arXiv subjects

K. A. Weaver

Publications and source records attributed to K. A. Weaver.

At least 37 records · Page 2Linked to original sources

arXiv2003

The quest for hot gas in the halo of NGC 1511

XMM-Newton observations of the starburst galaxy NGC 1511 reveal the presence of a previously unknown extended hot gaseous phase of its ISM, which partly extends out of the disk plane. The emission distribution is asymmetric, being brightest in the eastern half of the galaxy, where also radio continuum observations suggest the highest level of star formation. Spectral analysis of the integral 0.2-12 keV X-ray emission from NGC 1511 indicates a complex emission composition. A model comprising a power law plus thermal plasma component, both absorbed by foreground gas, cannot explain all details of the observed spectrum, requiring a third spectral component to be added. This component can be a second thermal plasma, but other spectral models can be fitted as well. Its X-ray properties characterize NGC 1511 as a starburst galaxy. The X-ray-to-infrared luminosity ratio is consistent with this result. Together with the X-ray data, XMM-Newton obtained UV images of NGC 1511, tracing massive stars heating the ambient gas, which is then seen in Hαemission. UV, Hαand near-infrared imagery suggest that NGC 1511 is disturbed, most likely by its two small companions, NGC 1511a and NGC 1511b.

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arXiv2002

Recent progress in understanding the hot and warm gas phases in the halos of star-forming galaxies

In this contribution we present a few selected examples of how the latest generation of space-based instrumentation -- NASA's Chandra X-ray Observatory and the Far-Ultraviolet Spectroscopic Explorer (FUSE) -- are finally answering old questions about the influence of massive star feedback on the warm and hot phases of the ISM and IGM. In particular, we discuss the physical origin of the soft thermal X-ray emission in the halos of star-forming and starburst galaxies, its relationship to extra-planar H-alpha emission, and plasma diagnostics using FUSE observations of O VI absorption and emission.

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arXiv2002

Extreme X-ray Iron Lines in Active Galactic Nuclei

We analyze X-ray spectra of heavily obscured (N_H > 10^{24} cm^{-2}) active galaxies obtained with Chandra, concentrating on the iron K alpha fluorescence line. We measure very large equivalent widths in most cases, up to 5 keV in the most extreme example. The geometry of an obscuring torus of material near the active galactic nucleus (AGN) determines the Fe emission, which we model as a function of torus opening angle, viewing angle, and optical depth. The starburst/AGN composite galaxies in this sample require small opening angles. Starburst/AGN composite galaxies in general therefore present few direct lines of sight to their central engines. These composite galaxies are common, and their large covering fractions and heavy obscuration effectively hide their intrinsically bright X-ray continua. While few distant obscured AGNs have been identified, we propose to exploit their signature large Fe K alpha equivalent widths to find more examples in X-ray surveys.

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arXiv2001

Chandra observations of NGC 253. II: On the origin of diffuse X-ray emission in the halos of starburst galaxies

We present a detailed case study of the diffuse X-ray and H-alpha emission in the halo of NGC 253, a nearby edge-on starburst galaxy driving a galactic superwind. The arcsecond spatial resolution of the Chandra ACIS instrument allows us to study the spatial and spectral properties of the diffuse X-ray emitting plasma with greatly superior spatial and spectral resolution compared to previous X-ray instruments. We find statistically significant structure within the X-ray diffuse emission on angular scales down to ~10 arcsec (~130 pc). There is no statistically significant evidence for any spatial variation in the spectral properties of the diffuse emission, over scales from ~400 pc to \~3 kpc. We show that the X-shaped soft X-ray morphology of the superwind, previously revealed by ROSAT, is matched by very similar X-shaped H-alpha emission, extending at least 8 kpc above the plane of the galaxy. In the northern halo the X-ray emission appears to lie slightly interior to the boundary marked by the H-alpha emission. The total 0.3-2.0 keV energy band X-ray luminosity of the northern halo, L_X ~ 5e38 erg/s, is very similar to the halo H-alpha luminosity of L_Ha ~ 4e38 erg/s, both of which are a small fraction of the estimated wind energy injection rate of ~1e42 erg/s from supernovae in the starburst. We show that there are a variety of models that can simultaneously explain spatially-correlated X-ray and H-alpha emission in the halos of starburst galaxies. These findings indicate that the physical origin of the X-ray-emitting million-degree plasma in superwinds is closely linked to the presence of much cooler and denser T ~ 1e4 K gas, not only within the central kpc regions of starbursts, but also on ~10 kpc-scales within the halos of these galaxies. (Abridged)

