The nature of the rare "Changing-look" (CL) phenomenon in active galactic nuclei (AGNs) is still under debate at current stage. We here present \it Swift/\rm XRT and UVOT follow-up observations of UGC\,3223, a newly discovered repeat CL-AGN with type transitions of $\mathrm{S1.5\rightarrow S2 \rightarrow S1.8}$ occurring in a period of about 30 years. By comparing the values previously reported in the \it ROSAT \rm All-sky Survey and in the second Swift-XRT Point Source catalog, we clearly find that the X-ray flux tightly follows the optical spectral transition, in which a spectral type closer to a Seyfert 1 type is associated with a higher X-ray flux. An invariable X-ray spectral shape is, however, found in the CL phenomenon of the object. An extremely low Eddington ratio of $\sim2\times10^{-4}$ can be obtained from the X-ray luminosity for its Seyfert 2 state, which suggests a favor of the disk-wind broad-line region model in explaining the CL phenomenon. A variation of the total UV emission is not revealed when compared to the previous \it GALEX \rm NUV observation, since the UVOT images indicate that $\sim90$\% UV emission comes from the intensive star formation in the host galaxy.
Changing-look phenomenon observed now in a growing number of active galaxies challenges our understanding of the accretion process close to a black hole. We propose a simple explanation for periodic outbursts in sources operating at a few per cent of the Eddington limit. The mechanism is based on two relatively well understood phenomena: radiation pressure instability and formation of the inner optically thin Advection-Dominated Accretion Flow. The limit cycle behaviour takes place in a relatively narrow transition zone between the standard disk and optically thin flow. Large changes in the co…
Changing-look phenomenon observed now in a growing number of active galaxies challenges our understanding of the accretion process close to a black hole. We propose a simple explanation for the sources where multiple semi-periodic outbursts are observed, and the sources are operating at a few per cent of the Eddington limit. The outburst are caused by the radiation pressure instability operating in the narrow ring between the standard gas-dominated outer disk and the hot optically thin inner Advection-Dominated Accretion Flow. The corresponding limit cycle is responsible for periodic outbursts…
The discovery of spectral type transition of active galactic nuclei (AGNs), the so-called changing-look(CL) phenomenon, challenges the widely accepted AGN paradigm, not only in the orientation based Unified Model, but also in the standard disk model. In past decades, only a couple of nearby repeat "changing-look" active galactic nuclei (CL-AGNs) have been identified. Here we report spectroscopic observations of UGC 3223 over the course of 18 years, from 2001 onwards. Combining the spectrum taken in 1987 by Stirpe, we have witnessed its type transitions from $1.5\rightarrow2.0\rightarrow1.8$ ov…
Apart from regular, low-level stochastic variability, some AGN occasionally show exceptionally large changes in the luminosity, spectral shape, and/or X-ray absorption. The most notable are the changes of the spectral type when the source classified as a Seyfert 1 becomes a Seyfert 2 galaxy or vice versa. Thus a name was coined of 'Changing-Look AGN' (CL AGN). The origin of this phenomenon is still unknown, but for most of the sources, there are strong arguments in favor of the intrinsic changes. Understanding the nature of such rapid changes is a challenge to the models of black hole accretio…
OQ~208: A New Fe~II Changing-look Active Galactic Nucleus and Implications for the Nature of the Changing-look Phenomenon
National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；Department of Astronomy, University of California, Berkeley（加州大学伯克利分校天文学系）；School of Astronomy and Space Science, University of Chinese Academy of Sciences（中国科学院大学天文与空间科学学院）；Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University（广西大学物理科学与技术学院相对论天体物理重点实验室）
The Changing-look Phenomenon Accompanied by an Accretion Mode Transition in NGC 3786
Kavli Institute for Astronomy and Astrophysics, Peking University（北京大学科维理天文与天体物理研究所）；Department of Astronomy, Yonsei University（延世大学天文学系）；Department of Astronomy, School of Physics, Peking University（北京大学物理学院天文学系）；Department of Earth Sciences, Pusan National University（釜山国立大学地球科学系）；Department of Astronomy and Atmospheric Sciences, Kyungpook National University（庆北国立大学天文学与大气科学系）
