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It is known that some active galactic nuclei (AGNs) transited from type 1 to type 2 or vice versa. There are two explanations for the so-called changing look AGNs: one is the dramatic change of the obscuration along the line-of-sight, the other is the variation of accretion rate. In this paper, we report the detection of large amplitude variations in the mid-infrared luminosity during the transitions in 10 changing look AGNs using WISE and newly released NEOWISE-R data. The mid-infrared light curves of 10 objects echoes the variability in the optical band with a time lag expected for dust reprocessing. The large variability amplitude is inconsistent with the scenario of varying obscuration, rather supports the scheme of dramatic change in the accretion rate., Published by ApjL, 7 pages, 3 figures, 2 tables

The origin of narrow line region (NLR) outflows remains unknown. In this paper, we explore the scenario in which these outflows are circumnuclear clouds driven by energetic accretion disk winds. We choose the well-studied nearby Seyfert galaxy NGC 4151 as an example. By performing 3D hydrodynamical simulations, we are able to reproduce the radial distributions of velocity, mass outflow rate and kinetic luminosity of NLR outflows in the inner 100 pc deduced from spatial resolved spectroscopic observations. The demanded kinetic luminosity of disk winds is about two orders of magnitude higher than that inferred from the NLR outflows, but is close to the ultrafast outflows (UFO) detected in X-ray spectrum and a few times lower than the bolometric luminosity of the Seyfert. Our simulations imply that the scenario is viable for NGC 4151. The existence of the underlying disk winds can be confirmed by their impacts on higher density ISM, e.g., shock excitation signs, and the pressure in NLR., 10 pages, 5 figures, accepted by ApJ

We present the mid-infrared (MIR) light curves (LCs) of a tidal disruption event (TDE) candidate in the center of a nearby ultraluminous infrared galaxy (ULIRG) F01004-2237 using archival {\it WISE} and {\it NEOWISE} data from 2010 to 2016. At the peak of the optical flare, F01004-2237 was IR quiescent. About three years later, its MIR fluxes have shown a steady increase, rising by 1.34 and 1.04 mag in $3.4$ and $4.6\mu$m up to the end of 2016. The host-subtracted MIR peak luminosity is $2-3\times10^{44}$\,erg\,s$^{-1}$. We interpret the MIR LCs as an infrared echo, i.e. dust reprocessed emission of the optical flare. Fitting the MIR LCs using our dust model, we infer a dust torus of the size of a few parsecs at some inclined angle. The derived dust temperatures range from $590-850$\,K, and the warm dust mass is $\sim7\,M_{\odot}$. Such a large mass implies that the dust cannot be newly formed. We also derive the UV luminosity of $4-11\times10^{44}$\,erg\,s$^{-1}$. The inferred total IR energy is $1-2\times10^{52}$\,erg, suggesting a large dust covering factor. Finally, our dust model suggests that the long tail of the optical flare could be due to dust scattering.

We analyze the X-ray, optical, and mid-infrared data of a “changing-look” Seyfert galaxy SDSS J155258+273728 at z ≃ 0.086. Over a period of one decade (2009–2018), its broad Hα line intensity increased by a factor of ∼4. Meanwhile, the X-ray emission in 2014 as observed by Chandra was about five times brighter than that in 2010 by Suzaku, and the corresponding emissions in the V-band, mid-infrared W1 band brighten by ∼0.18, 0.32 mag, respectively. Moreover, the absorption in X-rays is moderate and stable, i.e., N H ∼ 10 21 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 − 0.21 + 0.22 ). With an Eddington ratio being lower than a few percent, the inner region of the accretion disk in SDSS J155258+273728 is likely a hot accretion flow. We then compile from literature the X-ray data of “changing-look” active galactic nuclei (AGNs), and find that they generally follow the well-established “V”-shaped correlation in AGNs, that is, above a critical turnover luminosity the X-ray spectra soften with the increasing luminosity, and below that luminosity the trend is reversed in the way of “harder when brighter.” This presents 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.

