Amplification and variability of the AGN X‐ray emission due to microlensing

We consider the contribution of microlensing to the AGN Fe Kα line and X‐ray continuum amplification and variation. To investigate the variability of the line and X‐ray continuum, we studied the effects of microlensing on quasar X‐ray spectra produced by crossing of a microlensing pattern across a standard relativistic accretion disk. To describe the disk emission we used a ray tracing method considering both metrics, Schwarzschild and Kerr. We found that the Fe Kα and continuum may experience significant amplification by a microlensing event (even for microlenses of very small mass). Also, we investigate a contribution of microlensing to the X‐ray variability of high‐redshifted QSOs, finding that cosmologically distributed deflector may contribute significantly to the X‐ray variability of high‐redshifted QSOs (z > 2). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic flares in Active Galactic Nuclei: modeling the iron Kα line

The X‐ray spectra of Active Galactic Nuclei (AGN) are complex and vary rapidly in time as seen in recent observations. Magnetic flares above the accretion disk can account for the extreme variability of AGN. They also explain the observed iron Kα fluorescence lines. We present radiative transfer modeling of the X‐ray reflection due to emission from magnetic flares close to the marginally stable orbit. The hard X‐ray primary radiation coming from the flare source illuminates the accretion disk. A Compton reflection/reprocessed component coming from the disk surface is computed for different emission directions. We assume that the density structure remains adjusted to the hydrostatic equilibrium without external illumination because the flare duration is only a quarter‐orbit. The model takes into account the variations of the incident radiation across the hot spot underneath the flare source. The integrated spectrum seen by a distant observer is computed for flares at different orbital phases close to the marginally stable orbit of a Schwarzschild black hole and of a maximally rotating Kerr black hole. The calculations include relativistic and Doppler corrections of the spectra using a ray tracing technique. We explore the practical possibilities to map out the azimuthal irradiation pattern of the inner accretion disks and conclude that the next generation of X‐ray satellites should reveal this structure from iron Kα line profiles and X‐ray lightcurves. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An XMM‐Newton survey of broad iron lines in AGN

We report on the iron Kα line properties of a sample of Seyfert galaxies observed with the XMM‐Newton EPIC pn instrument. Using a systematic and uniform analysis, we find that complexity at iron‐K is extremely common in the XMM‐Newton spectra. Once appropriate soft X‐ray absorption, narrow 6.4 keV emission and associated Compton reflection are accounted for, ∼75% of the sample show an improvement when a further component is introduced. The typical properties of the broad emission are both qualitatively and quantitatively consistent with previous results from ASCA. The complexity is in general very well described by relativistic accretion disk models. In most cases the characteristic emission radius is constrained to be within ∼50R _g, where strong gravitational effects become important. We find in about 1/3 of the sample the accretion disk interpretation is strongly favoured over competing models. In a few objects no broad line is apparent. We find evidence for emission within 6R _g in only two cases, both of which exhibit highly complex absorption. Evidence for black hole spin based on the X‐ray spectra therefore remains tentative. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A short introduction to broad and variable iron lines around black holes

Accreting black holes often show iron line emission in their X‐ray spectra. When this line emission is very broad or variable then it is likely to originate from close to the black hole. The theory and observations of such broad and variable iron lines are briefly reviewed here. In order for a clear broad line to be found, one or more of the following have to occur: high iron abundance, dense disk surface and minimal complex absorption. Several excellent examples are found from observations of Seyfert galaxies and Galactic Black Holes. In several cases there is strong evidence that the black hole is rapidly spinning. Further examples are expected as more long observations are made with XMM‐Newton, Chandra and Suzaku. Intriguing instances of rapid variability of some narrow iron lines, both emission and absorption, have been reported. These may reflect variations in the irradiation or motion of physical structures on the accretion disk. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Relativistic Fe Kα features in the XMM‐Newton spectrum of the intermediate Seyfert galaxy 4U 1344‐60

We present the analysis of optical and X‐ray XMM‐Newton data of the source 4U 1344‐60. On the basis of the optical data we propose to classify 4U 1344‐60 as a Seyfert 1.5 galaxy and we measured a redshift value z = 0.012 ± 0.001. The observed X‐ray spectrum is complex. The continuum emission can be described as a power law obscured by two neutral absorption components. 4U 1344‐60 exhibits a broad and skewed iron line at 6.4 keV most likely originated in a few gravitational radius of an accretion disc. The analysis also reveals the presence of two narrow emission line‐like features at ∼4.9 keV and ∼5.3 keV. Assuming that hot spots on the surface of the accretion disc, orbiting very close to the black hole is responsible of these emission lines, the accretion disc would present an inclination of 20° and the active regions would be located in the 6–10 R _g radius range. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

