The superluminous transient ASASSN-15lh as a tidal disruption event from a Kerr black hole

Abstract

3 When a star passes within the tidal radius of a supermassive black hole, it will be torn apart 1 .For a star with the mass of the Sun (M ) and a non-spinning black hole with a mass < 10 8 M , the tidal radius lies outside the black hole event horizon 2 and the disruption results in a luminous flare 3,4,5,6 . Here we report observations over a period of 10 months of a transient, hitherto interpreted 7 as a superluminous supernova 8 . Our data show that the transient rebrightened substantially in the ultraviolet and that the spectrum went through three different spectroscopic phases without ever becoming nebular. Our observations are more consistent with a tidal disruption event than a superluminous supernova because of the temperature evolution 6 , the presence of highly ionised CNO gas in the line of sight 9 and our improved localisation of the transient in the nucleus of a passive galaxy, where the presence of massive stars is highly unlikely 10,11 . While the supermassive black hole has a mass > 10 8 M 12, 13 , a star with the same mass as the Sun could be disrupted outside the event horizon if the black hole were spinning rapidly 14 . The rapid spin and high black hole mass can explain the high luminosity of this event.ASASSN-15lh was discovered by the All-Sky Automated Survey for SuperNovae (ASAS-SN) on 14 June 2015 at a redshift of z = 0.2326. Its light curve peaked at V ∼ 17 mag implying an absolute magnitude of M = −23.5 mag, more than twice as luminous as any known supernova (SN) 7 . Our long-term spectroscopic follow-up reveals that ASASSN-15lh went through three different spectroscopic phases (Figure 1). During the first phase 7 , the spectra were dominated by two broad absorption features. While these features appear similar to those observed in superluminous supernovae (SLSNe; Supplementary Figure 1), their physical origin is different. The features in 4 SLSNe are due to O II 8, 15 , but this would produce an additional strong feature at ∼4,400Å (Supplementary Figure 2) . The feature at ∼4,100Å cannot be easily identified in the tidal disruption event (TDE) framework either. Two possibilities are that it could be due to absorption of Mg II or high-velocity He II 16 . After the initial broad absorption features disappeared, the spectra of ASASSN-15lh were dominated by two emission features. A possible identification for these features is He II λλ3, 202 and 4, 686Å, which are both consistently blueshifted by ∼15,000 km s −1 ( Supplementary Figure 3). He II emission is commonly seen in optically discovered TDEs 4, 5 at different blueshifts, albeit typically at lower velocities, but it has not been seen in H-poor SLSNe.These features disappeared after day +75 (measured in rest frame from peak) and the later spectra were mostly featureless, with the exception of two emission features at ∼4,000 and 5,200Å. The spectra remained much bluer than those of SLSNe 17 for many months after the peak and never revealed nebular features, even up to day +256.A UV spectrum obtained with the HST on day +168 does not sh...

X-Rays from the Location of the Double-humped Transient ASASSN-15lh

“…Both spectral models point to a very limited amount of neutral hydrogen in the host galaxy along our line of sight, consistent with the very low N(H I) inferred by Leloudas et al (2016) from Lyα and the very strong highionization lines (N V and O VI).…”

“…This observation, together with the location of the transient-astrometrically consistent with the host-galaxy nucleus-inspired a connection between ASASSN-15lh and the tidal disruption of a star by the supermassive black hole (SMBH) in the host galaxy (Brown et al 2016b;Godoy-Rivera et al 2016;Leloudas et al 2016;Perley et al 2016). In this context ASASSN-15lh would be the most luminous TDE ever observed, associated with an SMBH with massM M 10 • 8.6…”

“…The old, massive *~Ḿ M 2 10 11 ☉ host galaxy of ASASSN-15lh, with limited star formation rate ( < -M SFR 0.3 yr ; Leaman et al 2011) as well as of envelope-stripped SLSNe, which tend to be younger star-forming systems with significantly lower stellar mass (Lunnan et al 2014(Lunnan et al , 2015Leloudas et al 2016;Perley et al 2016). This observation, together with the location of the transient-astrometrically consistent with the host-galaxy nucleus-inspired a connection between ASASSN-15lh and the tidal disruption of a star by the supermassive black hole (SMBH) in the host galaxy (Brown et al 2016b;Godoy-Rivera et al 2016;Leloudas et al 2016;Perley et al 2016).…”

“…At ∼100 days, the X-ray emission at the location of ASASSN-15lh is more luminous than GRB-SNe. However, observations obtained around the same epoch by the ATCA in Figure 3 put deep limits on the radio emission from ASASSN-15lh (Kool et al 2015;Leloudas et al 2016), and rule out the presence of powerful jets seen on-axis (most of the parameter space associated with off-axis GRB-like jets is also ruled out). Also in this case, the luminous and not strongly variable X-ray emission at the location of ASASSN-15lh, which lacks a luminous radio counterpart, seems to be more in line with observations of non-jetted TDEs (recent examples are ASASSN-14li, Miller et al 2015;Alexander et al 2016;Holoien et al 2016b;or ASASSN-15oi, Holoien et al 2016a).…”

On Hybrid Wave Functions, Tidal Forces and Black Holes

“…This statement might well be correct for a particle of negligible mass, but it cannot hold for a particle whose mass is sufficient to distort the geometry of space-time in the vicinity of the crossing point. Thus a recent study [7] reports the tidal disruption of a star that evidently approached too close for comfort to a black hole, the disruption beginning to occur at a separation greater than the radius of the event horizon, and a significant amount of the mass-energy of the disrupted star being ejected in the form of a jet extending for several light years in a direction away from the black hole. (Note that, when there is only a single jet, conservation of momentum requires a balancing amount of mass-energy to be directed unseen into the black hole core, which provides a clear avenue for growth of the black hole.)…”