidate MAXI J1836-194

We present a detailed spectral analysis of the black hole candidate MAXI J1836-194. The source was caught in the intermediate state during its 2011 outburst by Suzaku and RXTE. We jointly fit the X-ray data from these two missions using the relxill model to study the reflection component, and a steep inner emissivity profile indicating a compact corona as the primary source is required in order to achieve a good fit. In addition, a reflection model with a lamp-post configuration (relxilllp), which is normally invoked to explain the steep emissivity profile, gives a worse fit and is excluded at 99% confidence level compared to relxill. We also explore the effect of the ionization gradient on the emissivity profile by fitting the data with two relativistic reflection components, and it is found that the inner emissivity flattens. These results may indicate that the ionization state of the disc is not constant. All the models above require a supersolar iron abundance higher than 4.5. However, we find that the high-density version of reflionx can describe the same spectra even with solar iron abundance well. A moderate rotating black hole (a* = 0.84-0.94) is consistently obtained by our models, which is in agreement with previously reported values.

astro-ph.HE↗
arXiv2019

The Large Amplitude X-ray Variability in NGC 7589: Possible Evidence for Accretion Mode Transition

We report the discovery of large amplitude X-ray variability in the low luminosity AGN (LLAGN) MGC 7589, and present possible observational evidence for accretion mode transition in this source. Long-term X-ray flux variations by a factor of more than 50 are found using X-ray data obtained by Swift/XRT and XMM-Newton over 17 years. Results of long-term monitoring data in the UV, optical and infrared bands over ~20 years are also presented. The Eddington ratio increased from $10^{-3}$ to $\sim0.13$, suggesting a transition of the accretion flow from an ADAF to a standard thin accretion disc. Further evidence supporting the thin disc in the high luminosity state is found by the detection of a significant soft X-ray component in the X-ray spectrum. The temperature of this component ($\sim19^{+15}_{-7}$eV, fitted with a blackbody model) is in agreement with the predicted temperature of the inner region for a thin disc around a black hole (BH) with mass of $\sim10^{7}\,M_{\mathrm{Sun}}$. These results may indicate that NGC 7589 had experienced accretion mode transition over a timescale of a few years, suggesting the idea that similar accretion processes are at work for massive black hole and black hole X-ray binaries.

astro-ph.HE↗
arXiv2019

Effect of accreting tidally disrupted stars on the spin evolution of $\sim 10^6M_{\odot}$ black holes

Accretion of tidally disrupted stars (TDSs) is expected to contribute significantly to the growth of massive black holes (MBHs) with mass $\sim 10^6 M_{\odot}$ in galactic centers. In this paper, we quantitatively investigate the effect of the TDS accretion on the spin evolution of these relatively small MBHs, by also considering the accretion of gas-clouds with (many) chaotic episodes. We find that the accretion of TDSs can play an important role or even a dominant role in shaping the spin distribution of $\sim 10^6 M_{\odot}$ MBHs, depending on the contribution fraction ($f_{\rm TDE}$) of the TDS accretion to the MBH growth. If $f_{\rm TDE}$ is as large as $\gtrsim 0.9$, most $\sim 10^6M_{\odot}$ MBHs have low spins ($|a| \lesssim 0.3$); if $f_{\rm TDE}$ is as small as $\lesssim 0.1$, most $\sim 10^6 M_{\odot}$ MBHs have high spins ($|a| \gtrsim 0.7$). We also find that (1) the fraction of highly spinning $\sim 10^6 M_{\odot}$ MBHs in the TDS accretion states is smaller than that in the gas-cloud (AGN) accretion states, which is a consequence of more rapid spin decrease during the period of consecutive TDS accretion than the spin increase during the AGN periods when the spin is large; (2) the fraction of retrograde spin accretion in the TDS accretion states is almost the same as that of prograde spin accretion, while it is negligible in the gas-cloud (AGN) accretion states. Current scarce sample of AGNs ($\sim 10^6 M_{\odot}$) with spin measurements hints an insignificant contribution from TDS accretion to MBH growth. Future measurements on spins of $\sim 10^6 M_{\odot}$ MBHs may provide stronger constraints on the importance of both AGN and TDS accretion states in their growth history.

