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S. Vaughan

Publications and source records attributed to S. Vaughan.

At least 73 records · Page 4Linked to original sources
arXiv2004

The discovery of an evolving dust scattered X-ray halo around GRB 031203

We report the first detection of a time-dependent, dust-scattered X-ray halo around a gamma-ray burst. GRB 031203 was observed by XMM-Newton starting six hours after the burst. The halo appeared as concentric ring-like structures centered on the GRB location. The radii of these structures increased with time as t^{1/2}, consistent with small-angle X-ray scattering caused by a large column of dust along the line of sight to a cosmologically distant GRB. The rings are due to dust concentrated in two distinct slabs in the Galaxy located at distances of 880 and 1390 pc, consistent with known Galactic features. The halo brightness implies an initial soft X-ray pulse consistent with the observed GRB.

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arXiv2004

Chandra observations of five X-ray transient galactic nuclei

We report on exploratory Chandra observations of five galactic nuclei that were found to be X-ray bright during the ROSAT all-sky survey (with L_X > 10^43 erg s^-1) but subsequently exhibited a dramatic decline in X-ray luminosity. Very little is known about the post-outburst X-ray properties of these enigmatic sources. In all five cases Chandra detects an X-ray source positionally coincident with the nucleus of the host galaxy. The spectrum of the brightest source (IC 3599) appears consistent with a steep power-law (Gamma~3.6). The other sources have too few counts to extract individual, well-determined spectra, but their X-ray spectra appear flatter (Gamma~2) on average. The Chandra fluxes are ~10^2-10^3 fainter than was observed during the outburst (up to 12 years previously). That all post-outburst X-ray observations showed similarly low X-ray luminosities is consistent with these sources having `switched' to a persistent low-luminosity state. Unfortunately the relative dearth of long-term monitoring and other data mean that the physical mechanism responsible for this spectacular behaviour is still highly unconstrained.

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arXiv2003

IZw1 observed with XMM-Newton: Low-energy spectral complexity, iron lines, and hard X-ray flares

We present a 20 ks XMM-Newton observation of the prototypical Narrow-Line Seyfert 1 galaxy IZw1. The best-fit model to the data is a double blackbody plus a dominant power-law, on which complex soft absorption (possibly a blended edge or absorption lines) and/or OVII emission are superimposed, as well as strong Fe Kalpha emission. The iron feature in the high-energy spectra appears broad; however, on close examination of the EPIC pn data, there exists the possibility that the broad emission feature can be attributed to a neutral Fe Kalpha line in addition to a blend of He- and H-like Fe Kalpha lines. The light curve shows a strong, hard X-ray flare concentrated in the 3-12 keV band. The flare appears to induce spectral variability, showing spectral hardening to be occuring as the flare intensifies. A detailed examination suggests that the spectral variability is most likely due to an increase in the 3-12 keV flux relative to the soft flux during the flare. A difference spectrum and complete modelling of the flare and non-flare spectra show intrinsic changes only in the normalisation of the continuum components and not in their shape parameters. The timing results are consistent with the flare originating in the accretion disc corona. The iron emission line(s) do not appear to respond to changes in the continuum flux during the flare; the iron lines are stronger in equivalent width during the low-flux (non-flare) states, and weaker during the flare.

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arXiv2003

A long, hard look at MCG-6-30-15 with XMM-Newton II: detailed EPIC analysis and modelling

The bright Seyfert 1 galaxy MCG-6-30-15 has provided some of the best evidence to date for the existence of supermassive black holes in active galactic nuclei. Observations with ASCA revealed an X-ray iron line profile shaped by strong Doppler and gravitational effects. In this paper the shape of the iron line, its variability characteristics and the robustness of this spectral interpretation are examined using the long XMM-Newton observation taken in 2001. A variety of spectral models, both including and excluding the effects of strong gravity, are compared to the data in a uniform fashion. The results strongly favour models in which the spectrum is shaped by emission from a relativistic accretion disc. It is far more difficult to explain the 3-10 keV spectrum using models dominated by absorption (either by warm or partially covering cold matter), emission line blends, curved continua or additional continuum components. These provide a substantially worse fit to the data and fail to explain other observations (such as the simultaneous BeppoSAX spectrum). This reaffirms the veracity of the relativistic `disc line' interpretation. The short term variability in the shape of the energy spectrum is investigated and explained in terms of a two-component emission model. Using a combination of spectral variability analyses the spectrum is successfully decomposed into a variable power-law component (PLC) and a reflection dominated component (RDC). The former is highly variable while the latter is approximately constant throughout the observation, leading to the well-known spectral variability patterns. (Abridged)

