arXiv subjects

Stefanie Komossa

Publications and source records attributed to Stefanie Komossa.

At least 55 records · Page 3Linked to original sources

arXiv2002

The X-ray evolving universe: (ionized) absorption and dust, from nearby Seyfert galaxies to high-redshift quasars

(Abridged) Cold and warm absorbers have beeen detected in all types of active galaxies (AGN) from low to high redshift. This gas, located in the black hole region of AGN, is thought to play an important role in AGN unification scenarios, in explaining the X-ray background, in black hole growth and AGN evolution. We provide a review of the observations of dusty and dust-free warm and cold absorbers at low and high redshift, including most recent results and exciting questions still open. Emphasis is on the science issues that we will be able to address with XEUS for the first time, particularly at high redshift, including: (i) determination of metal abundances of X-ray (cold) absorbers by detection of metal absorption edges, (ii) analysis of the composition of dust mixed with cold and ionized gas (K-edges of metals in cold dust and cold gas will be resolvable from each other for the first time), (iii) measurement of the velocity field of the gas, (iv) utilization of these results to investigate the evolution of gas and dust in AGN from high to low redshift: the evolution of abundances, dust content, ionization state, amount and velocity of gas, and its role in feeding the black hole. We emphasize the importance of iron absorption measurements with XEUS at high redshift for two key issues of cosmology: the early star formation history of the universe, and the measurement of cosmological parameters. As an example, we discuss recent XMM-Newton observations of the high-redshift BAL quasar APM 08279+5255.

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arXiv2001

X-ray Evidence for Supermassive Black Holes in Non-Active Galaxies. Subtitle: Detection of X-ray Flare Events, Interpreted as Tidal Disruptions of Stars by SMBHs

It has long been suggested that supermassive black holes in non-active galaxies might be tracked down by occasional tidal disruptions of stars on nearly radial orbits. A tidal disruption event would reveal itself by a luminous flare of electromagnetic radiation. Theorists argued that the convincing detection of such a tidal disruption event would be the observation of an event which fulfills the following three criteria: (1) the event should be of finite duration (a `flare'), (2) it should be very luminous (up to L_{max} ~ 10^{45} erg/s in maximum), and (3) it should reside in a galaxy that is otherwise perfectly {\em non-active} (to be sure to exclude an upward fluctuation in gaseous accretion rate of an {\em active} galaxy). During the last few years, several X-ray flare events were detected which match exactly the above criteria. We therefore consider these events to be excellent candidates for the occurrences of the theoretically predicted tidal disruption flares. In this contribution, we review the previous observations of giant X-ray flares from normal galaxies, present new results on these objects, critically discuss alternatives to the favored outburst scenario, and report results from our ongoing search for further tidal disruption flares based on the ROSAT all-sky survey database.

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arXiv2001

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

In the last few years, giant-amplitude, non-recurrent X-ray flares have been observed from several non-active galaxies (NGC 5905, RXJ1242-11, RXJ1624+75, RXJ1420+53, RXJ1331-32). All of them share similar properties, namely: extreme X-ray softness in outburst, huge peak luminosity (up to ~10^{44} erg/s), and the absence of optical signs of Seyfert activity. Tidal disruption of a star by a supermassive black hole is the favored explanation of these unusual events. We present a review of the previous results, a search for radio emission from all outbursters, based on the NVSS database, and dedicated radio observations of NGC 5905 carried out with the VLA. These provide important constraints on the presence of an (obscured) active nucleus (AGN) at the center of each flaring galaxy. We rigorously explore AGN scenarios to account for the unusual X-ray outbursts from the optically `normal' galaxies and find AGN-related models highly unlikely. We conclude that the previously favored scenario - tidal disruption of a star by a supermassive black hole at the center of each of the outbursters - provides the best explanation for the X-ray observations. Finally, we present results from our on-going search for further X-ray flares from a sample of ~140 nearby active and non-active galaxies, using the ROSAT data base. While we do not find another X-ray flaring normal galaxy among this sample - entirely consistent with the predictions of the tidal disruption scenario - several highly variable active galaxies are detected. Their variability is not linked to tidal disruption, but best explained in terms of absorption or accretion-disk-related models.

