We report the discovery of a possible close binary system of quasars in SDSS J1048+0055. The [OII... more We report the discovery of a possible close binary system of quasars in SDSS J1048+0055. The [OIII]λλ4959,5007 emission lines are clearly double-peaked, and two discrete radio sources with a projected physical separation of ∼ 20 pc are found in the VLBA milliarcsec resolution image at 8.4 GHz. Each of the [O III]λλ4959, 5007 doublets and Hβ can be well modelled by two Gaussians and the line ratio, [OIII]λ5007/Hβ ∼ 7, is typical of Seyfert 2 galaxies. No broad component of Hβ was detected and its [OIII]λ5007 luminosity, L[OIII] ≈ 9.2 × 10 42 erg s −1, is comparable to luminous quasars and is a few ten times more luminous than typical Seyfert galaxies. One natural interpretation is that SDSS J1048+0055 contains two close quasar-like nuclei and the BLR around them are obscured. Other possible models are also discussed. We suggest that double-peaked narrow emission line profile may be an effective way of selecting candidates of binary black holes with intermediate separation.
We present empirical relations between the global dust reddening and other physical galaxy proper... more We present empirical relations between the global dust reddening and other physical galaxy properties including the Hα luminosity, Hα surface brightness, metallicity and axial ratio for star-forming disc galaxies. The study is based on a large sample of ∼ 22 000 well-defined star-forming galaxies selected from the Sloan Digital Sky Survey (SDSS). The reddening parameterized by color excess E(B − V) is derived from the Balmer decrement. Besides the dependency of reddening on Hα luminosity / surface brightness and gas phase metallicity, it is also correlated with the galaxy inclination, in the sense that edge-on galaxies are more attenuated than face-on galaxies at a give intrinsic luminosity. In light of these correlations, we present the empirical formulae of E(B −V) as a function of these galaxy properties, with a scatter of only 0.07 mag. The empirical relation can be reproduced if most dust attenuation to the H ii region is due to diffuse interstellar dust distributing in a disc thicker than that of H ii regions. The empirical formulae can be incorporated into semi-analytical models of galaxy formation and evolution to estimate the dust reddening and enable comparison with observations more practically.
We report the discovery of a dwarf Seyfert 1 active galactic nucleus (AGN) with a candidate inter... more We report the discovery of a dwarf Seyfert 1 active galactic nucleus (AGN) with a candidate intermediate-mass black hole hosted by the dwarf galaxy SDSS J160531.84+174826.1 at z = 0.032. A broad component of the Hα line with FWHM=781km s −1 is detected in its optical spectrum, and a bright, point-like nucleus is evident from a HST imaging observation. Non-thermal X-ray emission is also detected from the nucleus. The black hole mass, as estimated from the luminosity and width of the broad Hα component, is about 7 ×10 4 M⊙. The host galaxy appears to be a disk galaxy with a boxy bulge or nuclear bar; with an absolute magnitude of MR = −17.8, it is among the least luminous host galaxies ever identified for a Seyfert 1.
PG 1411+442, and PG 0043+039. All three objects are found to be X-ray quiet, possibly due to subs... more PG 1411+442, and PG 0043+039. All three objects are found to be X-ray quiet, possibly due to substantial intrinsic absorption with N-H > 10(23) cm(-2) but might have intrinsically a more typical spectral energy distribution. This picture is supported by explicit spectral fits to the combined ROSAT/ASCA data of the nearby bright object PG 1411+442. An analysis of the large number of radio-quiet quasars not detected in the ROSAT All-Sky Survey indicates that perhaps only a small fraction of BAL quasars have not been recognized before and that the number of intrinsically X-ray quiet quasars must be small.
We have conducted a systematic investigation of the origin and underlying physics of the line--li... more We have conducted a systematic investigation of the origin and underlying physics of the line--line and line--continuum correlations of AGNs, particularly the Baldwin effect. Based on the homogeneous sample of Seyfert 1s and QSOs in the SDSS DR4, we find the origin of all the emission-line regularities is Eddington ratio (L/Ledd). The essential physics is that L/Ledd regulates the distributions of the properties (particularly column density) of the clouds bound in the line-emitting region.
