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Register a new userThe Eddington ratio-dependent changing look events in NGC 2992
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We present an analysis of historical multi-wavelength emission of the Changing Look (CL) Active Galactic Nucleus (AGN) in NGC 2992, covering epochs ranging from 1978 to 2021, as well as new X-ray and optical spectra. The galaxy presents multiple Seyfert type transitions from type 2 to intermediate-type, losing and regaining its H$alpha$ BEL recurrently. In X-rays, the source shows intrinsic variability with the absorption corrected luminosity varying by a factor of $sim$ 40. We rule out tidal disruption events or variable obscuration as causes of the type transitions and show that the presence and the flux of the broad H$alpha$ emission line are directly correlated with the 2-10 keV X-ray luminosity (L$_{2-10}$): the component disappears at L$_{2-10} leq 2.6times10^{42}$ergcms, this value translates into an Eddington ratio ($lambda_{rm Edd}$) of $sim$ 1%. The $lambda_{rm Edd}$ in which the BEL transitions occur is the same as the critical value at which there should be a state transition between a radiatively inefficient accretion flow (RIAF) and a thin accretion disk, such similarity suggests that the AGN is operating at the threshold mass accretion rate between the two accretion modes. We find a correlation between the narrow Fe K$alpha$ flux and $lambda_{rm Edd}$, and an anti-correlation between full-width at half maximum of H$alpha$ BEL and $lambda_{rm Edd}$, in agreement with theoretical predictions. Two possible scenarios for type transitions are compatible with our results: either the dimming of the AGN continuum, which reduces the supply of ionising photons available to excite the gas in the Broad Line Region (BLR), or the fading of the BLR structure itself occurs as the low accretion rate is not able to sustain the required cloud flow rate in a disk-wind BLR model.
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We present a study of optical, UV and X-ray light curves of the nearby changing look active galactic nucleus in the galaxy NGC 1566 obtained with the Neil Gehrels Swift Observatory and the MASTER Global Robotic Network over the period 2007 - 2018. We also report on our optical spectroscopy at the South African Astronomical Observatory with the 1.9-m telescope on the night 2018 August 2-3. A substantial increase in X-ray flux by 1.5 orders of magnitude was observed following the brightening in the UV and optical bands during the last year. After a maximum was reached at the beginning of 2018 July the fluxes in all bands decreased with some fluctuations. The amplitude of the flux variability is strongest in the X-ray band and decreases with increasing wavelength. Low-resolution spectra reveal a dramatic strengthening of the broad emission as well as high-ionization [FeX]6374 A lines. These lines were not detected so strongly in the past published spectra. The change in the type of the optical spectrum was accompanied by a significant change in the X-ray spectrum. All these facts confirm NGC 1566 to be a changing look Seyfert galaxy.
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.
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 events (TDE) or supernova events. If BLRs have a flat geometry and NLS1s are seen face-on then we have to see CL cases only if the orientation of the BLR changes as a result of a TDE or a close encounter of a star without a TDE. If NLS1s include both high Eddington accretion rate and low-inclination AGNs then a significant fraction of NLS1s could be obscured and would not be identified as NLS1s. CL cases might happen more in such objects if dust sublimation occurs following a strong increase in the optical luminosity.
Mrk 590 was originally classified as a Seyfert 1 galaxy, but then it underwent dramatic changes: the nuclear luminosity dropped by over two orders of magnitude and the broad emission lines all but disappeared from the optical spectrum. Here we present followup observations to the original discovery and characterization of this changing look active galactic nucleus (AGN). The new Chandra and HST observations from 2014 show that Mrk 590 is awakening, changing its appearance again. While the source continues to be in a low state, its soft excess has re-emerged, though not to the previous level. The UV continuum is brighter by more than a factor of two and the broad MgII emission line is present, indicating that the ionizing continuum is also brightening. These observations suggest that the soft excess is not due to reprocessed hard X-ray emission. Instead, it is connected to the UV continuum through warm Comptonization. Variability of the Fe K-alpha emission lines suggests that the reprocessing region is within about 10 light years or 3 pc of the central source. The AGN type change is neither due to obscuration, nor due to one-way evolution from type-1 to type-2, as suggested in literature, but may be related to episodic accretion events.
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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