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Register a new userRBS 1032: A Tidal Disruption Event in Another Dwarf Galaxy?
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RBS 1032 is a supersoft ($Gammasim5$), luminous ($sim10^{43}$ erg/s) ROSAT PSPC source which has been associated with an inactive dwarf galaxy at $z=0.026$, SDSS J114726.69+494257.8. We have analyzed an XMM-Newton observation which confirms that RBS 1032 is indeed associated with the dwarf galaxy. Moreover, RBS 1032 has undergone a factor of $sim100-300$ decay since November 1990. This variability suggests that RBS 1032 may not be a steadily accreting intermediate-mass black hole, but rather an accretion flare from the tidal disruption of a star by the central black hole (which may or may not be intermediate-mass). We suggest that additional tidal disruption events may remain unidentified in archival ROSAT data, such that disruption rate estimates based upon ROSAT All-Sky Survey data may need reconsideration.
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The bright transient AT2018cow has been unlike any other known type of transient. Its high brightness, rapid rise and decay and initially nearly featureless spectrum are unprecedented and difficult to explain using models for similar burst sources. We present evidence for faint gamma-ray emission continuing for at least 8 days, and featureless spectra in the ultraviolet bands -- both unusual for eruptive sources. The X-ray variability of the source has a burst-like character. The UV-optical spectrum does not show any CNO line but is well described by a blackbody. We demonstrate that a model invoking the tidal disruption of a 0.1 - 0.4 Msun Helium White Dwarf (WD) by a 100,000 to one million solar mass Black Hole (BH) located in the outskirts of galaxy Z~137-068 could provide an explanation for most of the characteristics shown in the multi-wavelength observations. A blackbody-like emission is emitted from an opaque photosphere, formed by the debris of the WD disruption. Broad features showing up in the optical/infrared spectra in the early stage are probably velocity broadened lines produced in a transient high-velocity outward moving cocoon. The asymmetric optical/infrared lines that appeared at a later stage are emission from an atmospheric layer when it detached from thermal equilibrium with the photosphere, which undergoes more rapid cooling. The photosphere shrinks when its temperature drops, and the subsequent infall of the atmosphere produced asymmetric line profiles. Additionally, a non-thermal jet might be present, emitting X-rays in the 10-150 keV band.
[Abridged] We present observations of PS16dtm, a luminous transient that occurred at the nucleus of a known Narrow-line Seyfert 1 galaxy hosting a 10$^6$ M$_odot$ black hole. The transient was previously claimed to be a Type IIn SLSN due to its luminosity and hydrogen emission lines. The light curve shows that PS16dtm brightened by about two magnitudes in ~50 days relative to the archival host brightness and then exhibited a plateau phase for about 100 days followed by the onset of fading in the UV. During the plateau PS16dtm showed no color evolution, maintained a blackbody temperature of 1.7 x 10$^4$ K, and radiated at approximately $L_{Edd}$ of the SMBH. The spectra exhibit multi-component hydrogen emission lines and strong FeII emission, show little evolution with time, and closely resemble the spectra of NLS1s while being distinct from those of Type IIn SNe. Moreover, PS16dtm is undetected in the X-rays to a limit an order of magnitude below an archival X-ray detection of its host galaxy. These observations strongly link PS16dtm to activity associated with the SMBH and are difficult to reconcile with a SN origin or any known form of AGN variability, and therefore we argue that it is a TDE in which the accretion of the stellar debris powers the rise in the continuum and excitation of the pre-existing broad line region, while providing material that obscures the X-ray emitting region of the pre-existing AGN accretion disk. A detailed TDE model fit to the light curve indicates that PS16dtm will remain bright for several years; we further predict that the X-ray emission will reappear on a similar timescale as the accretion rate declines. Finally, we place PS16dtm in the context of other TDEs and find that TDEs in AGN galaxies are an order of magnitude more efficient and reach Eddington luminosities, likely due to interaction of the stellar debris with the pre-existing accretion disk.
We present ground-based and textit{Swift} observations of iPTF16fnl, a likely tidal disruption event (TDE) discovered by the intermediate Palomar Transient Factory (iPTF) survey at 66.6 Mpc. The lightcurve of the object peaked at absolute $M_g=-17.2$ mag. The maximum bolometric luminosity (from optical and UV) was $L_p~simeq~(1.0,pm,0.15) times 10^{43}$ erg/s, an order of magnitude fainter than any other optical TDE discovered so far. The luminosity in the first 60 days is consistent with an exponential decay, with $L propto e^{-(t-t_0)/tau}$, where $t_0$=~57631.0 (MJD) and $tausimeq 15$ days. The X-ray shows a marginal detection at $L_X=2.4^{1.9}_{-1.1}times 10^{39}$ erg/s (textit{Swift} X-ray Telescope). No radio counterpart was detected down to 3$sigma$, providing upper limits for monochromatic radio luminosity of $ u L_{ u} < 2.3times10^{36}$ erg/s and $ u L_{ u}<1.7times 10^{37}$ erg/s (VLA, 6.1 and 22 GHz). The blackbody temperature, obtained from combined textit{Swift} UV and optical photometry, shows a constant value of 19,000 K. The transient spectrum at peak is characterized by broad He II and H$alpha$ emission lines, with an FWHM of about 14,000 km/s and 10,000 km/s respectively. He I lines are also detected at $lambdalambda$ 5875 and 6678. The spectrum of the host is dominated by strong Balmer absorption lines, which are consistent with a post-starburst (E+A) galaxy with an age of $sim$650 Myr and solar metallicity. The characteristics of iPTF16fnl make it an outlier on both luminosity and decay timescales, as compared to other optically selected TDEs. The discovery of such a faint optical event suggests a higher rate of tidal disruptions, as low luminosity events may have gone unnoticed in previous searches.
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.
Aims. We investigate the evolution of X-ray selected tidal disruption events. Methods. New events are found in near-real time data from XMM-Newton slews and are monitored by multi-wavelength facilities. Results. In August 2016, X-ray emission was detected from the galaxy XMMSL2 J144605.0+685735 (a.k.a. 2MASX 14460522+6857311), a factor 20 times higher than an upper limit from 25 years earlier. The X-ray flux was flat for ~100 days and then fell by a factor 100 over the following 500 days. The UV flux was stable for the first 400 days before fading by a magnitude, while the optical (U,B,V bands) have been roughly constant for 850 days. Optically, the galaxy appears to be quiescent, at a distance of $127pm{4}$ Mpc (z=$0.029pm{0.001}$) with a spectrum consisting of a young stellar population of age 1-5 Gyr, an older population and a total stellar mass of ~6 x $10^{9}$ solar masses. The bolometric luminosity peaked at L bol ~ $10^{43}$ ergs s$^{-1}$ with an X-ray spectrum that may be modeled by a power-law of $Gamma$~2.6 or Comptonisation of a low-temperature thermal component by thermal electrons. We consider a tidal disruption event to be the most likely cause of the flare. Radio emission was absent in this event down to < 10$mu$Jy, which limits the total energy of a hypothetical off-axis jet to E < 5 x $10^{50}$ ergs. The independent behaviour of the optical, UV and X-ray light curves challenges models where the UV emission is produced by reprocessing of thermal nuclear emission or by stream-stream collisions. We suggest that the observed UV emission may have been produced from a truncated accretion disk and the X-rays from Compton upscattering of these disk photons.
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