Based on the spectroscopic and shear catalogs for SDSS galaxies in the local Universe, we compare... more 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 h−1M , 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 o...
Based on the spectroscopic and shear catalogs for SDSS galaxies in the local Universe, we compare... more Based on the spectroscopic and shear catalogs for SDSS galaxies in the local Universe, we compared optically selected active galactic nuclei (AGNs) with control star-forming and quiescent galaxies on galactic and inter-halo scales, and larger. We find that AGNs are preferentially found in two specific stages of galaxy evolution: in the starburst and ‘green valley’ phases. We also find that the stellar population of their host galaxies is quite independent of stellar mass, which is not the case for more typical galaxies. Combining galaxy-galaxy lensing and galaxy clustering on large scales, we measured the mass of AGN host halos. The typical halo mass is about 1012 h−1 M⊙, similar to the characteristic mass in the stellar mass-halo mass relation (SHMR). For a given stellar mass, AGN host galaxies and star-forming galaxies share the same SHMR, while quiescent galaxies have more massive halos. Clustering analyses on halo scales reveals that AGNs are surrounded by a larger number of sat...
Space Telescopes and Instrumentation 2020: Ultraviolet to Gamma Ray
The Hot Universe Baryon Surveyor (HUBS) mission is proposed to study "missing" baryons in the uni... more The Hot Universe Baryon Surveyor (HUBS) mission is proposed to study "missing" baryons in the universe. Unlike dark matter, baryonic matter is made of elements in the periodic table, and can be directly observed through the electromagnetic signals that it produces. Stars contain only a tiny fraction of the baryonic matter known to be present in the universe. Additional baryons are found to be in diffuse (gaseous) form, in or between galaxies, but a significant fraction has not yet been seen. The latter ("missing" baryons) are thought to be hiding in low-density warm-hot ionized medium (WHIM), based on results from theoretical studies and recent observations, and be distributed in the vicinity of galaxies (i.e., circumgalactic medium) and between galaxies (i.e., intergalactic medium). Such gas would radiate mainly in the soft X-ray band and the emission would be very weak, due to its very low density. HUBS is optimized to detect the X-ray emission from the hot baryons in the circumgalactic medium, and thus fill a void in observational astronomy. The goal is not only to detect the "missing" baryons, but to characterize their physical and chemical properties, as well as to measure their spatial distribution. The results would establish the boundary conditions for understanding galaxy evolution. Though highly challenging, detecting "missing" baryons in the intergalactic medium could be attempted, perhaps in the outskirts of galaxy clusters, and could shed significant light on the large-scale structures of the universe. The current design of HUBS will be presented, along with the status of technology development.
The optical time-domain astronomy has grown rapidly in the past decade but the dynamic infrared s... more 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 4.6µm > 10 42 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 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.
The past decade has experienced an explosive increase of optically-discovered tidal disruption ev... more 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 −1 , corresponding to a dust covering factor f c ∼ 0.01 at scale of sub-parsec. The only exception is the disputed source ASASSN-15lh, which shows an ultra-high dust luminosity (∼ 10 43.5 erg s −1) 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.
GSN 069 is an ultra-soft X-ray active galactic nucleus that previously exhibited a huge X-ray out... more 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.
The combination of the linear size from reverberation mapping (RM) and the angular distance of th... more 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., Hβ for RM and Paα for SA). Here we evaluate the size deviations measured by six pairs of hydrogen lines (e.g., Hβ, Hα and Paα) via the locally optimally emitting cloud (LOC) models of BLR. We find that the radius ratios K(=R SA /R 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 Paα(SA)/Hβ(RM) and Hα(SA)/Hβ(RM) pairs not only are closest to 1 but also have considerably smaller uncertainty. Considering the current infrared interferometry technology, the Paα(SA)/Hβ(RM) pair is the ideal choice for the low redshift objects in the SARM project. In the future, the Hα(SA)/Hβ(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.