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arXiv2001

Another intermediate mass black hole in a starburst galaxy?: The luminous X-ray source in NGC 3628 reappears

In a 52 ks-long Chandra ACIS-S observation of the nearby starburst galaxy NGC 3628, obtained to study the starburst-driven outflow from this galaxy, we have detected a very luminous (L_X = 1.1e40 erg/s in the 0.3-8.0 keV energy band) point source located at least 20 arcsec (~970 pc) from the nucleus of the galaxy. No radio, optical or near-IR counterpart to this source has been found. This is most probably the reappearance of the strongly-variable X-ray-luminous source discovered by Dahlem et al (1995), which faded by a factor >27 between December 1991 and March 1994 (at which point it had faded below the detection limit in a ROSAT HRI observation). This source is clearly a member of an enigmatic class of X-ray sources that are considerably more luminous than conventional X-ray binaries but less luminous than AGN, and which are not found at the dynamical center of the host galaxy. The Chandra spectrum is best-fit by an absorbed power law model with a photon index of Gamma = 1.8+/-0.2, similar to that seen in Galactic BH binary candidates in their hard state. Bremsstrahlung models or multi-color disk models (the favored spectral model for objects in this class based on ASCA observations) can provide statistically acceptable fits only if the data at energies E > 5 keV is ignored. This is one of the first X-ray spectra of such an object that is unambiguously that of the source alone, free from the spectral contamination by X-ray emission from the rest of the galaxy that affects previous spectral studies of these objects using ASCA.

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arXiv2001

The Obscuring Starburst of NGC 6221 and Implications for the Hard X-ray Background

We present NGC 6221 as a case study of ``X-ray loud composite galaxies,'' which appear similar to starbursts at optical wavelengths and resemble traditional active galactic nuclei in X-rays. The net optical spectrum of NGC 6221 is properly characterized as a starburst galaxy, but in X-rays, NGC 6221 is similar to Seyfert 1 galaxies, exhibiting a power-law continuum spectrum, a broad Fe K alpha line, and continuum variability on timescales of days and years. High-resolution images reveal that the detected active nucleus is relatively weak, not only at optical, but also at near-infrared wavelengths. An obscuring starburst, in which the interstellar gas and dust associated with the starburst conceal the active nucleus, accounts for these peculiar features. We demonstrate quantitatively that obscuration by column density N_H = 10^{22} cm^{-2} combined with relatively weak intrinsic nuclear activity can produce an optical spectrum characteristic of the surrounding starburst alone. While optical surveys would not identify the active nuclei that make these galaxies significant X-ray sources, such galaxies may in fact be important contributors to the X-ray background.

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arXiv2000

The Seyfert-Starburst Connection in X-rays. II. Results and Implications

We present the results of X-ray imaging and spectroscopic analysis of a sample of Seyfert 2 galaxies that contain starbursts, based on their optical and UV characteristics. These composite galaxies exhibit extended, soft, thermal X-ray emission, which we attribute to their starburst components. Comparing their X-ray and far-infrared properties with ordinary Seyfert and starburst galaxies, we identify the spectral characteristics of their various intrinsic emission sources. The observed far-infrared emission of the composite galaxies may be associated almost exclusively with star formation, rather than the active nucleus. The ratio of the hard X-ray luminosity to the far-infrared and [O III] 5007 luminosity distinguishes most of these composite galaxies from ``pure'' Seyfert 2 galaxies, while their total observed hard X-ray luminosity distinguishes them from ``pure'' starbursts. The hard nuclear X-ray source is generally heavily absorbed (N_H > 10^{23} cm^{-2}) in the composite galaxies. Based on these results, we suggest that the interstellar medium of the nuclear starburst is a significant source of absorption. The majority of the sample are located in groups or are interacting with other galaxies, which may trigger the starburst or allow rapid mass infall to the central black hole, or both. We conclude that starbursts are energetically important in a significant fraction of active galaxies, and starbursts and active galactic nuclei may be part of a common evolutionary sequence.

