arXiv subjects

Hongyan Zhou

Publications and source records attributed to Hongyan Zhou.

At least 37 records · Page 2Linked to original sources

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

A Broad Absorption Line Outflow Associated with the Broad Emission Line Region in the Quasar SDSS J075133.35+134548.3

We report the discovery of unusual broad absorption lines (BALs) in the bright quasar SDSS J075133.35+134548.3 at z~1, and present a detailed study of BAL and emission lines using the archived and newly obtained optical and NIR spectroscopic data. The BALs are detected tentatively in Mg II, Al III and reliably in HeI*3889, HeI*10830, which show complexed velocity structures splitting into two components: a high-velocity component (HV), with a blueshifted velocity range of -9300 ~ -3500 km/s, can be reliably detected tentatively in Mg II, Al III and reliably in HeI* 10830, whereas it is undetectable in HeI* 3889; and low-Velocity component (LV), with the velocity of -3500 ~ -1800 km/s, is only detected in HeI*3889 and HeI* 10830. With the BALs from different ions, the HV outflowing gas can be constrained to have a density n_H~10^10.3-10^11.4 cm^-3, a column density N_H~10^21 cm^-2 and an ionization parameter U~10^-1.83 - 10^-1.72; and thus inferring a distance of R_HV~0.5 pc from the central continuum source with a monochromatic luminosity of 7.0*10^45 erg/s at 5100 A. This distance is remarkable similar to that of normal broad emission line region (BLR) estimated according to the luminosity-distance from the reverberation experiments, suggesting the association between the BLR and the HV BAL outflowing gas. Interestingly enough, a blueshifted component is also significantly detected in Mg II and Al III broad emission lines (BELs), and the Al III/Mg II of such a BEL component can be reproduced by the physical parameters inferred from the HV BAL gas. The LV BAL gas very likely have a larger column density and a higher ionization level than the HV BAL gas, and hence a smaller distance than that of the latter. Further spectroscopy with high S/N ratio and broader wavelength coverage is needed to confirm this and shed new light on the possible connection between BALs and BELs.

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arXiv2020

Extended radio AGN at z ~ 1 in the ORELSE survey: The confining effect of dense environments

Recent hydrodynamic simulations and observations of radio jets have shown that the surrounding environment has a large effect on their resulting morphology. To investigate this we use a sample of 50 Extended Radio Active Galactic Nuclei (ERAGN) detected in the Observations of Redshift Evolution in Large Scale Environments (ORELSE) survey. These sources are all successfully cross-identified to galaxies within a redshift range of $0.55 \leq z \leq 1.35$, either through spectroscopic redshifts or accurate photometric redshifts. We find that ERAGN are more compact in high-density environments than those in low-density environments at a significance level of 4.5$σ$. Among a series of internal properties under our scrutiny, only the radio power demonstrates a positive correlation with their spatial extent. After removing the possible radio power effect, the difference of size in low- and high-density environments persists. In the global environment analyses, the majority (86\%) of high-density ERAGN reside in the cluster/group environment. In addition, ERAGN in the cluster/group central regions are preferentially compact with a small scatter in size, compared to those in the cluster/group intermediate regions and fields. In conclusion, our data appear to support the interpretation that the dense intracluster gas in the central regions of galaxy clusters plays a major role in confining the spatial extent of radio jets.

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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

Ultradense Gases beyond Dusty Torus in a Partially Obscured Quasar

The co-evolution between black holes and galaxies suggests that feedback of active galactic nuclei influence host galaxies through ejecting radiative and kinetic energies to surroundings. Larger scale outflow in local universe are frequently observed by spatially resolved spectroscopy, while smaller scale outflow cannot be directly resolved by current observations. At the scale of the dusty torus, radiative and kinetic energies ejected from the central active nucleus interact with the materials. However, observations of such outflow are rarely reported due to the lack detection of unambiguously gas emission. Here we report the detection of clear and rich emission lines origin from the scale of dusty tours in an partially obscured quasar. The lines share a common intermediate width with full width at half maximum about 1900 \kmps\ and are shown in two systems: a major system is unshifted and a minor system has a blue-shifts of about 2600 \kmps. The line intensity ratios, combining photo-ionization simulations, indicates an ultradense line-emitting region with the density as high as $\sim$ $10^{13}~\rm cm^{-3}$. We interpret this as the lines being excited by a shock induced by the high-density and high-temperature gases at the scale of dusty torus, rather than photo-ionized by the central accretion disk. We speculate that the outflow, launched from the accretion disk, collides onto the inner wall of the dusty torus and shock-heat the gases to cause the major emission lines. The outflowing gases may also collide onto surrounding isolated clouds, and give rise to blue-shifted minor emission lines.