We present an analysis of the variability of broad absorption lines (BALs) in a quasar SDSS J1419... more 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 ∆T between two observations is longer than the recombination timescale t rec , the BAL variations can be detected. This can be characterized as a "sharp rise" in the detection rate of BAL variation at ∆T = t 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 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 rec for half an order of magnitude in two individual troughs. This result indicates that there may be multiple components with different t rec but the same velocity in an individual trough.
We report the discovery of a mid-infrared (MIR) flare using WISE data in the center of the nearby... more 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 × 10 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 ∼ 10 51 erg, most of which was released in less than ∼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.
We analyze the X-ray, optical, and mid-infrared data of a “changing-look” Seyfert galaxy SDSS J15... more 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 ...
Changing look active-galatic-nuclei (CL AGNs) can yield considerable insight into accretion physi... more Changing look active-galatic-nuclei (CL AGNs) can yield considerable insight into accretion physics as well as the co-evolution of black holes and their host galaxies. A large sample of these CL AGNs is essential to achieve the latter goal. We propose an effective method to select CL candidates from spectroscopic quasar catalogs using the mid-infrared (MIR) variability information derived from ALL-WISE/NEOWISE data releases. Our primary selection criteria include both a large amplitude flux variation and a transition of MIR color from an AGN to a normal galaxy. A pilot spectroscopic followup of 7 candidates among about 300 candidates selected from Sloan Digital Sky Survey low-redshift (z<0.5) AGN sample results in 6 new turn-off CL AGNs. We expect to obtain hundreds of CL AGNs once full spectroscopic follow-up of the sample is carried out.
The broad Mg ii line in quasars has distinct variability properties compared with broad Balmer li... more 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 Hα/Hβ gas, and responds with a lower amplitude to continuum variations. If the broad-line region (BLR) is truncated at a physical radius of ∼ 0.3 pc (for a 10 8.5 M 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 Hα/Hβ 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 Hα/Hβ, and the rare population of broad Mg ii emitters in the spectra of massive inactive galaxies.
We report the discovery of a possible close binary system of quasars in SDSS J1048+0055. The [OII... more We report the discovery of a possible close binary system of quasars in SDSS J1048+0055. The [OIII]λλ4959,5007 emission lines are clearly double-peaked, and two discrete radio sources with a projected physical separation of ∼ 20 pc are found in the VLBA milliarcsec resolution image at 8.4 GHz. Each of the [O III]λλ4959, 5007 doublets and Hβ can be well modelled by two Gaussians and the line ratio, [OIII]λ5007/Hβ ∼ 7, is typical of Seyfert 2 galaxies. No broad component of Hβ was detected and its [OIII]λ5007 luminosity, L[OIII] ≈ 9.2 × 10 42 erg s −1, is comparable to luminous quasars and is a few ten times more luminous than typical Seyfert galaxies. One natural interpretation is that SDSS J1048+0055 contains two close quasar-like nuclei and the BLR around them are obscured. Other possible models are also discussed. We suggest that double-peaked narrow emission line profile may be an effective way of selecting candidates of binary black holes with intermediate separation.
We present empirical relations between the global dust reddening and other physical galaxy proper... more We present empirical relations between the global dust reddening and other physical galaxy properties including the Hα luminosity, Hα surface brightness, metallicity and axial ratio for star-forming disc galaxies. The study is based on a large sample of ∼ 22 000 well-defined star-forming galaxies selected from the Sloan Digital Sky Survey (SDSS). The reddening parameterized by color excess E(B − V) is derived from the Balmer decrement. Besides the dependency of reddening on Hα luminosity / surface brightness and gas phase metallicity, it is also correlated with the galaxy inclination, in the sense that edge-on galaxies are more attenuated than face-on galaxies at a give intrinsic luminosity. In light of these correlations, we present the empirical formulae of E(B −V) as a function of these galaxy properties, with a scatter of only 0.07 mag. The empirical relation can be reproduced if most dust attenuation to the H ii region is due to diffuse interstellar dust distributing in a disc thicker than that of H ii regions. The empirical formulae can be incorporated into semi-analytical models of galaxy formation and evolution to estimate the dust reddening and enable comparison with observations more practically.
