The Destruction and Recreation of the X-Ray Corona in a Changing-look Active Galactic Nucleus

Abstract

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 (kT ∼ 80 − 120 eV). This implies that the X-ray corona, ubiquitously found in AGN, was destroyed in the event. Our dense ∼ 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 ∼ 4 dex in ∼ 100 days, while on short timescales up to ∼ 2 dex in ∼ 8 hours. The luminosity of the source is found to first show a strong dip down to ∼ 10 40 erg s −1 , and then a constant increase in luminosity to levels exceeding the pre-outburst level 300 days after the optical event detection, rising up asymptotically to ∼ 2 × 10 44 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 (Γ ≃ 3) reappears, and dominates the emission for 0.3-2 keV luminosities 10 43.7 erg s −1 , ∼ 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.

A status report on AGN variability

“…Things can never be that simple, however, as complex behaviour is appearing in some of the best studied CL AGN, as shown in Trakhtenbrot et al (2019) and Ricci et al (2020). They present results from the CL nucleus in 1ES 1927 + 654 which changed from a "true" Type-II to a Type-I system in the time scale of months.…”

A light echo from the warm outflow in the ULIRG F01004-2237 following a major flare in its optical continuum emission

“…In this case, it is possible that the prolonged nature of the flare-related activity is due to the disruption of the AGN accretion disk by the debris stream of the TDE, subsequent re-settling of the AGN accretion disk, as well as re-processing of the light of the TDE flare by extended AGN-related gas structures such as the outer part of the accretion disk or the circum-nuclear torus. In this context, it is interesting that some "changing-look" AGN have also been explained in terms of hybrid AGN/TDE models (Merloni et al 2015;Ricci et al 2020). However, the latter cases do not show the strong Bowen lines visible in F01004-2237 and the two similar objects.…”

NuSTAR Non-detection of a Faint Active Galactic Nucleus in an Ultraluminous Infrared Galaxy with Kpc-scale Fast Wind

“…In addition to the fading AGN scenario where the central AGN activity indicated by X-ray and MIR radiation is finally quenched, there is another possibility that the AGN in J0916a may brighten up again as shown in the "changing-look AGNs" (CLAGNs), which have strong variability by an order of magnitude and can rebrighten when their accretion states change from faint phase to bright phase in a timescale of 1-10 years (e.g., Ricci et al 2020). If J0916a has a CLAGN and its X-ray emitting corona would recover L 2−10 keV of ∼ 9 × 10 44 erg s −1 as expected from the corrected L [OIII] , then the AGN can be significantly detected by NuSTAR, e.g., S / N > 7 in 3-20 keV with an exposure of 100 ksec in a Comptonthick case (N H ∼ 10 24 cm −2 ).…”