arXiv subjects

D. R. Ballantyne

Publications and source records attributed to D. R. Ballantyne.

At least 127 records · Page 7Linked to original sources

arXiv2004

X-ray Reflection from Inhomogeneous Accretion Disks: II. Emission Line Variability and Implications for Reverberation Mapping

One of the principal scientific objectives of the upcoming Constellation-X mission is to attempt to map the inner regions of accretion disks around black holes in Seyfert galaxies by reverberation mapping of the Fe K fluorescence line. This area of the disk is likely radiation pressure dominated and subject to various dynamical instabilities. Here, we show that density inhomogeneities in the disk atmosphere resulting from the photon bubble instability (PBI) can cause rapid changes in the X-ray reflection features, even when the illuminating flux is constant. Using a simulation of the development of the PBI, we find that, for the disk parameters chosen, the Fe K and O VIII Lyαlines vary on timescales as short as a few hundredths of an orbital time. In response to the changes in accretion disk structure, the Fe K equivalent width (EW) shows variations as large as ~100 eV. The magnitude and direction (positive or negative) of the changes depends on the ionization state of the atmosphere. The largest changes are found when the disk is moderately ionized. The O VIII EW varies by tens of eV, as well as exhibiting plenty of rapid, low-amplitude changes. This effect provides a natural explanation for some observed instances of short timescale Fe K variability which was uncorrelated with the continuum (e.g., Mrk 841). New predictions for Fe K reverberation mapping should be made which include the effects of this accretion disk driven line variability and a variable ionization state. Reflection spectra averaged over the evolution of the instability are well fit by constant density models in the 2-10 keV region.

astro-ph↗

arXiv2004

Continuum Acceleration of Black Hole Winds

Motivated by recent observations of high-velocity, highly ionized winds in several QSOs, models of purely continuum-driven winds launched from approximately 200 GM_{BH}/c^2 are presented. Launching conditions are investigated, as well as the observational signatures for a variety of initial conditions and illuminating continua. While we verify that continuum-driven, highly-ionized outflows reach the observed velocities for L/L_{Edd} >~ 1, independent of the incident spectral shape, such winds are too highly ionized to exhibit the observed absorption features when launched with an AGN continuum (in fact, such winds are so ionized that they are driven primarily by electron scattering). If the wind is instead illuminated with a blackbody continuum originating from an optically thick shield, the gas is too weakly ionized and does not produce high energy absorption features. If high-velocity, high-ionization winds are truly launched from very near the black hole, such winds must be launched under other conditions or via other processes; we summarize some possibilities.

astro-ph↗

arXiv2004

The XMM-Newton View of the Broad-Line Radio Galaxy 3C 120

We present the results of a 127 ks XMM-Newton observation of the broad-line radio galaxy 3C 120 performed simultaneously with RXTE. The time-averaged spectrum is Seyfert-like, with a reflection amplitude R ~ 0.5, and a neutral Fe K line with equivalent width ~53 eV. The line is slightly broadened with a FWHM$\approx 10^4$ km s$^{-1}$. This is consistent with arising from an accretion disc radius of $\ga 75$ $GM/c^2$ at an inclination angle of ~10 degrees, consistent with the limit of < 14 degree derived from the radio jets. At low energies the spectrum requires excess absorption above the Galactic value and a soft excess which is best fit with a bremsstrahlung model (kT=0.3-0.4 keV). The total luminosity in the bremsstrahlung component is just under half of the total hard X-ray luminosity. Weak O VII and O VIII edges are detected with high precision, suggesting the presence of a warm absorber component. A two-component ionized disc model, with a very highly ionized reflector presumably arising from very close to the black hole, is only a small improvement over a truncated disc model. The strength of the soft X-ray emission features produced by the distant neutral reflector are overpredicted in our solar abundance model, implying that the heavy metal abundance in 3C 120 is subsolar. The total count-rate declined by 20 per cent over the course of the long observation, while small-scale rapid variability was present at the level of a few percent. A possible increase in the Fe K line flux, significant at the 90 per cent level, was identified at ~80 ks in the observation. The total unabsorbed luminosity of 3C 120 implies that it is accreting close to its Eddington rate, consistent with a model of an ionized thick disc. A connection between accretion disc thickness and radio jet production is discussed.

