Warm absorbers in active galactic nuclei. The giant X-ray outbursts from nearby, non-active galaxies: tidal disruption flares?

We first provide a review of the properties of warm absorbers concentrating on what we have learned from ROSAT and ASCA. This includes dusty and dust-free warm absorbers, non-X-ray emission and absorption features of warm absorbers, and the possible warm absorber interpretation of the peculiar 1.1 keV features. We then discuss facets of warm absorbers by a more detailed investigation of individual objects: In a first part, we discuss several candidates for dusty warm absorbers. In a second part, we review and extend our earlier study of a possible relation between warm absorber and CLR in NGC4051, and confirm that both components are of different origin (the observed coronal lines are underpredicted by the models, the warm absorber is too highly ionized). We then suggest that a potential overprediction of these lines in more lowly ionized absorbers can be avoided if these warm absorbers are dusty. In a third part, we present first results of an analysis of a deep ROSAT PSPC observation of the quasar MR2251-178, the first one discovered to host a warm absorber. Finally, we summarize our scrutiny under which conditions a warm absorber could account for the dramatic spectral variability of the narrow-line Seyfert 1 galaxy RXJ0134-4258. (orig.)Available from TIB Hannover: RN 9303(484) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

A two-phase model for the narrow line region of NGC 4151

SIGLEAvailable from TIB Hannover: RN 9303(496) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

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 ULIRGs 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. (orig.)Available from TIB Hannover: RN 9303(495) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The giant X-ray outbursts in NGC 5905 and IC 3599: follow-up observations and outburst scenerios

Huge amplitude X-ray outbursts in a few galaxies were reported in the last few years. As one of the exciting possibilities to explain these observations, tidal disruption of a star by a central supermassive black hole has been proposed. In the present paper, we perform a detailed discussion of this and other possible scenarios for the X-ray outburst in NGC 5905, and a comparison of NGC 5905 and IC 3599 in outburst as well as in quiescence. To this end we present (i) a thorough analysis of all ROSAT PSPC X-ray observations of NGC 5905 and new HRI data, (ii) optical photometry of NGC 5905 quasi-simultaneous to the X-ray outburst and on longer terms, (iii) the first post-outburst optical spectra of NGC 5905 and high-resolution post-outburst spectra of IC 3599, and (iv) photoionization models for the high-excitation emission lines that were discovered in the optical outburst spectrum of IC 3599. The investigated outburst models include, besides the tidal disruption event, a supernova in dense medium, an accretion-disk instability, an event of extreme gravitational lensing, the X-ray afterglow of a GRB, and the possibility of a warm-absorbed hidden Seyfert nucleus in the center of the galaxy. The successful models, all involving the presence of a central supermassive black hole, are selected and implications are discussed. The optical spectra of both galaxies in quiescence are carefully examined for signs of permanent low-level Seyfert-activity. Whereas IC 3599 shows several signs of activity, none is revealed for NGC 5905. At present, and among the X-ray bursts, this makes NGC 5905 the only safe candidate for a tidal disruption event in an otherwise non-active galaxy. The physical conditions in the HII emission-line gas are also investigated. We briefly comment on a search for further highly variable objects on the basis of ROSAT observations. Several with factors 10-20 are found; all of them are known to harbour warm absorbers. (orig.)Available from TIB Hannover: RN 9303(458) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Discovery of a giant and luminous X-ray outburst from the optically inactive galaxy pair RX J1242.6-1119

We report the discovery of large-amplitude X-ray variability from the direction of the previously unknown, optically inactive galaxy pair RX J1242.6-1119. The X-ray source shows variability by a factor >or#approx# 20 between the ROSAT all-sky survey and a later pointed observation separated by #propor to# 1.5 yr. Its spectrum is extremely soft with photon index #GAMMA#_x#approx =# -5, among the steepest ever observed among galaxies. Based on the redshift derived from the optical spectra, z = 0.05, the source's intrinsic luminosity is large, L_x >or#approx# 10"4"3 erg s"-"1. Surprisingly, the optical spectra of both galaxies are characterized by absorption lines and do not show signs of (Seyfert) activity. This makes RX J1242-11 the third candidate for giant-amplitude variability in an otherwise non-active galaxy, the first two being NGC 4552 (in the UV: Renzini et al. 1995) and NGC 5905 (in X-rays: Bade et al. 1996, Komossa and Bade 1999). Several mechanisms to explain this unexpected and peculiar behavior are investigated. The most likely one seems to be an accretion event onto an otherwise dormant supermassive black hole (SMBH), e.g., by a tidal disruption eventSIGLEAvailable from TIB Hannover: RN 9303(472) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

