arXiv subjects

R. F. Mushotzky

Publications and source records attributed to R. F. Mushotzky.

107 records · Page 6Linked to original sources

arXiv1998

The X-ray Spectral Variability of the Seyfert Galaxy NGC 3227

(Abridged) We present the results from {\it ASCA} observations of NGC 3227 along with those from a {\it ROSAT} observation. The 0.6--10 keV continuum is consistent with a powerlaw with $Γ\sim 1.6$, flatter than that typically observed in Seyfert galaxies. Fe $K$-shell emission is observed with an EW and profile typical of Seyfert 1 galaxies. The {\it ASCA} observations in 1993 reveal absorption by a screen $N_{H,z}^{ion} \simeq 3\times10^{21} cm^{-2}}$ of ionized material with $U_X \simeq 0.01$. Joint analysis of the {\it ASCA} and {\it ROSAT} data show an additional screen of neutral material instrinsic to NGC 3227 with $N_{H,z}^{neu} \sim $few$\times10^{20} cm^{-2}}$. NGC 3227 exhibits spectral variability both within and between the observations, due to short-term variability in the continuum emission and longer-term variability in the absorption. Time-resolved spectroscopy show the continuum steepened by $ΔΓ\simeq 0.1$ during a flare of duration $\sim 10^4$s. The column increased by a factor of $\sim10$ by the 1995 epoch, while the continuum is consistent with that observed in 1993. The 1995 data also show evidence that the warm absorber allows $\sim$10% of the nuclear emission to escape without attenuation.

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arXiv1998

New constraints on the continuum-emission mechanism of AGN: Intensive monitoring of NGC 7469 in the X-ray and ultraviolet

We have undertaken near-continuous monitoring of the Seyfert 1 galaxy NGC 7469 in the X-ray with RXTE over a ~30d baseline. The source shows strong variability with a root-mean-square (rms) amplitude of ~16 per cent, and peak-to-peak variations of a factor of order 2. Simultaneous data over this period were obtained in the ultraviolet (UV) using IUE, making this the most intensive X-ray UV/X-ray variability campaign performed for any active galaxy. Comparison of the continuum light curves reveals very similar amplitudes of variability, but different variability characteristics, with the X-rays showing much more rapid variations. The data are not strongly correlated at zero lag. The largest absolute value of the correlation coefficient occurs for an anticorrelation between the two bands, with the X-ray variations leading the UV by ~4d. The largest positive correlation is for the ultraviolet to lead the X-rays by ~4d. Neither option appears to be compatible with any simple interband transfer function. The peak positive correlation at ~4d occurs because the more prominent peaks in the UV light curve appear to lead those in the X-rays by this amount. However, the minima of the light curves are near-simultaneous. These observations provide new constraints on theoretical models of the central regions of active galactic nuclei. Models in which the observed UV emission is produced solely by re-radiation of absorber X-rays are ruled out by our data, as are those in which the X-rays are produced solely by Compton upscattering of the observed UV component by a constant distribution of particles.

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arXiv1998

An ASCA-ROSAT Study of the Distant, Lensing Cluster A2390

We present the results of a combined study of ASCA and ROSAT observations of the distant cluster Abell 2390.We determine the surface brightness profile and the gas density distribution of the cluster from the ROSAT PSPC and HRI data. A combined spatially resolved spectral analysis of the ASCA and ROSAT data show that the temperature distribution of the intracluster medium of A2390 is consistent with an isothermal temperature distribution in the range 9 to 12 keV except for the central region for which we find strong evidence of a multi-temperature structure. We determine the gravitational mass profile of the cluster and find a mass value of 2e15 solar masses for a radius 3 Mpc. A comparison of the projected mass profiles of the cluster shows an excellent agreement between the mass determined from X-ray data and the mass determined from the models for the gravitational arc and the weak lensing results.

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arXiv1998

ASCA Observations of the Ionized Gas in the Seyfert Galaxy NGC 3783

We present the results from ASCA observations of NGC 3783 carried out during 1993 and 1996. Variability is observed both between the two epochs and within the individual observations, the latter due to a non-stationary process. We find the spectra at both epochs to contain features due to absorption by ionized material, predominantly due to Ovii and Oviii. We find the opacity for such material to decrease by a factor approx 2 in the approx 0.7--1.0 keV band between the epochs while the photoionizing flux increases by approx25%. By comparison with detailed photoionization models we show this behaviour is inconsistent with that expected from a single-zone of photoionized gas. A deficit in the data compared to such models, consistent with excess absorption by Oviii, supports the suggestion that the material consists of two or more zones. Significant Fe K-shell emission is also observed during both epochs. We show this emission is dominated by the asymmetric line profile expected from the innermost regions of the accretion flow. We find no evidence that the Fe emission varied in either shape or equivalent width between the two epochs.

