A population of highly energetic transient events in the centres of active galaxies

Kankare, E.; Kotak, R.; Mattila, S.; Lundqvist, Peter; Ward, M. J.; Fraser, M.; Lawrence, Andy; Smartt, S. J.; Meikle, W. P. S.; Bruce, Alastair; Harmanen, J.; Hutton, S.; Inserra, C.; Kangas, T.; Pastorello, A.; Reynolds, T.; Romero-Cañizales, C.; Smith, K.; Valenti, S.; Chambers, K.; Hodapp, Klaus W.; Huber, M. E.; Kaiser, N.; Kudritzki, Rolf‐Peter; Magnier, E. A.; Tonry, J.; Wainscoat, R. J.; Waters, C.

doi:10.1038/s41550-017-0290-2

Cited by 78 publications

(106 citation statements)

References 43 publications

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“…This suggests a connection between all these events around AGN and AT2018fyk, which occurred in a quiescent SMBH. This strengthens the arguments in favour of a TDE interpretation for PS16dtm, the Kankare et al (2017) events and the coronal line emitters.…”

Section: Discussionsupporting

confidence: 85%

Evidence for rapid disc formation and reprocessing in the X-ray bright tidal disruption event candidate AT 2018fyk

Wevers

Pasham

Velzen

et al. 2019

Monthly Notices of the Royal Astronomical Society

147

149

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We present optical spectroscopic and Swift UVOT/XRT observations of the X-ray and UV/optical bright tidal disruption event (TDE) candidate AT 2018fyk/ASASSN-18ul discovered by ASAS-SN. The Swift lightcurve is atypical for a TDE, entering a plateau after ∼40 days of decline from peak. After 80 days the UV/optical lightcurve breaks again to decline further, while the X-ray emission becomes brighter and harder. In addition to broad H, He and potentially O/Fe lines, narrow emission lines emerge in the optical spectra during the plateau phase. We identify both high ionisation (O iii) and low ionisation (Fe ii) lines, which are visible for ∼45 days. We similarly identify Fe ii lines in optical spectra of ASASSN-15oi 330 d after discovery, indicating that a class of Fe-rich TDEs exists. The spectral similarity between AT 2018fyk, narrow-line Seyfert 1 galaxies and some extreme coronal line emitters suggests that TDEs are capable of creating similar physical conditions in the nuclei of galaxies. The Fe ii lines can be associated with the formation of a compact accretion disk, as the emergence of low ionisation emission lines requires optically thick, high density gas. Taken together with the plateau in X-ray and UV/optical luminosity this indicates that emission from the central source is efficiently reprocessed into UV/optical wavelengths. Such a twocomponent lightcurve is very similar to that seen in the TDE candidate ASASSN-15lh, and is a natural consequence of a relativistic orbital pericenter.

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Section: Discussionsupporting

confidence: 85%

Evidence for rapid disc formation and reprocessing in the X-ray bright tidal disruption event candidate AT 2018fyk

Wevers

Pasham

Velzen

et al. 2019

Monthly Notices of the Royal Astronomical Society

147

149

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“…2) and the slower nuclear event PS1-10adi (dashed grey line, plotted here to match its absolute V -band magnitudes; ref. 34). Error bars in these light-curves are 1 − σ-equivalent.…”

