arXiv subjects

Stefanie Komossa

Publications and source records attributed to Stefanie Komossa.

At least 73 records · Page 4Linked to original sources

arXiv1999

X-ray emission mechanisms of LINERs

We present here an analysis of the X-ray properties of a sample of LINER galaxies observed with the ROSAT PSPC and HRI instruments. A spatial analysis shows that the bulk of the X-ray emission is consistent with arising from a point source; some extended emission appears at weak emission levels. The X-ray spectra are formally best described by a powerlaw with photon index Gamma_x ~ -2 or thermal emission from a Raymond-Smith plasma with highly subsolar abundances (Z < 0.1). Several emission mechanisms that might contribute to the observed X-ray spectra are discussed. In particular, we take the very subsolar abundances derived from Raymond-Smith fits as an indication of a more complex emission mechanism, like the presence of a second hard component or plasma out of collisional-ionization equilibrium.

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arXiv1999

Dusty warm absorbers: The case of IRAS 13349+2438

Warm absorbers are an important new probe of the central region of active galaxies (AGN). Observing and modeling this component provides a wealth of information on the nature of the warm absorber itself, its relation to other components of the active nucleus, and the intrinsic AGN X-ray spectral shape. We briefly review the general properties of dusty warm gas. For the first time, we then apply such a model to the IR loud quasar IRAS 13349+2438. It was the first to be suggested to host a dusty warm absorber (Brandt et al. 1996), but has not yet been modeled as such.

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arXiv1999

The exceptional X-ray properties of the NLSy1 galaxy RXJ 0134.3-4258

The Narrow-line Seyfert 1 galaxy RXJ0134.3-4258 was detected during the ROSAT all-sky survey with an ultrasoft spectrum (Greiner 1996). Our later pointed observation led to the discovery of drastic spectral variability. Here, we present the first detailed analysis of the soft X-ray properties of this peculiar source.

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arXiv1999

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 (~ 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 NGC 6240 which we advertize as a prototypical active ULIRG, obscured in X-rays by a high column density absorber.

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arXiv1999

The central radio source in the NGC 383 group: jet - IGM interaction ?

The NGC 383 group is a rich group of galaxies that harbours the central radio source 3C 31 = NGC 383. After presenting results from the X-ray analysis of the extended intra-group medium and the central radio galaxy, we discuss the spatial morphology of X-ray emitting gas as compared to the radio plasma and perform a comparison of the pressures of both components.

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arXiv1999

The nature of the luminous X-ray emission of NGC 6240

We briefly review and extend our discussion of the ROSAT detection of the extraordinarily luminous (> 10^{42} erg/s) partly extended (> 30 kpc diameter) X-ray emission from the ultraluminous infrared galaxy NGC 6240. The `standard'-model of starburst outflow is contrasted with alternatives and a comparison with the X-ray properties of ellipticals is performed.

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arXiv1999

X-ray study of the NGC 383 group of galaxies and the source 1E0104+3153

(abridged) We present results from an analysis of the X-ray properties of the NGC 383 galaxy group based on ROSAT PSPC and HRI data. X-ray emission can be traced out to ~ 1 Mpc, the estimated virial radius of the system. We determine a total mass of 6x10^{13} M_sun for the group inside this radius with a gas mass fraction of 21%. The intragroup-gas temperature of 1.5 keV is both consistent with the galaxy velocity dispersion and the X-ray luminosity-temperature relation of groups and clusters suggesting that the group is fairly relaxed. This is also indicated by the almost spherically symmetric appearance of the group's X-ray halo. The X-ray properties of the radio galaxy NGC 383 (3C 31) which is located near the center of the group are discussed. Its spectrum is best described by a two-component model, consisting of emission from a low-temperature Raymond-Smith plasma, and a hard tail. The possible interaction of the radio jets of 3C 31 with the IGM is studied. A spatial, spectral and temporal analysis of the Einstein source 1E0104+3153 located within the field of view is performed, one goal being the identification of the optical counterpart (with both, a high-redshift BAL quasar and a nearby elliptical galaxy, member of a small group, located within the Einstein X-ray error circle). We find evidence that the IGM of the small group contributes significantly to the X-ray emission of 1E0104, which can be described by a Raymond-Smith model of kT ~ 2 keV and a soft X-ray luminosity of L_x ~ 3 10^{43} erg/s.

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arXiv1999

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

(abridged) 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 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 observations of NGC 5905, (ii) optical photometry of NGC 5905 quasi-simultaneous to the X-ray outburst, (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, and the X-ray afterglow of a GRB. 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. We briefly comment on a search for further highly variable objects on the basis of ROSAT observations. Several with factors 10-20 are found.

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arXiv1999

The giant X-ray outburst in NGC 5905 - a tidal disruption event ?

NGC 5905 is one of the very few galaxies that underwent a giant X-ray outburst, with a change in ROSAT photon countrate of a factor ~100. The outburst spectrum is both, very soft and luminous. Our high-resolution follow-up optical spectroscopy of NGC 5905 does not reveal any signs of Seyfert activity. At present, this makes NGC 5905 the only non-active galaxy among the X-ray outbursting ones. We discuss several scenarios to account for the exceptional properties of NGC 5905, including a supernova in dense medium, an accretion-disk instability, an event of extreme gravitational lensing, and the X-ray afterglow of a GRB to account for the X-ray outburst. We find that the most likely model to explain the observations seems to be tidal disruption of a star by a central SMBH, a scenario proposed by Rees (1988) as a tracer of SMBHs in nearby non-active galaxies. The X-ray outburst in this HII galaxy then lends further support to the scenario that all galaxies passed through an active phase, leaving dormant SMBHs in their centers.

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arXiv1998

The complex X-ray spectra of active galaxies with warm absorbers

Warm absorbers are an important new probe of the central regions of active galaxies. So far, they revealed their existence mainly in the soft X-ray spectral region. In observing and modeling this component, we can learn a lot about the nature of the warm absorber itself, its relation to other components of the active nucleus, and the intrinsic AGN X-ray spectral shape. Here, we briefly review the basic X-ray spectral features of warm absorbers (dust-free WAs, dusty WAs, peculiar ~1.1 keV absorption, emission and reflection components) and then discuss these in more detail based on analyses of individual objects observed with the X-ray satellite ROSAT. The importance of XMM in improving our knowledge of the warm material is discussed.

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arXiv1998

The ROSAT view of NGC 1365 and the luminous highly variable off-nuclear X-ray source NGC 1365 - X1

We present an X-ray spectral and spatial analysis of the composite starburst/Seyfert galaxy NGC 1365. Excellent fits of the ROSAT PSPC spectrum are obtained by combining a soft thermal component with a hard powerlaw. The hard component may be either seen directly or can be explained by scattering of the AGN powerlaw at circumnuclear warm high-column-density gas. A compilation of the multi-wavelength properties of NGC 1365 and comparison with hard X-ray selected AGNs shows that the hard component of NGC 1365 is too faint compared to its broad Balmer line components challenging simple unified models. According to analytical estimates, supernova driven outflow can fully account for the X-ray luminosity in the Raymond-Smith component if the observed IR emission is mainly provided by the central starburst. We do not find obvious optical counterparts for three faint PSPC sources south of the nucleus. In particular, there is no coincidence with the two supernovae reported in NGC 1365. With the ROSAT HRI data, we have precisely located the extraordinary southwest X-ray source NGC1365-X1 which falls on one of the subordinate spiral arms. The source is found to be highly variable (a factor about 10) on the timescale of months. Intrinsic to NGC 1365, its huge luminosity makes it exceptional among stellar X-ray sources. At present, the most likely interpretation seems to be an ultra-powerful X-ray binary.

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arXiv1998

Warm absorbers in Narrow-Line Seyfert 1 galaxies

Warm absorbers are an important new tool for investigating the conditions within the central regions of active galaxies. They have been observed in ~50% of the well-studied Seyfert galaxies and have also been detected in quite a number of Narrow-line Seyfert 1 galaxies (NLSy1). Here, we present a study of the X-ray properties of several NLSy1s with focus on their warm absorbers: (a) An analysis of all ROSAT PSPC observations of NGC 4051 including new ones is performed, which reveals variability by a factor ~30 in count rate and much less variability in the warm absorber parameters. (b) The possibility of a dusty warm absorber in IRAS 13349+2438 is explored on the basis of photoionization models for dusty warm gas and explicit ROSAT spectral fits. (c) The X-ray spectrum of the NLSy1 1E0117.2-2837 is analyzed. It can be successfully described by a very steep powerlaw of photon index G_x = -4, or alternatively by a warm-absorbed flat powerlaw. UV-EUV emission lines expected to arise from the warm material are predicted. (d) The strong spectral variability of RX J0134.3-4258 (from G_x = -4.4 in the ROSAT survey observation, to ~ -2.2$ in our subsequent pointing) is examined in terms of warm absorption.

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arXiv1998

The ROSAT view of NGC 1365: core emission, the highly variable source NGC 1365--X1 and alignments of surrounding X-ray sources

We present a spectral and spatial analysis of the nuclear and circumnuclear X-ray emission of the prominent southern starburst/Seyfert galaxy NGC 1365. To describe the X-ray spectrum of the core source, we favour a two-component model consisting of about equally strong contributions from a Raymond-Smith plasma (with ~cosmic abundances) and a powerlaw. The origin of both components is discussed, and a detailed comparison of the X-ray properties with multi-wavelength observations of NGC 1365 and a large sample of type-I AGNs is performed. Among the surrounding X-ray sources we focus on an analysis of the enigmatic source NGC 1365-X1, which is one of the most luminous and most highly variable off-nuclear X-ray sources known so far. Positions of further X-ray sources in the field of view are derived and it is briefly pointed out that all HRI sources (except one) are `aligned' relative to the central source.

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arXiv1998

The ULIRG NGC 6240: Luminous extended X-ray emission and evidence for an AGN

We briefly review and extend our discussion of the ROSAT detection of the extraordinarily luminous (>10^{42} erg/s) partly extended (> 30 kpc diameter) X-ray emission from the double-nucleus ultraluminous infrared galaxy NGC 6240. The ROSAT spectrum can be well fit by emission from two components in roughly equal proportions: a thermal optically thin plasma with kT \sim 0.6 keV and a hard component that can be represented by a canonical AGN powerlaw. Source counts appear to have dropped by 30% within a year. Altogether, these findings can be well explained by a contribution of radiation from an AGN essentially hidden at other wavelengths. Fits of ASCA spectra, conducted by various groups, corroborate this result, revealing a high-equivalent width FeK blend which again is straightforwardly interpreted by scattered AGN light. If radiating at the Eddington limit, the central black hole mass does not exceed 10^7 M_sun. We discuss implications for the formation of this AGN. However, the luminosity in the remaining extended thermal component is still at the limits of a pure starburst-wind-induced source. We suggest that the deeply buried starburst has switched to a partially dormant phase so that heating of the outflow is diminished and a cooling flow could have been established. This flow may account for the extended shock heating traced by LINER-like emission line ratios and the extremely luminous H_2 emission from the central region of this galaxy. Next-generation X-ray telescopes will be able to test this proposal.

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arXiv1998

The giant-amplitude X-ray outburst in NGC 5905 - a tidal disruption event ?