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arXiv2000

The Seyfert-Starburst Connection in X-rays. I. The Data

We analyze X-ray spectra and images of a sample of Seyfert 2 galaxies that unambiguously contain starbursts, based on their optical and UV characteristics. Although all sample members contain active galactic nuclei (AGNs), supermassive black holes or other related processes at the galactic centers alone cannot account for the total X-ray emission in all instances. Eleven of the twelve observed galaxies are significantly resolved with the ROSAT HRI, while six of the eight sources observed with the lower-resolution PSPC also appear extended on larger scales. The X-ray emission is extended on physical scales of 10 kpc and greater, which we attribute to starburst-driven outflows and supernova-heating of the interstellar medium. Spectrally, a physically-motivated composite model of the X-ray emission that includes a heavily absorbed (N_H > 10^{23} cm^{-2}) nuclear component (the AGN), power-law like scattered AGN flux, and a thermal starburst describes this sample well. Half the sample exhibit iron K alpha lines, which are typical of AGNs.

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arXiv2000

X-ray imaging of the Seyfert 2 galaxy Circinus with Chandra

We present results from the zeroth-order imaging of a Chandra HETGS observation of the nearby Seyfert 2 galaxy Circinus. Twelve X-ray sources were detected in the ACIS-S image of the galaxy, embedded in diffuse X-ray emission. The latter shows a prominent (~18arcsec) soft ``plume'' in the N-W direction, coincident with the [OIII] ionization cone. The radial profiles of the brightest X-ray source at various energies are consistent with an unresolved (FWHM ~0.8arcsec) component, which we identify as the active nucleus, plus two extended components with FWHMs ~ 2.3arcsec and 18arcsec, respectively. In a radius of 3arcsec, the nucleus contributes roughly the same flux as the extended components at the softest energies (< 2 keV). However, at harder energies (> 2 keV), the contribution of the nucleus is dominant. The zeroth-order ACIS spectrum of the nucleus exhibits emission lines at both soft and hard X-rays, including a prominent Fe Kalpha line at 6.4 keV, showing that most of the X-ray lines previously detected with ASCA originate in a compact region (<15 pc). Based on its X-ray spectrum, we argue that the 2.3arcsec extended component is scattered nuclear radiation from nearby ionized gas. The large-scale extended component includes the emission from the N-W plume and possibly from the outer starburst ring.

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arXiv2000

High-resolution X-ray spectroscopy of the Seyfert 2 galaxy Circinus with Chandra

Results from a 60 ks Chandra HETGS observation of the nearby Seyfert 2 Circinus are presented. The spectrum shows a wealth of emission lines at both soft and hard X-rays, including lines of Ne, Mg, Si, S, Ar, Ca, and Fe, and a prominent Fe Kalpha line at 6.4 keV. We identify several of the He-like components and measure several of the Lyman lines of the H-like ions. The lines' profiles are unresolved at the limited signal-to-noise ratio of the data. Our analysis of the zeroth-order image in a companion paper constrains the size of the emission region to be 20-60 pc, suggesting that emission within this volume is almost entirely due to the reprocessing of the obscured central source. Here we show that a model containing two distinct components can reproduce almost all the observed properties of this gas. The ionized component can explain the observed intensities of the ionized species, assuming twice-solar composition and an N \propto r^{-1.5} density distribution. The neutral component is highly concentrated, well within the 0.8arcsec point source, and is responsible for almost all of the observed Kalpha (6.4 keV) emission. Circinus seems to be different than Mkn~3 in terms of its gas distribution.

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arXiv2000

Variable Iron K-alpha Lines in Seyfert 1 Galaxies

We find that variability of the iron K-alpha line is common in Seyfert 1 galaxies. Using data from the ASCA archive for objects that have been observed more than once during the mission, we study the time-averaged spectra from individual observations, thereby probing variability on timescales that range from days to years. Since the statistics of the data do not warrant searches for line variability in terms of a complex physical model, we use a a simple Gaussian to model the gross shape of the line, and then use the centroid energy, intensity and equivalent width as robust indicators of changes in the line profile. We find that ~70% of Seyfert 1s (ten out of fifteen) show variability in at least one of these parameters: the centroid energy, intensity, and equivalent width vary in six, four, and eight sources respectively. Due to the low S/N, limited sampling and time averaging, we consider these results to represent lower limits to the rate of incidence of variability. In most cases changes in the line do not appear to track changes in the continuum. In particular, we find no evidence for variability of the line intensity in NGC 4151, suggesting an origin in a region larger than the putative accretion disk, where most of the iron line has been thought to originate. Mkn 279 is investigated on short timescales. The time-averaged effective line energy is 6.5 keV in the galaxy rest frame. As the continuum flux increases by 20% in a few hours, the Fe K line responds with the effective line energy increasing by 0.22 keV (~10,500 km s^-1). Problems with the ASCA and Rosat calibration that affect simultaneous spectral fits are discussed in an appendix.