astro-ph↗

arXiv2004

An XMM-Newton observation of Ark 120: the X-ray spectrum of a `bare' Seyfert 1 nucleus

We report on a long (100 ks) XMM-Newton observation of the bright Seyfert 1 galaxy Arakelian 120. The source previously showed no signs of intrinsic reddening in its infrared-ultraviolet continuum and previous observations had shown no evidence for ionized absorption in either the ultraviolet or X-ray bands. The new XMM-Newton RGS data place tight limits on the presence of an ionized X-ray absorber and confirm that the X-ray spectrum of Ark 120 is essentially unmodified by intervening matter. Thus Ark 120 can be considered a `bare' Seyfert 1 nucleus. This observation therefore offers a clean view of the X-ray spectrum of a `normal' Seyfert galaxy free from absorption effects. The spectrum shows a Doppler broadened iron emission line (FWHM ~ 3*10^4 km/s) and a smooth, continuous soft excess which appears to peak at an energy ~0.5 keV. This adds weight to the claim that genuine soft excesses (i.e. those due to a real steepening of the underlying continuum below ~2 keV) are ubiquitous in Seyfert 1 spectra. However, the detailed shape of the excess could not be reproduced by any of the simple models tested (power-laws, blackbodies, Comptonised blackbodies, accretion disc reflection). This observation therefore demonstrates both the need to understand the soft excess (as a significant contributor to the luminosity of most Seyfert 1s) and the inability of the existing, simple models to explain it.

astro-ph↗

arXiv2004

Reflection spectra from an accretion disc illuminated by a neutron star X-ray burst

Recent time-resolved X-ray spectra of a neutron star undergoing a superburst revealed an Fe K line and edge consistent with reprocessing from the surrounding accretion disc. Here, we present models of X-ray reflection from a constant density slab illuminated by a blackbody, the spectrum emitted by a neutron star burst. The calculations predict a prominent Fe K line and a rich soft X-ray line spectrum which is superimposed on a strong free-free continuum. The lines slowly vanish as the ionization parameter of the slab is increased, but the free-free continuum remains dominant at energiesless than 1 keV. The reflection spectrum has a quasi-blackbody shape only at energies greater than 3 keV. If the incident blackbody is added to the reflection spectrum, the Fe K equivalent width varies between 100 and 300 eV depending on the ionization parameter and the temperature, kT, of the blackbody. The equivalent width is correlated with kT, and therefore we predict a strong Fe K line when an X-ray burst is at its brightest (if iron is not too ionized or the reflection amplitude too small). Extending the study of reflection features in the spectra of superbursts to lower energies would provide further constraints on the accretion flow. If the Fe K line or other features are relativistically broadened then they can determine the system inclination angle (which leads to the neutron star mass), and, if the mass is known, a lower-limit to the mass/radius ratio of the star.

astro-ph↗

arXiv2004

The Evolution of the Accretion Disk around 4U 1820-30 During a Superburst

Accretion from a disk onto a collapsed, relativistic star -- a neutron star or black hole -- is the mechanism widely believed to be responsible for the emission from compact X-ray binaries. Because of the extreme spatial resolution required, it is not yet possible to directly observe the evolution or dynamics of the inner parts of the accretion disk where general relativistic effects are dominant. Here, we use the bright X-ray emission from a superburst on the surface of the neutron star 4U 1820-30 as a spotlight to illuminate the disk surface. The X-rays cause iron atoms in the disk to fluoresce, allowing a determination of the ionization state, covering factor and inner radius of the disk over the course of the burst. The time-resolved spectral fitting shows that the inner region of the disk is disrupted by the burst, possibly being heated into a thicker, more tenuous flow, before recovering its previous form in ~1000 s. This marks the first instance that the evolution of the inner regions of an accretion disk has been observed in real-time.

astro-ph↗

arXiv2003

X-ray Reflection from Inhomogeneous Accretion Disks: I. Toy Models and Photon Bubbles