XMM‐Newton and broad iron lines

Iron line emission is common in the X‐ray spectra of accreting black holes. When the line emission is broad or variable then it is likely to originate from close to the black hole. X‐ray irradiation of the accretion flow by the power‐law X‐ray continuum produces the X‐ray ‘reflection’ spectrum which includes the iron line. The shape and variability of the iron lines and reflection can be used as a diagnostic of the radius, velocity and nature of the flow. The inner radius of the dense flow corresponds to the innermost stable circular orbit and thus can be used to determine the spin of the black hole. Studies of broad iron lines and reflection spectra offer much promise for understanding how the inner parts of accretion flows (and outflows) around black holes operate. There remains great potential for XMM‐Newton to continue to make significant progress in this work. The need for high quality spectra and thus for long exposure times is paramount. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Results and perspectives of young stellar object long look programs

Both Chandra and XMM‐Newton have performed long look programs for studying the YSO physics. I will discuss recent results on the controversial issue of Class 0 YSO X‐ray emission, the observational evidence of magnetic funnels interconnecting the YSO with its circumstellar disk and the Fe 6.4 keV fluorescent line emission and its origin. While recent results of the XMM‐Newton DROXO program challenge the “standard” interpretation of the Fe 6.4 kev line origin as due to photoionized fluorescing disk material, the discovery of X‐ray excited Ne 12.81 μ m line is a clear evidence of the interaction between X‐rays and disk material. Future long look observations with XMM‐Newton are required to clarify the X‐ray effects on YSO disk. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

X‐ray observations of classical novae: Theoretical implications

Detection of X‐rays from classical novae, both in outburst and post‐outburst, provides unique and crucial information about the explosion mechanism. Soft X‐rays reveal the hot white dwarf photosphere, whenever hydrogen (H) nuclear burning is still on and expanding envelope is transparent enough, whereas harder X‐rays give information about the ejecta and/or the accretion flow in the reborn cataclysmic variable. The duration of the supersoft X‐ray emission phase is related to the turn‐off of the classical nova, i.e., of the H‐burning on top of the white dwarf core. A review of X‐ray observations is presented, with a special emphasis on the implications for the duration of post‐outburst steady H‐burning and its theoretical explanation. The particular case of recurrent novae (both the “standard” objects and the recently discovered ones) will also be reviewed, in terms of theoretical feasibility of short recurrence periods, as well as regarding implications for scenarios of type Ia supernovae (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The iron emission line complex of MCG‐5‐23‐16: the long XMMNewton look

We present the results of the simultaneous XMM‐Newton and Chandra observations of the bright Seyfert 1.9 galaxy MCG–5‐23‐16, which is one of the best known examples of a relativistically broadened iron Kα line. We find that: a) the soft X‐ray emission is likely to be dominated by photoionized gas, b) the complex iron emission line is best modelled with a narrow and a broad component with a FWHM ∼44000 km/s. This latter component has an EW ∼50 eV and its profile is well described with an emission line mainly originating from the accretion disk a few tens of gravitational radii from the central black hole and viewed with an inclination angle ∼40°. We found evidence of a possible sporadic absorption line at ∼7.7 keV which, if associated with Fe XXVI Kα resonance absorption, is indicative of a possible high velocity (v ∼ 0.1c) outflow. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Suzaku observations of iron lines and reflection in AGN

Initial results on the iron K‐shell line and reflection component in several AGN observed as part of the Suzaku Guaranteed Time program are reviewed. This paper discusses a small sample of Compton‐thin Seyferts observed to date with Suzaku; namely MCG‐5‐23‐16, MCG‐6‐30‐15, NGC4051, NGC3516, NGC2110, 3C 120 and NGC2992. The broad iron Kα emission line appears to be present in all but one of these Seyfert galaxies, while the narrow core of the line from distant matter is ubiquitous in all the observations. The iron line in MCG‐6‐30‐15 shows the most extreme relativistic blurring of all the objects, the red‐wing of the line requires the inner accretion disk to extend inwards to within 2.2R _g of the black hole, in agreement with the XMM‐Newton observations. Strong excess emission in the Hard X‐ray Detector (HXD) above 10 keV is observed in many of these Seyfert galaxies, consistent with the presence of a reflection component from reprocessing in Compton‐thick matter (e.g. the accretion disk). Only one Seyfert galaxy (NGC 2110) shows neither a broad iron line nor a reflection component. The spectral variability of MCG‐6‐30‐15, MCG‐5‐23‐16 and NGC 4051 is also discussed. In all 3 cases, the spectra appear harder when the source is fainter, while there is little variability of the iron line or reflection component with source flux. This agrees with a simple two component spectral model, whereby the variable emission is the primary power‐law, while the iron line and reflection component remain relatively constant. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Swift observations of CSS081007:030559+054715

