arXiv subjects

Tinggui Wang

Publications and source records attributed to Tinggui Wang.

At least 91 records · Page 5Linked to original sources

arXiv2021

Mid-InfraRed Outbursts in Nearby Galaxies (MIRONG). II. Optical Spectroscopic Follow-up

Infrared echo has proven to be an effective means to discover transient accretion events of supermassive black holes (SMBHs), such as tidal disruption events (TDEs) and changing-look active galactic nuclei (AGNs), in dusty circumnuclear environments. To explore the dusty populations of SMBH transient events, we have constructed a large sample of Mid-infrared Outbursts in Nearby Galaxies (MIRONG) and performed multiwavelength observations. Here we present the results of multiepoch spectroscopic follow-up observations of a subsample of 54 objects spanning a time scale of 4 yr. Emission-line variability was detected in 22 of them with either emergence or enhancement of broad Balmer emission lines in comparison with pre-outburst spectra. Coronal lines, HeIIλ4686 and Bowen line NIIIλ4640 appeared in the spectra of nine,seven and two sources, respectively. These results suggest that MIRONG is a mixed bag of different transient sources. We have tentatively classified them into different subclass according to their spectral evolution and light curves. Two sources have been in a steady high broad Hα flux up to the latest observation and might be turn-on AGNs. Broad lines faded out in the remaining sources, indicating a transient ionizing source ignited by TDE or sporadic gas accretion. Thirty-one sources do not show noticeable spectral change with respect to their pre-outburst spectra. They have a statistically redder MIR color and lower MIR luminosity of the outbursts,which are consistent with heavily obscured events.

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arXiv2021

Evidence of a tidal disruption event in GSN 069 from the abnormal carbon and nitrogen abundance ratio

GSN 069 is an ultra-soft X-ray active galactic nucleus that previously exhibited a huge X-ray outburst and a subsequent long-term decay. It has recently presented X-ray quasi-periodic eruptions (QPEs). We report the detection of strong nitrogen lines but weak or undetectable carbon lines in its far ultraviolet spectrum. With a detailed photoionization model, we use the C IV/N IV] ratio and other ratios between nitrogen lines to constrain the [C/N] abundance of GSN 069 to be from -3.33 to -1.91. We argue that a partially disrupted red giant star can naturally explain the abnormal C/N abundance in the UV spectrum, while the surviving core orbiting the black hole might produce the QPEs.

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arXiv2021

Hosts and triggers of AGNs in the Local Universe

Based on the spectroscopic and shear catalogs for SDSS galaxies in the local Universe, we compare optically-selected active galactic nuclei (AGNs) with control star-forming and quiescent galaxies on galactic, inter-halo and larger scales. We find that AGNs are preferentially found in two specific stages of galaxy evolution: star-burst and `green valley' phases, and that the stellar population of their host galaxies is quite independent of stellar mass, different from normal galaxies. Combining galaxy-galaxy lensing and galaxy clustering on large scales, we measure the mass of AGN host halos. The typical halo mass is about $10^{12}h^{-1}\rm M_{\odot}$, similar to the characteristic mass in the stellar mass-halo mass relation (SHMR). For given stellar mass, AGN host galaxies and star-forming galaxies share the same SHMR, while quiescent galaxies have more massive halos. Clustering analysis on halo scales reveals that AGNs are surrounded by a larger number of satellites (with stellar mass down to 1/1000 of the mass of the central galaxy) than star-forming galaxies, and that galaxies with larger stellar velocity dispersion have more satellites. The number of satellites also increase with halo mass, reaching unity around $10^{12}h^{-1}\rm M_{\odot}$. Our results suggest a scenario, in which the interaction of the central galaxy with the satellites triggers an early episode of star burst and AGN activities, followed by multiple AGN cycles driven by the non-axisymmetric structure produced by the interaction. The feedback from the starburst and AGN reduces the amount of cold gas for fueling the central black hole, producing a characteristic halo mass scale, $\sim 10^{12}h^{-1}\rm M_{\odot}$, where the AGN fraction peaks.