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arXiv2020

Multiwavelength Study of an X-ray Tidal Disruption Event Candidate in NGC 5092

We present multiwavelength studies of a transient X-ray source, XMMSL1 J131952.3+225958, associated with the galaxy NGC 5092 at $z=0.023$ detected in the XMM-Newton SLew survey (XMMSL). The source brightened in the 0.2--2 keV band by a factor of $>20$ in 2005 as compared with previous flux limits and then faded by a factor of $>200$ as observed with it XMM-Newton in 2013 and with it Swift in 2018. At the flaring state, the X-ray spectrum can be modeled with a blackbody at a temperature of $\sim$ 60 eV and an overall luminosity of $\sim$ $1.5 \times 10^{43}$ erg s$^{-1}$. A UV flare and optical flare were also detected with the Galaxy Evolution Explorer and the Sloan Digital Sky Survey, respectively, within several months of the X-ray flare, whose nonstellar UV--optical spectrum can be described with a blackbody at a temperature of $\sim$ $(1-2) \times 10^4$ K and a luminosity of $\sim$ $(2-6) \times 10^{43}$ erg s$^{-1}$. Interestingly, mid-infrared monitoring observations of NGC 5092 with the Wide-field Infrared Survey Explorer 5--13 yr later show a continuous flux decline. These dramatic variability properties, from the X-ray through UV and optical to infrared, appear to be orderly, suggestive of a stellar tidal disruption event (TDE) by a massive black hole, confirming the postulation by Kanner et al.(2013). This TDE candidate belongs to a rare sample with contemporaneous bright emission detected in the X-ray, UV, and optical, which are later echoed by dust-reprocessed light in the mid-infrared. The black hole has a mass of $\sim$ $5 \times 10^{7} \rm M_{\odot}$, residing in a galaxy that is dominated by a middle-aged stellar population of 2.5 Gyr.

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arXiv2019

A Deeply Buried Narrow-Line Seyfert 1 Nucleus Uncovered in Scattered Light

We present spectropolarimetric and spectrophotometric observations of the peculiar active galactic nucleus (AGN) SDSS J120300.19+162443.7 (hereafter J1203+1624) at $z=0.1656$. Its optical total flux spectra clearly show broad emission lines (BELs) in H$α$ and H$β$. After removal of narrow emission lines (NELs), the full width at half maximum of the lorentzian BEL is $FWHM_{\rm BEL} \approx 1,270$ km s$^{-1}$, fulfilling the conventional definition of a narrow line Seyfert 1 (NLS1) galaxy. However, its NELs are unprecedentedly strong when compared to type 1AGNs. This, together with its large MIR excess ($g-W_4 = 13.172$ mag), implies that the observer and the narrow emission line region (NELR) might see a different ionization continuum. Our optical spectropolarimetry confirms its type 2 nature by detecting a polarized blue continuum and Balmer BELs ($FWHM_{\rm Polarized~BEL} \approx 1,183$ km s$^{-1}$), with a high polarization degree of $> 20$% in the blue wing. The mass and Eddington rate of the central black hole is estimated based on both transmitted and scattered AGN radiation, which is $M_{\bullet} < 2.9 \times 10^7 M_\odot$ and $L_{\rm bol}/L_{\rm Edd} > 1.5$. Severe extinction of the AGN emission also enables a clear view of the compact host galaxy. Discovery of J1203+1624 suggests that NLS1 follows AGN unfication scheme, studying of its analogs could blaze a new trail for exploring the connection between black hole growth and star formation in the host galaxy. Interesting features of J1203+1624 like the peculiar NELs, inflowing scattering clouds within sublimation radius are worth detailed follow-ups in the future.

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arXiv2019

Discovery of Metastable He I* $λ$10830 Mini-broad Absorption Lines and Very Narrow Paschen $α$ Emission Lines in the ULIRG Quasar IRAS F11119+3257