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arXiv2001

The X-ray properties of the low-redshift quasar MRC 2251-178 and its environment: some surprises

MR 2251-178 was the first quasar initially discovered in X-rays, and the first one found to host a warm absorber. Despite its many outstanding multi-wavelength properties, MR 2251-178 has not yet been studied in detail with recent X-ray observatories. Here, we present results from an analysis of the X-ray spectral, temporal, and spatial properties of this source and its environment based on deep observations carried out with the X-ray satellite {\sl ROSAT}. The derived mean soft X-ray luminosity of MR 2251-178, ~10^{45} erg/s, places the quasar among the most X-ray luminous AGN in the local universe. The complete results of this study appeared in A&A 367, 801 (see http://www.xray.mpe.mpg.de/~skomossa/).

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arXiv2001

Supersolar metal abundances and the Broad Line Region of Narrow-line Seyfert 1 galaxies

Narrow-line Seyfert 1 galaxies (NLSy1) are intriguing due to their extreme continuum and emission line properties which are not yet well understood. This paper is motivated by the recent suggestion that NLSy1 galaxies might be young active galactic nuclei (AGNs) and that they may have supersolar metal abundances. We have examined the stability of broad emission line region (BLR) clouds under isobaric perturbation and its dependence on gas metal abundances. We show that supersolar metallicity increases the range where multiple phases in pressure balance may exist. This is particularly important for NLSy1s with steep X-ray spectra where it delays the trend of hindrance of a multi-phase equilibrium. Finally, the role of warm absorbers in the context of this scenario is assessed.

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arXiv2001

A two-phase model for the Narrow Line Region of NGC 4151

NGC 4151 is one of the brightest and best-studied Seyfert galaxies. Here, we present a two-phase model of the narrow-line region (NLR) of NGC 4151. This study is motivated by (i) the fact that the X-ray spectrum of NGC 4151 is among the flattest known for Seyferts, and (ii) the recent Chandra detection of an X-ray narrow-line region in this galaxy. X-ray spectra as flat as that of NGC 4151 (Gamma_x = -1.5) are expected to favor the presence of two gas phases in pressure equilibrium (Krolik, McKee & Tarter 1981). In the present study, we show that a pronounced two-phase equilibrium develops in the extended emission-line region if we use the observed multi-wavelength spectrum of NGC 4151 to ionize the clouds. The material is stable to isobaric perturbations over a wide range of temperatures. We therefore propose that such a condition has arisen in the NLR of NGC 4151, and that it explains the detection of hot, extended X-ray gas which we identify as the NLR-cloud confining medium.

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arXiv2001

The X-ray view of the quasar MR2251-178 and its host cluster: variability, absorption, and intracluster gas emission

(abridged) MR2251-178 was the first quasar initially discovered in X-rays, and the first one found to host a warm absorber. The quasar turned out to be an outstanding object in many respects. It has a high ratio of X-ray/optical luminosity, is surrounded by the largest quasar emission-line nebula known, and is located in the outskirts of a cluster of galaxies. Here, we present results from an analysis of the X-ray spectral, temporal, and spatial properties of this source and its environment based on deep ROSAT observations. Remarkably, we do not detect any excess X-ray cold absorption expected to originate from the giant gas nebula surrounding MR2251-178. This excludes the presence of a huge HI envelope around the quasar. The X-ray spectrum of MR2251-178 is best fit by a warm-absorbed powerlaw with an ionization parameter log U = 0.5 and a column density log N_w = 22.6 which, however, cannot be the same material as the giant optical emission line nebula. The mean (0.1--2.4)keV X-ray luminosity amounts to 10^{45} erg/s. A spatial analysis shows that the bulk of the X-ray emission from the quasar is consistent with a point source, as expected in view of the powerlaw-shaped X-ray spectrum and the rapid X-ray variability we detect. In addition, extended emission appears at weak emission levels, including a bridge between the quasar and the cD galaxy of the cluster. The X-ray emission from the intra-cluster medium is weak or absent. None of the other member galaxies of the cluster to which MR 2251-178 belongs, are detected in X-rays. However, east of the quasar there is a significant excess of X-ray sources, several of them without optical counterparts on the UK Schmidt plates.

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arXiv2001

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.