Numerical simulations of the interiors of radiation dominated accretion disks show that significant density inhomogeneities can be generated in the gas. Here, we present the first results of our study on X-ray reflection spectra from such heterogeneous density structures. We consider two cases: first, we produce a number of toy models where a sharp increase or decrease in density of variable width is placed at different depths in a uniform slab. Comparing the resulting reflection spectra to those from an unaltered slab shows that the inhomogeneity can affect the emission features, in particular the Fe K and O VIII Lyalpha lines. The magnitude of any differences depends on both the parameters of the density change and the ionizing power of the illuminating radiation, but the inhomogeneity is required to be within ~2 Thomson depths of the surface to cause an effect. However, only relatively small variations in density (on the order of a few) are necessary for significant changes in the reflection features to be possible. Our second test was to compute reflection spectra from the density structure predicted by a simulation of the non-linear outcome of the photon bubble instability. The resulting spectra also exhibited differences from the constant density models, caused primarily by a strong 6.7 keV iron line. Nevertheless, constant density models can provide a good fit to simulated spectra, albeit with a low reflection fraction, between 2 and 10 keV. Below 2 keV, differences in the predicted soft X-ray line emission result in very poor fits with a constant density ionized disk model. The results indicate that density inhomogeneities may further complicate the relationship between the Fe K equivalent width and the X-ray continuum.

astro-ph↗

arXiv2003

On the Location and Composition of the Dust in the MCG-6-30-15 Warm Absorber

Hubble Space Telescope images of MCG-6-30-15 show a dust lane crossing the galaxy just below the nucleus. In this paper, we argue that this dust lane is responsible for the observed reddening of the nuclear emission and the Fe I edge hinted at in the Chandra spectrum of MCG-6-30-15. We further suggest that the gas within the dust lane can comprise much of the low ionization component (i.e., the one contributing the O VII edge) of the observed warm absorber. Moreover, placing the warm absorbing material at such distances (hundreds of pc) can account for the small outflow velocities of the low ionization absorption lines as well as the constancy of the O VIII edge. Photoionization models of a dusty interstellar gas cloud (with a column appropriate for the reddening toward MCG-6-30-15) using a toy Seyfert 1 spectral energy distribution show that it is possible to obtain a significant O VII edge (τ~0.2) if the material is ~150 pc from the ionizing source. For MCG-6-30-15, such a distance is consistent with the observed dust lane. The current data on MCG-6-30-15 is unable to constrain the dust composition within the warm absorber. Astronomical silicate is a viable candidate, but there are indications of a very low O abundance in the dust, which is inconsistent with a silicate origin. If true, this may indicate that there were repeated cycles of grain destruction and growth from shocks in the interstellar medium of MCG-6-30-15. Pure iron grains are an unlikely dust constituent due to the limit on their abundance in the Galaxy, yet they cannot be ruled out. The high column densities inferred from the highly ionized zone of the warm absorber implies that this gas is dust-free.

astro-ph↗

arXiv2003

The Contribution of Particle Impact to the Production of Fe K Emission from Accreting Black Holes

The iron K line is perhaps the most important spectral diagnostic available in the study of accreting black holes. The line is thought to result from the reprocessing of external X-rays by the surface of the accretion disk. However, as is observed in the solar corona, illumination by energetic particles may also produce line emission. In principle, such a process may be uncorrelated with the observed X-rays and could explain some of the unexpected variability behavior of the Fe line. This paper compares predictions of iron K flux generated by impacting electrons and protons to that from photoionization. Non-thermal power-laws of electrons are considered as well as thermal distributions of electrons and virialized protons. The electrons are thought to originate in a magnetically dominated accretion disk corona, while the protons are considered in the context of a two phase (hot/cold) accretion scenario. In each case, the Fe K flux from particle impact is found to be < 1% of that produced by photoionization by a hard X-ray power-law (normalized to the same energy flux as the particles). Thus, the electrons or protons must strike the disk with 100--10,000 times more energy flux than radiation for particle impact to be a significant producer of Fe K flux. This situation is difficult to reconcile with the observations of hard X-ray spectra, or the proposed particle acceleration mechanisms in the accretion disk corona. Truncated accretion flows must be externally illuminated by hard X-rays in order to produce the Fe line, as proton impact is very inefficient in generating line emission. In contrast to the Sun, our conclusion is that, with the possible exception for localized regions around magnetic footpoints, particle impact will not be an important contributor to the X-ray emission in accreting black holes.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

arXiv2002

Soft X-ray emission lines from photoionized accretion discs: constraints on their strength and width