astro-ph.HE↗
arXiv2019

X-ray properties of two transient ULX candidates in galaxy NGC 7090

We report the X-ray data analysis of two transient ultraluminous X-ray sources (ULXs, hereafter X1 and X2) located in the nearby galaxy NGC 7090. While they were not detected in the 2004 XMM-Newton and 2005 Chandra observations, their 0.3-10 keV X-ray luminosities reached $>3\times10^{39}\,\mathrm{erg\,s^{-1}}$ in later XMM-Newton or Swift observations, showing increases in flux by a factor of $>80$ and $>300$ for X1 and X2, respectively. X1 showed indications of spectral variability: at the highest luminosity, its X-ray spectra can be fitted with a powerlaw ($Γ=1.55\pm0.15$), or a multicolour disc model with $T_{\mathrm{in}}=2.07^{+0.30}_{-0.23}$ keV; the X-ray spectrum became softer ($Γ=2.67^{+0.69}_{-0.64}$), or cooler ($T_\mathrm{in}=0.64^{+0.28}_{-0.17}$ keV) at lower luminosity. No strong evidence for spectral variability was found for X2. Its X-ray spectra can be fitted with a simple powerlaw model ($Γ=1.61^{+0.55}_{-0.50}$), or a multicolour disc model ($1.69^{+1.17}_{-0.48}$ keV). A possible optical counterpart for X1 is revealed in HST imaging. No optical variability is found, indicating that the optical radiation may be dominated by the companion star. Future X-ray and optical observations are necessary to determine the true nature of the compact object.

astro-ph.HE↗
arXiv2018

Accretion in Strong Field Gravity with eXTP

In this paper we describe the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to accretion flows in the strong field gravity regime around both stellar-mass and supermassive black-holes. eXTP has the unique capability of using advanced 'spectral-timing-polarimetry' techniques to analyze the rapid variations with three orthogonal diagnostics of the flow and its geometry, yielding unprecedented insight into the inner accreting regions, the effects of strong field gravity on the material within them and the powerful outflows which are driven by the accretion process.

astro-ph.HE↗
arXiv2018

New dynamics of energy use and CO2 emissions in China

Global achievement of climate change mitigation will heavy reply on how much of CO2 emission has and will be released by China. After rapid growth of emissions during last decades, China CO2 emissions declined since 2014 that driven by decreased coal consumption, suggesting a possible peak of China coal consumption and CO2 emissions. Here, by combining a updated methodology and underlying data from different sources, we reported the soaring 5.5% (range: +2.5% to +8.5% for one sigma) increase of China CO2 emissions in 2018 compared to 2017, suggesting China CO2 is not yet to peak and leaving a big uncertain to whether China emission will continue to rise in the future. Although our best estimate of total emission (9.9Gt CO2 in 2018) is lower than international agencies in the same year, the results show robust on a record-high energy consumption and total CO2 emission in 2018. During 2014-2016, China energy intensity (energy consumption per unit of GDP) and total CO2 emissions has decreased driven by energy and economic structure optimization. However, the decrease in emissions is now offset by stimulates of heavy industry production under economic downturn that driving coal consumption (+5% in 2018), as well as the surging of natural gas consumption (+18% in 2018) due to the government led coal-to-gas energy transition to reduce local air pollutions. Timing policy and actions are urgent needed to address on these new drivers to turn down the total emission growth trend.

econ.GN↗
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.

astro-ph.GA↗
arXiv2018

The efficient, the intensive, and the productive: insights from urban Kaya scaling

Urban areas play an unprecedented role in potentially mitigating climate change and supporting sustainable development. In light of the rapid urbanisation in many parts on the globe, it is crucial to understand the relationship between settlement size and CO2 emission efficiency of cities. Recent literature on urban scaling properties of emissions as a function of population size have led to contradictory results and more importantly, lacked an in-depth investigation of the essential factors and causes explaining such scaling properties. Therefore, in analogy to the well-established Kaya Identity, we develop a relation combining the involved exponents. We demonstrate that application of this Urban Kaya Relation will enable a comprehensive understanding about the intrinsic factors determining emission efficiencies in large cities by applying it to a global dataset of 61 cities. Contrary to traditional urban scaling studies which use Ordinary Least Squares (OLS) regression, we show that the Reduced Major Axis (RMA) is necessary when complex relations among scaling exponents are to be investigated. RMA is given by the geometric mean of the two OLS slopes obtained by interchanging the dependent and independent variable. We discuss the potential of the Urban Kaya Relation in main-streaming local actions for climate change mitigation.

physics.soc-ph↗
arXiv2017

Automatic Question-Answering Using A Deep Similarity Neural Network

Automatic question-answering is a classical problem in natural language processing, which aims at designing systems that can automatically answer a question, in the same way as human does. In this work, we propose a deep learning based model for automatic question-answering. First the questions and answers are embedded using neural probabilistic modeling. Then a deep similarity neural network is trained to find the similarity score of a pair of answer and question. Then for each question, the best answer is found as the one with the highest similarity score. We first train this model on a large-scale public question-answering database, and then fine-tune it to transfer to the customer-care chat data. We have also tested our framework on a public question-answering database and achieved very good performance.