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arXiv1997

ASCA observations of type-2 Seyfert Galaxies. III. Orientation and X-ray Absorption

We discuss the spectral properties of a sample of type-2 Seyfert galaxies based upon the analysis of \asca data. In this paper we consider the sources for which the X-ray spectra appear to be dominated by the nuclear continuum, transmitted through a large column of absorbing material. We find that both Seyfert-2 galaxies and NELGs show iron K$α$ line profiles indicative of reprocessing of nuclear X-rays in a face-on accretion disk. Such line profiles are also observed in Seyfert-1 galaxies. This result is contrary to unification models, which would predict the inner regions of Seyfert-2 galaxies to be observed edge-on. This raises some questions as to the orientation of the circumnuclear absorber. If the observed differences between Seyfert type-1 and type-2 galaxies, and NELGs are not due to differences in the orientation of the absorbing material, then we suggest that differences in dust composition and grain size, and in the density of the circumnuclear gas could be of primary importance.

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arXiv1997

ASCA Observations of Seyfert 1 galaxies: III. The Evidence for Absorption & Emission due to Photoionized Gas

(Abridged) We present the results from a detailed analysis of the 0.6 - 10 keV spectra of 23 ASCA observations of 18 objects. We find that in most cases the underlying continuum can be well-represented by a powerlaw with a photon index Gamma approx 2. However we find strong evidence for photoionized gas in the line-of-sight to 13/18 objects. We present detailed modelling of this gas based upon the ION photoionization code. We find the X-ray ionization parameter for the ionized material is strongly peaked at Ux approx 0.1. The column densities of ionized material are typically in the range N_{H,z} approx 10^{21}--10^{23} cm^{-2}, although highly ionized column densities up to 10^{24} cm^{-2} cannot be excluded in some cases. We also investigate the importance of the emission-spectrum from the ionized gas, finding that it significantly improves the fit to many sources with an intensity consistent with material subtending a large solid angle at the central source. A deficit of counts is observed at approx 1 keV in the sources exhibiting the strongest absorption features. We suggest this is likely to be the signature of a second zone of (more highly) ionized gas, which might have been seen previously in the deep Fe K-shell edges observed in some Ginga observations. We find evidence that the ionized material in NGC 3227 and MCG-6-30-15 contains embedded dust, whilst there is no such evidence in the other sources

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arXiv1997

Evidence for Absorption due to Highly-Ionized Gas in the Radio-quiet Quasar PG 1114+445

We present results on the X-ray spectrum of the quasar PG 1114+445 from an ASCA observation performed in 1996 June, and a ROSAT observation performed 3 years earlier. We show good agreement between all the datasets can be obtained if the underlying continuum in the 0.2--10 keV band is assumed to be a powerlaw (photon index Gamma approx 1.8) absorbed by photoionized material. The ionized gas imprints deep absorption edges in the observed spectrum <~ 2 keV due to Ovii and Oviii, from which we determine its column density (approx 2E22 cm^{-2}) and ionization parameter (Ux approx 0.1) to be similar to that observed in Seyfert-I galaxies. Unfortunately these data do not allow any strong constraints to be placed on the location, or solid angle subtended by the material at the ionizing source. We also find evidence for absorption in the Fe K-shell band in excess of that predicted from the lower energy features. This implies an Fe/O abundance ratio approx 10 times the cosmic value, or an additional screen of more-highly ionized gas, possibly out-flowing from the nucleus. We briefly compare our results with those obtained from other active galaxies.

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arXiv1997

On the dependence of the iron K-line profiles with luminosity in Active Galactic Nuclei

We present evidence for changes in the strength and profile of the iron K-alpha line in Active Galactic Nuclei (AGN), based on X-ray observations with ASCA. There is a clear decrease in the strength of the line with increasing luminosity. This relation is is not due solely to radio power, as it persists when only radio-quiet AGN are considered and therefore cannot be fully explained by relativistic beaming. In addition to the change in strength, the line profile also appears to be different in higher luminosity sources. We discuss these results in terms of a model where the accretion disk becomes ionized as a function of the accretion rate.

