潮汐瓦解事件（TDEs）可能是矮星系中大质量黑洞的重要增长渠道。理论研究认为，矮星系中观测到的活动星系核（AGNs）主要由TDE提供能量。为评估这一说法，我们对7 Ms钱德拉深场南天区（CDF-S）巡天中探测到的矮星系宿主AGN进行了光变分析，观测时间跨度约为16年。基于X-CIGALE的光谱能量分布（SED）建模，我们选取了由矮星系（恒星质量低于10^10 M_⊙）宿主AGN。我们聚焦于具有全波段探测的X射线源，最终得到78个AGN样本（0.122 ≤ z ≤ 3.515）。我们用t^{-5/3}的经典TDE模型和常数模型拟合X射线光变曲线。若前者在拟合质量上优于后者，则将该源视为潜在TDE。我们识别出五个潜在TDE，仅占样本的一小部分。利用对模拟光变曲线拟合模型得到的真阳性率和假阳性率，我们进行贝叶斯分析，以获得样本中TDE比例的后验分布。后验分布峰值接近于零（2.56%），我们得到2-σ上限为9.80%。因此，我们的结果表明，矮星系中观测到的AGN并非主要由TDE提供能量。
英文摘要：
Tidal disruption events (TDEs) could be an important growth channel for massive black holes in dwarf galaxies. Theoretical work suggests that the observed active galactic nuclei (AGNs) in dwarf galaxies are predominantly TDE-powered. To assess this claim, we perform variability analyses on the dwarf-hosted AGNs detected in the $7$ Ms Chandra Deep Field-South (CDF-S) survey, with observations spanning $\approx 16$ years. Based on the spectral energy distribution (SED) modeling with X-CIGALE, we select AGNs hosted by dwarf galaxies (stellar mass below $10^{10}\ M_\odot$). We focus on X-ray sources with full-band detections, leading to a sample of $78$ AGNs (0.122 $\leq$ $z$ $\leq$ 3.515). We fit the X-ray light curves with a canonical TDE model of $t^{-5/3}$ and a constant model. If the former outperforms the latter in fitting quality for a source, we consider the source as a potential TDE. We identify five potential TDEs, constituting a small fraction of our sample. Using true- and false-positive rates obtained from fitting models to simulated light curves, we perform Bayesian analysis to obtain the posterior of the TDE fraction for our sample. The posterior peaks close to zero ($2.56\%$), and we obtain a $2$-$σ$ upper limit of $9.80\%$. Therefore, our result indicates that the observed AGNs in dwarf galaxies are not predominantly powered by TDEs.
Tidal disruption events (TDEs) in active galactic nuclei (AGNs) have been overlooked for a long time but tentatively been investigated recently. We report the discovery of a long-lasting luminous mid-infrared (mid-IR) flare in PS1-10adi, which is a newly-identified highly energetic transient event occurred in AGN. The IR luminosity of PS1-10adi, as well as other analogous events, are at least one order of magnitude higher than all known supernova, but can be well interpreted as the dust echoes of TDEs, whose ultra-high IR energy is reprocessed from the dusty torus around the black hole. The to…
We explore the contributions of Tidal Disruption Events (TDEs) to the flares in Active Galactic Nuclei (AGN) at high redshifts. Using the latest data available from X-ray and optical observations of high-redshift galaxies, in combination with the evolution of their central supermassive black holes, we calculate the contribution of TDE to AGNs as a function of their luminosities. We find that at low redshifts ($z < 1$), a few percent of all AGN with bolometric luminosities $L_{\rm Bol} \lesssim 10^{44}$ erg s$^{-1}$ may be attributable to possible TDEs. However, this fraction can significantly …
We present results of spectroscopic monitoring observations of the Ultra-Luminous Infra Red Galaxy F01004-2237. This galaxy was observed to undergo changes in its optical spectrum, detected by comparing a spectrum from 2015 with one from 2000. These changes were coincident with photometric brightening. The main changes detected in the optical spectrum are enhanced He II $λ$4686 emission and the appearance of He I $λ$3898,$λ$5876 emission lines. The favoured interpretation of these changes was that of a tidal disruption event (TDE) happening in 2010. However, subsequent work suggested that thes…
The massive black hole (BH) population in dwarf galaxies ($M_{\rm BH} \lesssim 10^5~M_\odot$) can provide strong constraints on the origin of BH seeds. However, traditional optical searches for active galactic nuclei (AGNs) only reliably detect high-accretion, relatively high-mass BHs in dwarf galaxies with low amounts of star formation, leaving a large portion of the overall BH population in dwarf galaxies relatively unexplored. Here, we present a sample of 81 dwarf galaxies ($M_\star \le 3 \times 10^9~M_\odot$) with detectable [Fe X]$λ$6374 coronal line emission indicative of accretion onto …