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arXiv2000

An optical and X-ray study of the peculiar narrow-line quasar QSO 0117-2837

We present an optical and X-ray study of the quasar QSO 0117-2837. It exhibits an interesting combination of optical and X-ray properties: Despite its extremely steep observed X-ray spectrum (Gamma_x ~ -4 when fit by a simple powerlaw), its Balmer lines are fairly broad. A two-component Gaussian fit to H_beta yields FWHM_{Hbeta, broad} ~ 4000 km/s, and places QSO 0117-2837 in the `zone of avoidance' in the FWHM_{Hbeta}-Gamma_x diagram. A time variability analysis shows that QSO 0117-2837 is another case of Narrow-line Sy1 galaxy (NLSy1 hereafter) which does not show any X-ray variability during the observation. The results are discussed in view of the NLSy1 character of QSO 0117-2837.

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arXiv2000

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.

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arXiv2000

RXTE observation of NGC 6240: a search for the obscured active nucleus

The wide-band energy spectrum of NGC6240 over the range 0.5-200keV is investigated using the RXTE and ASCA data. The RXTE data provide the spectrum beyond the ASCA range (0.5-10 keV) with significant detection of signals up to 20keV and the upper limits above 20keV. The spectrum above 10keV is found to be very flat. A strong iron-K emission line discovered in the previous ASCA observation is also confirmed with the RXTE PCA. These results provide further evidence for the dominance of a reflection component, i.e. emission from cool material illuminated by an AGN. By fitting the spectra obtained with RXTE and ASCA simultaneously, we satisfactorily modeled the AGN spectrum with a Compton reflection component and probably a transmitted AGN component penetrating through a thick absorber. The X-ray luminosity of the AGN is estimated to be in the range 4x10^43 - 6x10^44 ergs/s in the range 2-10keV, which categorizes NGC6240 among the most luminous Seyfert nuclei. The ratio of the 2-10keV X-ray luminosity to the infrared luminosity, L_X(2-10keV)/L_IR, is 0.01 - 0.1, which implies a quite substantial, if not dominant, contribution of AGN to the infrared luminosity.

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arXiv2000

Warm Absorbers in Active Galactic Nuclei

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 NGC 4051, 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.

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arXiv1999

X-ray emission/absorption mechanisms of 4 NLSy-1-like AGN and a radio quasar: QSO 0117-2837, RXJ0134.3-4258, NGC 4051, Mrk 1298, and 4C +74.26

We present a study of the X-ray variability properties and spectral shapes of five active galaxies all of which show extreme or enigmatic X-ray properties. We focus on QSO 0117-2837, RXJ0134.3-4258, and NGC 4051, and briefly comment on Mrk 1298 and 4C +74.26. The individual objects were originally partly selected as candidates to host warm absorbers on the basis of (i) characteristic X-ray absorption features (NGC 4051, Mrk 1298, 4C +74.26), (ii) extreme X-ray spectral steepness (QSO 0117-2837; this object is found to be located in the `zone of avoidance' when plotted in the Gamma_x - FWHM_Hbeta diagram), and (iii) drastic spectral variability (RXJ0134.3-4258). The temporal analysis reveals large-amplitude variability by a factor ~30 in the long-term X-ray lightcurve of NGC 4051, very rapid variability of Mrk 1298, constant X-ray flux of the NLSy1 galaxy QSO 0117-2837, and constant mean countrate of RXJ0134.3-4258 despite huge spectral changes. Besides the warm absorber, several further mechanisms and their merits/shortcomings are investigated to explain the spectral characteristics of the individual objects. Different models are favored for different sources. Consequences for Narrow-line Seyfert 1s in general are discussed and we present results from photoionization models to distinguish between different suggested NLSy1 scenarios.

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arXiv1999

Discovery of a giant and luminous X-ray outburst from the optically inactive galaxy pair RXJ1242.6-1119

We report the discovery of large-amplitude X-ray variability from the direction of the previously unknown, optically inactive galaxy pair RXJ1242.6-1119. The X-ray source shows variability by a factor > 20 between the ROSAT all-sky survey and a later pointed observation separated by ~1.5 yr. Its spectrum is extremely soft with photon index Gamma_x ~ -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 > 9x10^{43} erg/s. Surprisingly, the optical spectra of both galaxies are characterized by absorption lines and do not show signs of (Seyfert) activity. This makes RXJ1242-11 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 & 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 event.

