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Register a new userLate-time UV observations of tidal disruption flares reveal unobscured, compact accretion disks
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The origin of thermal optical and UV emission from stellar tidal disruption flares (TDFs) remains an open question. We present Hubble Space Telescope far-UV (FUV) observations of eight optical/UV selected TDFs 5-10 years post-peak. Six sources are cleanly detected, showing point-like FUV emission from the centers of their host galaxies. We discover that the light curves of TDFs from low-mass black holes ($<10^{6.5} M_odot$) show significant late-time flattening. Conversely, FUV light curves from high-mass black hole TDFs are generally consistent with an extrapolation from the early-time light curve. The observed late-time emission cannot be explained by existing models for early-time TDF light curves (i.e. reprocessing or circularization shocks), but is instead consistent with a viscously spreading, unobscured accretion disk. These disk models can only reproduce the observed FUV luminosities, however, if they are assumed to be thermally and viscously stable, in contrast to the simplest predictions of alpha-disk theory. For one TDF in our sample, we measure an upper limit to the UV luminosity that is significantly lower than expectations from theoretical modeling and an extrapolation of the early-time light curve. This dearth of late-time emission could be due to a disk instability/state change absent in the rest of the sample. The disk models that explain the late-time UV detections solve the TDF missing energy problem by radiating a rest-mass energy of ~0.1 solar mass over a period of decades, primarily in extreme UV wavelengths.
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We propose a model to explain the time delay between the peak of the optical and X-ray luminosity, dt hereafter, in UV/optically-selected tidal disruption events (TDEs). The following picture explains the observed dt in several TDEs as a consequence of the circularization and disk accretion processes as long as the sub-Eddington accretion. At the beginning of the circularization, the fallback debris is thermalized by the self-crossing shock caused by relativistic precession, providing the peak optical emission. During the circularization process, the mass fallback rate decreases with time to form a ring around the supermassive black hole (SMBH). The formation timescale corresponds to the circularization timescale of the most tightly bound debris, which is less than a year to several decades, depending mostly on the penetration factor, the circularization efficiency, and the black hole mass. The ring will subsequently evolve viscously over the viscous diffusion time. We find that it accretes onto the SMBH on a fraction of the viscous timescale, which is $2$ years for given typical parameters, leading to X-ray emission at late times. The resultant dt,is given by the sum of the circularization timescale and the accretion timescale and significantly decreases with increasing penetration factor to several to $sim10$ years typically. Since the X-ray luminosity substantially decreases as the viewing angle between the normal to the disk plane and line-of-sight increases from $0^circ$ to $90^circ$, a low late-time X-ray luminosity can be explained by an edge-on view. We also discuss the super-Eddington accretion scenario, where dt,is dominated by the circularization timescale.
We construct a time-dependent relativistic accretion model for tidal disruption events (TDEs) with an $alpha-$viscosity and the pressure dominated by gas pressure. We also include the mass fallback rate $dot{M}_f$ for both full and partial disruption TDEs, and assume that the infalling debris forms a seed disc in time $t_c$, which evolves due to the mass addition from the infalling debris and the mass loss via accretion onto the black hole. Besides, we derive an explicit form for the disc height that depends on the angular momentum parameter in the disc. We show that the surface density of the disc increases at an initial time due to mass addition, and then decreases as the mass fallback rate decreases, which results in a decrease in the disc mass $M_{rm d}$ with a late-time evolution of $M_{rm d} propto t^{-1.05}$ and $M_{rm d} propto t^{-1.38}$ for full and partial disruption TDEs respectively, where $t$ is the time parameter. The bolometric luminosity $L$ shows a rise and decline that follows a power-law at late times given by $L propto t^{-1.8}$ and $L propto t^{-2.3}$ for full and partial disruption TDEs respectively. Our obtained luminosity declines faster than the luminosity inferred using $L propto dot{M}_f$. We also compute the light curves in various spectral bands.
We present late-time follow-up of the relativistic tidal disruption flare candidate Swift J1112.2-8238. We confirm the previously determined redshift of $z=0.8900pm0.0005$ based on multiple emission line detections. {em HST} imaging of the host galaxy indicates a complex and distorted morphology with at least two spatially distinct components. These are offset in velocity space by less than 350,km,s$^{-1}$ in VLT/X-Shooter observations, suggesting that the host is undergoing interaction with another galaxy. The transient position is consistent to 2.2$sigma$ with the centre of a bulge-like component at a distance of 1.1$pm$0.5,kpc from its centre. Luminous, likely variable radio emission has also been observed, strengthening the similarities between Swift J1112.2-8238 and other previously identified relativistic tidal disruption flares. While the transient location is $sim2sigma$ from the host centroid, the disrupted nature of the host may provide an explanation for this. The tidal disruption model remains a good description for these events.
Stars that pass within the Roche radius of a supermassive black hole will be tidally disrupted, yielding a sudden injection of gas close to the black hole horizon which produces an electromagnetic flare. A few dozen of these flares have been discovered in recent years, but current observations provide poor constraints on the bolometric luminosity and total accreted mass of these events. Using images from the Wide-field Infrared Survey Explorer (WISE), we have discovered transient 3.4 micron emission from several previously known tidal disruption flares. The observations can be explained by dust heated to its sublimation temperature due to the intense radiation of the tidal flare. From the break in the infrared light curve we infer that this hot dust is located ~0.1 pc from the supermassive black hole. Since the dust has been heated by absorbing UV and (potentially) soft X-ray photons of the flare, the reprocessing light curve yields an estimate of the bolometric flare luminosity. For the flare PTF-09ge, we infer that the most likely value of the luminosity integrated over frequencies at which dust can absorb photons is $8times 10^{44}$ erg/s, with a factor of 3 uncertainty due to the unknown temperature of the dust. This bolometric luminosity is a factor ~10 larger than the observed black body luminosity. Our work is the first to probe dust in the nuclei of non-active galaxies on sub-parsec scales. The observed infrared luminosity implies a covering factor ~1% for the nuclear dust in the host galaxies.
We present radio observations of the tidal disruption event candidate (TDE) XMMSL1 J0740$-$85 spanning 592 to 875 d post X-ray discovery. We detect radio emission that fades from an initial peak flux density at 1.6 GHz of $1.19pm 0.06$ mJy to $0.65pm 0.06$ mJy suggesting an association with the TDE. This makes XMMSL1 J0740$-$85 at $d=75$ Mpc the nearest TDE with detected radio emission to date and only the fifth TDE with radio emission overall. The observed radio luminosity rules out a powerful relativistic jet like that seen in the relativistic TDE Swift J1644+57. Instead we infer from an equipartition analysis that the radio emission most likely arises from a non-relativistic outflow similar to that seen in the nearby TDE ASASSN-14li, with a velocity of about $10^4$ km s$^{-1}$ and a kinetic energy of about $10^{48}$ erg, expanding into a medium with a density of about $10^2$ cm$^{-3}$. Alternatively, the radio emission could arise from a weak initially-relativistic but decelerated jet with an energy of $sim 2times 10^{50}$ erg, or (for an extreme disruption geometry) from the unbound debris. The radio data for XMMSL1 J0740$-$85 continues to support the previous suggestion of a bimodal distribution of common non-relativistic isotropic outflows and rare relativistic jets in TDEs (in analogy with the relation between Type Ib/c supernovae and long-duration gamma-ray bursts). The radio data also provide a new measurement of the circumnuclear density on a sub-parsec scale around an extragalactic supermassive black hole.
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