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arXiv2003

Arakelian 564: An XMM-Newton view

We report on two XMM-Newton observations of the bright narrow-line Seyfert 1 galaxy Ark 564 taken one year apart (2000 June and 2001 June). The 0.6-10 keV continuum is well described by a soft blackbody component (kT~140-150 eV) plus a steep power law (Gamma~2.50-2.55). No significant spectral changes are observed between the two observations, although the X-ray flux in the second observation is ~40-50 per cent lower. In both observations we detect a significant absorption edge at a rest-frame energy of ~0.73 keV, corresponding to OVII. The presence of the absorption feature is confirmed by a simultaneous Chandra grating observation in 2000 June, although the best-fitting edge threshold is at a slightly lower energy in the Chandra data, possibly because of a different parameterisation of the underlying X-ray continuum. We find tentative evidence for a broad iron emission line in the 2000 June observation. The results from an analysis of the power spectral density (PSD) function are also presented. The present XMM-Newton data support the idea that the PSD shows two breaks, although the location of the high-frequency break requires further constraints.

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arXiv2003

Long-Term X-ray Spectral Variability in Seyfert 1 Galaxies

Direct time-resolved spectral fitting has been performed on continuous RXTE monitoring of seven Seyfert 1 galaxies in order to study their broadband spectral variability and Fe K alpha variability characteristics on time scales of days to years. Variability in the Fe K alpha line is not detected in some objects but is present in others, e.g., in NGC 3516, NGC 4151 and NGC 5548 there are systematic decreases in line flux by factors of ~2-5 over 3-4 years. The Fe K alpha line varies less strongly than the broadband continuum, but, like the continuum, exhibits stronger variability towards longer time scales. Relatively less model-dependent broadband fractional variability amplitude (Fvar) spectra also show weaker line variability compared to the continuum variability. Comparable systematic long-term decreases in the line and continuum are present in NGC 5548. Overall, however, there is no evidence for correlated variability between the line and continuum, severely challenging models in which the line tracks continuum variations modified only by a light-travel time delay. Local effects such as the formation of an ionized skin at the site of line emission may be relevant. The spectral fitting and Fvar spectra both support spectral softening as continuum flux increases.

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arXiv2003

On characterising the variability properties of X-ray light curves from active galaxies

We review some practical aspects of measuring the amplitude of variability in `red noise' light curves typical of those from Active Galactic Nuclei (AGN). The quantities commonly used to estimate the variability amplitude in AGN light curves, such as the fractional rms variability amplitude, F_var, and excess variance, sigma_XS^2, are examined. Their statistical properties, relationship to the power spectrum, and uses for investigating the nature of the variability processes are discussed. We demonstrate that sigma_XS^2 (or similarly F_var) shows large changes from one part of the light curve to the next, even when the variability is produced by a stationary process. This limits the usefulness of these estimators for quantifying differences in variability amplitude between different sources or from epoch to epoch in one source. Some examples of the expected scatter in the variance are tabulated for various typical power spectral shapes, based on Monte Carlo simulations. The excess variance can be useful for comparing the variability amplitudes of light curves in different energy bands from the same observation. Monte Carlo simulations are used to derive a description of the uncertainty in the amplitude expected between different energy bands (due to measurement errors). Finally, these estimators are used to demonstrate some variability properties of the bright Seyfert 1 galaxy Markarian 766. The source is found to show a strong, linear correlation between rms amplitude and flux, and to show significant spectral variability.

