arXiv subjects

C. S. Reynolds

Publications and source records attributed to C. S. Reynolds.

161 records · Page 9Linked to original sources

arXiv1997

A multi-wavelength study of the Seyfert 1 galaxy MCG-6-30-15

We present a multiwaveband spectroscopic study of the nearby Seyfert 1 galaxy MCG-6-30-15. New optical spectra from the Anglo-Australian Telescope are presented which clearly show the effects of dust extinction/reddening on both the emission line spectrum and the non-stellar AGN continuum. The reddening is constrained to be in the range E(B-V)=0.61-1.09. Spectroscopy in the X-ray band, with both ROSAT and ASCA, reveal absorption by the warm absorber but little or no neutral absorption expected to accompany the dust responsible for the optical reddening. The dusty warm absorber solution to this discrepancy is discussed and photoionization models of such warm absorbers are constructed. The optical spectrum also displays the relatively strong `coronal' lines of [FeX]6375, [FeXI]7892 and [FeXIV]5303. We show that these lines may plausibly originate from the outer regions of the warm absorber, although better calculations of the collision strengths for these transitions are required in order to conclusively address this issue. We also present new ultraviolet data from the International Ultraviolet Explorer and suggest that much of the observed UV flux is scattered into our line of sight (with a scattering fraction of 1-5 per cent). We conclude with a discussion of the global energetics of this system.

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arXiv1997

Special relativistic effects on the strength of the fluorescent K-alpha iron line from black hole accretion disks

The broad iron K$α$ emission line, commonly seen in the X-ray spectrum of Seyfert nuclei, is thought to originate when the inner accretion disk is illuminated by an active disk-corona. We show that relative motion between the disk and the X-ray emitting material can have an important influence on the observed equivalent width (EW) of this line via special relativistic aberration and Doppler effects. We suggest this may be relevant to understanding why the observed EW often exceeds the prediction of the standard X-ray reflection model. Several observational tests are suggested that could disentangle these special relativistic effects from iron abundance effects.

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arXiv1997

The profile and equivalent width of the X-ray iron emission-line from a disk around a Kerr black hole

Recent X-ray observations have shown broad, skewed iron line emission from Seyfert 1 galaxies which is explained by the emission being fluorescence on a disk close to a black hole. During one interval, the line in MCG--6-30-15 was so broad and redshifted that a Kerr black hole is implied. We are therefore studying the effects of the Kerr metric on the line profile, and extending the work by Laor and Kojima which dealt only with extreme values of the spin parameter. Here we report that the spin parameter of the black hole in MCG--6-30-15 is high (a/M>0.94), and invert the line profile to obtain the disk emissivity profile, which approximates a power-law. Continuum radiation returning to the disk because of the Kerr metric does not enhance the equivalent width of the line seen above 3 keV by more than about 20 per cent if the continuum source corotates with the disk.

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arXiv1997

Warm absorbers in Seyfert 1 galaxies

X-ray spectroscopy of Seyfert 1 galaxies often reveal absorption edges resulting from photoionized gas along the line-of-sight to the central engine, the so-called warm absorber. I discuss how recent ASCA observations of warm absorber variability in MCG-6-30-15 can lead us to reject a one-zone model and, instead, have suggested a multi-zone warm absorber. The evidence for dust within the warm absorbers of MCG-6-30-15 and IRAS 13349+2438 is also addressed. These dusty warm absorbers reveal themselves by significantly reddening the optical flux without heavily absorbing the soft X-ray photons. Thermal emission from this warm/hot dust may be responsible for the infra-red bump commonly seen in the broad band spectrum of many Seyfert galaxies.

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arXiv1996

ASCA observations of the nearby galaxies Dwingeloo 1 and Maffei 1

We present ASCA observations of the nearby galaxies Dwingeloo 1 (Dw1) and Maffei 1 (Mf1). X-ray sources are clearly detected within 3 arcminutes of the optical nuclei of both galaxies. Despite the low Galactic latitude of these fields ($|b|<1\degmark$) we conclude, on probability and spectral grounds, that the detected sources are intrinsic to these galaxies rather than foreground or background interlopers. The Dw1 source, designated Dw1-X1, is interpreted as being either a hyper-luminous X-ray binary (with a 0.5--10\,keV luminosity of more than $10^{39}\ergps$) or an X-ray bright supernova. The Mf1 emission is hard and extended, suggesting that it originates from a population of X-ray binaries. Prompted by the Dw1-X1 results, we discuss the nature of hyper-luminous X-ray binary systems. Such sources are commonly seen in nearby galaxies with a frequency of approximately one per galaxy. We present a possible connection between these luminous systems and Galactic superluminal sources.

