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Register a new userBroadband X-Ray Observations of the 2018 Outburst of the Changing-Look Active Galactic Nucleus NGC 1566
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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.
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We present the drastic transformation of the X-ray properties of the active galactic nucleus 1ES 1927+654, following a changing-look event. After the optical/UV outburst the power-law component, produced in the X-ray corona, disappeared, and the spectrum of 1ES 1927+65 instead became dominated by a blackbody component ($kTsim 80-120$ eV). This implies that the X-ray corona, ubiquitously found in AGN, was destroyed in the event. Our dense $sim 450$ day long X-ray monitoring shows that the source is extremely variable in the X-ray band. On long time scales the source varies up to $sim 4$ dex in $sim 100$ days, while on short timescales up to $sim2$ dex in $sim 8$ hours. The luminosity of the source is found to first show a strong dip down to $sim 10^{40}rm,erg,s^{-1}$, and then a constant increase in luminosity to levels exceeding the pre-outburst level $gtrsim $300 days after the optical event detection, rising up asymptotically to $sim 2times10^{44}rm,erg,s^{-1}$. As the X-ray luminosity of the source increases, the X-ray corona is recreated, and a very steep power-law component ($Gammasimeq 3$) reappears, and dominates the emission for 0.3-2 keV luminosities $gtrsim 10^{43.7}rm,erg,s^{-1}$, $sim 300$ days after the beginning of the event. We discuss possible origins of this event, and speculate that our observations could be explained by the interaction between the accretion flow and debris from a tidally disrupted star. Our results show that changing-look events can be associated with dramatic and rapid transformations of the innermost regions of accreting SMBHs.
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.
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 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.
We present two NuSTAR observations of the local Seyfert 2 active galactic nucleus (AGN) and an ultraluminous X-ray source (ULX) candidate in NGC 5643. Together with archival data from Chandra, XMM-Newton and Swift-BAT, we perform a high-quality broadband spectral analysis of the AGN over two decades in energy ($sim$0.5-100 keV). Previous X-ray observations suggested that the AGN is obscured by a Compton-thick (CT) column of obscuring gas along our line-of-sight. However, the lack of high-quality $gtrsim$ 10 keV observations, together with the presence of a nearby X-ray luminous source, NGC 5643 X-1, had left significant uncertainties in the characterization of the nuclear spectrum. NuSTAR now enables the AGN and NGC 5643 X-1 to be separately resolved above 10 keV for the first time and allows a direct measurement of the absorbing column density toward the nucleus. The new data show that the nucleus is indeed obscured by a CT column of $N_{rm{H}}$ $gtrsim$ 5 $times$ 10$^{24}$ cm$^{-2}$. The range of 2-10 keV absorption-corrected luminosity inferred from the best fitting models is $L_{2-10,rm{int}} =$ (0.8-1.7) $times$ 10$^{42}$ erg s$^{-1}$, consistent with that predicted from multiwavelength intrinsic luminosity indicators. We also study the NuSTAR data for NGC 5643 X-1, and show that it exhibits evidence for a spectral cut-off at energy, $E$ $sim$ 10 keV, similar to that seen in other ULXs observed by NuSTAR. Along with the evidence for significant X-ray luminosity variations in the 3-8 keV band from 2003-2014, our results further strengthen the ULX classification of NGC 5643 X-1.
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