The complex X-ray spectra of active galaxies with warm absorbers. Warm absorbers in Narrow-Line Seyfert 1 galaxies

SIGLEAvailable from TIB Hannover: RN 9303(453) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Formation of massive black holes in early mergers?

We review theoretical and observational arguments favoring a scenario in which a typical massive black hole (MBH) is formed in the merger core of colliding disk systems at high z during the build-up of a spheroid. Low-mass (#propor to#10"5"-"6 M_sun) seed black holes are assumed to have been formed earlier. The most massive black holes giving rise to the most luminous active galactic nuclei (AGN) in active phases are expected to grow in violent mergers of large disk-plus-bulge systems that lead to giant elliptical galaxies, the typical hosts of radio galaxies and quasars. We consider current ultraluminous infrared galaxies (ULIRG) as systems closely resembling the predecessors of early formed massive ellipticals. As an example, we discuss the evidence for an AGN in the merging system NGC6240 which we advertize as a prototypical active ULIRG, obscured in X-rays by a high column density absorber. (orig.)95 refs.SIGLEAvailable from TIB Hannover: RN 9303(464) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

X-Ray outbursts from nearby 'normal' and active galaxies A review, new radio observations, and an X-ray search for further tidal disruption flares

SIGLEAvailable from TIB Hannover: RN 9303(498) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

A ROSAT observation of the warm-absorbed soft X-ray spectrum of NGC 4051

We present and analyze a pointed ROSAT PSPC observation of the Seyfert galaxy NGC 4051. The X-ray spectrum consists of a powerlaw modified by absorption edges and an additional soft excess during the high-state in source flux. Modeling the spectrum in terms of warm absorption yields a large column density of the ionized material of log N_W=22.7 and an ionization parameter of logU=0.4. These properties are essentially constant throughout the observation, whereas the luminosity changes by more than a factor of 4. The underlying powerlaw is in its steepest observed state, with a photon index #GAMMA#_x=-2.3. Consulting information from optical observations, evidence for a separate EUV bump component in NGC 4051 is provided. The impact of several parameters on the deduced properties of the ionized material is critically assessed. In particular, the influence of dust mixed with the warm gas is explored and shown to constrain the history or density of the absorber. The absorber-intrinsic optical-UV emission (and absorption) line spectrum is predicted and the possibility of a warm absorber origin of one of the observed emission line regions in NGC 4051 is investigated. Consequences for the narrow-line Seyfert 1 character of NGC 4051 are discussed. (orig.)Available from TIB Hannover: RN 9303(381) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

ROSAT observations of warm absorbers in AGN

We study the properties of warm absorbers in several active galaxies (AGN) and present new candidates, using ROSAT X-ray observations. Several aspects of the characteristics of warm absorbers are discussed, constraints on the density and location of the ionized material are provided; the impact of the presence of the warm gas on other observable spectral regions is investigated, particularly with respect to the possibility of a warm-absorber origin of one of the known high-ionization emission-line region in AGN; the possibility of dust mixed with the warm material is critically assessed; and the thermal stability of the ionized gas is examined. Based on the properties of known warm absorbers, we then address the question of where else ionized absorbers might play a role in determining the traits of a class of objects or individual peculiar objects. In this context, the possibility to produce the steep soft X-ray spectra of narrow-line Seyfert-1 galaxies by warm absorption is explored, as compared to the alternative of an accretion-produced soft excess. The potentiality of explaining the strong spectral variability in RX J0134-42 and the drastic flux variability in NGC 5905 via warm absorption is scrutinized. (orig.)67 refs.Available from TIB Hannover: RN 9303(387) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman