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

The giant-amplitude X-ray outburst in NGC 5905 - a tidal disruption event? The ROSAT view of NGC 1365: core emission, the highly variable source NGC 1365-X1, and alignments of surrounding X-ray sources. Properties of warm absorbers in active galaxies. The ULIRG NGC 6240: luminous extended X-ray emission and evidence for an AGN

SIGLEAvailable from TIB Hannover: RN 9303(448) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Four new active galaxies with steep soft X-ray spectra

We have discovered four AGN in the ROSAT all-sky-survey data with very steep X-ray spectra. We apply several models to these X-ray spectra with emphasis on warm absorber models which give an adequate description of the data. We report on the follow-up optical and radio observations which allow the identification of three of these objects as Narrow Line Seyfert 1 galaxies, and the fourth as BL Lac object. We have measured small FWHM H#beta# lines, strong FeII emission and weak [OIII] emission in the three Narrow Line Seyfert 1 galaxies, in line with known correlations with respect to the steepness of the X-ray spectra. We have discovered strong optical variability in the BL Lac object and two of the Seyfert galaxies using photographic plates of the Sonneberg Observatory field patrol. We finally discuss the statistical implications of our search algorithm on the expected number density of soft X-ray selected AGN and conclude that up to 30% of X-ray selected AGN might have supersoft X-ray spectra. (orig.)Available from TIB Hannover: RN 9303(342) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Rapid follow-up ROSAT obsevation of GRB 940301

The strong #gamma#-ray burst of March 1, 1994 was imaged by COMPTEL and found to have an identical location within the errors as a burst which occurred on July 4, 1993. This location coincidence had prompted speculations on a possible single source origin for both bursts. We have performed a ROSAT PSPC mosaic observation within four weeks of GRB 940301. The results of these observations and the comparison of the intensities of the detected sources with those detected during the ROSAT all-sky-survey in September 1990 are presented. We neither find a flaring X-ray source in the April 1994 observation nor any variability of the X-ray soruces detected in the all-sky-survey. We discuss the consequences of our negative result on both, the possibility of the location coincidence being due to a repeating burst source as well as due to two independent sources. In the former case the source could either be a Soft Gamma Repeater similar to SGR 1806-20 and SGR 0525-66, or a sofar unknown classical burst repeater. We conclude that a quiescent luminous X-ray source as is found for the above mentioned Soft Gamma Repeaters is very unlikely to be present in the case of GRB 930704 / GBR 940301. (orig.)Available from TIB Hannover: RN 9303(366) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman