While the possibility of low-level AGN activity in both ASASSN14ae and ASASSN14li is interesting, the flare and host properties remain most consistent with a tidal dis- ruption flare origin. Further, based on the anlysis of ASASSN15lh in (Leloudas et al., 2016) and the continuing nuclear position of the flare, it seems most likely that ASASSN15lh is also associated with a tidal disruption flare. However, its mas- sive host (and likely correspondingly massive SMBH) and extreme luminosity would likely require a rapidly spinning black hole for this interpretation to be viable. The peculiar asymmetric morphology of the centre of the host may be indicative of a post-merger host, possibly providing a further link with the tidal disruption flare interpretation given the recent discovery of a preference for E+A galaxies, a rare galaxy class that may have undergone a recent merger (Arcavi et al., 2014; French et al., 2016, 2017).
The change in the host emission in CSS100217 is evidence that the flare may have directly impacted the AGN emission. A change in the luminosity of AGN can occur in so-called changing-look AGN (e.g. Matt et al., 2003; Puccetti et al., 2007) where the observed X-ray emission can be seen to change on relatively short timescales of months to years, though these situations have for the most part been explained as absorption effects from intervening gas clouds along the line of sight. However a number of sources have also been shown to transition between Seyfert 1 and Seyfert 2 classifications, or vice versa, based on optical line emission. Such
−100 −50 0 50 100 150
Rest frame days since peak
−24.0 −23.5 −23.0 −22.5 −22.0 −21.5 −21.0 −20.5 Absolute magnitude
Figure 5.8: The lightcurves of the 5 most luminous supernovae detected to date: iPTF13ajg (R-band, cyan squares; Vreeswijk et al., 2014), SN2008es (V-band, ma- genta triangles; Miller et al., 2009), CSS121015 (V-band, green stars; Benetti et al., 2014), CSS100217 (V-band, blue triangles; Drake et al., 2011) and ASASSN15lh (V- band, red circles; Dong et al., 2016). CSS100217 and ASASSN15lh peak somewhat above the other candidates and exhibit similar durations and late time decay rates while the other flares decline at a somewhat faster rate.
situations may be explained in a similar absorption case due to patchy coverage of the torus (Elitzur, 2012), or be due to changes in accretion rate (Elitzur et al., 2014). Perhaps most interestingly for this study, it has also been suggested that such changes could be triggered by short-lived transient events, such as tidal disruption flares. In the case of SDSS J015957.64+003310.5, for example, the AGN has evolved from Type 1 to Type 1.9 within the space of ten years, a change that accompanied a drop in AGN emission of a factor 6 (LaMassa et al., 2015), though it has also been suggested that this change is the result of long-term evolution of a tidal dis- ruption event (Merloni et al., 2015). Importantly, this change in optical flux was accompanied by a corresponding drop in X-ray emission, a situation that does not appear to have occurred in the case of CSS100217 considering the apparent rise in X-ray emission at late times. Nonetheless, it is possible that the host of CSS100217 is undergoing a similar change that may become apparent in its spectrum over time, thus providing motivation for spectroscopic observations of the host over the coming years.
Strong AGN flares reminiscent of TDFs have been observed in at least one other event. One early interpretation of a flare from the known Seyfert 1.9 galaxy IC3599 was that of tidal disruption flare, in part due to its apparent decline similar to the expectedt−5/3 for TDFs (Grupe et al., 1995). Approximately 10 and 20 years later, the host appeared to display similarly evolving flares, and it was suggested that this may be evidence of repeat partial disruptions from a star on a 10 year orbit of its host’s supermassive black hole (Campana et al., 2015). However, analysis of optical emission based on CSS observations showed that optical flaring with a long duration and slow rise preceded the third X-ray flare by hundreds of days, a situation that was difficult to reconcile with the quickly rising nature of tidal disruption flares (Grupe et al., 2015). Thus it seems most likely that the flaring is as a result of AGN variability, possibly due to episodic feeding through interaction in a black hole binary, or due to disk instability. While the magnitude of the change in this case (∼0.2 mag increase in the optical emission) is considerably smaller than that observed in CSS100217, and the timescale of optical emission was longer by several hundred days, it is possible that the host has undergone a similar flare. It is therefore also possible that each may undergo quasiperiodic flaring activity as IC3599 may have done, in which case further significant flares may be expected in the future.
A further possibility, suggested by (Wyrzykowski et al., 2017), is that of a new class of tidal disruption flare that originates from within active hosts. Along with CSS100217 and ASASSN14li being likely candidates for this class examined within this work, other examples include OGLE16aaa (an optical flare with a re-
markably similar temperature evolution to both ASASSN14ae and ASASSN14li; Wyrzykowski et al., 2017), SDSS J095209.56+214313.3 (SDSS J0952+21; Komossa et al., 2008), SDSS J074820.67+471214.3 (SDSS J0748+47; Wang et al., 2011a) and IGR J12580+0134 (IGR J1258+01, a likely disruption of a super-Jupiter; Nikolajuk and Walter, 2013). While the observations of each flare are quite different, with SDSS J0952+21 and SDSS J0748+47 identified through transient coronal line emis- sion that exceeds that expected from supernovae, IGR J1258+01 detected in high energy X-rays with INTEGRAL and OGLE16aaa (and of course CSS100217 and ASASSN14li) found through the more typical optical imaging survey method, all have been identified as being most likely associated with a tidal disruption event despite residing in hosts with evidence of AGN activity. The suggestion then is that these events could represent a missing link between the more typical TDFs from quiescent hosts and a fraction of “changing-look” QSOs with transient broad emission lines and an increase in blue continuum emission (MacLeod et al., 2016). It is therefore possible that the bias towards searching for TDFs in inactive hosts could be resulting in an underestimate of the true TDF rate.
The possible detection of this new class of AGN flare and possible sub-class of TDF further complicates an already complex situation in the classification of nuclear flares. While continued observations of each host may yet shed light on their as yet uncertain origins, the possible existence of these new classes can only be confirmed in the continued search for further examples.
5.6
Summary
Here I summarise the key findings of this study:
• Both ASASSN14ae and ASASSN14li have very strong ties to the centres of their hosts, with offsets of less than∼10 pc, making their association with the
central supermassive black hole quite likely.
• While the lightcurves of ASASSN14ae and ASASSN14li are somewhat unusual, their temperature evolution, host properties and central position meas they remain strong candidate for tidal disruption flares.
• CSS100217 has exhibited a marked change in its host level emission, likely a change in accretion rate of the AGN, that occurred contemporaneously with the flare, indicating a possible link.
• Given the similarities between the events, it is conceivable that both CSS100217 and ASASSN15lh belong to a new class of AGN flare. However the lack of
obvious signs of AGN activity in the host of ASASSN15lh may make a TDF flare origin more likely. Alternatively, ASASSN14li and CSS100217 could rep- resent members of a new sub-class of TDF that fills a possible gap between inactive host TDFs and “changing-look” QSOs.