A new class of flares from accreting supermassive black holes

Abstract

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...

The Sloan Digital Sky Survey Reverberation Mapping Project: Accretion and Broad Emission Line Physics from a Hypervariable Quasar

“…Nonetheless, broad MgII and Hα emission are detected at all times. We also note that Figure 4 provides evidence for increased O III λ3133 emission in high flux states, which could be produced by Bowen fluorescence (Trakhtenbrot et al 2019b luminosity L (erg s 1 ) 57100 57600 58100 MJD Figure 3. Host galaxy subtracted, rest frame, optical to X-ray SEDs.…”

To TDE or not to TDE: the luminous transient ASASSN-18jd with TDE-like and AGN-like qualities

“…Two other rapid turn-on events, OGLE17aaj (Gromadzki et al 2019) and ASASSN-17cv/AT 2017bgt (Trakhtenbrot et al 2019), were especially interesting because of the presence of strong He ii λ4686, N iii λ4640, and N iii λ4100 emission. OGLE17aaj and ASASSN-17cv had relatively quick rises to peak brightness over a few months and had UV/optical SEDs that were well fit by T ∼ 10 4 K blackbodies.…”

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“…The Bowen fluorescence mechanism (Bowen 1934(Bowen , 1935 has been proposed and widely discussed to explain the prominent O III and N III optical emission lines observed in various and diverse astrophysical systems such as planetary nebulae (e.g., Unno 1955;Weymann & Williams 1969), X-ray binary stars (e.g., McClintock et al 1975), and Wolf-Rayet stars (e.g., Crowther 2007). It has also been proposed in the context of supermassive black hole accretion disks by Netzer et al (1985), but only recently identified in a flaring AGN for the first time (Trakhtenbrot et al 2019). Since elements heavier than H and He should be rare in these systems, the only plausible explanation for seeing strong O III and N III fluorescent lines is that these lines are excited by some large sources of energy not available to the predominant H and He.…”