arXiv subjects

Stefanie Komossa

Publications and source records attributed to Stefanie Komossa.

78 records · Page 5Linked to original sources

arXiv1997

ROSAT evidence for AGN and superwind activity in NGC 6240 and NGC 2782

We present ROSAT observations of the starburst galaxy NGC 2782 (HRI plus a weak PSPC frame) and the ultraluminous infrared galaxy NGC 6240 (PSPC). The (0.1-2.4) keV spectra of both objects appear similar. However, due to better sampling spectral modeling is only warranted in case of NGC 6240 for which both a single thermal Raymond-Smith model (kT = 0.44) or a hybrid model consisting of 80% power-law with the canonical photon index -1.9 plus 20% Raymond-Smith contribution (kT = 0.63) lead to good fits. However, the single thermal model turns out to be unlikely because it yields a luminosity of 3.8 10^{43} erg/s, which is hard to reach in a starburst superwind-scenario. The hybrid model leads to a more moderate luminosity of 5.2 10^{42} erg/s, of which 1.0 10^{42} erg/s can be attributed to shocked superwind gas. We link the remaining 4.2 10^{42} erg/s powerlaw to an AGN component because the alternative of inverse-compton scattering of the FIR radiation leads to a too low flux when estimated with available data. The result appears to be consistent with preliminarily announced ASCA observations. For NGC 2782 we find L_x = 4 10^{41} erg/s which can be explained by emission from a shocked superwind region with a high preshock density in agreement with earlier optical evidence for an outflowing supershell.

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arXiv1997

Soft X-ray properties of the Seyfert 1.8 galaxy NGC 3786

An analysis of survey and pointed ROSAT PSPC observations of the Seyfert 1.8 galaxy NGC 3786 has revealed interesting spectral and temporal behaviour. The spectrum is found to show clear signs of excess absorption, and there is evidence that part (or all) of it is caused by the presence of a warm absorber. The soft X-ray spectral properties are discussed in the context of the Sy 1.8 classification of NGC 3786, and are combined with published optical data in an effort to discriminate between several possible absorption models. The one involving a warm absorber with internal dust, located between BLR and NLR, is favoured and shown to account successfully for the high observed broad line reddening as well as for the strong X-ray variability (a factor $\sim$ 10) we detect between the 1990 and 1992 observation.

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arXiv1997

Evidence for a dusty warm absorber in NGC 3227 ?

We have analyzed survey and pointed \ros PSPC observations of the Seyfert galaxy NGC 3227. Large amplitude X-ray variability is detected, with a factor $\sim$ 15 change in count rate within about 3 years. Smaller changes are seen on the timescale of days, the largest being a factor of 3.5. No strong spectral variability is found throughout the pointed observation. The X-ray spectrum is modeled in terms of warm absorption and both, a dust-free warm absorber and one with internal dust, give an excellent spectral fit. Additionally using multi-wavelength observations of NGC 3227, the dusty warm absorber is favored and suggested as an explanation for the lack of a large cold absorbing column in the soft X-ray region despite excess reddening along the line of sight. Self-consistent re-calculation of the models including Galactic-ISM-like dust results in an ionization parameter log U = -0.25 and a column density log N_w = 21.8 of the warm absorber. X-ray variability and dust survival arguments are used to constrain the density and further properties of the ionized material. The influence of dust and other parameters on the thermal stability of the warm absorber is investigated. Predictions are given for the absorber-intrinsic optical--UV line emission and absorption.

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arXiv1997

Interpretation of the emission line spectra of Seyfert 2 galaxies by multi-component photoionization models

We present multi-component photoionization models allowing for local density inhomogeneities in the NLR to interpret the emission line spectra of Seyfert 2 galaxies. This approach leads to a successful match of a large set of line intensities from the UV to the NIR. In particular, the hitherto elusive NIR features [SIII]9062+9531 as well as high-ionization lines like [FeVII] are consistently fitted. The predictions of CIII] and CIV are considerably improved. From the detailed analysis of single-component photoionization models we derive the minimal radial extent of the NLR and the necessary span in density. Furthermore, we determine constraints on suggestions made about the role of matter-bounded clouds, and on proposed explanations for large [OIII]4363/5007 ratios (the so-called `temperature problem'), and assess the usability of some emission-line ratios as indicators of the ionization parameter. We find that a systematic variation of the cloud column densities in a population of matter-bounded clouds is inconsistent with the trends and correlations exhibited by the emission lines in the diagnostic diagrams. Concerning the temperature problem, the only possibility that leads to an overall consistency with the strengths of all other observed emission lines is subsolar metal abundances (as compared to e.g. the presence of dust, the existence of a high-density component, or matter-bounded clouds). In addition, the consequences of the presence of (Galactic-ISM-like) dust internal to the clouds were investigated. These models alleviate the [OIII]-ratio problem but did not lead to overall consistent fits. In our final model series, the NLR is composed of a mixture of metal-depleted (0.5 x solar) clouds with a radius-independent range in densities (10^2 to 10^5 cm^-3) distributed over a range of distances from the nucleus (galactocentric radii from at least $\sim$ 10^{20} cm to 10^{21.5} cm, for $Q_{tot} = 10^{54}$ s^{-1}). In order to encompass the observed range of each line intensity relative to H$β$, it turns out to be necessary to vary the spectral energy distribution incident on the clouds, qualitatively confirming the findings of Ho et al. (1993). We found a successful continuum sequence by adding an increasing contribution of a hot black body ($T \approx 200000$ K) to a steep powerlaw ($α_{uv-x} \approx -2$). These continua imply that low and high-excitation objects differ in the strength but not in the basic shape of the EUV bump.

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arXiv1996

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 RXJ0134-42 and the drastic flux variability in NGC 5905 via warm absorption is scrutinized.

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arXiv1996

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 log U = 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.

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