NGC 5905 is one of the very few galaxies that underwent a giant X-ray outburst, with a change in photon countrate of a factor 100. The outburst spectrum is both, very soft and luminous (Bade, Komossa & Dahlem 1996). Remarkably, the optical pre-outburst emission line spectrum of NGC 5905 is that of an HII-type galaxy, i.e. does not show any signs of Seyfert activity. One exciting explanation of the X-ray observation is that we may have witnessed the tidal disruption of a star by a supermassive black hole (SMBH) residing in the nucleus of this galaxy. The expected flare of electromagnetic radiation being produced when the stellar debris is swallowed by the black hole was proposed by Rees (1988) as a means of tracing SMBHs in nearby non-active galaxies. In the present work, we discuss this and other possible outburst scenarios through an analysis of all ROSAT PSPC and HRI X-ray observations of NGC 5905. We also present simultaneous and long-term optical photometry of NGC 5905 as well as follow-up optical spectroscopy.

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arXiv1998

Properties of warm absorbers in active galaxies

We present a study of the nature of warm absorbers on the basis of ROSAT X-ray observations and photoionization calculations carried out with the code Cloudy. We focus on `non-standard' warm absorbers: (i) the proposed dusty warm absorbers in IRAS 13349+2438 and IRAS 17020+4544, and possibly in 4C +74.26; (ii) we explore several scenarios to account for the recently observed peculiar absorption features around 1.1 keV on the basis of detailed photoionization models, and apply these to PG 1404+226.

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arXiv1998

ROSAT HRI discovery of luminous extended X-ray emission in NGC 6240

We report the detection of luminous extended X-ray emission in NGC 6240 on the basis of ROSAT HRI observations of this ultraluminous IR galaxy. The spatial structure and temporal behavior of the X-ray source were analyzed. We find that >= 70% of the soft X-ray emission is extended beyond a radius of 5 arcsec. Strong emission can be traced out to a radius of 20 arcsec and weaker emission extends out to 50 arcsec. With a luminosity of at least 10^{42} erg/s this makes NGC 6240 one of the most luminous X-ray emitters in extended emission known. Evidence for a nuclear compact variable component is indicated by a drop of 32% in the HRI count rate as compared to the PSPC data taken one year earlier. No short-timescale variability is detected. The HRI data, which represent the first high-resolution study of the X-ray emission from NGC 6240, complement previous spectral fits to ROSAT PSPC data (astro-ph/9710098; A&A 330, 823) that suggested a two-component model consisting of thermal emission from shocked gas immersed in a starburst wind plus a powerlaw source attributed to scattered light from an obscured AGN. We discuss several models to account for the extended and compact emission. Although pushed to its limits the starburst outflow model is tenable for the essential part of the extended emission. For the AGN-type component we propose a model consisting of a near-nuclear `warm scatterer' that explains the apparent fading of the X-ray flux within a year as well as the strong FeKalpha complex seen in an ASCA spectrum.

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arXiv1998

Properties of dusty warm absorbers and the case of IRAS 17020+4544

We present a study of the properties of dusty warm absorbers and point to some important consequences for the resulting X-ray absorption spectra of AGN. Pronounced effects of the presence of dust in warm material are (i) an apparent flattening of the observed X-ray spectrum in the ROSAT band due to a sequence of absorption edges and a shift to lower gas ionization, (ii) the presence of a strong carbon edge at 0.28 keV, and (iii) the expectation of increased time variability of the warm absorber parameters. The first two effects can be drastic and may completely hamper an X-ray spectral fit with a dusty warm absorber even if a dust-free one was successfully applied to the data. In order to demonstrate facets of the dusty warm absorber model and test the recently reported important, albeit indirect, evidence for dusty warm material in the Narrow-Line Seyfert 1 galaxy IRAS 17020+4544 we have analyzed ROSAT PSPC and HRI observations of this galaxy. The X-ray spectrum can be successfully described by a single powerlaw with index Gamma_x = -2.4 plus small excess cold absorption, or alternatively by a steeper intrinsic powerlaw (G_x = -2.8) absorbed by a dusty warm absorber. The findings are discussed in light of the NLSy1 character of IRAS 17020 and consequences for NLSy1s in general are pointed out. In particular, the presence of dusty warm gas results in a steeper intrinsic powerlaw than observed, thus exaggerating the `FeII problem'. It also implies weaker potential warm-absorber contribution to high-ionization Fe coronal lines.

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