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arXiv2021

The Deviation of the Broad-line Region Size Between Reverberation Mapping and Spectroastrometry

The combination of the linear size from reverberation mapping (RM) and the angular distance of the broad line region (BLR) from spectroastrometry (SA) in active galactic nuclei (AGNs) can be used to measure the Hubble constant $H_0$. Recently, Wang et al. (2020) successfully employed this approach and estimated $H_0$ from 3C 273. However, there may be a systematic deviation between the response-weighted radius (RM measurement) and luminosity-weighted radius (SA measurement), especially when different broad lines are adopted for size indicators (e.g., \hb\ for RM and \pa\ for SA). Here we evaluate the size deviations measured by six pairs of hydrogen lines (e.g., \hb, \ha\ and \pa) via the locally optimally emitting cloud (LOC) models of BLR. We find that the radius ratios $K$(=$R_{\rm SA}$/$R_{\rm RM}$) of the same line deviated systematically from 1 (0.85-0.88) with dispersions between 0.063-0.083. Surprisingly, the $K$ values from the \pa(SA)/\hb(RM) and \ha(SA)/\hb(RM) pairs not only are closest to 1 but also have considerably smaller uncertainty. Considering the current infrared interferometry technology, the \pa(SA)/\hb(RM) pair is the ideal choice for the low redshift objects in the SARM project. In the future, the \ha(SA)/\hb(RM) pair could be used for the high redshift luminous quasars. These theoretical estimations of the SA/RM radius pave the way for the future SARM measurements to further constrain the standard cosmological model.

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arXiv2021

Infrared Echoes of Optical Tidal Disruption Events: ~1% Dust Covering Factor or Less at sub-parsec Scale

The past decade has experienced an explosive increase of optically-discovered tidal disruption events (TDEs) with the advent of modern time-domain surveys. However, we still lack a comprehensive observational view of their infrared (IR) echoes in spite of individual detections. To this end, we have conducted a statistical study of IR variability of the 23 optical TDEs discovered between 2009 and 2018 utilizing the full public dataset of Wide-field Infrared Survey Explorer. The detection of variability is performed on the difference images, yielding out 11 objects with significant (>$3σ$) variability in at least one band while dust emission can be only fitted in 8 objects. Their peak dust luminosity is around $10^{41}$-$10^{42}$ erg/s, corresponding to a dust covering factor $f_c\sim0.01$ at scale of sub-parsec. The only exception is the disputed source ASASSN-15lh, which shows an ultra-high dust luminosity ($\sim10^{43.5}$ erg/s) and make its nature even elusive. Other non-detected objects show even lower $f_c$, which could be one more order of magnitude lower. The derived $f_c$ is generally much smaller than those of dusty tori in active galactic nuclei (AGNs), suggesting either a dearth of dust or a geometrically thin and flat disk in the vicinity of SMBHs. Our results also indicate that the optical TDE sample (post-starburst galaxies overrepresented) is seriously biased to events with little dust at sub-pc scale while TDEs in dusty star-forming systems could be more efficiently unveiled by IR echoes.

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arXiv2021

An Extraordinary Response of Iron Emission to the Central Outburst in a Tidal Disruption Event Candidate

Understanding the origin of \feii\ emission is important because it is crucial to construct the main sequence of Active Galactic Nuclei (AGNs). Despite several decades of observational and theoretical effort, the location of the optical iron emitting region and the mechanism responsible for the positive correlation between the \feii\ strength and the black hole accretion rate remain open questions as yet. In this letter, we report the optical \feii\ response to the central outburst in PS1-10adi, a candidate tidal disruption event (TDE) taking place in an AGN at $z = 0.203$ that has aroused extensive attention. For the first time, we observe that the \feii\ response in the rising phase of its central luminosity is significantly more prominent than that in the decline phase, showing a hysteresis effect. We interpret this hysteresis effect as a consequence of the gradual sublimation of the dust grains situating at the inner surface of the torus into gas when the luminosity of the central engine increases. It is the iron element released from the sublimated dust that contributes evidently to the observed \feii\ emission. This interpretation, together with the weak response of the \hb\ emission as we observe, naturally explains the applicability of relative \feii\ strength as a tracer of the Eddington ratio. In addition, optical iron emission of this origin renders the \feii\ time lag a potential "standard candle" with cosmological implications.