Section: Competing Interestsmentioning

confidence: 98%

A new class of flares from accreting supermassive black holes

et al. 2019

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Accreting supermassive black holes (SMBHs) can exhibit variable emission across the electromagnetic spectrum and over a broad range of time-scales. The variability of active galactic nuclei (AGN) in the ultra-violet (UV) and optical is usually at the few tens of percent level over time-scales of hours to weeks 1 . Recently, rare, more dramatic changes to the emission from accreting SMBHs have been observed, including tidal disruption events (TDEs) 2-5 , "changing look" AGN 6-9 , and other extreme variability objects 10,11 . The physics behind the "re-ignition", enhancement, and "shutdown" of accretion onto SMBHs is not entirely understood. Here we present a rapid increase in ultraviolet-optical emission in the centre of a nearby galaxy marking the onset of sudden increased accretion onto a SMBH. The optical spectrum of this flare, dubbed AT 2017bgt, exhibits a mix of emission features. Some are typical of luminous, unobscured AGN, but others are likely driven by Bowen fluorescence -robustly linked here, for the first time, with highvelocity gas in the vicinity of the accreting SMBH. The spectral features and increased UV flux show little evolution over a period of at least 14 months. This disfavours the tidal disruption of a star as their origin, and instead suggests a longer-term event of intensified accretion. Together with two other recently reported events with similar properties, we define a new class of SMBH-related flares. This has important implications for the classification of different types of enhanced accretion onto SMBHs.AT 2017bgt was discovered by the All Sky Automated Survey for Supernovae (ASAS-SN 12 ) as ASASSN-17cv on 2017 February 21 in the early-type galaxy 2MASX J16110570+0234002, at z=0.064 (ref. 13; see Methods §). The long-term ASAS-SN optical data show that the total emission from the galaxy brightened by a factor of ∼50% over a period of about two months, with half of the rise occurring within three weeks (see Supplementary Fig. 1). Follow-up Swift observations show that the UV emission increased by a factor of ∼75 compared to GALEX data from 2004, reaching a luminosity of νLν (NUV) 8.9×10 44 erg s −1 , and that the X-ray emission increased by a factor of ∼2−3 compared to ROSAT data from 1990 August (see 'Detection and photometric monitoring' and 'Archival multi-wavelength data' in Methods for details on all new and archival data). The archival X-ray luminosity, of L(2−10 keV) 7×10 42 erg s −1 , and the archival UV to X-ray luminosity ratio are consistent with what is commonly observed in AGN (that is, a UV-to-X-ray spectral slope of αox≈ − 1.2; see Methods §). Archival detections in the radio (from 1998; obtained by the Very Large Array) and in the mid-infrared (from 2010; obtained with the Wide-field Infrared Survey Explorer) can be accounted for by star formation in the host galaxy. Thus, AT 2017bgt experienced a dramatic increase in its UV emission, accompanied by a smaller increase in optical and X-ray emission, sometime between 2004 and 2017.The X-ray spectral energy distribution...

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“…This number is made uncertain to the degree that the distribution of stellar orbits near an AGN is systematically different from the center of an inactive galaxy, and that the black hole in a galaxy with an AGN tends to be more massive than a galaxy without one (see also Karas & Šubr 2007;Kennedy et al 2016). Because both TDEs and AGNs vary on timescales of weeks to months, and because a TDE in an AGN presents less contrast against the prior state of the system than a TDE in an inactive galaxy, deciding whether an increase in brightness is due to a TDE or is merely AGN variability is not trivial (Komossa 2015;Kankare et al 2017;Auchettl et al 2018;Trakhtenbrot et al 2019b); indeed, there are a number of cases in which the correct identification of a particular episode of variation is disputed (e.g., Campana et al 2015;Grupe et al 2015;Merloni et al 2015;Saxton et al 2015;Blanchard et al 2017;Lin et al 2017;Wyrzykowski et al 2017;Mattila et al 2018;Shu et al 2018). It is therefore of interest to see if TDEs in AGNs have distinctive observational characteristics that allow us to recognize them more reliably.…”

Section: Introductionmentioning

confidence: 99%

Tidal Disruption Events in Active Galactic Nuclei

Chan

Piran

Krolik

et al. 2019

ApJ

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A fraction of tidal disruption events (TDEs) occur in active galactic nuclei (AGNs) whose black holes possess accretion disks; these TDEs can be confused with common AGN flares. The disruption itself is unaffected by the disk, but the evolution of the bound debris stream is modified by its collision with the disk when it returns to pericenter. The outcome of the collision is largely determined by the ratio of the stream mass current to the azimuthal mass current of the disk rotating underneath the stream footprint, which in turn depends on the mass and luminosity of the AGN. To characterize TDEs in AGNs, we simulated a suite of stream-disk collisions with various mass current ratios. The collision excites shocks in the disk, leading to inflow and energy dissipation orders of magnitude above Eddington; however, much of the radiation is trapped in the inflow and advected into the black hole, so the actual bolometric luminosity may be closer to Eddington. The emergent spectrum may not be thermal, TDE-like, or AGN-like. The rapid inflow causes the disk interior to the impact point to be depleted within a fraction of the mass return time. If the stream is heavy enough to penetrate the disk, part of the outgoing material eventually hits the disk again, dissipating its kinetic energy in the second collision; another part becomes unbound, emitting synchrotron radiation as it shocks with surrounding gas.

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A population of highly energetic transient events in the centres of active galaxies

Cited by 78 publications

References 43 publications

Evidence for rapid disc formation and reprocessing in the X-ray bright tidal disruption event candidate AT 2018fyk

Evidence for rapid disc formation and reprocessing in the X-ray bright tidal disruption event candidate AT 2018fyk

A new class of flares from accreting supermassive black holes

Tidal Disruption Events in Active Galactic Nuclei

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