Small ultra high frequency (UHF) radio frequency identification (RFID) tags of fragment- type structure can be designed for broadband operation and for versatile impedance matching to different chips. The fragment-type tag structure can be optimized by using genetic algorithm. In our design, multi-objective evolutionary algorithm based on decomposition combined with enhanced genetic operators (MOEA/D-GO) is used for optimization searching. The multiple objectives are defined in terms of power transmission coefficients for operation in multiple RFID bands and for impedance matching to several prevailing RFID chips. For demonstration, a fragmented tiny square UHF tag of dimensions of 5. 5m m∗ 5.5 mm is designed for multi-band operation over the 433 MHz, 869 MHz and 915 MHz RFID bands, and a fragmented round tiny RFID tag of radius of 4.5 mm is also designed for versatile connection to five prevailing RFID chips at 915 MHz. The tiny round versatile tag is tested by connecting two chips, the IMPIMJ Monza-4 chip (11 − 143j) and ALIEN Higgs-3 chip (27 − 195j), respectively. Effects of input impedance and adjunct fragments on versatility of the design are further discussed.

Very few low-ionization broad absorption line (LoBAL) QSOs have been found at high redshifts to date. One high-redshift LoBAL QSO, J0122+1216, was recently discovered at the Lijiang 2.4-m Telescope with an initial redshift determination of 4.76. Aiming to investigate its physical properties, we carried out follow-up observations in the optical and near-IR spectroscopy. Near-IR spectra from UKIRT and P200 confirms that it is a LoBAL, with a new redshift determination of $4.82\pm0.01$ based on the \mgii~ emission-line. The new \mgii~ redshift determination reveals strong blueshifts and asymmetry of the high-ionization emission lines. We estimated a black hole mass of $\sim 2.3\times 10^9 M_\odot$ and Eddington ratio of $\sim 1.0$ according to the empirical \mgii-based single-epoch relation and bolometric correction factor. It is possible that strong outflows are the result of an extreme quasar environment driven by the high Eddington ratio. A lower limit on the outflowing kinetic power ($>0.9\% L_{Edd}$) was derived from both emission and absorption lines, indicating these outflows play a significant role in the feedback process to regulate the growth of its black hole as well as host galaxy evolution., 12 pages, 10 figures. Accepted for publication in The Astrophysical Journal

In this paper we present an analysis of absorption line variability in mini-BAL quasar LBQS 1206+1052. The SDSS spectrum demonstrates that the absorption troughs can be divided into two components of blueshift velocities of $\sim$700 km s$^{-1}$ and $\sim$1400 km s$^{-1}$ relative to the quasar rest-frame. The former component shows rare Balmer absorption, which is an indicator of high density absorbing gas, thus the quasar is worth follow-up spectroscopic observations. Our follow-up optical and near-infrared spectra using MMT, YFOSC, TripleSpec and DBSP reveal that the strengths of the absorption lines vary for both of the two components, while the velocities do not change. We reproduce all of the spectral data by assuming that only the ionization state of the absorbing gas is variable and that all other physical properties are invariable. The variation of ionization is consistent with the variation of optical continuum from the V-band light-curve. Additionally, we can not interpret the data by assuming that the variability is due to a movement of the absorbing gas. Therefore, our analysis strongly indicates that the absorption line variability in LBQS 1206+1052 is photoionization-driven. As shown from photo-ionization simulations, the absorbing gas with blueshift velocity of $\sim$700 km s$^{-1}$ has a density in the range of $10^9$ to $10^{10}$ cm$^{-3}$ and a distance of $\sim$1 pc, and the gas with blueshift velocity of $\sim$1400 km s$^{-1}$ has a density of $10^3$ cm$^{-3}$ and a distance of $\sim$1 kpc., Accepted by ApJ, 47 pages, 15 figures and 5 tables

