A likely tidal disruption of a star by the intermediate-mass black hole (IMBH) of a dwarf galaxy was recently identified in association with Abell 1795. Without deep spectroscopy for this very faint object, however, the possibility of a more massive
background galaxy or even a disk-instability flare from a weak AGN could not be dismissed. We have now obtained 8 hours of Gemini spectroscopy which unambiguously demonstrate that the host galaxy is indeed an extremely low-mass $(M_astsim 3times 10^8; {rm M}_{odot})$ galaxy in Abell 1795, comparable to the least-massive galaxies determined to host IMBHs via other studies. We find that the spectrum is consistent with the X-ray flare being due to a tidal disruption event rather than an AGN flare. We also set improved limits on the black hole mass $({rm log}[M_{bullet}/{rm M}_{odot}] sim 5.3 - 5.7)$ and infer a 15-year X-ray variability of a factor of $> 10^4$. The confirmation of this galaxy-black hole system provides a glimpse into a population of galaxies that is otherwise difficult to study, due to the galaxies low masses and intrinsic faintness, but which may be important contributors to the tidal disruption rate.
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.
As part of our ongoing archival X-ray survey of galaxy clusters for tidal flares, we present evidence of an X-ray transient source within 1 arcmin of the core of Abell 1795. The extreme variability (a factor of nearly 50), luminosity (> 2 x 10^42 erg
s^{-1}), long duration (> 5 years) and supersoft X-ray spectrum (< 0.1 keV) are characteristic signatures of a stellar tidal disruption event according to theoretical predictions and to existing X-ray observations, implying a massive >~10^5 M_sun black hole at the centre of that galaxy. The large number of X-ray source counts (~700) and long temporal baseline (~12 years with Chandra and XMM-Newton) make this one of the best-sampled examples of any tidal flare candidate to date. The transient may be the same EUV source originally found contaminating the diffuse ICM observations of Bowyer et al. (1999), which would make it the only tidal flare candidate with reported EUV observations and implies an early source luminosity 1-2 orders of magnitude greater. If the host galaxy is a cluster member then it must be a dwarf galaxy, an order of magnitude less massive than the quiescent galaxy Henize 2-10 which hosts a massive black hole that is difficult to reconcile with its low mass. The unusual faintness of the host galaxy may be explained by tidal stripping in the cluster core.
Theory suggests that a star making a close passage by a supermassive black hole at the center of a galaxy can under most circumstances be expected to emit a giant flare of radiation as it is disrupted and a portion of the resulting stream of shock-he
ated stellar debris falls back onto the black hole itself. We examine the first results of an ongoing archival survey of galaxy clusters using Chandra and XMM-selected data, and report a likely tidal disruption flare from SDSS J131122.15-012345.6 in Abell 1689. The flare is observed to vary by a factor of >30 over at least 2 years, to have maximum L_X(0.3-3.0 keV)> 5 x 10^{42} erg s^{-1} and to emit as a blackbody with kT~0.12 keV. From the galaxy population as determined by existing studies of the cluster, we estimate a tidal disruption rate of 1.2 x 10^{-4} galaxy^{-1} year^{-1} if we assume a contribution to the observable rate from galaxies whose range of luminosities corresponds to a central black hole mass (M_bh) between 10^6 and 10^8 M_sun.