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arXiv1997

Rosat, Asca and Osse Observations of the Broad Line Radio Galaxy 3C120

We present simultaneous observations of the superluminal radio galaxy 3C120 performed with the ASCA and GRO (OSSE) satellites on February-March 1994, as well as an analysis of all the ROSAT archival data. The ASCA spectrum of this object can be described by an absorbed (N_H=1.6e^21 cm^-2) power law with photon index Gamma_ASCA=2 and a very broad sigma>0.8 keV intense iron line (EW>400 eV) at ~ 6 keV. The combined ASCA--OSSE data do not exclude the presence of a narrower (sigma=0.4 keV) and less intense (EW<300 eV) iron line plus a hard component, corresponding either to reflection from an accretion disk or to a flatter power law from a jet. However a single power law plus broad Fe line is preferred from a statistical point of view by the ASCA data. The ROSAT data yield a column density in excess of the Galactic value. The spectral slopes, ranging from Gamma_ROSAT=2.5 to 3.3, are steeper than that measured by ASCA, suggesting the presence of a soft excess. The 0.1-2 keV power-law slope is variable and softer at higher intensity. These results show that the combined soft and hard X-ray spectrum of 3C120 is rather complex. The intrinsic absorption, the soft excess, and the iron line indicate that the X-ray emission from this blazar-like radio galaxy is dominated by a Seyfert-like component, at least in the 0.1-10keV energy band. The jet contribution, if present, becomes important only at higher energies.

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arXiv1997

The Galaxy Cluster Luminosity-Temperature Relationship and Iron Abundances - A Measure of Formation History ?

Both the X-ray luminosity-temperature (L-T) relationship and the iron abundance distribution of galaxy clusters show intrinsic dispersion. Using a large set of galaxy clusters with measured iron abundances we find a correlation between abundance and the relative deviation of a cluster from the mean L-T relationship. We argue that these observations can be explained by taking into account the range of cluster formation epochs expected within a hierarchical universe. The known relationship of cooling flow mass deposition rate to luminosity and temperature is also consistent with this explanation. From the observed cluster population we estimate that the oldest clusters formed at z>~2. We propose that the iron abundance of a galaxy cluster can provide a parameterization of its age and dynamical history.

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arXiv1997

ASCA observations of type-2 Seyfert galaxies: II. The Importance of X-ray Scattering and Reflection

We discuss the importance of X-ray scattering and Compton reflection in type-2 Seyfert galaxies, based upon the analysis of ASCA observations of 25 such sources. Consideration of the iron Kalpha, [O III] line and X-ray variability suggest that NGC 1068, NGC 4945, NGC 2992, Mrk 3, Mrk 463E and Mrk 273 are dominated by reprocessed X-rays. We examine the properties of these sources in more detail. We find that the iron Kalpha complex contains significant contributions from neutral and high-ionization species of iron. Compton reflection, hot gas and starburst emission all appear to make significant contributions to the observed X-ray spectra. Mrk 3 is the only source in this subsample which does not have a significant starburst contamination. The ASCA spectrum below 3 keV is dominated by hot scattering gas with U_X ~ 5, N_H ~ 4 x 10^23 cm^-2. This material is more highly ionized than the zone of material comprising the warm absorber seen in Seyfert~1 galaxies, but may contain a contribution from shock-heated gas associated with the jet. Estimates of the X-ray scattering fraction cover 0.25 - 5%. The spectrum above 3 keV appears to be dominated by a Compton reflection component although there is evidence that the primary continuum component becomes visible close to 10 keV.

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arXiv1997

The X-ray View of AGN -- Overview

Recent ASCA and ROSAT X-ray observations of active galaxies have revealed a host of new data on the fundamental properties of active galaxies. Amongst these are the discovery and characterization of absorption by ionized gas in Seyfert-I galaxies (the "warm absorber") , the discovery and parameterization of broad Fe K lines which originate in the central 100 Schwarzschild radii, a substantial modification in the form of the ionization continuum from previous models and the absence of X-ray emission from broad absorption line quasars. We briefly summarize the present observational situation and indicate where this field might progress in the next few years with the enhanced capabilities of AXAF, XMM and Astro-E.

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arXiv1997

The Luminosity-Temperature Relation at z=0.4 for Clusters of galaxies

We have obtained the first large sample of accurate temperatures for clusters at z>0.14 from ASCA. We compare the luminosity temperature (L-T) distribution for these clusters with the low redshift sample of David et al (1993) and find that there is no evidence for evolution. We also find that the intrinsic variance in this relation is roughly constant with redshift. Additionally, there is no detectable change in the relationship of optical velocity dispersion to X-ray temperature with redshift. Most cosmological simulations driven primarily by gravity predict substantial changes in the L-T relation due to the recent rapid growth of clusters. Our results are consistent either with models in which the cluster core entropy is dominated by pre-heating, or with low Omega models in which cluster structure does not evolve strongly with time. The intrinsic variance in the L-T relation at a fixed redshift can be due a variety of possibilites e.g. a change in the baryonic fraction from cluster to cluster, variation in the fraction of the total energy in the system arising from shocks or supernova heating or variations in the emission measure distributions in multiphase gas.