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arXiv2020

X-ray flares from the stellar tidal disruption by a candidate supermassive black hole binary

Optical transient surveys have led to the discovery of dozens of stellar tidal disruption events (TDEs) by massive black hole in the centers of galaxies. Despite extensive searches, X-ray follow-up observations have produced no or only weak X-ray detections in most of them. Here we report the discovery of delayed X-ray brightening around 140 days after the optical outburst in the TDE OGLE16aaa, followed by several flux dips during the decay phase. These properties are unusual for standard TDEs and could be explained by the presence of supermassive black hole binary or patchy obscuration. In either scenario, the X-rays can be produced promptly after the disruption but are blocked in the early phase, possibly by a radiation-dominated ejecta which leads to the bulk of optical and ultraviolet emission. Our findings imply that the reprocessing is important in the TDE early evolution, and X-ray observations are promising in revealing supermassive black hole binaries.

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arXiv2020

High-redshift Extreme Variability Quasars from Sloan Digital Sky Survey Multi-Epoch Spectroscopy

We perform a systematic search for high-redshift ($z >$ 1.5) extreme variability quasars (EVQs) using repeat spectra from the Sixteenth Data Release of Sloan Digital Sky Survey, which provides a baseline spanning up to $\sim$18 yrs in the observed frame. We compile a sample of 348 EVQs with a maximum continuum variability at rest frame 1450 Angstrom of more than 100% (i.e., $δ$V $\equiv$ (Max$-$Min)/Mean $>$1). The EVQs show a range of emission line variability, including 23 where at least one line in our redshift range disappears below detectability, which can then be seen as analogous to low-redshift changing-look quasars (CLQs)". Importantly, spurious CLQs caused by SDSS problematic spectral flux calibration, e.g., fiber drop issue, have been rejected. The similar properties (e.g., continuum/line, difference-composite spectra and Eddington ratio) of normal EVQs and CLQs, implies that they are basically the same physical population with analogous intrinsic variability mechanisms, as a tail of a continuous distribution of normal quasar properties. In addition, we find no reliable evidence ($\lesssim$ 1$σ$) to support that the CLQs are a subset of EVQs with less efficient accretion. Finally, we also confirm the anti-breathing of C IV (i.e., line width increases as luminosity increases) in EVQs, and find that in addition to $\sim$ 0.4 dex systematic uncertainty in single-epoch C IV virial black hole mass estimates, an extra scatter of $\sim$ 0.3 dex will be introduced by extreme variability.

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arXiv2020

Mapping Diffuse Emission in Lyman UV band

The CAFE (Census of warm-hot intergalactic medium, Accretion, and Feedback Explorer) and LyRIC (Lyman UV Radiation from Interstellar medium and Circum-galactic medium) have been proposed to the space agencies in China respectively. CAFE was first proposed in 2015 as a joint scientific CAS-ESA small space mission. LyRIC was proposed in 2019 as an independent external payload operating on the Chinese Space Station. Both missions are dedicated to mapping the Lyman UV emission (ionized oxygen (O VI) resonance lines at 103.2 and 103.8 nm, and Lyman series) for the diffuse sources in our Galaxy and the circum-galactic mediums of the nearby galaxies. We present the primary science objectives, mission concepts, the enabling technologies, as well as the current status.

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arXiv2020

A sharp rise in the detection rate of broad absorption line variations in a quasar SDSS J141955.26+522741.1

We present an analysis of the variability of broad absorption lines (BALs) in a quasar SDSS J141955.26+522741.1 at $z=2.145$ with 72 observations from the Sloan Digital Sky Survey Data Release 16 (SDSS DR16). The strong correlation between the equivalent widths of BAL and the continuum luminosity, reveals that the variation of BAL trough is dominated by the photoionization. The photoionization model predicts that when the time interval $ΔT$ between two observations is longer than the recombination timescale $t_{\rm rec}$, the BAL variations can be detected. This can be characterized as a "sharp rise" in the detection rate of BAL variation at $ΔT=t_{\rm rec}$. For the first time, we detect such a "sharp rise" signature in the detection rate of BAL variations. As a result, we propose that the $t_{\rm rec}$ can be obtained from the "sharp rise" of the detection rate of BAL variation. It is worth mentioning that the BAL variations are detected at the time-intervals less than the $t_{\rm rec}$ for half an order of magnitude in two individual troughs. This result indicates that there may be multiple components with different $t_{\rm rec}$ but the same velocity in an individual trough.