cs.CL↗
arXiv2016

Dependence of the broad Fe K$α$ line on the physical parameters of AGN

In this paper, the dependence of the broad Fe K$α$ line on the physical parameters of AGN, such as the black hole mass $M_{BH}$, accretion rate (equivalently represented by Eddington ratio $λ_{Edd}$), and optical classification, is investigated by applying the X-ray spectra stacking method to a large sample of AGN which have well measured optical parameters. A broad line feature is detected ($>3σ$) in the stacked spectra of the high $λ_{Edd}$ sub-sample ($\logλ_{Edd}>-0.9$). The profile of the broad line can be well fitted with relativistic broad line model, with the line energy consistent with highly ionized Fe K$α$ line (i.e. Fe xxvi). A model consisting of multiple narrow lines cannot be ruled out, however. We found hints that the Fe K line becomes broader as the $λ_{Edd}$ increases. No broad line feature is shown in the sub-sample of broad-line Seyfert 1 (BLS1) galaxies and in the full sample, while a broad line might be present, though at low significance, in the sub-sample of narrow-line Seyfert 1 (NLS1) galaxies. We find no strong dependence of the broad line on black hole masses. Our results indicate that the detection/properties of the broad Fe K$α$ line may strongly depend on $λ_{Edd}$, which can be explained if the ionization state and/or truncation radius of the accretion disc changes with $λ_{Edd}$. The non-detection of the broad line in the BLS1 sub-sample can be explained if the the average EW of the relativistic Fe K$α$ line is weak or/and the fraction of sources with relativistic Fe K$α$ line is small in BLS1 galaxies.

astro-ph.HE↗
arXiv2016

Deep Hashing: A Joint Approach for Image Signature Learning

Similarity-based image hashing represents crucial technique for visual data storage reduction and expedited image search. Conventional hashing schemes typically feed hand-crafted features into hash functions, which separates the procedures of feature extraction and hash function learning. In this paper, we propose a novel algorithm that concurrently performs feature engineering and non-linear supervised hashing function learning. Our technical contributions in this paper are two-folds: 1) deep network optimization is often achieved by gradient propagation, which critically requires a smooth objective function. The discrete nature of hash codes makes them not amenable for gradient-based optimization. To address this issue, we propose an exponentiated hashing loss function and its bilinear smooth approximation. Effective gradient calculation and propagation are thereby enabled; 2) pre-training is an important trick in supervised deep learning. The impact of pre-training on the hash code quality has never been discussed in current deep hashing literature. We propose a pre-training scheme inspired by recent advance in deep network based image classification, and experimentally demonstrate its effectiveness. Comprehensive quantitative evaluations are conducted on several widely-used image benchmarks. On all benchmarks, our proposed deep hashing algorithm outperforms all state-of-the-art competitors by significant margins. In particular, our algorithm achieves a near-perfect 0.99 in terms of Hamming ranking accuracy with only 12 bits on MNIST, and a new record of 0.74 on the CIFAR10 dataset. In comparison, the best accuracies obtained on CIFAR10 by existing hashing algorithms without or with deep networks are known to be 0.36 and 0.58 respectively.

cs.CV↗
arXiv2016

X-ray spectral properties of the AGN sample in the northern XMM-XXL field

In this paper we describe and publicly release a catalogue consisting of 8445 point-like X-ray sources detected in the XMM-XXL north survey. For the 2512 AGN which have reliable spectroscopy, we present the X-ray spectral fitting which has been computed with a Bayesian approach. We have also applied an X-ray spectral stacking method to different sub-samples. We confirm the well-known Iwasawa-Taniguchi effect in our luminosity-redshift sub-samples, and argue that such an effect is due to a decrease in the covering factor of a distant obscuring torus with increasing X-ray luminosity. By comparing the distribution of the reflection fraction, the ratio of the normalization of the reflected component to the direct radiation, we find that the low-luminosity, low-redshift sub-sample had systematically higher reflection fraction values than the high-redshift, high-luminosity one. On the other hand, no significant difference is found between samples having similar luminosity but different redshift, suggesting that the structure of the torus does not evolve strongly with redshift. Contrary to previous works, we do not find evidence for an increasing photon index at high Eddington ratio. This may be an indication that the structure of the accretion disc changes as the Eddington ratio approaches unity. Comparing our X-ray spectral analysis results with the optical spectral classification, we find that $\sim20$ per cent of optical type-1 AGN show an X-ray absorbing column density higher than $10^{21.5}\,\rm{cm^{-2}}$, and about 50 per cent of type-2 AGN have an X-ray absorbing column density less than $10^{21.5}\,\rm{cm^{-2}}$. We suggest that the excess X-ray absorption shown in the high-luminosity optical type-1 AGN can be due to small-scale dust-free gas within (or close to) the broad line region, while in the low-luminosity ones it can be due to a clumpy torus with a large covering factor.