We report the discovery of a dwarf Seyfert 1 active galactic nucleus (AGN) with a candidate inter... more We report the discovery of a dwarf Seyfert 1 active galactic nucleus (AGN) with a candidate intermediate-mass black hole hosted by the dwarf galaxy SDSS J160531.84+174826.1 at z = 0.032. A broad component of the Hα line with FWHM=781km s −1 is detected in its optical spectrum, and a bright, point-like nucleus is evident from a HST imaging observation. Non-thermal X-ray emission is also detected from the nucleus. The black hole mass, as estimated from the luminosity and width of the broad Hα component, is about 7 ×10 4 M⊙. The host galaxy appears to be a disk galaxy with a boxy bulge or nuclear bar; with an absolute magnitude of MR = −17.8, it is among the least luminous host galaxies ever identified for a Seyfert 1.
PG 1411+442, and PG 0043+039. All three objects are found to be X-ray quiet, possibly due to subs... more PG 1411+442, and PG 0043+039. All three objects are found to be X-ray quiet, possibly due to substantial intrinsic absorption with N-H > 10(23) cm(-2) but might have intrinsically a more typical spectral energy distribution. This picture is supported by explicit spectral fits to the combined ROSAT/ASCA data of the nearby bright object PG 1411+442. An analysis of the large number of radio-quiet quasars not detected in the ROSAT All-Sky Survey indicates that perhaps only a small fraction of BAL quasars have not been recognized before and that the number of intrinsically X-ray quiet quasars must be small.
We have conducted a systematic investigation of the origin and underlying physics of the line--li... more We have conducted a systematic investigation of the origin and underlying physics of the line--line and line--continuum correlations of AGNs, particularly the Baldwin effect. Based on the homogeneous sample of Seyfert 1s and QSOs in the SDSS DR4, we find the origin of all the emission-line regularities is Eddington ratio (L/Ledd). The essential physics is that L/Ledd regulates the distributions of the properties (particularly column density) of the clouds bound in the line-emitting region.
Based on the spectroscopic and shear catalogs for SDSS galaxies in the local Universe, we compare... more 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 h−1M , 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 o...
Based on the spectroscopic and shear catalogs for SDSS galaxies in the local Universe, we compare... more Based on the spectroscopic and shear catalogs for SDSS galaxies in the local Universe, we compared optically selected active galactic nuclei (AGNs) with control star-forming and quiescent galaxies on galactic and inter-halo scales, and larger. We find that AGNs are preferentially found in two specific stages of galaxy evolution: in the starburst and ‘green valley’ phases. We also find that the stellar population of their host galaxies is quite independent of stellar mass, which is not the case for more typical galaxies. Combining galaxy-galaxy lensing and galaxy clustering on large scales, we measured the mass of AGN host halos. The typical halo mass is about 1012 h−1 M⊙, similar to the characteristic mass in the stellar mass-halo mass relation (SHMR). For a given stellar mass, AGN host galaxies and star-forming galaxies share the same SHMR, while quiescent galaxies have more massive halos. Clustering analyses on halo scales reveals that AGNs are surrounded by a larger number of sat...
Space Telescopes and Instrumentation 2020: Ultraviolet to Gamma Ray
The Hot Universe Baryon Surveyor (HUBS) mission is proposed to study "missing" baryons in the uni... more The Hot Universe Baryon Surveyor (HUBS) mission is proposed to study "missing" baryons in the universe. Unlike dark matter, baryonic matter is made of elements in the periodic table, and can be directly observed through the electromagnetic signals that it produces. Stars contain only a tiny fraction of the baryonic matter known to be present in the universe. Additional baryons are found to be in diffuse (gaseous) form, in or between galaxies, but a significant fraction has not yet been seen. The latter ("missing" baryons) are thought to be hiding in low-density warm-hot ionized medium (WHIM), based on results from theoretical studies and recent observations, and be distributed in the vicinity of galaxies (i.e., circumgalactic medium) and between galaxies (i.e., intergalactic medium). Such gas would radiate mainly in the soft X-ray band and the emission would be very weak, due to its very low density. HUBS is optimized to detect the X-ray emission from the hot baryons in the circumgalactic medium, and thus fill a void in observational astronomy. The goal is not only to detect the "missing" baryons, but to characterize their physical and chemical properties, as well as to measure their spatial distribution. The results would establish the boundary conditions for understanding galaxy evolution. Though highly challenging, detecting "missing" baryons in the intergalactic medium could be attempted, perhaps in the outskirts of galaxy clusters, and could shed significant light on the large-scale structures of the universe. The current design of HUBS will be presented, along with the status of technology development.