IRAS F11119+3257 is a quasar-dominated Ultra-Luminous InfraRed Galaxy, with a partially obscured narrow-line seyfert 1 nucleus. In this paper, we present the NIR spectroscopy of F11119+3257, in which we find unusual Paschen emission lines, and metastable He I* $λ$10830 absorption associated with the previously reported atomic sodium and molecular OH mini-BAL (Broad Absorption Line) outflow. Photo-ionization diagnosis confirms previous findings that the outflows are at kilo-parsec scales. Such large-scale outflows should produce emission lines. We indeed find that high-ionization emission lines ([O III], [Ne III], and [Ne V]) are dominated by blueshifted components at similar speeds to the mini-BALs. The blueshifted components are also detected in some low-ionization emission lines, such as [O II] $λ$3727 and some Balmer lines (H$α$, H$β$, and H$γ$), even though their cores are dominated by narrow ($FWHM_{\rm NEL} = 570\pm40$km s$^{-1}$) or broad components at the systemic redshift of $z=0.18966\pm0.00006$. The mass flow rate (230-730$~M_\odot \rm yr^{-1}$) and the kinetic luminosity ($\dot{E}_k \sim 10^{43.6-44.8} $erg s$^{-1}$) are then inferred jointly from the blueshifted emission and absorption lines. In the NIR spectrum of F11119+3257, we also find that the Paschen emission lines are unique, in which a very narrow ($FWHM=260\pm20~$km s$^{-1}$) component is shown in only Pa$α$. This narrow component most probably comes from heavily obscured star formation. Based on the Pa$α$ and Pa$β$ emissions, we obtain an extinction at the $H$ band, $A_H~>~2.1$ (or a reddenning of $E_{B-V}~>~$3.7), and a star formation rate of $SFR~>~130\rm M_\odot yr^{-1}$ that resembles the estimates inferred from the FIR emissions ($SFR_{\rm FIR} = 190\pm90$ $M_\odot$ yr$^{-1}$).

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arXiv2019

An On-going Mid-infrared Outburst in the White Dwarf 0145+234: Catching in Action of Tidal Disruption of an Exoasteroid?

We report the detection of a large amplitude MIR outburst in the white dwarf (WD) 0145+234 in the NEOWISE Survey data. The source had a stable MIR flux before 2018, and was brightened by about 1.0 magnitude in the W1 and W2 bands within half a year and has been continuously brightening since then. No significant variations are found in the optical photometry data during the same period. This suggests that this MIR outburst is caused by recent replenishing or redistribution of dust, rather than intrinsic variations of the WD. SED modeling of 0145+234 suggests that there was already a dust disk around the WD in the quiescent state, and both of the temperature and surface area of the disk evolved rapidly since the outburst. The dust temperature was about 1770K in the initial rising phase, close to the sublimation temperature of silicate grains, and gradually cooled down to around 1150K, while the surface area increased by a factor of about 6 during the same period. The inferred closest distance of dust to the WD is within the tidal disruption radius of a gravitationally bounded asteroid. We estimated the dust mass to be between $3\times10^{15}$ to $3\times10^{17}$ $ρ/(1 \mathrm g~cm^{-3})$ kg for silicate grains of a power-law size distribution with a high cutoff size from 0.1 to 1000 microns. We interpret this as a possible tidal breakup of an exo-asteroid by the WD. Further follow-up observations of this rare event may provide insights on the origin of dust disk and metal pollution in some white dwarfs.

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arXiv2019

Profile of and Variability in Double-Peaked Balmer Emission Lines in 3C 445

We extract the multiple-epoch Balmer-line profiles of the heavily obscured quasar 3C 445 from the spectral curves in the literature, and analyze the emission-line profiles of the H$α$ and H$β$ lines and the profile variability in the H$α$ line in the large time interval of more than three decades. The profile comparison between the H$α$ and H$β$ lines shows that both Balmer lines share the profile with the same form, while the blue system of the H$β$ line is seriously weaker than that of the H$α$ line. Moreover, the blue system of the H$α$ line suddenly disappeared completely and then did not appear again, however the other two components did not exhibit significant variation in the velocity or the amplitude. These findings suggest that the blue system of 3C 445, as with SDSS J153636.22+044127.0 and its analogs, is probable the result of the shock-heated outflowing gases. The observation angle of almost edge-on which the previous studies suggested can easily produce the high-speed and high-temperature shock in the collision between the massive outflow and the inner surface of the dusty torus.