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arXiv1996

An X-ray spectral study of 24 type-1 AGN

I present a study of the X-ray spectral properties of a sample containing 24 type-1 active galactic nuclei using the medium spectral resolution of ``ASCA''. The sample consists of 20 radio-quiet objects and 4 radio-loud objects. A simple power-law continuum absorbed by Galactic material provides a very poor description of the spectra of most objects. Deviations from the power-law form are interpreted in terms of X-ray reprocessing/absorption processes. In particular, at least half of the objects show K-shell absorption edges of warm oxygen (O vii and O viii) characteristic of optically-thin, photoionized material along the line-of-sight to the central engine, the so-called warm absorber. It is found that objects with significant optical reddening display deep O vii edges. Coupled with other evidence, this suggests the existence of dusty warm plasma. A radiatively driven outflow originating from the molecular torus is probably the source of this plasma. Rapid variability of the warm absorber also points to there being another component closer to the central source and probably situated within the broad line region. Spectral features at energies characteristic of cold fluorescent iron K$α$ emission are common. Radio-quiet objects have iron emission well described as originating from either the inner regions of an accretion disk or, in a small number of cases, from the molecular torus. Two of the radio-loud objects (3C~120 and 3C~382) have a much broader feature which presents problems for the relativistic disk model. The presence of radio-jets may be important in forming this spectral feature.

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arXiv1996

The `quiescent' black hole in M87

It is believed that most giant elliptical galaxies possess nuclear black holes with masses in excess of $10^8\Msun$. Bondi accretion from the interstellar medium might then be expected to produce quasar-like luminosities from the nuclei of even quiescent elliptical galaxies. It is a puzzle that such luminosities are not observed. Motivated by this problem, Fabian & Rees have recently suggested that the final stages of accretion in these objects occurs in an advection-dominated mode with a correspondingly small radiative efficiency. Despite possessing a long-known active nucleus and dynamical evidence for a black hole, the low radiative and kinetic luminosities of the core of M87 provide the best illustration of this problem. We examine an advection-dominated model for the nucleus of M87 and show that accretion at the Bondi rate is compatible with the best known estimates for the core flux from radio through to X-ray wavelengths. The success of this model prompts us to propose that FR-I radio galaxies and quiescent elliptical galaxies accrete in an advection dominated mode whereas FR-II type radio-loud nuclei possess radiatively efficient thin accretion disks.

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arXiv1996

The matter content of the jet in M87: evidence for an electron-positron jet

Recent observations have allowed the geometry and kinematics of the M87 jet to be tightly constrained. We combine these constraints with historical Very Long Baseline Interferometry (VLBI) results and the theory of synchrotron self-absorbed radio cores in order to investigate the physical properties of the jet. Our results strongly suggest the jet to be dominated by an electron-positron (pair) plasma. Although our conservative constraints cannot conclusively dismiss an electron-proton plasma, the viability of this solution is extremely vulnerable to further tightening of VLBI surface brightness limits. The arguments presented, coupled with future high-resolution multi-frequency VLBI studies of the jet core, will be able to firmly distinguish these two possibilities.

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arXiv1996

A powerful and highly variable off-nuclear X-ray source in the composite starburst/Seyfert 2 galaxy NGC 4945

We report on a powerful and variable off-nuclear X-ray source in the nearby spiral galaxy NGC 4945. Two ROSAT PSPC observations show the source to brighten in 0.5--2.0 keV flux by a factor of about 9 on a time-scale of 11 months or less. It is seen by ASCA about one month after the second PSPC pointing, and is seen to have dimmed by a factor of > 7 in a ROSAT HRI pointing about one year after the second PSPC pointing. Its maximum observed 0.8--2.5 keV luminosity is about 8E38 erg/s, making it brighter than any known persistent X-ray binary in the Milky Way. Its total X-ray luminosity is probably larger than 1.2E39 erg/s. The observed variability argues against a superbubble interpretation, and the off-nuclear position argues against a low-luminosity active galactic nucleus. The source is therefore probably either an ultra-powerful X-ray binary or an ultra-powerful supernova remnant. Optical monitoring has not identified any supernovae in NGC 4945 during the time of the X-ray observations, and any supernova would have had to have been either very highly absorbed or intrinsically optically faint.