astro-ph.HE↗
arXiv2015

X-ray fluorescent lines from the Compton-thick AGN in M51

The cold disk/torus gas surrounding active galactic nuclei (AGN) emits fluorescent lines when irradiated by hard X-ray photons. The fluorescent lines of elements other than Fe and Ni are rarely detected due to their relative faintness. We report the detection of K$α$ lines of neutral Si, S, Ar, Ca, Cr, and Mn, along with the prominent Fe K$α$, Fe K$β$, and Ni K$α$ lines, from the deep Chandra observation of the low-luminosity Compton-thick AGN in M51. The Si K$α$ line at 1.74 keV is detected at $\sim3σ$, the other fluorescent lines have a significance between 2 and 2.5 $σ$, while the Cr line has a significance of $\sim1.5σ$. These faint fluorescent lines are made observable due to the heavy obscuration of the intrinsic spectrum of M51, which is revealed by Nustar observation above 10 keV. The hard X-ray continuum of M51 from Chandra and Nustar can be fitted with a power-law spectrum with an index of 1.8, reprocessed by a torus with an equatorial column density of $N_{\rm H}\sim7\times10^{24}$ cm$^{-2}$ and an inclination angle of $74$ degrees. This confirms the Compton-thick nature of the nucleus of M51. The relative element abundances inferred from the fluxes of the fluorescent lines are similar to their solar values, except for Mn, which is about 10 times overabundant. It indicates that Mn is likely enhanced by the nuclear spallation of Fe.

astro-ph.HE↗
arXiv2015

Obscuration-dependent evolution of Active Galactic Nuclei

We aim to constrain the evolution of AGN as a function of obscuration using an X-ray selected sample of $\sim2000$ AGN from a multi-tiered survey including the CDFS, AEGIS-XD, COSMOS and XMM-XXL fields. The spectra of individual X-ray sources are analysed using a Bayesian methodology with a physically realistic model to infer the posterior distribution of the hydrogen column density and intrinsic X-ray luminosity. We develop a novel non-parametric method which allows us to robustly infer the distribution of the AGN population in X-ray luminosity, redshift and obscuring column density, relying only on minimal smoothness assumptions. Our analysis properly incorporates uncertainties from low count spectra, photometric redshift measurements, association incompleteness and the limited sample size. We find that obscured AGN with $N_{H}>{\rm 10^{22}\, cm^{-2}}$ account for ${77}^{+4}_{-5}\%$ of the number density and luminosity density of the accretion SMBH population with $L_{\rm X}>10^{43}\text{ erg/s}$, averaged over cosmic time. Compton-thick AGN account for approximately half the number and luminosity density of the obscured population, and ${38}^{+8}_{-7}\%$ of the total. We also find evidence that the evolution is obscuration-dependent, with the strongest evolution around $N_{H}\thickapprox10^{23}\text{ cm}^{-2}$. We highlight this by measuring the obscured fraction in Compton-thin AGN, which increases towards $z\sim3$, where it is $25\%$ higher than the local value. In contrast the fraction of Compton-thick AGN is consistent with being constant at $\approx35\%$, independent of redshift and accretion luminosity. We discuss our findings in the context of existing models and conclude that the observed evolution is to first order a side-effect of anti-hierarchical growth.

astro-ph.HE↗
arXiv2014

Universal scaling of the 3:2 twin-peak quasi-periodic oscillation frequencies with black hole mass and spin revisited

We discuss further observational support of an idea formulated a decade ago by Abramowicz, Klu{ź}niak, McClintock and Remillard. They demonstrated that the 3:2 pairs of frequencies of the twin-peak black hole (BH) high-frequency quasi-periodic oscillations (QPOs) scale inversely with the BH masses and that the scaling covers the entire range from stellar to supermassive BHs. For this reason, they believed that the QPOs may be used for accurate measurements of masses and spins of BHs.

astro-ph.HE↗