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Register a new userASASSN-14ae: A Tidal Disruption Event at 200 Mpc
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ASASSN-14ae is a candidate tidal disruption event (TDE) found at the center of SDSS J110840.11+340552.2 ($dsimeq200$~Mpc) by the All-Sky Automated Survey for Supernovae (ASAS-SN). We present ground-based and Swift follow-up photometric and spectroscopic observations of the source, finding that the transient had a peak luminosity of $Lsimeq8times10^{43}$~erg~s$^{-1}$ and a total integrated energy of $Esimeq1.7times10^{50}$ ergs radiated over the $sim5$ months of observations presented. The blackbody temperature of the transient remains roughly constant at $Tsim20,000$~K while the luminosity declines by nearly 1.5 orders of magnitude during this time, a drop that is most consistent with an exponential, $Lpropto e^{-t/t_0}$ with $t_0simeq39$~days. The source has broad Balmer lines in emission at all epochs as well as a broad He II feature emerging in later epochs. We compare the color and spectral evolution to both supernovae and normal AGN to show that { ame} does not resemble either type of object and conclude that a TDE is the most likely explanation for our observations. At $z=0.0436$, ASASSN-14ae is the lowest-redshift TDE candidate discovered at optical/UV wavelengths to date, and we estimate that ASAS-SN may discover $0.1 - 3$ of these events every year in the future.
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We present ground-based and Swift photometric and spectroscopic observations of the tidal disruption event (TDE) ASASSN-15oi, discovered at the center of 2MASX J20390918-3045201 ($dsimeq216$ Mpc) by the All-Sky Automated Survey for SuperNovae (ASAS-SN). The source peaked at a bolometric luminosity of $Lsimeq1.3times10^{44}$ ergs s$^{-1}$ and radiated a total energy of $Esimeq6.6times10^{50}$ ergs over the first $sim3.5$ months of observations. The early optical/UV emission of the source can be fit by a blackbody with temperature increasing from $Tsim2times10^4$ K to $Tsim4times10^4$ K while the luminosity declines from $Lsimeq1.3times10^{44}$ ergs s$^{-1}$ to $Lsimeq2.3times10^{43}$ ergs s$^{-1}$, requiring the photosphere to be shrinking rapidly. The optical/UV luminosity decline during this period is most consistent with an exponential decline, $Lpropto e^{-(t-t_0)/tau}$, with $tau simeq46.5$ days for $t_0simeq57241.6$ (MJD), while a power-law decline of $Lpropto (t-t_0)^{-alpha}$ with $t_0simeq57212.3$ and $alpha=1.62$ provides a moderately worse fit. ASASSN-15oi also exhibits roughly constant soft X-ray emission that is significantly weaker than the optical/UV emission. Spectra of the source show broad helium emission lines and strong blue continuum emission in early epochs, although these features fade rapidly and are not present $sim3$ months after discovery. The early spectroscopic features and color evolution of ASASSN-15oi are consistent with a TDE, but the rapid spectral evolution is unique among optically-selected TDEs.
When a star passes within the tidal radius of a supermassive black hole, it will be torn apart. For a star with the mass of the Sun ($M_odot$) and a non-spinning black hole with a mass $<10^8 M_odot$, the tidal radius lies outside the black hole event horizon and the disruption results in a luminous flare. Here we report observations over a period of 10 months of a transient, hitherto interpreted as a superluminous supernova. 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, the presence of highly ionised CNO gas in the line of sight and our improved localisation of the transient in the nucleus of a passive galaxy, where the presence of massive stars is highly unlikely. While the supermassive black hole has a mass $> 10^8 M_odot$, a star with the same mass as the Sun could be disrupted outside the event horizon if the black hole were spinning rapidly. The rapid spin and high black hole mass can explain the high luminosity of this event.
We present late-time observations by Swift and XMM-Newton of the tidal disruption event (TDE) ASASSN-15oi that reveal that the source brightened in the X-rays by a factor of $sim10$ one year after its discovery, while it faded in the UV/optical by a factor of $sim 100$. The XMM-Newton observations measure a soft X-ray blackbody component with $kT_{rm bb} sim 45$ eV, corresponding to radiation from several gravitational radii of a central $sim 10^6 M_odot$ black hole. The last Swift epoch taken almost 600 days after discovery shows that the X-ray source has faded back to its levels during the UV/optical peak. The timescale of the X-ray brightening suggests that the X-ray emission could be coming from delayed accretion through a newly forming debris disk, and that the prompt UV/optical emission is from the prior circularization of the disk through stream-stream collisions. The lack of spectral evolution during the X-ray brightening disfavors ionization breakout of a TDE veiled by obscuring material. This is the first time a TDE has been shown to have a delayed peak in soft X-rays relative to the UV/optical peak, which may be the first clear signature of the real-time assembly of a nascent accretion disk, and provides strong evidence for the origin of the UV/optical emission from circularization, as opposed to reprocessed emission of accretion radiation.
We present late-time optical spectroscopy and X-ray, UV, and optical photometry of the nearby ($d=214$ Mpc, $z=0.0479$) tidal disruption event (TDE) ASASSN-15oi. The optical spectra span 450 days after discovery and show little remaining transient emission or evolution after roughly 3 months. In contrast, the Swift and XMM-Newton observations indicate the presence of evolving X-ray emission and lingering thermal UV emission that is still present 600 days after discovery. The thermal component of the X-ray emission shows a unique, slow brightening by roughly an order of magnitude to become the dominant source of emission from the TDE at later times, while the hard component of the X-ray emission remains weak and relatively constant throughout the flare. The TDE radiated $(1.32pm0.06)times10^{51}$ ergs across all wavelengths, and the UV and optical emission is consistent with a power law decline and potentially indicative of a late-time shift in the power-law index that could be caused by a transition in the dominant emission mechanism.
We survey the properties of stars destroyed in TDEs as a function of BH mass, stellar mass and evolutionary state, star formation history and redshift. For Mbh<10^7Msun, the typical TDE is due to a M*~0.3Msun M-dwarf, although the mass function is relatively flat for $M*<Msun. The contribution from older main sequence stars and sub-giants is small but not negligible. From Mbh~10^7.5-10^8.5Msun, the balance rapidly shifts to higher mass stars and a larger contribution from evolved stars, and is ultimately dominated by evolved stars at higher BH masses. The star formation history has little effect until the rates are dominated by evolved stars. TDE rates should decline very rapidly towards higher redshifts. The volumetric rate of TDEs is very high because the BH mass function diverges for low masses. However, any emission mechanism which is largely Eddington-limited for low BH masses suppresses this divergence in any observed sample and leads to TDE samples dominated by Mbh~10^6.0-10^7.5Msun BHs with roughly Eddington peak accretion rates. The typical fall back time is relatively long, with 16% having Tfb<10^(-1) years (37 days), and 84% having longer time scales. Many residual rate discrepancies can be explained if surveys are biased against TDEs with these longer Tfb, which seems very plausible if Tfb has any relation to the transient rise time. For almost any BH mass function, systematic searches for fainter, faster time scale TDEs in smaller galaxies, and longer time scale TDEs in more massive galaxies are likely to be rewarded.
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