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arXiv1996

The variable iron K emission line in MCG-6-30-15

We report on the variability of the iron K emission line in the Seyfert 1 galaxy MCG--6-30-15 during a four-day ASCA observation. The line consists of a narrow core at an energy of about 6.4 keV, and a broad red wing extending to below 5 keV, which are interpreted as line emission arising from the inner parts of an accretion disk. The narrow core correlates well with the continuum flux whereas the broad wing weakly anti-correlates. When the source is brightest, the line is dominated by the narrow core, whilst during a deep minimum, the narrow core is very weak and a huge red tail appears. However, at other times when the continuum shows rather rapid changes, the broad wing is more variable than the narrow core, and shows evidence for correlated changes contrary to its long time scale behaviour. The peculiar line profile during the deep minimum spectrum suggests that the line emitting region is very close to a central spinning (Kerr) black hole where enormous gravitational effects operate.

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arXiv1995

The variable OVIII Warm Absorber in MCG-6-30-15

We present the results of a 4 day ASCA observation of the Seyfert galaxy MCG-6-30-15, focussing on the nature of the X-ray absorption by the warm absorber, characterizd by the K-edges of the intermediately ionized oxygen, OVII and OVIII. We confirm that the column density of OVIII changes on a timescale of $\sim 10^4$~s when the X-ray continuum flux decreases. The significant anti-correlation of column density with continuum flux gives direct evidence that the warm absorber is photoionized by the X-ray continuum. From the timescale of the variation of the OVIII column density, we estimate that it originates from gas within a radius of about $10^{17}\cm$ of the central engine. In contrast, the depth of the OVII edge shows no response to the continuum flux, which indicates that it originates in gas at larger radii. Our results strongly suggest that there are two warm absorbing regions; one located near or within the Broad Line Region, the other associated with the outer molecular torus, scattering medium or Narrow Line Region.

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arXiv1995

ROSAT PSPC observations of Cygnus-A : X-ray spectra of the cooling flow and hot spots

We present a {\it ROSAT} Position Sensitive Proportional Counter (PSPC) observation of the powerful radio galaxy Cygnus-A. The X-ray emission in the {\it ROSAT} band is dominated by thermal emission from the hot intracluster medium of the associated cluster. Image deprojection confirms the existence of a significant cluster cooling flow with total mass deposition rate of $\sim 250\,{\rm M}_\odot\,{\rm yr}^{-1}$ and a (Hubble time) cooling radius of $\sim 180$\,kpc. Spectral data show the gradient in the emission-weighted mean temperature with the temperature decreasing towards the centre of the cluster. We also find signatures of the radio source: in particular, we detect the X-ray emission from the western radio hot spot previously found by the {\it ROSAT} High Resolution Imager (HRI). We find the emission from the hot spot to be hard and discuss the physical implications of this result.

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arXiv1995

On broad iron K-alpha lines in Seyfert 1 galaxies

The X-ray spectrum obtained by Tanaka et al from a long observation of the active galaxy MCG$-6-30-15$ shows a broad iron K$α$ line skewed to low energies. The simplest interpretation of the shape of the line is that it is due to doppler and gravitational redshifts from the inner parts of a disk about a massive black hole. Similarly broad lines are evident in shorter observations of several other active galaxies. In this paper we investigate other line broadening and skewing mechanisms such as Comptonization in cold gas and doppler shifts from outflows. We have also fitted complex spectral models to the data of MCG$-6-30-15$ to see whether the broad skewed line can be mimicked well by other absorption or emission features. No satisfactory mechanism or spectral model is found, thus strengthening the relativistic disk line model.

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arXiv1995

ASCA PV observations of the Seyfert 1 galaxy MCG-6-30-15 : rapid variability of the warm absorber

We present a detailed re-analysis of the two {\it ASCA} Performance Verification observations of the nearby Seyfert 1 galaxy MCG-6-30-15. Confirming the results of Fabian et al. (1994), we find definite evidence for the {\sc O\,vii} and {\sc O\,viii} K-shell absorption edges of the warm absorber and a doubling of the warm absorber column density within the 3 weeks separating the two observations. No intra-day {\it flux-correlated} variability of the warm absorber is found. However, we report the discovery of an `event' in which the warm absorber parameters temporarily change for \sim10\,000\thinspace s before returning to their original values. Possible interpretations are discussed but a contradiction remains: the constancy of the ionization state of the warm absorber argues that it lies at large distances from the central source whereas the short term change in column density argues for small distances. Fluorescent iron emission is examined. As found by Fabian et al. (1994), the iron line is broad and strong (equivalent width \sim300\thinspace eV). The line profile is also suggestive of it being skewed. Such a line would be expected from a relativistic accretion disk. We also find very rapid primary X-ray variability. Assuming relativistic beaming to be unimportant, the derived efficiency is comparable to the maximum obtainable from accretion onto a Schwarzschild black hole. Correlated variability outside of the energy range of {\it ASCA} might exceed this maximum, thus requiring efficient accretion onto a Kerr hole.