The uncertain origin of the recently-discovered `changing-looking' quasar phenomenon -- in which a luminous quasar dims significantly to a quiescent state in repeat spectroscopy over ~10 year timescales -- may present unexpected challenges to our understanding of quasar accretion. To better understand this phenomenon, we take a first step to building a sample of changing-look quasars with a systematic but simple archival search for these objects in the Sloan Digital Sky Survey Data Release 12. By leveraging the >10 year baselines for objects with repeat spectroscopy, we uncover two new changin…
"Changing-look" quasars are a new class of highly variable active galactic nuclei that have changed their spectral type over surprisingly short timescales of just a few years. The origin of this phenomenon is debated, but is likely to reflect some change in the accretion flow. To investigate the disk-corona systems in these objects, we measure optical/UV-X-ray spectral indices ($α_{\rm OX}$) and Eddington ratios ($λ_{\rm Edd}$) of ten previously-discovered changing-look quasars at two or more epochs. By comparing these data with simulated results based on the behavior of X-ray binaries, we fin…
NGC 1566 is a changing look AGN known to exhibit recurrent X-ray outbursts with each lasting for several years. The most recent X-ray outburst is observed on 2018, with a substantial increase of 2--10 keV flux by a factor of ~24 than the historical minimum. We re-analyze the XMM-Newton and NuSTAR observations covering the pre-outburst, outburst and post-outburst epochs, and confirm the discovery of the broad feature in the ~5--7 keV band during the period of outburst that could be interpreted as a relativistic Fe K_alpha emission line. Our analysis suggests that its flux has increased in tande…
The Seyfert-1 galaxy NGC 3516 has undergone major spectral changes in recent years. In 2017 we obtained Chandra, NuSTAR, and Swift observations during its new low-flux state. Using these observations we model the spectral energy distribution (SED) and the intrinsic X-ray absorption, and compare the results with those from historical observations taken in 2006. We thereby investigate the effects of the changing-look phenomenon on the accretion-powered radiation and the ionized outflows. Compared to its normal high-flux state in 2006, the intrinsic bolometric luminosity of NGC 3516 was lower by …
The widely accepted active galactic nucleus (AGN) paradigm has been recently challenged by the discovery of the so-called ``changing-look'' (CL) phenomenon characterized by spectral-type transitions. By comparing the SDSS-V and SDSS DR16 spectroscopic datasets, here we report the identification of 14 new CL-AGNs (redshift $z<0.5$) exhibiting spectral-type changes on a timescale of $\sim 10$yr. Follow-up spectroscopy was conducted with the Lick Shane 3m and Keck 10m telescopes for three of the objects. Detailed analysis of these spectra enables us to arrive at the following two main results. (1…
Radio and X-ray flux rebrightening six years after outburst in a partially-obscured extreme changing-look AGN
Anhui Normal University（安徽师范大学）；University of Science and Technology of China（中国科学技术大学）；Nanjing Normal University（南京师范大学）；National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）
Changing-look active galactic nuclei (CL-AGNs) as a new subpopulation challenge some fundamental physics of AGNs because the timescales of the phenomenon can hardly be reconciled with accretion disk models. In this Letter{\textit{}}, we demonstrate the extreme case: close binaries of supermassive black holes (CB-SMBHs) with high eccentricities are able to trigger the CL transition through one orbit. In this scenario, binary black holes build up their own mini-disks by peeling gas off the inner edges of the circumbinary disk during the apastron phase, after which they tidally interact with the …
Polarization properties of changing-look active galactic nuclei: NGC 1365 and NGC 2992
Foundation for Research and Technology-Hellas (FORTH)（希腊研究与技术基金会）；Instituto de Astrofisica de Canarias（加那利群岛天体物理研究所）；Universidad de La Laguna（拉古纳大学）；National Astronomical Observatory of Japan（日本国立天文台）
The 180-day Space Telescope and Optical Reverberation Mapping campaign on NGC 5548 discovered an anomalous period, the broad-line region (BLR) holiday, in which the emission lines decorrelated from the continuum variations. This is important since the correlation between the continuum-flux variations and the emission-line response is the basic assumption for black hole (BH) mass determinations through reverberation mapping. During the BLR holiday, the high-ionization intrinsic absorption lines also decorrelated from the continuum as a result of variable covering factor of the line of sight (LO…