Close encounters between stellar-mass black holes (BHs) and stars occur frequently in dense star clusters and in the disks of active galactic nuclei (AGNs). Recent studies have shown that in highly eccentric close encounters, the star can be tidally disrupted by the BH (micro-tidal disruption event, or micro-TDE), resulting in rapid mass accretion and possibly bright electromagnetic signatures. Here we consider a scenario in which the star might approach the stellar-mass BH in a gradual, nearly circular inspiral, under the influence of dynamical friction on a circum-binary gas disk or three-bo…
UV and optically selected candidates for stellar tidal disruption events (TDE) often exhibit broad spectral features (HeII emission, H$α$ emission, or absorption lines) on a blackbody-like continuum (1e4K<T<1e5K). The lines presumably emit from TDE debris or circumnuclear clouds photoionized by the flare. Line velocities however are much lower than expected from a stellar disruption by supermassive black hole (SMBH), and are somewhat faster than expected for the broad line region (BLR) clouds of a persistently active galactic nucleus (AGN). The distinctive spectral states are not strongly rela…
We study the stellar dynamics of the first star clusters after intermediate-mass black holes (IMBHs) are formed via runaway stellar collisions. We use the outputs of cosmological simulations of Sakurai et al. (2017) to follow the star cluster evolution in a live dark matter (DM) halo. Mass segregation within a cluster promotes massive stars to be captured by the central IMBH occasionally, causing tidal disruption events (TDEs). We find that the TDE rate scales with the IMBH mass as $\dot{N}_{\rm TDE}\sim0.3\,{\rm Myr}^{-1}(M_{\rm IMBH}/1000\,{\rm M}_{\odot})^2$. The DM component affects the st…
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 (>$3σ$) varia…
Two major challenges to unification schemes for active galactic nuclei (AGN) are the existence of Narrow-Line Seyfert 1s (NLS1s) and the existence of changing-look (CL) AGNs. AGNs can drastically change their spectral appearance in the optical (changing their Seyfert type) and/or in the X-ray region. We illustrate the CL phenomenon with our multi-wavelength monitoring of NGC 2617 and discuss its properties compared with NLS1s. There are few examples of CL NLS1s and the changes are mostly only in the X-ray region. It has been proposed that some of these could be cases of a tidal-disruption even…
AT2019aalc: An obscured tidal disruption event candidate in an active galactic nucleus revealed by its first flare?
College of Physics and Electronic Engineering, Northwest Normal University（西北师范大学物理与电子工程学院）；School of Physical Science and Technology, Guangxi University（广西大学物理科学与技术学院）
Size measurements and characterization of the astrophysical properties of multiple-component radio-AGNs in the ROGUE I catalog
Astronomical Observatory of the Jagiellonian University（雅盖隆大学天文台）；National Center for Nuclear Research（国家核研究中心）；Wits Centre for Astrophysics, School of Physics, University of the Witwatersrand（威特沃特斯兰德大学物理学院天体物理中心）；Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University（马萨里克大学理学院理论物理与天体物理学系）；LUX, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS（巴黎天文台，PSL大学，索邦大学，法国国家科学研究中心）；Departamento de Fìsica–CFM, Universidade Federal de Santa Catarina（圣卡塔琳娜联邦大学物理系）；Department of Physics, Indian Institute of Technology Madras（印度金奈理工学院物理系）
Radio Activity Across Accretion State Changes in Changing-look AGNs: Insights from FIRST and VLASS over Two Decades
School of Physics and Technology, Nanjing Normal University（南京师范大学物理科学与技术学院）；Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台光学天文重点实验室）；European Southern Observatory（欧洲南方天文台）；Shanghai Astronomical Observatory, Chinese Academy of Sciences（中国科学院上海天文台）；School of Astronomy and Space Science, Nanjing University（南京大学天文与空间科学学院）；Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education（教育部现代天文与天体物理学重点实验室（南京大学））
We study the properties of the host galaxies of Changing-Look Active Galactic Nuclei (CL AGNs) with the aim of understanding the conditions responsible for triggering CL activity. We find that CL AGN hosts primarily reside in the so-called green valley that is located between spiral-like star-forming galaxies and dead ellipticals, implying that CL AGNs are activated during distinct periods of quenching and galaxy transformation processes. CL AGN hosts have low galaxy asymmetry indicators, suggesting that secular evolutionary processes (the influence of bars and spirals, and possibly minor merg…