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arXiv1995

ASCA observations of the Seyfert 1 galaxies Mrk1040 and MS0225.5+3121

We present {\it ASCA} observations of the Seyfert 1 galaxies Mrk~1040 and MS~0225.5+3121. Mrk~1040 was found to have decreased in flux by almost a factor of 4 since an {\it EXOSAT} observation 10\,years ago. The energy spectrum of Mrk~1040 displays complexity both at soft energies (below 0.8\thinspace keV) and at hard energies (6--7\thinspace keV). The latter is readily interpreted as fluorescent K$α$ emission from cold iron expected when the primary X-ray source illuminates cold optically-thick material. This line is both broad (with FWHM 16\,000--70\,000 km\,s$^{-1}$) and strong (equivalent width $\sim 550\pm 250$\, eV) suggesting that it originates from material close to the compact object with non-solar abundances. Abundance effects on the equivalent width of such a line are investigated via Monte Carlo simulations. We find that strong lines can be produced with physically plausible abundances. The effect of abundances on the associated reflection continuum is also discussed. The soft spectral complexity implies either a strong soft excess together with intrinsic absorption, or a complex absorber. Various models for the nature of such a complex absorber are discussed. MS~0225.5+3121 shows no evidence for any variability and has a spectrum that is well described by a power law with Galactic absorption.

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arXiv1995

Warm absorbers in active galactic nuclei

Recent {\it ASCA} observations confirm the presence of X-ray absorption due to partially ionized gas in many Seyfert 1 galaxies; the so-called warm absorber. Constraints on the location of the warm material are presented with the conclusion that this material lies at radii coincident with, or just outside, the broad-line region. The stability of this warm material to isobaric perturbations under the assumptions of thermal and photoionization equilibrium is also studied. It is shown that there is a remarkably small range of ionization parameter, $ξ$, for which the warm absorber state is stable. The robustness of this result to changes in the shape of the primary continuum, the assumed density and optical depth is investigated. Given the constraints on the location and the stability properties of the material, several models for the environments of Seyfert nuclei are discussed. These attempt to explain the presence of significant amounts of partially ionized material. In particular, various models of the broad-line region are discussed. The simple two-phase model of the broad-line region proves to be unsatisfactory. A model of the broad-line region is presented in which a turbulent, hot intercloud medium is mechanically heated. Turbulent mixing layers could then give rise to warm absorption features. Finally, a model is discussed in which the warm absorber is due to a steady state, radiatively driven outflow.

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arXiv1994

ROSAT PSPC Observations of the Seyfert 1 Galaxies Ark 564, NGC 985, Kaz 163, MRK 79 and RX J 2256.6+0525

We present spatial, temporal and spectral analyses of ROSAT Position Sensitive Proportional Counter (PSPC) observations of the Seyfert 1 galaxies Ark 564, NGC 985, Kaz 163, Mrk 79 and RX J2256.6+0525. Ark 564 is a powerful narrow-line Seyfert 1 with strong Fe II emission. Several similar narrow-line Seyfert 1 galaxies have recently been found to have remarkably steep soft X-ray continua as well as rapid X-ray variability. We find that Ark 564 also has a very steep ($Γ> 3$) 0.1--2.5 keV spectrum and varies by $\sim 20$ per cent in 1500 s. We examine models for Ark 564 in light of both its X-ray and optical characteristics, and suggest a possible connection between the steep X-ray spectra of narrow-line Seyfert 1 galaxies and their narrow lines. NGC 985 has a large soft excess, a warm absorber, or both. The three other Seyferts have systematically steeper spectra than are typically observed for Seyferts in higher energy Ginga data, indicating that they also harbour further spectral complexity. Kaz 163 shows $\sim 45$ per cent intensity variability in the 0.1--2.5 keV band, and Mrk 79 shows evidence for variability as well.

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