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arXiv2003

A softer look at MCG--6-30-15 with XMM-Newton

We present analysis and results from the Reflection Grating Spectrometer during the 320 ks XMM observation of the Seyfert 1 galaxy MCG-6-30-15. The spectrum is marked by a sharp drop in flux at 0.7 keV which has been interpreted by Branduardi-Raymont et al. as the blue wing of a relativistic OVIII emission line and by Lee at al. as a dusty warm absorber. We find that the drop is well explained by the FeI L2,3 absorption edges and obtain reasonable fits over the 0.32-1.7 keV band using a multizone, dusty warm absorber model. Some residuals remain which could be due to emission from a relativistic disc, but at a much weaker level than from any model relying on relativistic emission lines alone. A model based on such emission lines can be made to fit if sufficient (warm) absorption is added, although the line strengths exceed those expected. The EPIC pn difference spectrum between the highest and lowest flux states of the source indicates that this is a power-law in the 3-10 keV band which, if extrapolated to lower energies, reveals the absorption function acting on the intrinsic spectrum, provided that any emission lines do not scale exactly with the continuum. We find that this function matches our dusty warm absorber model well. The soft X-ray spectrum is therefore dominated by absorption structures, with the equivalent width of any individual emission lines in the residuals being below about 30 eV. (abridged)

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arXiv2003

XMM-Newton spectral properties of the Narrow-Line Seyfert 1 galaxy IRAS 13224--3809

We report on the first XMM-Newton observation of the highly X-ray variable, radio-quiet, Narrow-Line Seyfert 1 galaxy IRAS 13224-3809 obtained during the guaranteed time programme with a 64 ks exposure. The most remarkable spectral feature is a sharp drop, by a factor ~5, in the spectrum at 8 keV. This is a similar, but stronger, feature to that which we found in 1H0707-495. Significant flattening of the hard X-ray spectrum occurs when the source flux decreases. The flattening of the spectrum can be modelled as an increase in the column density of the absorbing material and/or its covering fraction. Below ~ 1.5 keV the spectrum is dominated by a giant soft X-ray excess, and at around ~ 1.2 keV there is a significant absorption feature detected, most probably due to ionized Fe L absorption. The new X-ray spectral properties detected with XMM-Newton in IRAS 13224-3809 support a partial covering interpretation, i.e. the presence of dense material inside the accretion-disc region partially obscuring the emission from the accretion disc. However, the sharpness of the feature, if due to photoelectric absorption, is surprising and may require an alternative explanation. One possibility which does fit the whole spectrum is that it is dominated by ionized reflection rather than absorption. The unusual spectral properties detected with XMM-Newton from Narrow-Line Seyfert 1 galaxies increase the known spectral complexity of active galactic nuclei and should further stimulate a combined theoretical and observational effort to achieve a better understanding of the physics of the innermost regions of AGN.

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arXiv2003

X-ray Fluctuation Power Spectral Densities of Seyfert 1 Galaxies

By combining complementary monitoring observations spanning long, medium and short time scales, we have constructed power spectral densities (PSDs) of six Seyfert~1 galaxies. These PSDs span $\gtrsim$4 orders of magnitude in temporal frequency, sampling variations on time scales ranging from tens of minutes to over a year. In at least four cases, the PSD shows a "break," a significant departure from a power law, typically on time scales of order a few days. This is similar to the behavior of Galactic X-ray binaries (XRBs), lower mass compact systems with breaks on time scales of seconds. NGC 3783 shows tentative evidence for a doubly-broken power law, a feature that until now has only been seen in the (much better-defined) PSDs of low-state XRBs. It is also interesting that (when one previously-observed object is added to make a small sample of seven), an apparently significant correlation is seen between the break time scale $T$ and the putative black hole mass $M_{\rm BH}$, while none is seen between break time scale and luminosity. The data are consistent with the linear relation $ T = M_{\rm BH}/10^{6.5} \Msun$; extrapolation over 6--7 orders of magnitude is in reasonable agreement with XRBs. All of this strengthens the case for a physical similarity between Seyfert~1s and XRBs.

