Narrow-Line Seyfert 1 Galaxies

I provide a short review of the properties of Narrow-line Seyfert 1 (NLS1) galaxies across the electromagnetic spectrum and of the models to explain them. Their continuum and emission-line properties manifest one extreme form of Seyfert activity. As such, NLS1 galaxies may hold important clues to the key parameters that drive nuclear activity. Their high accretion rates close to the Eddington rate provide new insight into accretion physics, their low black hole masses and perhaps young ages allow us to address issues of black hole growth, their strong optical FeII emission places strong constraints on FeII and perhaps metal formation models and physical conditions in these emission-line clouds, and their enhanced radio quiteness permits a fresh look at causes of radio loudness and the radio-loud radio-quiet bimodality in AGN.Comment: To appear in the proceedings of "The nuclear region, host galaxy and environment of AGN", E. Benitez, I. Cruz-Gonzalez & Y. Krongold (eds), RevMexA

The giant X-ray outbursts from nearby, non-active galaxies: tidal disruption flares ?

One efficient method to probe the direct vicinity of SMBHs in nearby galaxies is to make use of the detection of flares from tidally disrupted stars (e.g., Lidskii & Ozernoi 1979, Rees 1988). The first few excellent candidates for the occurrence of this process in non-active galaxies have emerged recently. Here, we present a review of these observations, compare with variability in AGN, and discuss theoretical implications. We concentrate on the cases of NGC 5905 and RXJ1242-1119, and report results from a systematic search for further X-ray flares from a sample of >100 nearby galaxies.Comment: 6 pages incl. 2 figures, needs agnsymp.cls; to appear in `ASCA/ROSAT Workshop on AGN and the X-ray Background' (Tokyo, Nov. 1-3, 1999), T. Takahashi and H. Inoue (eds). Typo in equation (1) corrected. Preprint and related papers also available at http://www.xray.mpe.mpg.de/~skomossa

Recoiling black holes: electromagnetic signatures, candidates, and astrophysical implications

Supermassive black holes (SMBHs) may not always reside right at the centers of their host galaxies. This is a prediction of numerical relativity simulations, which imply that the newly formed single SMBH, after binary coalescence in a galaxy merger, can receive kick velocities up to several 1000 km/s due to anisotropic emission of gravitational waves. Long-lived oscillations of the SMBHs in galaxy cores, and in rare cases even SMBH ejections from their host galaxies, are the consequence. Observationally, accreting recoiling SMBHs would appear as quasars spatially and/or kinematically off-set from their host galaxies. The presence of the "kicks" has a wide range of astrophysical implications which only now are beginning to be explored, including consequences for black hole and galaxy assembly at the epoch of structure formation, black hole feeding, and unified models of Active Galactic Nuclei (AGN). Here, we review the observational signatures of recoiling SMBHs and the properties of the first candidates which have emerged, including follow-up studies of the candidate recoiling SMBH of SDSSJ092712.65+294344.0.Comment: 10 pages, Advances in Astronomy 2012, one chapter of the special issue "Seeking for the Leading Actor on the Cosmic Stage: Galaxies versus Supermassive Black Holes

Tidal disruption of stars by supermassive black holes: Status of observations

Stars in the immediate vicinity of supermassive black holes (SMBHs) can be ripped apart by the tidal forces of the black hole. The subsequent accretion of the stellar material causes a spectacular flare of electromagnetic radiation. Here, we provide a review of the observations of tidal disruption events (TDEs), with an emphasis on the important contributions of Swift to this field. TDEs represent a new probe of matter under strong gravity, and have opened up a new window into studying accretion physics under extreme conditions. The events probe relativistic effects, provide a new means of measuring black hole spin, and represent signposts of intermediate-mass BHs, binary BHs and recoiling BHs. Luminous, high-amplitude X-ray flares, matching key predictions of the tidal disruption scenario, have first been discovered with ROSAT, and more recently with other missions and in other wavebands. The Swift discovery of two gamma-ray emitting, jetted TDEs, never seen before, has provided us with a unique probe of the early phases of jet formation and evolution, and SwiftJ1644+75 has the best covered lightcurve of any TDE to date. Further, Swift has made important contributions in providing well-covered lightcurves of TDEs discovered with other instruments, setting constraints on the physics that govern the TDE evolution, and including the discovery of the first candidate binary SMBH identified from a TDE lightcurve.Comment: Review, 12 pages, to appear in Journal of High-Energy Astrophysics. Proceedings (review contributions) of "Swift: 10 years of discovery

X-rays from the environment of supermassive black holes in active galaxies

X-rays are a powerful probe of the physical conditions in the nuclei of active galaxies. We review the X-ray properties of radio-quiet AGN, LINERs and ultraluminous IR galaxies based on observations carried out with the X-ray satellite ROSAT. We then summarize the observations of giant X-ray flares from non-active galaxies, interpreted as stellar tidal disruptions by supermassive black holes.Comment: invited review, IX. Marcel Grossmann Meeting on General Relativity, Gravitation and Relativistic Field Theories (Rome, July 2000), session: `Astrophysics of Neutron Stars and Black Holes: Observations'; to appear in the online proceedings at http://www.icra.it/MG/mg9/mg9.htm, a 2-page condensate will be published in World Scientific; long and short version are also available at http://www.xray.mpe.mpg.de/~skomossa