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arXiv2020

Mid-InfraRed Outburst in Nearby Galaxies (MIRONG) I: Sample Selection and Characterization

The optical time-domain astronomy has grown rapidly in the past decade but the dynamic infrared sky is rarely explored. Aiming to construct a sample of mid-infrared outburst in nearby galaxies (MIRONG), we have conducted a systematical search of low-redshift ($z<0.35$) SDSS spectroscopic galaxies that have experienced recent MIR flares using their Wide-field Infrared Survey Explorer (WISE) light curves. A total of 137 galaxies have been selected by requiring a brightening amplitude of 0.5 magnitude in at least one WISE band with respect to their quiescent phases. Only a small faction (10.9%) has corresponding optical flares. Except for the four supernova (SNe) in our sample, the MIR luminosity of remaining sources ($L_{\rm 4.6μm}>10^{42}~\rm erg~s^{-1}$) are markedly brighter than known SNe and their physical locations are very close to the galactic center (median <0.1"). Only four galaxies are radio-loud indicating that synchrotron radiation from relativistic jets could contribute MIR variability. We propose that these MIR outburst are dominated by the dust echoes of transient accretion onto supermassive black holes, such as tidal disruption events (TDEs) and turn-on (changing-look) AGNs. Moreover, the inferred peak MIR luminosity function is generally consistent with the X-ray and optical TDEs at high end albeit with large uncertainties. Our results suggest that a large population of transients have been overlooked by optical surveys, probably due to dust obscuration or intrinsically optical weakness. Thus, a search in the infrared band is crucial for us to obtain a panoramic picture of nuclear outburst. The multiwavength follow-up observations of the MIRONG sample are in progress and will be presented in a series of subsequent papers.

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arXiv2020

Years Delayed X-ray Afterglows of TDEs Originated from Wind-Torus Interactions

Tidal disruption events (TDEs) occurred in active galactic nuclei (AGNs) are a special class of sources with outstanding scientific significance. TDEs can generate ultrafast winds, which should almost inevitably collide with the preexisting AGN dusty tori. We perform analytical calculations and simulations on the wind-torus interactions and find such a process can generate considerable X-ray afterglow radiation several years or decades later after the TDE outburst. This provides a new origin for the years delayed X-rays in TDEs. The X-ray luminosity can reach 10^{41-42} erg/s, and the light curve characteristics depend on the parameters of winds and tori. We apply the model to two TDE candidates, and provide lower limits on the masses of the disrupted stars, as well as rigorous constraints on the gas densities of tori. Our results suggest that the observations of the time delay, spectral shape, luminosity and the light curve of the X-ray afterglow can be used to constrain the physical parameters of both TDE winds and tori, including the wind velocity, wind density and torus density.

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arXiv2020

Bright Debris Disk Candidates Observed with AKARI/Far-Infrared Surveyor (FIS)

We cross-correlate \hip\ main-sequence star catalog with \fis\ catalog, and identify 136 stars (at $>90$% reliability) with far-infrared detections at least in one band. After rejecting 51 stars classified as young stellar objects, Be stars, other type stars with known dust disks or with potential contaminations and 2 stars without infrared excess emission, we obtain a sample of 83 candidate stars with debris disks. Stars in our sample cover spectral types from B to K-types with most being early types. This represents an unique sample of luminous debris disks that derived uniformly from all sky survey with a spatial resolution a factor of two better than the previous such survey by \iras. Moreover, by collecting the infrared photometric data from other public archives, 85% of them have infrared excesses in more than one bands, allowing the estimate of the dust temperatures. We fit the blackbody model to the broad band spectral energy distribution of these stars to derive the statistical distribution of the disk parameters. 7 stars require an additional warm component of temperature around 200 K. While a substantial fraction of our sample(58 stars) have weak 12 \micron\ excess indicating that a warm dust component maybe common among these bright debris disk systems.

