arXiv subjects

K. Nandra

Publications and source records attributed to K. Nandra.

At least 235 records · Page 13Linked to original sources

arXiv1999

The properties of the relativistic iron K-line in NGC 3516

We present an analysis of the relativistic iron K-alpha line in the Seyfert 1 galaxy NGC 3516, based on a continuous, five-day ASCA observation. The broad profile which has been found in several other AGN is confirmed in NGC 3516 with unprecedented signal-to-noise ratio. Disk-line models with either a Kerr or Schwarzschild metric fit the integrated profile, but both require emission very strongly concentrated in the inner disk. We find tentative evidence for the line signatures of Ni K-alpha and/or Fe K-beta. The continuum flux varied by \~50 per cent during the observation and time-resolved analysis shows that the line also changes. The line core seems to follow the continuum, but the blue wing is unrelated and shows a greater amplitude (factor ~2) of variability. The red wing is formally consistent with a constant but appears to be correlated with the blue wing. We interpret this as evidence for independent variability of the broadest parts of the line. There also appears to be an absorption feature in the profile, consistent with resonance scattering in infalling material. This variable feature may be the signature of material being accreted by the central black hole.

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arXiv1999

On X-ray Variability in Seyfert Galaxies

This paper presents a quantification of the X-ray variability amplitude for 79 ASCA observations of 36 Seyfert 1 galaxies. We find that consideration of sources with the narrowest permitted lines in the optical band introduces scatter into the established correlation between X-ray variability and nuclear luminosity. Consideration of the X-ray spectral index and variability properties together shows distinct groupings in parameter space for broad and narrow-line Seyfert 1 galaxies, confirming previous studies. A strong correlation is found between hard X-ray variability and FWHM Hbeta. A range of nuclear mass and accretion rate across the Seyfert population can explain the differences observed in X-ray and optical properties. An attractive alternative model, which does not depend on any systematic difference in central mass, is that the circumnuclear gas of NLSy1s is different to BLSy1s in temperature, optical depth, density or geometry.

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arXiv1999

ASCA and ROSAT observations of NGC5548: discrepant spectral indices

We report on simultaneous ASCA and ROSAT observations of the Seyfert galaxy NGC5548 made during the ASCA Performance Verification phase. Spectral features due to a warm absorber and reflection are clearly seen in the X-ray spectra. We find that the continuum spectral shape differs between the ASCA and ROSAT datasets. The photon-index obtained from the ROSAT PSPC exceeds that from the ASCA SIS about 0.4. The discrepancy is clear even in the 0.5-2 keV energy band over which both detectors are sensitive. The spectra cannot be made consistent by choosing a more complex model. The problem likely lies in the response curve (estimated effective area) of one, or both, detectors. There may be important consequences for a wide range of published results.

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arXiv1998

A Peculiar Emission Line Feature in the X-ray Spectrum of the Quasar PKS 0637-752

We report the results from an ASCA observation of the high-luminosity, radio-loud quasar PKS 0637-752 (redshift 0.654), covering the 0.8-15 keV band in the quasar-frame. We find the source to have a luminosity ~10^46 erg/s in the 2-10 keV band, a factor of ~3 lower than during a previous GINGA observation. The continuum appears to be well modeled by a simple power-law with a photon index of 1.64 +/- 0.07, with no evidence for absorption by material intrinsic to the quasar, or Fe-K emission (with an equivalent width < 80 eV at 90% confidence). However we do find evidence for a narrow emission line at an energy 1.60 +/- 0.07 keV and equivalent width 59 [+38,-34] eV (both in the quasar frame). Line emission at these energies has not been observed in any other active galaxy or quasar to date. We reject the possibility that this line is the result of instrumental artifacts, and briefly explore possible identifications.

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arXiv1998

X-ray Observations of the Seyfert galaxy LB 1727 (1H 0419-577)

We discuss the properties of the Seyfert 1.5 galaxy LB 1727, also known as 1H 0419-577, from X-ray observations obtained by ASCA and ROSAT along with optical observations from earlier epochs. ASCA shows only modest (< 20%) variations in X-ray flux within or between the observations. In contrast, a daily monitoring campaign over 1996 Jun - Sept by the ROSAT HRI instrument reveals the soft X-ray (0.1-2 keV) flux to have increased by a factor ~3. The 2 - 10 keV continuum can be parameterized as a power-law with a photon index Gamma ~ 1.45-1.68 across ~0.7 - 11 keV in the rest-frame. We also report the first detection of iron Kalpha line emission in this source. Simultaneous ASCA and ROSAT data show the X-ray spectrum to steepen sharply at a rest-energy \~0.75 keV, the spectrum below this energy can be parameterized as a power-law of slope Gamma ~3.6. We show that LB 1727 is one of the few Seyferts for which we can rule out the possibility that the presence of a warm absorber is solely responsible for the spectral steepening in the soft X-ray regime. Consideration of the overall spectral-energy-distribution for this source indicates the presence of a pronounced XUV-bump visible in optical, ultraviolet and soft X-ray data.

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arXiv1998

X-ray Signatures of an Ionized Reprocessor in the Seyfert galaxy Ton S 180

We discuss the hard X-ray properties of the Seyfert galaxy Ton S 180, based upon the analysis of ASCA data. We find the X-ray flux varied by a factor ~2 on a time scale of a few thousand seconds. The source showed significantly higher amplitude of variability in the 0.5-2 keV band than in the 2-10 keV band. The continuum is adequately parameterized as a Gamma ~ 2.5 power-law across the 0.6--10 keV band . We confirm the recent discovery of an emission line of high equivalent width, due to Fe K-shell emission from highly-ionized material. These ASCA data show the Fe line profile to be broad and asymmetric and tentatively suggest it is stronger during the X-ray flares, consistent with an origin from the inner parts of an accretion disk. The X-ray spectrum is complex below 2 keV, possibly due to emission from a blend of soft X-ray lines, which would support the existence of an ionized reprocessor, most likely due to a relatively high accretion rate in this source.

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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

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

A Simple New Method For Analysing Gapped Time-Series: Search For a High Frequency Cut-off In The X-ray Power Spectrum of MCG -6-30-15

We present a new, simple method for analysing gapped time-series which is particularly suited to probe the X-ray power spectra of active galactic nuclei (AGN).The method is applied to a four-day observation of the Seyfert 1 galaxy MCG -6-30-15 with ASCA. MCG -6-30-15 is well-known for rapid, large amplitude X-ray variability. In this source, our method probes the power spectrum in the range 0.001 Hz to 0.05 Hz, thus extending the high-frequency coverage by a factor of over 50 compared to previous studies. The ASCA data out a cessation of variability up to 0.016 Hz at greater than 90% confidence. The power-law index of the power spectrum is consistent with -1.4.At 0.021 Hz we can place an upper limit of 7% on the intrinsic fluctuations of the source luminosity at greater than 99% confidence. Above this frequency Poisson noise becomes dominant but we can place an upper limit of 10% on intrinsic fluctuations up to 0.05 Hz at 98.9% confidence. The data argue against orbital modulation as the origin of the variability since it would imply upper limits on the mass of the putative black hole of less than half a million solar solar masses and therefore super-Eddington luminosities.

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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

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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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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