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arXiv1999

Galactic Fountains and Galactic Winds

The development of galactic fountain theories is reviewed with special emphasis on the different approaches and concepts that have been used in the past. In particular the outstanding contribution of Franz Kahn to our physical understanding is appreciated. It is argued that galactic outflows represent an essential phase during galaxy evolution. The dynamics of the outflow imprints its signature on the emission spectra of soft X-rays, which may well be observable with AXAF and XMM. Finally, some remarks about winds in starburst galaxies and AGN are made.

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arXiv1999

Testing Narrow-line Seyfert 1 scenarios with photoionization models

Narrow Line Seyfert 1 (NLSy1) galaxies have recently received a lot of attention due to their unusual optical--X-ray properties which are not yet well understood. Photoionization models of the circum-nuclear emission/absorption regions allow to investigate scenarios to explain the main characteristics of NLSy1s, i.e., (i) extremely steep X-ray spectra within the ROSAT energy range, (ii) narrow Balmer lines and (iii) weak forbidden lines except for some relatively strong high-ionization iron lines. Here, we focus on (iii). In particular, we study the influence of different EUV - soft-X-ray spectral shapes (a giant soft excess, a steep X-ray powerlaw, presence of a warm absorber) and NLR cloud properties on the predicted optical emission-line ratios. The calculations were carried out with Ferland's code Cloudy.

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arXiv1999

The Steep Spectrum Quasar PG1404+226 with ASCA, HST and ROSAT

(abridged) We present and discuss our observations of the NL quasar PG1404+226 with ASCA and HST, and a re-analysis of our earlier observations with ROSAT. The soft X-ray spectrum is very steep and displays an absorption feature (edge or line at ~1.1 keV). We have applied a variety of models to the ASCA and ROSAT spectra without finding a completely satisfactory fit, and the identification of the edge remains uncertain. A satisfactory fit of the ASCA spectrum assuming that the edge is produced by highly ionized iron (using the code absori in XSPEC) is obtained with an overabundance of iron by a factor > 25 compared to solar, a suggestion supported by the extremely high equivalent width of the Fe K_alpha line at 6.4 keV. A warm absorber model fitting the absorption feature with NeVII-NeX edges and assuming a peculiar oxygen/neon abundance ratio is consistent with the ROSAT data but not the ASCA data. Finally, it is also possible that the observed edge is caused by a OVIII or OVII edge or line, blueshifted by z_abs=0.2 to 0.5 depending on the specific identification, as has been suggested previously for 2 other NL quasars, but there are no other features in the UV and X-ray spectra in support of this suggestion. Two systems of UV absorption lines, one nearly at rest in the source frame, the other blueshifted by ~1900 km/s are identified in the HST/FOS spectra. Photoionization models indicate that the UV absorption and the ~1 keV absorption are probably caused by absorbers with different physical conditions. PG1404+226 is one more case of AGN where both UV and X-ray absorption features are detected, thereby increasing further the significance of the previously noted statistical association of the two types of absorbers.

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arXiv1999

X-ray properties of LINERs

We present an investigation of the X-ray properties of 13 LINERs based on ROSAT all-sky survey and pointed PSPC and HRI observations. Several sources are studied for the first time in X-rays. The X-ray spectra are best described by a powerlaw with photon index Gamma ~ -2.0 or thermal emission from gas with very subsolar abundances. The luminosities range between log L_x = 37.7 (NGC 404) and 40.8 (NGC 4450). No X-ray variability on the timescale of hours/days is detected. This is in line with the suggestion that LINERs may accrete in the advection-dominated mode. On a longer term, one of the objects, NGC 2768, turns out to be slightly variable. Some sources appear to be extended at weak emission levels whereas the bulk of the X-ray emission is consistent with arising from a point source within the PSPC instrumental resolution. L_x/L_B ratios are derived and emission mechanisms that potentially contribute to the observed X-ray luminosities are discussed. We also examine the presence of second X-ray sources near the target sources both in terms of instrumental effects and in terms of `real' sources within the LINER galaxies.

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