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arXiv2019

Fast Inflow Directly Feeding Black Hole Accretion Disk in Quasars

Quasars are high-luminosity active galactic nuclei believed to be powered by accretion of interstellar matter onto a super-massive black hole (SMBH) therein. Most of the observed energy is released in an accretion disk of inspiralling gas surrounding the SMBH. An enormous amount of fueling material is expected to be transported inwards. However, basic questions remain unanswered as to whether and how the accretion disks are supplied with external gas, since no disk-feeding inflow has hitherto been observed clearly. Here we report the discovery of highly redshifted broad absorption lines arising from neutral hydrogen and helium atoms in a small sample of quasars. Their absorption troughs show a broad range of Doppler velocities from zero extending continuously inward up to as high as $\sim 5,000$ km s$^{-1}$. We thus see through streams of cold gas moving with a radially inward velocity component that spans an immense gradient---a result of gravitational acceleration by the central SMBH. Extensive photo-ionization modeling for the archetypical object SDSS J103516.20+142200.6 indicates the inflowing gas to be dense, thick and moderately ionized, with a characteristic distance to the SMBH of $\sim 1,000$ gravitational radii, possibly overlapping or close to the outer accretion disk. Our results present the first compelling evidence for the long-sought inflow directly feeding quasars' accretion disks with external materials, likely originating from the dusty torus at a parsec scale. Our approach provides a new tool to probe the bulk of the so far elusive fueling inflows in quasars. Their studies may help address some of the fundamental questions concerning accretion physics, the onset and sustainment of quasar activity, and the SMBH growth at centers of most galaxies.

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arXiv2019

Model Adaptation via Model Interpolation and Boosting for Web Search Ranking

This paper explores two classes of model adaptation methods for Web search ranking: Model Interpolation and error-driven learning approaches based on a boosting algorithm. The results show that model interpolation, though simple, achieves the best results on all the open test sets where the test data is very different from the training data. The tree-based boosting algorithm achieves the best performance on most of the closed test sets where the test data and the training data are similar, but its performance drops significantly on the open test sets due to the instability of trees. Several methods are explored to improve the robustness of the algorithm, with limited success.

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arXiv2019

A comprehensive and uniform sample of broad-line active galactic nuclei from the SDSS DR7

A new, complete sample of 14,584 broad-line AGNs at $z<0.35$ is presented, which are uncovered homogeneously from the complete database of galaxies and quasars observed spectroscopically in the Sloan Digital Sky Survey Seventh Data Release. The stellar continuum is properly removed for each spectrum with significant host absorption line features, and careful analyses of the emission-line spectra, particularly in the H$\rm α$ and H$\rm β$ wavebands, are carried out. The broad Balmer emission line, particularly, H$\rm α$, is used to indicate the presence of an AGN. The broad H$\rm α$ lines have luminosities in a range of $10^{38.5}$-$10^{44.3}$ erg s$^{-1}$, and line widths (FWHMs) of 500-34,000 km s$^{-1}$. The virial black hole masses, estimated from the broad line measurements, span a range of $10^{5.1}$-$10^{10.3}$ $M_\odot$, and the Eddington ratios vary from $-3.3$ to $1.3$ in logarithmic scale. Other quantities such as multi-wavelength photometric properties and flags denoting peculiar line profiles are also included in this catalog. We describe the construction of this catalog and briefly discuss its properties. The catalog is publicly available online. This homogeneously selected AGN catalog, along with the accurately measured spectral parameters, provide the most updated, largest AGN sample data, which will enable further comprehensive investigations of the properties of the AGN population in the low-redshift universe.

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arXiv2019

SDSS J094635.06+101706.1: a redshift one, very radio-loud, γ-ray emitting narrow-line Seyfert 1 galaxy

As hybrids of narrow-line Seyfert 1 (NLS1) galaxies and blazars, γ-ray emitting NLS1s are important probes of jet physics in the high Eddington-ratio regime. Only very few of them are known to date; the majority of them below redshift z = 0.5. Here we present the identification of the γ-ray emitting AGN TXS 0943+105 (SDSS J094635.06+101706.1) as a high-redshift NLS1 galaxy. It turns out to be one of the radio-loudest NLS1s known, highly variable at all wavelengths, and shows widely extended radio emission at a (projected) > 100 kpc scale. It is a known strong γ-ray emitter with a luminous flare reported previously. At redshift z=1.004, this is the most distant γ-NLS1 known to date.

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arXiv2019

SDSS J153636.22+044127.0 and its analogues: shocked outflows, not active binary black holes

The binary emission-line system, particularly the binary broad-line emission system, is considered the most effective indicator of the black hole binary. A plausible example of such a system, SDSS J153636.22+044127.0, was reported as the first known object with two hydrogen Balmer broad-line systems, which are interpreted to be the result of broad-line regions around a pair of black holes (Boroson \& Lauer 2009). Here, we show the follow-up optical and near-infrared spectral observations of SDSS J153636.22+044127.0 and its analogues. In these objects, the broad hydrogen Balmer and Paschen, He I and Mg II lines share the same peculiar emission-line profile (including a blue system, a red system and a double-peaked disk-line component); however, the invariance in the large time interval, the absence of the blue system in He I $λ$10830 profile and the abnormally strong emission of the hydrogen Pa$β$ blue system oppose the binary proposal. We suggest that these unique broad lines arise from the AGN emission-line region and the shock-heated outflowing gases rather than a binary system of two active black holes.