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arXiv2020

A Mid-infrared Flare in the Active Galaxy MCG-02-04-026: Dust Echo of a Nuclear Transient Event

We report the discovery of a mid-infrared (MIR) flare using WISE data in the center of the nearby Seyfert 1.9 galaxy MCG-02-04-026. The MIR flare began in the first half of 2014, peaked around the end of 2015, and faded in 2017. During these years, energy more than $7\times10^{50}$ erg was released in the infrared, and the flare's MIR color was generally turning red. We detected neither optical nor ultraviolet (UV) variation corresponding to the MIR flare based on available data. We explained the MIR flare using a dust echo model in which the radiative transfer is involved. The MIR flare can be well explained as thermal reradiation from dust heated by UV-optical photons of a primary nuclear transient event. Although the transient event was not seen directly due to dust obscuration, we can infer that it may produce a total energy of at least $\sim10^{51}$ erg, most of which was released in less than $\sim$3 years. The nature of the transient event could be a stellar tidal disruption event by the central supermassive black hole (SMBH), or a sudden enhancement of the existing accretion flow onto the SMBH, or a supernova which was particularly bright.

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arXiv2020

X-ray spectral shape variations in changing-look Seyfert galaxy SDSSJ155258+273728

We analyze the X-ray, optical, and mid-infrared data of a "changing-look" Seyfert galaxy \sdssj15 at $z\simeq0.086$. Over a period of one decade (2009 - 2018), its broad H$α$ line intensity increased by a factor of $\sim$4. Meanwhile, the X-ray emission in 2014 as observed by \chandra\ was about five times brighter than that in 2010 by {\it Suzaku}, and the corresponding emissions in V-band, mid-infrared W1 band brighten by $\sim$ 0.18, 0.32 mag, respectively. Moreover, the absorption in X-rays is moderate and stable, i.e. ${\rm N_{H}}\sim 10^{21} {\rm cm^{-2}}$, but the X-ray spectrum becomes harder in the 2014 \chandra\ bright state (i.e. photon index $Γ= 1.52^{+0.06}_{-0.06}$) than that of the 2010 \suzaku\ low state ($Γ=2.03^{+0.22}_{-0.21}$). With an Eddington ratio being lower than a few percent, the inner region of the accretion disk in \sdssj15\ is likely a hot accretion flow. We then compile from literature the X-ray data of "changing-look" AGNs, and find that they generally follow the well-established "V"-shaped correlation in AGNs, that is, above a critical turn-over luminosity the X-ray spectra soften with the increasing luminosity, and below that luminosity the trend is reversed in a way of "harder when brighter". This presents a direct evidence that CL-AGNs have distinctive changes in not only the optical spectral type, but also the X-ray spectral shape. The similarity in the X-ray spectral evolution between CL-AGNs and black hole X-ray binaries indicates that the observed CL-AGNs phenomena may relate to the state transition in accretion physics.

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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

A wide star-black-hole binary system from radial-velocity measurements

All stellar mass black holes have hitherto been identified by X-rays emitted by gas that is accreting onto the black hole from a companion star. These systems are all binaries with black holes below 30 M$_{\odot}$$^{1-4}$. Theory predicts, however, that X-ray emitting systems form a minority of the total population of star-black hole binaries$^{5,6}$. When the black hole is not accreting gas, it can be found through radial velocity measurements of the motion of the companion star. Here we report radial velocity measurements of a Galactic star, LB-1, which is a B-type star, taken over two years. We find that the motion of the B-star and an accompanying H$α$ emission line require the presence of a dark companion with a mass of $68^{+11}_{-13}$ M$_{\odot}$, which can only be a black hole. The long orbital period of 78.9 days shows that this is a wide binary system. The gravitational wave experiments have detected similarly massive black holes$^{7,8}$, but forming such massive ones in a high-metallicity environment would be extremely challenging to current stellar evolution theories$^{9-11}$.

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