CSS081007:030559+054715 was discovered by the Catalina Real‐time Transient Survey. Optical spectroscopy revealed a multi‐peaked Hα emission line profile with radial velocities exceeding 1500 km/s, as well as strong Ne emission, suggestive of a neon nova. We monitored the source extensively with the Swift satellite, obtaining a unique dataset spanning 270 days in the soft X‐ray and UV bands. The data reveal a soft, blackbody‐like spectrum with a temperature around 55 eV (though dependent on the modelling), variable X‐ray and UV light curves with a 1.77 day period in both the X‐ray and UV bands, a longer timescale modulation of ∼ 50 days, followed by a slowly declining trend in the soft X‐ray and UV flux. We highlight the Swift observations and their implications for the SSS nature of this object (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Discovery of a double peaked Fe emission line in the Cloverleaf quasar H1413+117

Our spatial and spectral analysis of a recent deep Chandra observation of H1413+117 confirms a microlensing event in a previous Chandra observation of this object performed about 5 years earlier. We present constraints on the structure of H1413+117 based on the time‐scale of this microlensing event. The analysis of the combined spectrum of the images indicates the presence of two emission line peaks at rest‐frame energies of 5.35 keV and 6.32 keV and detected at the ≳97% and ≳99% confidence levels, respectively. The double peaked Fe emission is fit well with an accretion disk‐line model, however, the best‐fit accretion disk model parameters are neither well constrained nor unique. Another possible interpretation of the Fe emission is fluorescent Fe emission from the back‐side of the wind. Additional observations are required to constrain better the model parameters. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Accreting white dwarfs as supersoft X‐ray sources

I review various phenomena associated with mass‐accreting white dwarfs (WDs) in the view of supersoft X‐ray sources. When the mass‐accretion rate is low (Ṁ_acc < a few × 10^–7 M⊙yr^–1), hydrogen nuclear burning is unstable and nova outbursts occur. A nova is a transient supersoft X‐ray source (SSS) in its later phase which timescale depends strongly on the WD mass. The X‐ray turn on/off time is a good indicator of the WD mass. At an intermediate mass‐accretion rate an accreting WD becomes a persistent SSS with steady hydrogen burning. For a higher mass‐accretion rate, the WD undergoes “accretion wind evolution” in which the WD accretes matter from the equatorial plane and loses mass by optically thick winds from the other directions. Two SSS, namely RX J0513‐6951 and V Sge, are corresponding objects to this accretion wind evolution. We can specify mass increasing WDs from light‐curve analysis based on the optically thick wind theory using multiwavelength observational data including optical, IR, and supersoft X‐rays. Mass estimates of individual objects give important information for the binary evolution scenario of type Ia supernovae (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Boundary layer emission in luminous LMXBs

We show that aperiodic and quasiperiodic variability of bright LMXBs – atoll and Z‐sources – on ∼ sec‐msec time scales is caused primarily by variations of the luminosity of the boundary layer. The emission of the accretion disk is less variable on these time scales and its power density spectrum follows P _disk(f ) ∝ f ^–1 law, contributing to observed flux variation at low frequencies and low energies only. The kHz QPOs have the same origin as variability at lower frequencies, i.e. independent of the nature of the “clock”, the actual luminosity modulation takes place on the neutron star surface. The boundary layer spectrum remains nearly constant in the course of the luminosity variations and is represented to certain accuracy by the Fourier frequency resolved spectrum. In the investigated range of ∼ (0.1 – 1) _Edd it depends weakly on the global mass accretion rate and in the limit ∼ _Edd is close toWien spectrum with kT ∼ 2.4 keV. Its independence on the global value of lends support to the theoretical suggestion by Inogamov & Sunyaev (1999) that the boundary layer is radiation pressure supported. Based on the knowledge of the boundary layer spectrum we attempt to relate the motion along the Z‐track to changes of physically meaningful parameters. Our results suggest that the contribution of the boundary layer to the observed emission decreases along the Z‐track from conventional ∼50% on the horizontal branch to a rather small number on the normal branch. This decrease can be caused, for example, by obscuration of the boundary layer by the geometrically thickened accretion disk at ∼ _Edd. Alternatively, this can indicate significant change of the structure of the accretion flow at ∼ _Edd and disappearance of the boundary layer as a distinct region of the significant energy release associated with the neutron star surface. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Accretion,jets and winds: High‐energy emission from young stellar objects – Doctoral Thesis Award Lecture 2010

