Dongyue Li 2018/4/27 Ref: Saxton et al. 2015 Esquej et al. 2008, 2012 Was the soft X-ray flare in NGC 3599 due to an AGN disk instability or delayed tidal disruption Dongyue Li 2018/4/27 Ref: Saxton et al. 2015 Esquej et al. 2008, 2012

Background: NGC 3599 (XMMSL1 J111527.3+180638) An early type galaxy in local universe: z=0.0028

X-ray observation Detected in 2003-11-22 with an EP-pn 0.2-12 keV count rate of 5.5 cts/s RASS: ratio of 88 (upper limit) ROSAT pointed: ratio of : 297 and 100 (upper limit) Follow-up observations Flux estimation: PIMMS typical near maximum spectrum: KT=0.07 keV, modified with Galactic absorption Unabsorbed luminosity in slew:

X-ray analysis Best fit model：

X-ray light curve (0.2-2 keV)

Optical observation (pre outburst) The long-slit optical spectrum of NGC 3599 was taken in 2000−02−05 at the Whipple Observatory with the FAST spectrograph located in the 1.5 m Tillinghast telescope

Optical observation stellar population of 2.1 Gy Intensity ratios of: [O III] 5007 Å/Hβ = 3.5(±1.0), [N II] 6584 Å/Hα = 1.1(±0.2) The dotted curve shows the demarcation between starburst galaxies and AGN defined by Kewley et al. (2001). The dashed curve shows Kauffmann et al 2003 demarcation Kauffmann et al.2003

X-ray light curve 2002-05-22: 0.2-2 keV: 19 photons in 4.0 s exposure Flare phases are marked as: quiescent (Q), rise (R; ≤107 months), plateau (P; ≥18 months) and decay (D; ∼36 months).

X-ray spectral analysis fittedwith a blackbody of kT∼45 eV plus a steep power law, with ∼2.7, both absorbed by the Galactic column (1.42 × 1020 cm−2; Kalberla et al. 2005). We fit this model, with free blackbody temperature and power-law slope

TDE candidate Classical model of TDE: fast rise, days to months -5/3 decay if the 2002-05-22 measurement lies on the rise of the luminosity curve and the 2003-11-22 measurement on the decline Rise time: 1.14 year

TDE candidate Recently, numerical simulations and new analytical work have shown that the development of a TDE light curve is dependent on when the streams of tidal debris intersect each other (Guillochon & Ramirez-Ruiz). Early interactions appear to be rare and in the majority of cases circularisation occurs late and at a large distance from the BH, 5–10 times further away than predicted by the classical model (Piran et al. 2015). This leads to a longer viscous time-scale and a rise to peak that takes months, years or even decades. For lower black hole masses (Mh 10  6), we find that flares are typically slowed down by about an order of magnitude, resulting in the majority of TDEs being sub-Eddington at peak. This also implies that current searches for TDEs are biased toward prompt flares, with slowed flares likely having been unidentified.

Evolved stars have weakly bound envelopes which may be stripped in black hole encounters without destroying the star (MacLeod, Guillochon & Ramirez-Ruiz 2012). These events, which result in light curves with a long rise to peak and a plateau phase lasting years

ULX The peak luminosity of NGC 3599, LX = 5 E41 erg/s1, falls within the range attained by ULX Stellar-mass accreting binaries and ULX have significantly harder low-state spectra

A highly variable AGN (i) an increase of factor 130 in X-ray flux within ≤9 yr; (ii) an unusually soft peak spectrum ( index∼ 4/kT ∼ 90 eV); (iii) A spectrum which remained soft while the flux dropped by a factor of 100 between 2003 and 2008 No obvious intrinsic absorption Warm absorber: poorly fitted Can not explain observation An increase in intrinsic emission A change in the disc structure filling and emptying of the inner accretion disc (2) Lightman-Eardley disc instability

(2) Lightman-Eardley (LE) disk instability IC 3599 Grupe et al. 2015 The instability begins when the radiation pressure becomes comparable with the gas pressure. A heating wave is generated at the inner edge of the disc which propagates backwards through the disc at the sound speed (cs). The heating increases the local viscosity, scaleheight of the disc and the local accretion rate (Cannizzo 1996). At this point the disc is no longer a thin Shakura–Sunyaev disc but still emits as a thermalized plasma. Rise time is longer than decay time

Summary Based on new data we have shown that it is difficult to reconcile the rise time of the soft X-ray flare seen in NGC 3599 with its previous classification as a classical, fast-rising, short plateau TDE. It does, however, fit into the emerging scheme of a disruption which has led to a late, distant, circularization where the rise is flatter and the peak lasts longer or alternatively to the tidal stripping of a red giant star. If the factor of 100 soft X-ray flare is interpreted as a change in the accretion from a persistent AGN, we find that the rise time of ≤9 yr is too short to be explained purely by the bulk motion of disc material about a 106M BH. The observed time-scales are compatible with the LE instability which boosts the thermal soft X-ray emission by heating the inner disk and raising its scaleheight. This mechanism predicts that flares will repeat on the viscous timescale of the truncation radius, i.e. every few decades. The behaviour of NGC 3599 would then be analogous to flares seen in certain Galactic binary systems, but is too rapid to be equivalent to the state changes which are seen in those systems.