Recent studies have shown that outflows in at least some broad absorption line (BAL) quasars are extended well beyond the putative dusty torus. Such outflows should be detectable in obscured quasars. We present four WISE selected infrared red quasars with very strong and peculiar ultraviolet Fe ii emission lines: strong UV Fe II UV arising from transitions to ground/low excitation levels, and very weak Fe II at wavelengths longer than 2800 {\AA}. The spectra of these quasars display strong resonant emission lines, such as C IV, Al III and Mg II but sometimes, a lack of non-resonant lines such as C III], S III and He II. We interpret the Fe II lines as resonantly scattered light from the extended outflows that are viewed nearly edge-on, so that the accretion disk and broad line region are obscured by the dusty torus, while the extended outflows are not. We show that dust free gas exposed to strong radiation longward of 912 {\AA} produces Fe II emission very similar to that observed. The gas is too cool to collisionally excite Fe II lines, accounting for the lack of optical emission. The spectral energy distribution from the UV to the mid-infrared can be modeled as emission from a clumpy dusty torus, with UV emission being reflected/scattered light either by the dusty torus or the outflow. Within this scenario, we estimate a minimum covering factor of the outflows from a few to 20% for the Fe II scattering region, suggesting that Fe II BAL quasars are at a special stage of quasar evolution., Comment: 24 pages, 6 figures

We report the detection of a significant infrared variability of the nearest tidal disruption event (TDE) ASASSN-14li using Wide-field Infrared Survey Explorer} and newly released Near-Earth Object WISE Reactivation} data. In comparison with the quiescent state, the infrared flux is brightened by 0.12 and 0.16 magnitude in the W1 ($3.4\mu$m) and W2 ($4.6\mu$m) bands at 36 days after the optical discovery (or $\sim110$ days after the peak disruption date). The flux excess is still detectable $\sim170$ more days later. Assuming that the flare-like infrared emission is from the dust around the black hole, its blackbody temperature is estimated to be $\sim2.1\times10^3$~K, slightly higher than the dust sublimation temperature, indicating that the dust is likely located close to the dust sublimation radius. The equilibrium between the heating and radiation of the dust claims a bolometric luminosity of $\sim10^{43}-10^{45}$~\lum, comparable with the observed peak luminosity. This result has for the first time confirmed the detection of infrared emission from the dust echoes of TDEs., Comment: published by ApJL

The sporadic accretion following the tidal disruption of a star by a super-massive black hole (TDE) leads to a bright UV and soft X-ray flare in the galactic nucleus. The gas and dust surrounding the black hole responses to such a flare with an echo in emission lines and infrared emission. In this paper, we report the detection of long fading mid-IR emission lasting up to 14 years after the flare in four TDE candidates with transient coronal lines using the WISE public data release. We estimate that the reprocessed mid-IR luminosities are in the range between $4\times 10^{42}$ and $2\times 10^{43}$ erg~s$^{-1}$ and dust temperature in the range of 570-800K when WISE first detected these sources three to five years after the flare. Both luminosity and dust temperature decreases with time. We interpret the mid-IR emission as the infrared echo of the tidal disruption flare. We estimate the UV luminosity at the peak flare to be 1 to 30 times $10^{44}$ erg s$^{-1}$ and for warm dust masses to be in the range of 0.05-1.3 Msun within a few parsecs. Our results suggest that the mid-infrared echo is a general signature of TDE in the gas-rich environment.

PS16dtm was classified as a candidate tidal disruption event (TDE) in a dwarf Seyfert 1 galaxy with low-mass black hole ($\sim10^6M\odot$) and has presented various intriguing photometric and spectra characteristics. Using the archival WISE and the newly released NEOWISE data, we found PS16dtm is experiencing a mid-infrared (MIR) flare which started $\sim11$ days before the first optical detection. Interpreting the MIR flare as a dust echo requires close pre-existing dust with a high covering factor, and suggests the optical flare may have brightened slowly for some time before it became bright detectable from the ground. More evidence is given at the later epochs. At the peak of the optical light curve, the new inner radius of the dust torus has grown to much larger size, a factor of 7 of the initial radius due to strong radiation field. At $\sim150$ days after the first optical detection, the dust temperature has dropped well below the sublimation temperature. Other peculiar spectral features shown by PS16dtm are the transient, prominent FeII emission lines and outflows indicated by broad absorption lines detected during the optical flare. Our model explains the enhanced FeII emission from iron newly released from the evaporated dust. The observed broad absorption line outflow could be explained by accelerated gas in the dust torus due to the radiation pressure., Comment: Accepted by ApJ, 5 figures