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arXiv2003

A two-component ionized reflection model of MCG--6-30-15

Ionized reflection has often been considered as the explanation for the unusual Fe K variability observed in MCG--6-30-15. In this paper, we test this model using a 325 ks observation of MCG--6-30-15 by XMM-Newton and BeppoSAX. The data are fit between 2.5 and 80 keV with the constant density models of Ross & Fabian. The best fit ionized reflection model requires the Fe K line to be split into two reprocessing events: one from the inner disc to build up the red wing, and the other from the outer accretion disc to fit the blue horn. The implied geometry is a disc which becomes strongly warped or flared at large radii. A good fit was obtained with a solar abundance of iron and a reflection fraction (R) of unity for the inner reflector. The combination of the two reflection spectra can appear to have R>2 as required by the BeppoSAX data. The inner reflector has an ionization parameter \log ξ=3.8, but the outer one is neutral with an inner radius ~70 gravitational radii (r_g), corresponding to a light crossing time of about an hour for a 10^7 Msun black hole. Applying this model to time-resolved spectra shows that the inner reflector becomes more ionized as the source brightens. This reduces the strength of the red wing at high flux states. The X-ray source is constrained to arise from a narrow annulus at ~5 r_g, with only 6 per cent of the 2-10 keV flux due to the outer reprocessor. This amount of localized energy generation is extremely difficult to produce without resorting to other energy sources such as the black hole spin. In fact, all the Fe K models fit to XMM-Newton spectra of MCG--6-30-15 require a large increase in energy production at the inner edge of the accretion disc.

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arXiv2003

The iron line in MCG-6-30-15 from XMM-Newton: evidence for gravitational light-bending?

The lack of variability of the broad iron line seen in the Seyfert 1 galaxy MCG-6-30-15 is studied using the EPIC pn data obtained from a 2001 observation of 325 ks, during which the count rate from the source varied by a factor of 5. The spectrum of 80 ks of data from when the source was bright, using the the lowest 10 ks as background, is well fitted over the 3-10 keV band with a simple power-law of photon index Gamma=2.11. The source spectrum can therefore be decomposed into an approximately constant component, containing a strong iron emission line, and a variable power law component. Assuming that the power-law continuum extends down to 0.5 keV enables us to deduce the absorption acting on the nucleus. A simple model involving Galactic absorption and 2 photoelectric edges (at 0.72 and 0.86 keV) is adequate for the EPIC spectrum above 1 keV. It still gives a fair representation of the spectrum at lower energies where absorption lines, UTAs etc are expected. Applying the absorption model to the low flux spectrum enables us to characterize it as reflection-dominated with iron about 3 times Solar. Fitting this model, with an additional power-law continuum, to 32 x 10 ks spectra of the whole observation reveals that Gamma lies mostly between 2 and 2.3 and the normalization of the reflection-dominated component varies by up to 25 per cent. The breadth of the iron line suggests that most of the illumination originates from about 2 gravitational radii. Gravitational light bending will be very strong there, causing the observed continuum to be a strong function of the source height and much of the continuum radiation to return to the disk. Together these effects can explain the otherwise puzzling disconnectedness of the continuum and reflection components of MCG-6-30-15.

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arXiv2003

The high frequency power spectrum of Markarian 766

An analysis is presented of the power spectrum of X-ray variability of the bright Seyfert 1 galaxy Mrk 766 as observed by XMM-Newton. Over the 0.2-10 keV energy range the power spectral density (PSD) is well-represented by a power-law with a slope of alpha_low ~ 1 at low frequencies, breaking to a slope of alpha_hi = 2.8 (-0.4/+0.2) at a frequency f_br ~ 5 x 10^-4 Hz. As has been noted before this broken power-law PSD shape is similar to that observed in the Galactic black hole candidate Cygnus X-1. If it is assumed that Mrk 766 shows a power spectrum similar in form to that of Cyg X-1, and that the break timescale scales linearly with black hole mass, then the mass of the black hole in Mrk 766 is inferred to be < 5 x 10^5 M_sun. This rather low mass would mean Mrk 766 radiates above the Eddington limit. The coherence between different energy bands is significantly below unity implying that variations in the different energy bands are rather poorly correlated. The low coherence can be explained in the framework of standard Comptonisation models if the properties of the Comptonising medium are rapidly variable or if there are several distinct emission sites.

