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Register a new userTracing the AGN/X-ray Binary Analogy with Light Curves of Individual Changing-Look AGN
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Physical models of X-ray binary outbursts can aid in understanding the origin of changing-look active galactic nuclei (AGN), if we can establish that these two black hole accretion phenomena are analogous. Previously, studies of the correlation between the UV-to-X-ray spectral index alpha_OX and Eddington ratio using single-epoch observations of changing-look AGN samples have revealed possible similarities to the spectral evolution of outbursting X-ray binaries. However, direct comparisons using multi-epoch UV/X-ray light curves of individual changing-look AGN undergoing dramatic changes in Eddington ratio have been scarce. Here, we use published Swift UV/X-ray light curves of two changing-look AGN (NGC 2617 and ZTF18aajupnt) to examine the evolution of their alpha_OX values during outburst. We show that the combination of these two changing-look AGN can trace out the predicted spectral evolution from X-ray binary outbursts, including the inversion in the evolution of alpha_OX as a function of Eddington ratio. We suggest that the spectral softening that is observed to occur below a critical Eddington ratio in both AGN and X-ray binaries is due to reprocessing of Comptonized X-ray emission by the accretion disk, based on the X-ray to UV reverberation lags previously observed in NGC 2617. Our results suggest that the physical processes causing the changing-look AGN phenomenon are similar to those in X-ray binary outbursts.
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We present simultaneous XMM-Newton and NuSTAR observations of the repeat changing-look AGN NGC 1566, which dramatically increased in brightness in the IR to X-ray bands in 2018. The broad-band X-ray spectrum was taken at the peak of the outburst and is typical of Seyfert 1 AGN. The spectrum shows a soft excess, Compton hump, warm absorption and reflection, ruling out tidal disruption as the cause of the outburst and demonstrating that a standard accretion disk can develop very rapidly. The high resolution grating spectrum reveals that the outburst has launched a ~ 500 km/s outflow, and shows photoionised emission lines from rest-frame gas. We discuss possible mechanisms for the outburst, and conclude that it is most likely caused by a disk instability.
We report the detection of high-amplitude X-ray flaring of the AGN HE 1136-2304, which is accompanied by a strong increase in the flux of the broad Balmer lines, changing its Seyfert type from almost type 2 in 1993 down to 1.5 in 2014. HE 1136-2304 was detected by the XMM-Newton slew survey at >10 times the flux it had in the ROSAT all-sky survey, and confirmed with Swift follow-up after increasing in X-ray flux by a factor of 30. Optical spectroscopy with SALT shows that the AGN has changed from a Seyfert 1.95 to a Seyfert 1.5 galaxy, with greatly increased broad line emission and an increase in blue continuum AGN flux by a factor of > 4. The X-ray spectra from XMM-Newton and NuSTAR reveal moderate intrinsic absorption and a high energy cutoff at 100 keV. We consider several different physical scenarios for a flare, such as changes in obscuring material, tidal disruption events, and an increase in the accretion rate. We find that the most likely cause of the increased flux is an increase in the accretion rate, although it could also be due to a change in obscuration.
1ES 1927+654 is a nearby active galactic nucleus (AGN) which underwent a changing-look event in early 2018, developing prominent broad Balmer lines which were absent in previous observations. We have followed up this object in the X-rays with an ongoing campaign that started in May 2018, and that includes 265 NICER (for a total of 678ks) and 14 Swift/XRT (26ks) observations, as well as three simultaneous XMM-Newton/NuSTAR (158/169 ks) exposures. In the X-rays, 1ES 1927+654 shows a behaviour unlike any previously known AGN. The source is extremely variable both in spectral shape and flux, and does not show any correlation between X-ray and UV flux on timescales of hours or weeks/months. After the outburst the power-law component almost completely disappeared, and the source showed an extremely soft continuum dominated by a blackbody component. The temperature of the blackbody increases with the luminosity, going from $kTsim 80$eV (for a 0.3--2keV luminosity of $L_{0.3-2}sim 10^{41.5}rm,erg,s^{-1}$) to $sim 200$eV (for $L_{0.3-2}sim 10^{44}rm,erg,s^{-1}$). The spectra show evidence of ionized outflows, and of a prominent feature at $sim 1$keV, which can be reproduced by a broad emission line. The unique characteristics of 1ES 1927+654 in the X-ray band suggest that it belongs to a new type of changing-look AGN. Future X-ray surveys might detect several more objects with similar properties.
We present the results of five NuSTAR observations of the type 2 active galactic nucleus (AGN) in IC 751, three of which were performed simultaneously with XMM-Newton or Swift/XRT. We find that the nuclear X-ray source underwent a clear transition from a Compton-thick ($N_{rm,H}simeq 2times 10^{24}rm,cm^{-2}$) to a Compton-thin ($N_{rm,H}simeq 4times 10^{23}rm,cm^{-2}$) state on timescales of $lesssim 3$ months, which makes IC 751 the first changing-look AGN discovered by NuSTAR. Changes of the line-of-sight column density at a $sim2sigma$ level are also found on a time-scale of $sim 48$ hours ($Delta N_{rm,H}sim 10^{23}rm,cm^{-2}$). From the lack of spectral variability on timescales of $sim 100$ ks we infer that the varying absorber is located beyond the emission-weighted average radius of the broad-line region, and could therefore be related either to the external part of the broad-line region or a clumpy molecular torus. By adopting a physical torus X-ray spectral model, we are able to disentangle the column density of the non-varying absorber ($N_{rm,H}sim 3.8times 10^{23}rm,cm^{-2}$) from that of the varying clouds [$N_{rm,H}sim(1-150)times10^{22}rm,cm^{-2}$], and to constrain that of the material responsible for the reprocessed X-ray radiation ($N_{rm,H} sim 6 times 10^{24}rm,cm^{-2}$). We find evidence of significant intrinsic X-ray variability, with the flux varying by a factor of five on timescales of a few months in the 2-10 and 10-50 keV band.
PG1535+547 is a bright NLS1 galaxy, whose high energy emission shows strong variability in shape and flux. ROSAT data classified it as soft X-ray weak QSO (objects whose X-ray-to-optical flux ratio is smaller than in typical QSO): their X-ray spectra are often characterized by highly ionized, complex absorbers and/or reflection from the disk, whose relative importance is currently debated. In both cases, the presence of such features implies that we are looking at matter located in the innermost regions of the AGN. In this paper we want to clarify the nature of the X-ray emission of PG1535+547, and constrain the physical properties of regions where the emission originates. We present new XMM observations, from which we obtained 2 spectra separated by about 1 week, that we compare with a previous XMM observation. The data support the complex and variable nature of the X-ray emission. The broad band flux increases by a factor ~2.3 in 3 years, and then decreases by a factor ~1.3 in about 1 week. In the new EPIC spectra strong absorption features at E<3keV and a complex spectral shape in the Fe line energy range are evident, coupled with a drop in the emission at higher energies. We describe all the states assuming either a warm absorber plus a relativistically blurred ionized reflection, or a two-phase warm absorber partially covering the source plus a scattered component. The variability is ascribed to the warm absorbers, that vary their physical properties on timescales of years and days. In the reflection scenario all the states require a high fraction of reflection. The strong variability in the X-ray band opposed to a more constant optical emission implies that PG1535+547 can not actually be classified as a soft X-ray weak AGN.
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