This article summarizes the processes of high‐energy emission in young stellar objects. Stars of spectral type A and B are called Herbig Ae/Be (HAeBe) stars in this stage, all later spectral types are termed classical T Tauri stars (CTTS). Both types are studied by high‐resolution X‐ray and UV spectroscopy and modeling. Three mechanisms contribute to the highenergy emission from CTTS: 1) CTTS have active coronae similar to main‐sequence stars, 2) the accreted material passes through an accretion shock at the stellar surface, which heats it to a few MK, and 3) some CTTS drive powerful outflows. Shocks within these jets can heat the plasma to X‐ray emitting temperatures. Coronae are already well characterized in the literature; for the latter two scenarios models are shown. The magnetic field suppresses motion perpendicular to the field lines in the accretion shock, thus justifying a 1D geometry. The radiative loss is calculated as optically thin emission. A mixture of shocked and coronal gas is fitted to X‐ray observations of accreting CTTS. Specifically, the model explains the peculiar line‐ratios in the He‐like triplets of Ne IX and O VII. All stars require only small mass accretion rates to power the X‐ray emission. In contrast, the HAeBe HD 163296 has line ratios similar to coronal sources, indicating that neither a high density nor a strong UV‐field is present in the region of the X‐ray emission. This could be caused by a shock in its jet. Similar emission is found in the deeply absorbed CTTS DG Tau. Shock velocities between 400 and 500 km s^–1 are required to explain the observed spectrum (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The X‐ray source population of the Andromeda galaxy M 31

First studies of the X‐ray source population of M 31 were performed with the Einstein Observatory and ROSAT. High resolution Chandra Observatory images not only spatially resolved the center area but also supernova remnants (SNRs) in the galaxy. Source catalogues of restricted areas were presented with high astrometric accuracy. Also luminosity function studies and studies of individual sources based on Chandra and XMM‐Newton observations led to a better knowledge of the X‐ray source population. An XMM‐Newton source catalog based on archival observations revealed more than 850 sources down to a 0.2–4.5 keV luminosity of 10³⁵ erg s^–1. EPIC hardness ratios as well as informations from earlier X‐ray, optical, and radio catalogues were used to distinguish between different source classes (SNRs, supersoft sources (SSSs), X‐ray binaries (XRBs), globular cluster sources within M 31, and foreground stars and objects in the background). However, many sources could only be classified as “hard”. These sources may either be XRBs or Crab‐like SNRs in M 31 or background sources. Two of the globular cluster sources could be identified as low mass XRBs with a neutron star as compact object as they showed type I X‐ray bursts. Many of the SSSs were identified as optical novae. Inspired by these results an XMM‐Newton survey of the entire D₂₅ disk of M 31 and a dedicated program to monitor X‐ray counterparts of optical novae in M 31 was started. We discuss implications for further nearby galaxy studies. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

QPO as the Rosetta Stone for understanding black hole accretion

Quasi‐periodic oscillations (QPO) seen in the X‐ray fluxes of individual neutron stars and black hole sources are one of most intriguing phenomena in today's astrophysics. The QPO nature is visibly determined by super‐strong Einstein's gravity. I argue here that it also profoundly depends on the MRI turbulence in accretion flows. Understanding the QPO physics may therefore guide accretion theory out of its present state of confusion. Readers will find here an up‐to‐date, comprehensible account of what is known, and what is not, about the QPO physics. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Global oscillations of a fluid torus as a modulation mechanism for black‐hole high‐frequency QPOs

We study strong‐gravity effects on modulation of radiation emerging from accreting compact objects as a possible mechanism for flux modulation in QPOs. We construct a toy model of an oscillating torus in the slender approximation assuming thermal bremsstrahlung for the intrinsic emissivity of the medium and we compute observed (predicted) radiation signal including contribution of indirect (higher‐order) images and caustics in the Schwarzschild spacetime. We show that the simplest oscillation mode in an accretion flow, axisymmetric up‐and‐down motion at the meridional epicyclic frequency, may be directly observable when it occurs in the inner parts of accretion flow around black holes. Together with the second oscillation mode, an in‐and‐out motion at the radial epicyclic frequency, it may then be responsible for the high‐frequency modulations of the X‐ray flux observed at two distinct frequencies (twin HF‐QPOs) in micro‐quasars. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Supersoft AGNs and their relations to Galactic binaries

I review some basic results on AGN with supersoft X‐ray spectra and their relations to Galactic binaries in their soft high states. This paper is based on a talk given at the Supersoft Sources Workshop at ESTEC in May 2009. Given the length of the talk and the number of pages the review cannot be complete and is biased towards my personal view. I demonstrate that at high accretion rates supersoft AGNs and Galactic binaries share steep soft X‐ray spectra, that the X‐ray variability of supersoft AGNs is more pronounced compared to Galactic binaries in their high states, that the X‐ray variability of supersoft novae and supersoft AGNs is similar, and that in Galactic binaries mostly positive time lags are seen, while negative time lags are observed in some supersoft AGN (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)