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arXiv2002

Ionized Disc Models of MCG-6-30-15

Ionized reflection is one of the possible mechanisms proposed to explain the small short-timescale variability of the MCG-6-30-15 Fe K line. In this contribution, we show fits of the Ross & Fabian ionized reflection models to the 325 ks XMM-Newton observation of MCG--6-30-15. It is found that the spectrum above 2.5 keV can be fitted with a combination of an ionized reflector from 5 r_g and a neutral reprocessor beyond about 70 r_g. The ionization parameter of the inner reflector increases as the source brightens.

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arXiv2002

X-ray continuum variability of MCG-6-30-15

This paper presents a comprehensive examination of the X-ray continuum variability of the bright Seyfert 1 galaxy MCG-6-30-15. The source clearly shows the strong, linear correlation between rms variability amplitude and flux first seen in Galactic X-ray binaries. The high frequency power spectral density (PSD) of MCG-6-30-15 is examined in detail using a Monte Carlo fitting procedure and is found to be well represented by a steep power-law at high frequencies (with a power-law index alpha ~ 2.5), breaking to a flatter slope (alpha ~ 1) below f_br ~ 0.6 - 2.0 x 10^-4 Hz, consistent with the previous results of Uttley, McHardy & Papadakis. The slope of the power spectrum above the break is energy dependent, with the higher energies showing a flatter PSD. At low frequencies the variations between different energy bands are highly coherent while at high frequencies the coherence is significantly reduced. Time lags are detected between energy bands, with the soft variations leading the hard. The magnitude of the lag is small (<200 s for the frequencies observed) and is most likely frequency dependent. These properties are remarkably similar to the temporal properties of the Galactic black hole candidate Cygnus X-1. The characteristic timescales in these two types of source differ by ~10^5; assuming that these timescales scale linearly with black hole mass then suggests a black hole mass ~10^6 M_sun for MCG-6-30-15. We speculate that the timing properties of MCG-6-30-15 may be analogous to those of Cyg X-1 in its high/soft state and discuss a simple phenomenological model, originally developed to explain the timing properties of Cyg X-1, that can explain many of the observed properties of MCG-6-30-15.

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arXiv2002

An XMM-Newton observation of Ton S180: Constraints on the continuum emission in ultrasoft Seyfert galaxies

We present an XMM-Newton observation of the bright, narrow-line, ultrasoft Seyfert 1 galaxy Ton S180. The 0.3-10 keV X-ray spectrum is steep and curved, showing a steep slope above 2.5 keV (Gamma ~ 2.3) and a smooth, featureless excess of emission at lower energies. The spectrum can be adequately parameterised using a simple double power-law model. The source is strongly variable over the course of the observation but shows only weak spectral variability, with the fractional variability amplitude remaining approximately constant over more than a decade in energy. The curved continuum shape and weak spectral variability are discussed in terms of various physical models for the soft X-ray excess emission, including reflection off the surface of an ionised accretion disc, inverse-Compton scattering of soft disc photons by thermal electrons, and Comptonisation by electrons with a hybrid thermal/non-thermal distribution. We emphasise the possibility that the strong soft excess may be produced by dissipation of accretion energy in the hot, upper atmosphere of the putative accretion disc.

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arXiv2002

A long hard look at MCG-6-30-15 with XMM-Newton

We present first results from a 325 ks observation of the Seyfert 1 galaxy MCG-6-30-15 with XMM-Newton and BeppoSAX. The strong, broad, skewed iron line is clearly detected and is well characterised by a steep emissivity profile within 6r_g (i.e. 6GM/c^2) and a flatter profile beyond. The inner radius of the emission appears to lie at about 2r_g, consistent with results reported from both an earlier XMM-Newton observation of MCG-6-30-15 by Wilms et al. and part of an ASCA observation by Iwasawa et al. when the source was in a lower flux state. The radius and steep emissivity profile do depend however on an assumed incident power-law continuum and a lack of complex absorption above 2.5 keV. The blue wing of the line profile is indented, either by absorption at about 6.7 keV or by a hydrogenic iron emission line. The broad iron line flux does not follow the continuum variations in a simple manner.

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