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arXiv2019

The properties of broad absorption line outflows based on a large sample of quasars

Quasar outflows carry mass, momentum and energy into the surrounding environment, and have long been considered a potential key factor in regulating the growth of supermassive black holes and the evolution of their host galaxies. A crucial parameter for understanding the origin of these outflows and measuring their influence on their host galaxies is the distance (R) between the outflow gas and the galaxy center. While R has been measured in a number of individual galaxies, its distribution remains unknown. Here we report the distributions of R and the kinetic luminosities of quasars outflows, using the statistical properties of broad absorption line variability in a sample of 915 quasars from the Sloan Digital Sky Surveys. The mean and standard deviation of the distribution of R are 10^{1.4+/-0.5} parsecs. The typical outflow distance in this sample is tens of parsec, which is beyond the theoretically predicted location (0.01 ~ 0.1 parsecs) where the accretion disc line-driven wind is launched, but is smaller than the scales of most outflows that are derived using the excited state absorption lines. The typical value of the mass-flow rate is of tens to a hundred solar masses per year, or several times the accretion rate. The typical kinetic-to-bolometric luminosity ratio is a few per cent, indicating that outflows are energetic enough to influence the evolution of their host galaxies.

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arXiv2019

Exoplanets in the Antarctic sky. II. 116 Transiting Exoplanet Candidates Found by AST3-II (CHESPA) within the Southern CVZ of TESS

We report first results from the CHinese Exoplanet Searching Program from Antarctica (CHESPA)---a wide-field high-resolution photometric survey for transiting exoplanets carried out using telescopes of the AST3 (Antarctic Survey Telescopes times 3) project. There are now three telescopes (AST3-I, AST3-II, and CSTAR-II) operating at Dome A---the highest point on the Antarctic Plateau---in a fully automatic and remote mode to exploit the superb observing conditions of the site, and its long and uninterrupted polar nights. The search for transiting exoplanets is one of the key projects for AST3. During the Austral winters of 2016 and 2017 we used the AST3-II telescope to survey a set of target fields near the southern ecliptic pole, falling within the continuous viewing zone of the TESS mission \citep{Ricker10}. The first data release of the 2016 data, including images, catalogs and lightcurves of 26578 bright stars ($7.5\le i \le15$) was presented in \citet{Zhang18}. The best precision, as measured by the RMS of the lightcurves at the optimum magnitude of the survey ($i=10$), is around 2\,mmag. We detect 222 objects with plausible transit signals from these data, 116 of which are plausible transiting exoplanet candidates according to their stellar properties as given by the TESS Input Catalog \citep{Stassun17}, Gaia DR2 \citep{Gaia18} and TESS-HERMES spectroscopy \citep{Sharma18}. With the first data release from TESS expected in late 2018, this candidate list will be a timely for improving the rejection of potential false positives.

astro-ph.EP↗

arXiv2018

Exoplanets in the Antarctic Sky. I. The First Data Release of AST3-II (CHESPA) and New Found Variables within the Southern CVZ of TESS

Located at Dome A, the highest point of the Antarctic plateau, the Chinese Kunlun station is considered to be one of the best ground-based photometric sites because of its extremely cold, dry, and stable atmosphere(Saunders et al. 2009). A target can be monitored from there for over 40 days without diurnal interruption during a polar winter. This makes Kunlun station a perfect site to search for short-period transiting exoplanets. Since 2008, an observatory has been built at Kunlun station and three telescopes are working there. Using these telescopes, the AST3 project has been carried out over the last six years with a search for transiting exoplanets as one of its key programs (CHESPA). In the austral winters of 2016 and 2017, a set of target fields in the Southern CVZ of TESS (Ricker et al. 2009) were monitored by the AST3-II telescope. In this paper, we introduce the CHESPA and present the first data release containing photometry of 26,578 bright stars (m_i < 15). The best photometric precision at the optimum magnitude for the survey is around 2 mmag. To demonstrate the data quality, we also present a catalog of 221 variables with a brightness variation greater than 5 mmag from the 2016 data. Among these variables, 179 are newly identified periodic variables not listed in the AAVSO databasea), and 67 are listed in the Candidate Target List(Stassun et al. 2017). These variables will require careful attention to avoid false-positive signals when searching for transiting exoplanets. Dozens of new transiting exoplanet candidates will be also released in a subsequent paper(Zhang et al. 2018b).

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