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arXiv2000

Probing Dense Matter in the cores of AGN: Observations with RXTE and ASCA

Preliminary results from an X-ray spectral study of Seyfert 1 galaxies with ASCA and RXTE are presented. From an analysis of X-ray reprocessing features of Compton reflection and Fe K-alpha fluorescence, it is found that iron line strength is not necessarily a good predictor of the amount of reflection. The variability properties of Fe K-alpha and reflection do not necessarily scale together and substantial decoupling of the behavior of the reprocessed flux with respect to continuum variability is common. Such trends suggest the presence of multiple and/or complex regions of dense matter in AGN cores and that standard accretion disk models drastically oversimplify reality.

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arXiv2000

Ambiguities in fits to the complex X-ray spectra of starburst galaxies

Spectral fits to X-ray data from both NGC 253 and M 82 provide ambiguous results. The so-called ``best fit'' results depend on the instrument with which the data were obtained and obviously on the choice of spectral model composition. We show that different spectral models can be fit equally well to BeppoSAX data of both galaxies. Metallicities are unreliable in general, with a strong dependence on the choice of model. Preference to one particular spectral model can only be given by combining spectroscopic and imaging X-ray data from all available satellites (ROSAT, ASCA, and BeppoSAX). Based on spectra of NGC 253, we demonstrate that a model consisting of two or more thermal plasma components plus a hard power law continuum and Fe K alpha line emission can explain all observations. These model components represent the integral spectrum of thermal gas and compact sources in starburst galaxies that are most likely supernova remnants and X-ray binaries. The same model can fit the X-ray data of M 82, but there the evidence, from ROSAT imaging, of the existence of compact sources which might represent high-mass X-ray binaries is weaker. This implies that its hard X-ray emission, which is extended in ROSAT images, might - if truly diffuse - be dominated by a very hot (several keV energy) thermal gas component.

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arXiv2000

The variability of the Seyfert galaxy NGC 2992: the case for a revived AGN

We report the transition to an active state of the nucleus in the Seyfert 1.9 galaxy NGC 2992, discovered by means of new hard X-ray data. While the 2-10 keV flux declined by a factor of ~ 20 from 1978 to 1994, two recent BeppoSAX observations in 1997 and in 1998 caught the nuclear emission raising back to the same level of activity observed in 1978. In both BeppoSAX observations the X-ray spectrum of the source is well represented by a power law with spectral index Gamma ~ 1.7, absorbed by a column density of N_H ~ 10^{22} cm^{-2} and characterized by a prominent iron Kαline. While in the second BeppoSAX data set the line properties appear to be consistent with those expected from accretion disc models, in the first BeppoSAX data set the iron feature is rather peculiar. The broadening is not significant and the line energy is E_{Kα}=6.62 +/- 0.07 keV, indicating emission from highly ionized iron. The line has too high equivalent width (~ 700 eV) to be produced by a hot scattering medium. By comparing these data with data previously in the literature, we interpret the spectral and flux changes in terms of different phases of rebuilding an accretion disc. The timescale for the disc rebuilding is estimated to range between 1 and 5 years. The X-ray data are complemented with optical and near-infrared followup spectra taken 1.5 months after the discovery of the X-ray burst. The spectra are characterized by prominent broad emission lines. There is also evidence for hot dust emission in the H and K bands that, however, is probably still in the process of increasing.

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arXiv1999

An X-ray Mini-survey of Nearby Edge-on Starburst Galaxies II. The Question of Metal Abundance

(abbreviated) We have undertaken an X-ray survey of a far-infrared flux limited sample of seven nearby edge-on starburst galaxies. Here, we examine the two X-ray-brightest sample members NGC 253 and M 82 in a self-consistent manner, taking account of the spatial distribution of the X-ray emission in choosing our spectral models. There is significant X-ray absorption in the disk of NGC 253. When this is accounted for we find that multi-temperature thermal plasma models with significant underlying soft X-ray absorption are more consistent with the imaging data than single-temperature models with highly subsolar abundances or models with minimal absorption and non-equilibrium thermal ionization conditions. Our models do not require absolute abundances that are inconsistent with solar values or unusually supersolar ratios of the alpha-burning elements with respect to Fe (as claimed previously). We conclude that with current data, the technique of measuring abundances in starburst galaxies via X-ray spectral modeling is highly uncertain. Based on the point-like nature of much of the X-ray emission in the PSPC hard-band image of NGC 253, we suggest that a significant fraction of the ``extended'' X-ray emission in the 3-10 keV band seen along the disk of the galaxy with ASCA and BeppoSAX (Cappi et al.) is comprised of discrete sources in the disk, as opposed to purely diffuse, hot gas. This could explain the low Fe abundances of ~1/4 solar derived for pure thermal models.