We present new optical spectra of the well-known broad absorption line (BAL) quasar PG 1411+ 442, using the DBSP spectrograph at the Palomar 200 inch telescope in 2014 and 2017 and the YFOSC spectrograph at the Lijiang 2.4 m telescope in 2015. A blueshifted narrow absorption line system is clearly revealed in 2014 and 2015 consisting of hydrogen Balmer series and metastable He I lines. The velocity of these lines is similar to the centroid velocity of the UV BALs, suggesting that both originate from the outflow. The Balmer lines vary significantly between the two observations and vanished in 2017. They were also absent in the archived spectra obtained before 2001. The variation is thought to be driven by photoionization change. Besides, the absorption lines show inversed Balmer decrement, i.e., the apparent optical depths of higher-order Balmer absorption lines are larger than those of lower-order lines, which is inconsistent with the oscillator strengths of the transitions. We suggest that such anomalous line ratios can be naturally explained by the thermal structure of a background accretion disk, which allows the obscured part of the disk to contribute differently to the continuum flux at different wavelengths. Highresolution spectroscopic and photometric monitoring would be very useful to probe the structure of the accretion disk as well as the geometry and physical conditions of the outflow.

The galactocentric distance of quasar absorption outflows are conventionally determined using absorption troughs from excited states, a method hindered by severely saturated or self-blended absorption troughs. We propose a novel method to estimate the size of a broad absorption line (BAL) region which partly obscures an emission line region by assuming virialized gas in the emission region surrounding a supermassive black hole with known mass. When a spiky Lya1216 line emission is present at the flat bottom of the deep Nv1240 absorption trough, the size of BAL region can be estimated. We have found 3 BAL quasars in the SDSS database showing such Lya lines. The scale of their BAL outflows are found to be 3-26 pc, moderately larger than the theoretical scale (0.01-0.1 pc) of trough forming regions for winds originating from accretion discs, but significantly smaller than most outflow sizes derived using the absorption troughs of the excited states of ions. For these three outflows, the lower limits of ratio of kinetic luminosity to Eddington luminosity are 0.02%-0.07%. These lower limits are substantially smaller than that is required to have significant feedback effect on their host galaxies., 10 pages, 9 figures, 2 tables, Accepted for publication in ApJ

We present results of follow-up optical spectroscopic observations of seven rare, extreme coronal line emitting galaxies reported by Wang et al. (2012) with Multi-Mirror Telescope (MMT). Large variations in coronal lines are found in four objects, making them strong candidates of tidal disruption events (TDE). For the four TDE candidates, all the coronal lines with ionization status higher than [Fe VII] disappear within 5-9 years. The [Fe VII] faded by a factor of about five in one object (J0952+2143) within 4 years, whereas emerged in other two without them previously. A strong increment in the [O III] flux is observed, shifting the line ratios towards the loci of active galactic nucleus on the BPT diagrams. Surprisingly, we detect a non-canonical [O III]5007/[O III]4959 2 in two objects, indicating a large column density of O$^{2+}$ and thus probably optical thick gas. This also requires a very large ionization parameter and relatively soft ionizing spectral energy distribution (e.g. blackbody with $T < 5\times 10^4$ K). Our observations can be explained as echoing of a strong ultraviolet to soft X-ray flare caused by tidal disruption events, on molecular clouds in the inner parsecs of the galactic nuclei. Re-analyzing the SDSS spectra reveals double-peaked or strongly blue-shouldered broad lines in three of the objects, which disappeared in the MMT spectra in two objects, and faded by a factor of ten in 8 years in the remaining object with a decrease in both the line width and centroid offset. We interpret these broad lines as arising from decelerating biconical outflows. Our results demonstrate that the signatures of echoing can persist for as long as ten years, and can be used to probe the gas environment in the quiescent galactic nuclei., 30 Pages, 10 Figures, 2 Tables, Accepted for Publication in ApJ

We present a statistical analysis of the variability of broad absorption lines (BALs) in quasars using the large multi-epoch spectroscopic dataset of the Sloan Digital Sky Survey Data Release 12 (SDSS DR12). We divide the sample into two groups according to the pattern of the variation of C iv BAL with respect to that of continuum: the equivalent widths (EW) of the BAL decreases (increases) when the continuum brightens (dims) as group T1; and the variation of EW and continuum in the opposite relation as group T2. We find that T2 has significantly (P_T, 10 pages, 9 figures, Accepted for publication in ApJS

