An ultraviolet–optical flare from the tidal disruption of a helium-rich stellar core

Abstract

The flare of radiation from the tidal disruption and accretion of a star can be used as a marker for supermassive black holes that otherwise lie dormant and undetected in the centres of distant galaxies 1 . Previous candidate flares 2-6 have had declining light curves in good agreement with expectations, but with poor constraints on the time of disruption and the type of star disrupted, because the rising emission was not observed. Recently, two 'relativistic' candidate tidal disruption events were discovered, each of whose extreme X-ray luminosity and synchrotron radio emission were interpreted as the onset of emission from a relativistic jet 7-10 . Here we report the discovery of a luminous ultraviolet-optical flare from the nuclear region of an inactive galaxy at a redshift of 0.1696. The observed continuum is cooler than expected for a simple accreting debris disk, but the well-sampled rise and decline of its light curve follows the predicted mass accretion rate, and can be modelled to determine the time of disruption to an accuracy of two days. The black hole has a mass of about 2 million solar masses, modulo a factor dependent on the mass and radius of the star disrupted. On the basis of the spectroscopic signature of ionized helium from the unbound debris, we determine that the disrupted star was a helium-rich stellar core.When the pericenter of a star's orbit (R p ) passes within the tidal disruption radius of a massive black hole, R T ≈ R ⋆ (M BH /M ⋆ ) 1/3 , tidal forces overcome the binding energy of the 1 star, which breaks up with roughly half of the stellar debris remaining bound to the black hole and the rest being ejected at high velocity 1 . For black holes above a critical mass,, the star becomes trapped within the event horizon of the black hole before being disrupted. The mass accretion rate (Ṁ ) in a tidal disruption event (TDE) can be calculated directly from the orbital return-times of the bound debris 1,11,12 . For the simplest case of a star of uniform density this yields,Ṁ = 2 3 ( f M⋆ t min )( t t min ) −5/3 , where f is the fraction of the star accreted and t min is the orbital period of the most tightly bound debris and, therefore, the time delay between the time of disruption and the start of the flare, which scales asThe radiative output of the accreted debris is less certain, and depends on the ratio of the accretion rate to the Eddington rate 13 . Table 2). No source is detected in a deep coadd of all the TDS epochs in 2009, with a 3σ upper limit of > 25.6 mag implying a peak amplitude of variability in the NUV of > 6.4 mag. See the Supplementary Information for details on the PS1 and GALEX photometry. PS1-10jh is coincident with the centre of a galaxy within the 3σ positional uncertainty (0.036 arcsec; Supplementary Information) with rest-frame u, g, r, i, and z photometry from SDSS 16 and K photometry from UKIDSS 17 fitted with a galaxy template 18 with M stars = (3.6 ± 0.2) × 10 9 M ⊙ and M r = −18.7 mag, where M stars is the galaxy stellar mass and M r is the absolute r-band...

XMMSL1 J074008.2-853927: a tidal disruption event with thermal and non-thermal components

“…This is in common with many TDE where the UV emission appears to be the high-energy part of a separate, lower-temperature, optical component (Gezari et al 2012;Holoien et al 2016a,b).…”

“…This radiation signature has been observed in the soft X-ray Bade et al 1996;Komossa & Greiner 1999b;Esquej et al 2007;Saxton et al 2012;Maksym et al 2010;Lin et al 2015) and UV (Gezari et al 2006(Gezari et al , 2008(Gezari et al , 2009 bands. Optical flares have also been detected (van Velzen et al 2011;Cenko et al 2012;Gezari et al 2012;Arcavi et al 2014) from candidate TDE, with an apparent temperature of a few ×10 4 K, cooler than that expected from an accretion disc (e.g. Bonning et al 2007).…”

Abundance anomalies in tidal disruption events

“…Arcavi et al (2014) noted that TDEs show a remarkable diversity in the relative strengths of the hydrogen Hα/Hβ Balmer lines and the HeII 4686Å emission line. In the most extreme cases, the broad HeII emission lines are significantly stronger than the Balmer lines (PS1-10jh, Gezari et al (2012); PTF09ge, Kasliwal et al (2009), , while others show both strong HeII and Balmer emission lines (SDSS J074820.66+471214.6, Wang et al (2012); ASASSN-14ae, Holoien et al (2014); ASASSN-14li, Holoien et al (2016)). There are also some that appear hydrogen dominated (PTF09djl, PTF09axc, Arcavi et al (2014);TDE2, van Velzen et al (2011), and possibly PS1-11af, Chornock et al (2014)).…”

“…Debates over the origin of strong HeII emission in TDEs have mainly focused on PS10jh, where Gezari et al (2012) proposed that the spectrum, with strong HeII 4686Å and undetected H Balmer emission ( > ∼ 5:1 ratio), could only be explained by disrupting a helium star. Guillochon et al (2014), based on the quasar broad line region models of Korista & Goad (2004), argued that there were parameter ranges that could almost reproduce the observed line ratios at Solar abundance.…”

Understanding extreme quasar optical variability with CRTS – I. Major AGN flares

“…Subsequent papers will deal with other phenomena exhibited by extreme variable quasars (such as discussed above). Although there have been several reports of significant optical/UV outbursts in quiescent galaxies, consistent with superluminous supernovae or candidate tidal disruption events (TDEs) (Gezari et al 2012;Chornock et al 2014;Liu et al 2014), those associated with active galaxies are much rarer as it can be difficult to distinguish a single significant event from more general variability. They have the potential, however, to inform about the structure and mechanics of the accretion disc and nuclear region.…”