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arXiv1999

X-ray Observations of AGN at Intermediate to High Redshift

The cores of active galactic nuclei (AGN) harbor some of the most extreme conditions of matter and energy in the Universe. One of the major goals of high-energy astrophysics is to probe these extreme environments in the vicinity of supermassive black holes, which are intimately linked to the mechanisms that produce the continuum emission in AGN. X-ray studies seek to understand the physics responsible for the continuum emission, its point of origin, how nuclear activity is fueled, and how supermassive black holes evolve. The key to finding answers to these questions lies in measuring the intrinsic luminosities and spectral shapes, the relation of these properties to other wavebands, and how the source properties change with redshift. This article reviews X-ray observations of AGN from redshifts of ~0.1-3 with the goal of summarizing our current knowledge of their X-ray spectral characteristics. Results are evaluated in terms of their robustness and are examined in the light of current theoretical predictions of energy release via processes associated with the accretion mechanism. A possible evolutionary scenario is discussed, along with the importance of AGN studies at high redshift as they relate to the total energetics of the Universe.

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arXiv1999

Very Extended X-ray and H-alpha Emission in M82: Implications for the Superwind Phenomenon

We discuss the properties and implications of a 3.7x0.9 kpc region of spatially-coincident X-ray and H-alpha emission about 11.6 kpc to the north of the galaxy M82 previously discussed by Devine and Bally (1999). The PSPC X-ray spectrum is fit by thermal plasma (kT=0.80+-0.17 keV) absorbed by only the Galactic foreground column density. We evaluate the relationship of the X-ray/H-alpha ridge to the M82 superwind. The main properties of the X-ray emission can all be explained as being due to shock-heating driven as the superwind encounters a massive ionized cloud in the halo of M82. This encounter drives a slow shock into the cloud, which contributes to the excitation of the observed H-alpha emission. At the same time, a fast bow-shock develops in the superwind just upstream of the cloud, and this produces the observed X-ray emission. This interpretation would imply that the superwind has an outflow speed of roughly 800 km/s, consistent with indirect estimates based on its general X-ray properties and the kinematics of the inner kpc-scale region of H-alpha filaments. The gas in the M82 ridge is roughly two orders-of-magnitude hotter than the minimum "escape temperature" at this radius, so this gas will not be retained by M82. (abridged)

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arXiv1999

X-ray Emission from the Prototypical LINER Galaxy NGC 1052

We examine the 0.1 to 10.0 keV X-ray spectrum of the bright nuclear LINER galaxy NGC 1052, one of two elliptical galaxies known to contain a luminous H2O maser. The observed 2.0-10.0 keV spectrum is unusually flat (photon index Gamma~0.2) and best described as intrinsically power-law shaped nuclear flux that is either (1) attenuated by a complex absorber with ~70% of the nuclear flux absorbed by a column density of NH ~ 3x10^23 cm^{-2} and ~30% absorbed by a column density of NH ~ 3-5x10^{22} cm^{-2}, or (2) reprocessed, with the nuclear source blocked and the X-rays Compton reflected in our direction by high column density (>=10^{24} cm^{-2}) gas. The moderate equivalent width of the Fe K-alpha line favors the dual absorption model as the most likely scenario. The 0.1-2.0 keV spectrum does not resemble the few times 10^6 to 10^7 K thermal emission typically found in other elliptical galaxies, but instead is best described as nuclear X-rays leaking through a patchy absorber or scattered in our direction by low-density, ionized gas with the thermal contribution limited to about 15% for solar abundances. The absorption-corrected 2-10 keV luminosity of the nuclear source is L_X ~ 8x10^{41} ergs s^{-1} or L_X ~ 2x10^{43} ergs s^{-1} for the dual-absorption and Compton-reflection models, respectively. The absorbing and H2O masing gases appear to be spatially separate in this galaxy.

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