The optical time-domain astronomy has grown rapidly in the past decade but the dynamic infrared s... more 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 4.6µm > 10 42 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 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.
The past decade has experienced an explosive increase of optically-discovered tidal disruption ev... more 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 −1 , corresponding to a dust covering factor f c ∼ 0.01 at scale of sub-parsec. The only exception is the disputed source ASASSN-15lh, which shows an ultra-high dust luminosity (∼ 10 43.5 erg s −1) 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.
GSN 069 is an ultra-soft X-ray active galactic nucleus that previously exhibited a huge X-ray out... more 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.
The combination of the linear size from reverberation mapping (RM) and the angular distance of th... more 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., Hβ for RM and Paα for SA). Here we evaluate the size deviations measured by six pairs of hydrogen lines (e.g., Hβ, Hα and Paα) via the locally optimally emitting cloud (LOC) models of BLR. We find that the radius ratios K(=R SA /R 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 Paα(SA)/Hβ(RM) and Hα(SA)/Hβ(RM) pairs not only are closest to 1 but also have considerably smaller uncertainty. Considering the current infrared interferometry technology, the Paα(SA)/Hβ(RM) pair is the ideal choice for the low redshift objects in the SARM project. In the future, the Hα(SA)/Hβ(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.
We present an analysis of the variability of broad absorption lines (BALs) in a quasar SDSS J1419... more 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 ∆T between two observations is longer than the recombination timescale t rec , the BAL variations can be detected. This can be characterized as a "sharp rise" in the detection rate of BAL variation at ∆T = t 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 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 rec for half an order of magnitude in two individual troughs. This result indicates that there may be multiple components with different t rec but the same velocity in an individual trough.
We report the discovery of a mid-infrared (MIR) flare using WISE data in the center of the nearby... more 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 × 10 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 ∼ 10 51 erg, most of which was released in less than ∼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.
We analyze the X-ray, optical, and mid-infrared data of a “changing-look” Seyfert galaxy SDSS J15... more 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 ...
Changing look active-galatic-nuclei (CL AGNs) can yield considerable insight into accretion physi... more Changing look active-galatic-nuclei (CL AGNs) can yield considerable insight into accretion physics as well as the co-evolution of black holes and their host galaxies. A large sample of these CL AGNs is essential to achieve the latter goal. We propose an effective method to select CL candidates from spectroscopic quasar catalogs using the mid-infrared (MIR) variability information derived from ALL-WISE/NEOWISE data releases. Our primary selection criteria include both a large amplitude flux variation and a transition of MIR color from an AGN to a normal galaxy. A pilot spectroscopic followup of 7 candidates among about 300 candidates selected from Sloan Digital Sky Survey low-redshift (z<0.5) AGN sample results in 6 new turn-off CL AGNs. We expect to obtain hundreds of CL AGNs once full spectroscopic follow-up of the sample is carried out.
The broad Mg ii line in quasars has distinct variability properties compared with broad Balmer li... more 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 Hα/Hβ gas, and responds with a lower amplitude to continuum variations. If the broad-line region (BLR) is truncated at a physical radius of ∼ 0.3 pc (for a 10 8.5 M 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 Hα/Hβ 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 Hα/Hβ, and the rare population of broad Mg ii emitters in the spectra of massive inactive galaxies.
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Papers by Tinggui Wang