Quasars with redshifts greater than 4 are rare, and can be used to probe the structure and evolution of the early universe. Here we report the discovery of six new quasars with $i$-band magnitudes brighter than 19.5 and redshifts between 2.4 and 4.6 from the YFOSC spectroscopy of the Lijiang 2.4m telescope in February, 2012. These quasars are in the list of $z>3.6$ quasar candidates selected by using our proposed $J-K/i-Y$ criterion and the photometric redshift estimations from the SDSS optical and UKIDSS near-IR photometric data. Nine candidates were observed by YFOSC, and five among six new quasars were identified as $z>3.6$ quasars. One of the other three objects was identified as a star and the other two were unidentified due to the lower signal-to-noise ratio of their spectra. This is the first time that $z>4$ quasars have been discovered using a telescope in China. Thanks to the Chinese Telescope Access Program (TAP), the redshift of 4.6 for one of these quasars was confirmed by the Multiple Mirror Telescope (MMT) Red Channel spectroscopy. The continuum and emission line properties of these six quasars, as well as their central black hole masses and Eddington ratios, were obtained., 7 pages, 2 figures, published in Research in Astronomy and Astrophysics (RAA) as a letter

Neutral Helium multiplets, HeI*3189,3889,10830 are very useful diagnostics to the geometry and physical conditions of the absorbing gas in quasars. So far only a handful of HeI* detections have been reported. Using a newly developed method, we detected HeI*3889 absorption line in 101 sources of a well-defined sample of 285 MgII BAL quasars selected from the SDSS DR5. This has increased the number of HeI* BAL quasars by more than one order of magnitude. We further detected HeI*3189 in 50% (52/101) quasars in the sample. The detection fraction of HeI* BALs in MgII BAL quasars is about 35% as a whole, and increases dramatically with increasing spectral signal-to-noise ratios, from 18% at S/N = 35. This suggests that HeI* BALs could be detected in most MgII LoBAL quasars, provided spectra S/N is high enough. Such a surprisingly high HeI* BAL fraction is actually predicted from photo-ionization calculations based on a simple BAL model. The result indicates that HeI* absorption lines can be used to search for BAL quasars at low-z, which cannot be identified by ground-based optical spectroscopic survey with commonly seen UV absorption lines. Using HeI*3889, we discovered 19 BAL quasars at z, Accepted for publication in ApJS. Tables 3, 4, 8, and 9 are now available in its entirety at the arXiv website along with the manuscript file, and will also be available via the link to the machine-readable tables on the ApJS website

Tidal disruption of stars by supermassive black holes at the centers of galaxies is expected to produce unique emission line signatures, which have not yet been explored adequately. Here we report the discovery of extremely strong coronal lines from [Fe X] up to [Fe XIV] in a sample of seven galaxies (including two recently reported cases), that we interpret as such signatures. This is the first systematic search for objects of this kind, by making use of the immense database of the Sloan Digital Sky Survey. The galaxies, which are non-active as evidenced by the narrow-line ratios, show broad emission lines of complex profiles in more than half of the sample. Both the high ionization coronal lines and the broad lines turn out to be fading on time scales of years in objects observed with spectroscopic follow-ups, suggesting their transient nature. Variations of inferred non-stellar continua, which have absolute magnitudes of at least -16 to -18 mag in the g band, are also detected in more than half of the sample. These extreme coronal line emitters reside in sub-L_* disk galaxies (-21.3 < M_i < -18.5) with small stellar velocity dispersions. The sample seems to form two distinct types based on the presence or absence of the [Fe VII] lines, with the latter having relatively low luminosities of [O III], [Fe XI], and the host galaxies. These characteristics can most naturally be understood in the context of transient accretion onto intermediate mass black holes at galactic centers following tidal disruption of stars in a gas-rich environment. We estimate the incidence of such events to be around 10^-5 per year for a galaxy with -21.5 < M_i < -18.5., 32 pages, 12 figures, ApJ accepted, typos corrected