We consider the properties of soft X-ray emission lines in the reprocessed emission from a photoionized accretion disc. Observations of these lines will be important in determining the ionization state and metallicity of the innermost regions of active galaxies. Calculations of reflection from a constant-density disc with an ionization parameter between 250 and 1000 erg cm/s show that emission from O VIII Lyαwill dominate the soft X-ray emission spectrum. There is also significant emission from C VI, N VII, O VII, as well as Fe XVII--XIX. As the ionization parameter is increased these lines become weaker and are broadened by Compton scattering. Significantly increasing the O abundance primarily strengthens the O VII line, making it as large or larger than the O VIII line. The nitrogen and carbon lines are quite weak with equivalent widths (EWs) <30 eV, even with an increase in the N abundance. A hydrostatic ionized disc model has a more realistic density structure and shows a similar spectrum, but with the lines weaker and broader. This is a result of the hot ionized skin at the surface of the disc. We apply these results to the controversial claim of soft X-ray relativistic lines in the XMM-Newton spectrum of MCG--6-30-15. We are unable to find a situation where O VIII has the required EW without substantial emission from O VII. Furthermore, Compton scattering results in the blue wing of the O VIII line to be much broader than the << 10 eV drop observed in the data. We conclude that soft X-ray accretion disc lines will, in general, be weak and broad features and are unlikely to produce sharp edges in the data.

astro-ph↗

arXiv2002

Extremely Weak Reflection Features in the X-ray Spectrum of XTE J1118+480: Possible Evidence for X-ray-Emitting Jets?

We have simultaneously fit Chandra and RXTE spectra of the Galactic black hole XTE J1118+480 with three models for X-ray reflection. We explored a range of accretion disc ionizations (log(xi)=1-4; xi=L_X/nR^{2}) and iron abundances (0.10-1.00). Our fits with the constant density ionized disc models of Ross & Fabian indicate that less than 0.5 per cent (90 per cent confidence upper-limit) of the observed flux is reflected. Fits with the "pexrav" of model Magdziarz & Zdziarski indicate that the two-dimensional solid angle (Omega/2pi) subtended by the disc relative to a central source of incident hard X-rays is 0.01 +0.06 -0.01. A combination of the high inclination (i=81 degrees), Comptonization, and bulk velocities may each contribute to the low reflection fractions we have measured. The results are also consistent with extended jets being the source of the hard X-ray flux, as the disc would then represent a small solid angle as seen from the emission region.

astro-ph↗

arXiv2002

Multiple X-ray reflection from ionized slabs

Multiple reflection of X-rays may be important when an accretion disc and its hot corona have a complicated geometry, or if returning radiation due to gravitational light bending is important, or in emission from a funnel such as proposed in some gamma-ray burst models. We simulate the effects of multiple reflection by modifying the boundary condition for an X-ray illuminated slab. Multiple reflection makes the soft X-ray spectrum steeper (softer) and strengthens broad emission and absorption features, especially the K-shell features of iron. This may be important in explaining the spectra of sources such as the Narrow-Line Seyfert 1 galaxy 1H0707-495.

astro-ph↗

arXiv2002

X-ray spectral diagnostics of the immediate environment of GRB 991216

The recent report of iron line features in the afterglow of the gamma-ray burst (GRB) 991216 has important implications for the properties of the radiating material and hence the nature of the immediate burst environment. We argue that the putative strong Fe emission line can be attributed to the reflected emission from Thomson-thick matter which is illuminated by a power-law continuum. The ionization parameter of the material (i.e., the flux to density ratio) is around 10^3, resulting in a Fe K line from He-like iron. A supersolar abundance of iron is not required by the data. Interestingly, the ionizing continuum must be harder than the observed one. We interpret this as due to the fact that the observed continuum is dominated by the standard blast wave emission, while the line is produced by reprocessing material located at much smaller radii.

astro-ph↗

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.

astro-ph↗

arXiv2002

Reflection Spectra from Photoionized Accretion Discs

We review recent progress on the modeling and use of reflection spectra from irradiated and ionized accretion discs. On the computational side, calculations of reflection spectra from discs with non-uniform density structure have shown that thermal instabilities can effect the predictions. Ionized reflection spectra have been used effectively in fitting data of Narrow-Line Seyfert 1 galaxies, and have placed constraints on the strength and shape of soft X-ray emission lines.

astro-ph↗