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arXiv1996

ASCA observations of Seyfert 1 galaxies: II. Relativistic Iron K-alpha emission

We present evidence for widespread relativistic effects in the central regions of active galactic nuclei. In a sample of 18 Seyfert 1 galaxies observed by \asca, 14 show an iron K$α$ line which is is resolved, with mean width $σ_{\rm Kα}=0.43\pm 0.12$~keV for a gaussian profile (Full Width at Half Maximum, FWHM$\sim 50,000$~km s$^{-1}$). However, many of the line profiles are asymmetric. A strong red wing is indicative of gravitational redshifts close to a central black hole and accretion disk models provide an excellent description of the data. The peak energy of the line is 6.4~keV, indicating that it arises by fluorescence in near-neutral material. Our fits imply a low inclination for the disk in these Seyfert 1 galaxies, with a mean of 30°, consistent with orientation-dependent unification schemes. Differences in the line profiles from source-to-source imply slight variations in geometry, which cannot be accounted for solely by inclination. In most cases, we require that the line emission arises from a range of radii. Although a small contribution to the emission from a region other than the disk is not ruled out, it is not generally required and has little effect on our conclusions regarding the disk line. Our data are fit equally well with rotating (Kerr) and non-rotating (Schwarzschild) black hole models. We find a mean spectral index in the 3-10 keV range of $<Γ_{3-10}>=1.91\pm 0.07$ after accounting for the effects of reflection. Such observations probe the innermost regions of AGN, and arguably provide the best evidence yet obtained for the existence of super-massive black holes in the centers of active galaxies.

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arXiv1996

ASCA observations of Seyfert 1 galaxies: I. Data Analysis, Imaging and Timing

We present the first in a series of papers describing the X-ray properties of a sample of 18 Seyfert 1 galaxies, using data obtained by \asca. The imaging data reveal a number of serendipitous hard X-ray sources in some source fields, but none contribute significantly to the hard X-ray flux of the AGN. All but one of the Seyferts show evidence for variability on timescales of minutes-hours, with the amplitude anti-correlated with the source luminosity, confirming previous results. In at least 8 sources, there is evidence that the variability amplitude below 2 keV is greater than that in the hard X-ray band, perhaps indicating variable components other than the power-law in the soft band. Ultra-rapid variability, implying significant power at frequencies $> 10^{-3}$~Hz is detected in at least 5 sources, but is difficult to detect in most cases, due to the sampling and signal-to-noise ratio. In Mrk 766 and MCG-6-30-15 there is also an indication that the high--frequency power--spectra are variable in shape and/or intensity. There is similar evidence in NGC 4151, but on longer time scales.

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arXiv1996

Variability of the iron K-emission line in the Seyfert 1 galaxy NGC 3516

We present strong evidence for variability of the flux of the iron K$α$ emission line in the Seyfert 1 galaxy, NGC 3516. Two ASCA observations, separated by $\sim 1$~yr, showed a marked decrease in continuum flux by $\sim 60$~per cent. The flux in the broad, iron K$α$ line decreased by the same factor in this time period, with no evidence for changes in the line profile. The line variability is significant at $>99$~per cent confidence and rules out models in which the line is produced in a molecular torus located at $>1$~pc from the nucleus. An accretion disk is considerably more likely.

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arXiv1996

The X-Ray Spectral Variability of Mrk 766

Analysis results from ASCA and ROSAT observations of the narrow-line Seyfert 1 galaxy Mrk 766 are reported. In the ASCA observation we observed rapid variability with a doubling time scale of 1000 seconds. A spectral variability event was observed in which the spectrum softened and hardened above and below ~1 keV, respectively, as the flux increased. The spectra could be modeled with 5 components: a power law, warm absorber, iron K(alpha) line and soft excess component flux. The spectral variability resulted from a highly significant change in the intrinsic photon law index from Gamma ~1.6 to ~2.0, an increase in the warm absorber ionization, and a marginally significant decrease in the soft component normalization. A ~100 eV equivalent width narrow iron K(alpha) line was detected in the high state spectrum. Spectral hardening during flux increases was observed in three ROSAT observations. The change in intrinsic photon index and disappearance of the soft excess component in the ASCA spectra can be explained as a transition from a first order pair reprocessed spectrum to a pair cascade brought about by a sudden increase in the injected electron Lorentz factor. The change in the ionization of the warm absorber, though model dependent, could correspond to the increase in flux at the oxygen edges resulting from the spectral index change. The ROSAT spectral variability can be interpreted by variable intensity hard power law and a relatively nonvarying soft component, possibly primary disk emission. These results are compared with those reported from other narrow-line Seyfert 1 galaxies.

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