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Register a new userX-ray spectra reveal the reawakening of the repeat changing-look AGN NGC 1566
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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.
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We study the nature of the changing-look Active Galactic Nucleus NGC 1566 during its June 2018 outburst. During the outburst, the X-ray intensity of the source rises up to ~25-30 times compared to its quiescent state intensity. We perform timing and spectral analysis of the source during pre-outburst, outburst, and post-outburst epochs using semi-simultaneous observations with the XMM-Newton, Nuclear Spectroscopic Telescope Array (NuSTAR), and Neil Gehrels Swift Observatories. We calculate variance, normalized variance, and fractional rms amplitude in different energy bands to study the variability. The broad-band 0.5-70 keV spectra are fitted with phenomenological models, as well as physical models. A strong soft X-ray excess is detected in the spectra during the outburst. The soft excess emission is found to be complex and could originate in the warm Comptonizing region in the inner accretion disc. We find that the increase in the accretion rate is responsible for the sudden rise in luminosity. This is supported by the q-shape of the hardness-intensity diagram that is generally found in outbursting black hole X-ray binaries. From our analysis, we find that NGC 1566 most likely harbours a low-spinning black hole with the spin parameter a* ~ 0.2. We also discuss a scenario where the central core of NGC 1566 could be a merging supermassive black hole.
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.
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.
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.
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