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Register a new userMechanism of the Changing Look phenomenon in Active Galactic Nuclei
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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 cold disk are due to the irradiation by the hot flow with accretion rate strongly varying during the cycle. The model gives quantitative predictions and works well for multiple outbursts of NGC 1566.
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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 mergers) might be the primary mechanism for transporting gas to the vicinity of the supermassive black hole (SMBH) rather than major mergers. Similar to tidal disruption events (TDEs) and highly variable AGNs, we find that CL AGN hosts are associated with SMBHs residing in high density pseudo-bulges and appear to overlap most significantly with the population of low-ionization nuclear emission-line region (LINER) galaxies. As such, CL AGN are likely fueled by strong episodic bursts of accretion activity, which appear to take place preferentially as the amount of material accessible for star formation and accretion dwindles. We also identify that CL AGN hosts are characterized by either large Sersic indices or high bulge fractions, which suggests a simple metric for identifying candidates for spectroscopic follow-up observations in forthcoming synoptic surveys.
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 disks during the periastron phase to efficiently exchange angular momentum within one orbital period. For mini-disks rotating retrograde to the orbit, the tidal torque rapidly squeezes the tidal parts of the mini-disks into a much smaller radius, which rapidly results in higher accretion and short flares before the disks decline into type-2 AGNs. Prograde-rotation mini-disks gain angular momentum from the binary and rotate outward, which causes a rapid turn-off from type-1 to type-2. Turn-on occurs around the apastron phase. CB-SMBHs control cycle transitions between type-1 and type-2 with orbital periods but allow diverse properties in CL-AGN light curves.
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 $mathrm{S1.5rightarrow S2 rightarrow S1.8}$ 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 $sim2times10^{-4}$ 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.
The changes of broad emission lines should be a crucial issue to understanding the physical properties of changing-look active galactic nucleus (CL-AGN). Here, we present the results of an intensive and homogeneous 6-month long reverberation mapping (RM) monitoring campaign during a low-activity state of the CL-AGN Seyfert galaxy NGC 3516. Photometric and spectroscopic monitoring was carried out during 2018--2019 with the Lijiang 2.4 m telescope. The sampling is 2 days in most nights, and the average sampling is $sim$3 days. The rest frame time lags of H$alpha$ and H$beta$ are $tau_{rm{H}alpha}=7.56^{+4.42}_{-2.10}$ days and $tau_{rm{H}beta}=7.50^{+2.05}_{-0.77}$ days, respectively. From a RMS H$beta$ line dispersion of $sigma_{rm{line}} = 1713.3 pm 46.7$ $rm{km}$ $rm{s^{-1}}$ and a virial factor of $f_{sigma}$ = 5.5, the central black hole mass of NGC 3516 is estimated to be $M_{rm{BH}}= 2.4^{+0.7}_{-0.3} times 10^{7} M_{odot}$, which is in agreement with previous estimates. The velocity-resolved delays show that the time lags increase towards negative velocity for both H$alpha$ and H$beta$. The velocity-resolved RM of H$alpha$ is done for the first time. These RM results are consistent with other observations before the spectral type change, indicating a basically constant BLR structure during the changing-look process. The CL model of changes of accretion rate seems to be favored by long-term H$beta$ variability and RM observations of NGC 3516.
We here report a spectroscopic identification of two new changing-look AGNs (CL-AGNs): SDSS,J104705.16+544405.8 and SDSS,J120447.91+170256.8 both with a turn-off type transition from type 1 to type 1.8/1.9. The identification is arrived by a follow-up spectroscopic observation of the five changing-look AGN (CL-AGN) candidates that are extracted from the sample recently released in Macleod et al. The candidates are extract by the authors from the Sloan Digit Sky Survey Data Release 7 spectroscopically confirmed quasars with large amplitude variability. By compiling a sample of 26 previously identified CL-AGNs, we confirm the claim in Macleod et al. that CL-AGNs tend to be biased against low Eddington ratio, and identify an overlap between the CL-AGNs at their dim state and the so-called intermediate-type AGNs. The overlap implies that there two populations of the intermediate-type AGNs with different origins. One is due to the torus orientation effect, and the another the intrinsic change of the accretion rate of the central supermassive blackholes.
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