Understanding the origin of \feii\ emission is important because it is crucial to construct the main sequence of Active Galactic Nuclei (AGNs). Despite several decades of observational and theoretical effort, the location of the optical iron emitting region and the mechanism responsible for the positive correlation between the \feii\ strength and the black hole accretion rate remain open questions as yet. In this letter, we report the optical \feii\ response to the central outburst in PS1-10adi, a candidate tidal disruption event (TDE) taking place in an AGN at $z = 0.203$ that has aroused ext…
Most massive galaxies contain a supermassive black hole (SMBH) at their center. When galaxies merge, their SMBHs sink to the center of the new galaxy where they are thought to eventually merge. During this process an SMBH binary is formed. The presence of two sets of broad emission lines in the optical spectrum of an active galactic nucleus (AGN) has been interpreted as evidence for two broad line regions (BLR), one surrounding each SMBH in a binary. We modeled the broad Balmer emission lines in SDSS spectra of 373 extreme variability AGNs using one broad and several narrow Gaussian components…
A High Incidence of Mid-infrared Variability in Local Ultraluminous Infrared Galaxies
National Astronomical Observatory of Japan（日本国立天文台）；The Graduate University for Advanced Studies, SOKENDAI（综合研究大学院大学）；Kitami Institute of Technology（北见工业大学）；Waseda University（早稻田大学）；Institute for Cosmic Ray Research, The University of Tokyo（东京大学宇宙射线研究所）；The University of Tokyo（东京大学）
Tidal disruption events (TDEs) occurred in active galactic nuclei (AGNs) are a special class of sources with outstanding scientific significance. TDEs can generate ultrafast winds, which should almost inevitably collide with the preexisting AGN dusty tori. We perform analytical calculations and simulations on the wind-torus interactions and find such a process can generate considerable X-ray afterglow radiation several years or decades later after the TDE outburst. This provides a new origin for the years delayed X-rays in TDEs. The X-ray luminosity can reach 10^{41-42} erg/s, and the light cu…
Infrared echo has proven to be an effective means to discover transient accretion events of supermassive black holes (SMBHs), such as tidal disruption events (TDEs) and changing-look active galactic nuclei (AGNs), in dusty circumnuclear environments. To explore the dusty populations of SMBH transient events, we have constructed a large sample of Mid-infrared Outbursts in Nearby Galaxies (MIRONG) and performed multiwavelength observations. Here we present the results of multiepoch spectroscopic follow-up observations of a subsample of 54 objects spanning a time scale of 4 yr. Emission-line vari…
We present an entirely new sample of 388 low-mass galaxies ($M_\star \leq 10^{10} M_\odot$) that have spectroscopic signatures indicating the presence of massive black holes (BHs) in the form of active galactic nuclei (AGNs) or tidal disruption events (TDEs). Of these, 70 have stellar masses in the dwarf galaxy regime with $10^8 \lesssim M_\star/M_\odot \lesssim 10^{9.5}$. We identify the active galaxies by analyzing optical spectra of a parent sample of $\sim$23,000 low-mass emission-line galaxies in the Galaxy and Mass Assembly (GAMA) Survey Data Release 4, and employing four different diagn…
Back from the dead: AT2019aalc as a candidate repeating TDE in an AGN
Ruhr Astroparticle and Plasma Physics Center (RAPP Center)（鲁尔天体物理与等离子体物理中心）；Leiden Observatory, Leiden University（莱顿天文台，莱顿大学）；Deutsches Elektronen-Synchrotron (DESY)（德国电子同步辐射中心）；Institut fur Physik, Humboldt-Universität zu Berlin（柏林洪堡大学物理研究所）；Division of Physics, Mathematics, and Astronomy, California Institute of Technology（加州理工学院物理、数学和天文学系）；University of Maryland, Department of Astronomy（马里兰大学天文学系）；NASA Goddard Space Flight Center, Astrophysics Science Division（美国宇航局戈达德太空飞行中心天体物理学科学部）；Center for Research and Exploration in Space Science and Technology, NASA/GSFC（NASA/GSFC空间科学与技术研究探索中心）
The optical time-domain astronomy has grown rapidly in the past decade but the dynamic infrared sky is rarely explored. Aiming to construct a sample of mid-infrared outburst in nearby galaxies (MIRONG), we have conducted a systematical search of low-redshift ($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%) h…
