Showing papers by "A. C. Fabian published in 2004"

A Comparison of Intermediate-Mass Black Hole Candidate Ultraluminous X-Ray Sources and Stellar-Mass Black Holes

Abstract: Cool thermal emission components have recently been revealed in the X-ray spectra of a small number of ultraluminous X-ray sources (ULXs) with LX ≥ 1040 ergs s-1 in nearby galaxies. These components can be well fitted with accretion disk models, with temperatures approximately 5-10 times lower than disk temperatures measured in stellar-mass Galactic black holes when observed in their brightest states. Because disk temperature is expected to fall with increasing black hole mass, and because the X-ray luminosity of these sources exceeds the Eddington limit for 10 M☉ black holes (LEdd 1.3 × 1039 ergs s-1), these sources are extremely promising intermediate-mass black hole candidates. In this Letter, we directly compare the inferred disk temperatures and luminosities of these ULXs, with the disk temperatures and luminosities of a number of Galactic black holes. The sample of stellar-mass black holes was selected to include different orbital periods, companion types, inclinations, and column densities. These ULXs and stellar-mass black holes occupy distinct regions of a LX-kT diagram, suggesting these ULXs may harbor intermediate-mass black holes. We briefly discuss the important strengths and weaknesses of this interpretation.

A Comparison of Intermediate Mass Black Hole Candidate ULXs and Stellar-Mass Black Holes

Abstract: Cool thermal emission components have recently been revealed in the X-ray spectra of a small number of ultra-luminous X-ray (ULX) sources with L_X > 1 E+40 erg/s in nearby galaxies. These components can be well fitted with accretion disk models, with temperatures approximately 5-10 times lower than disk temperatures measured in stellar-mass Galactic black holes when observed in their brightest states. Because disk temperature is expected to fall with increasing black hole mass, and because the X-ray luminosity of these sources exceeds the Eddington limit for 10 Msun black holes (L_Edd = 1.3 E+39 erg/s), these sources are extremely promising intermediate-mass black hole candidates (IMBHCs). In this Letter, we directly compare the inferred disk temperatures and luminosities of these ULXs, with the disk temperatures and luminosities of a number of Galactic black holes. The sample of stellar-mass black holes was selected to include different orbital periods, companion types, inclinations, and column densities. These ULXs and stellar-mass black holes occupy distinct regions of a L_X -- kT diagram, suggesting these ULXs may harbor IMBHs. We briefly discuss the important strengths and weaknesses of this interpretation.

Simultaneous Chandra and RXTE Spectroscopy of the Microquasar H~1743-322: Clues to Disk Wind and Jet Formation from a Variable Ionized Outflow

Abstract: We observed the bright phase of the 2003 outburst of the Galactic black hole candidate H 1743-322 in X-rays simultaneously with Chandra and RXTE on four occasions. The Chandra/HETGS spectra reveal narrow, variable (He-like) Fe XXV and (H-like) Fe XXVI resonance absorption lines. In the first observation, the Fe XXVI line has a FWHM of 1800 +/- 400 km/s and a blue-shift of 700 +/- 200 km/s, suggesting that the highly ionized medium is an outflow. Moreover, the Fe XXV line is observed to vary significantly on a timescale of a few hundred seconds in the first observation, which corresponds to the Keplerian orbital period at approximately 1 E+4 gravitational radii. Our models for the absorption geometry suggest that a combination of geometric effects and changing ionizing flux are required to account for the large changes in line flux observed between observations, and that the absorption likely occurs at a radius less than 1 E+4 radii for a 10 Msun black hole. Viable models for the absorption geometry include cyclic absorption due to an accretion disk structure, absorption in a clumpy outflowing disk wind, or possibly a combination of these two. If the wind in H 1743-322 has unity filling factor, the highest implied mass outflow rate is 20 percent of the Eddington mass accretion rate. This wind may be a hot precursor to the Seyfert-like, outflowing "warm absorber" geometries recently found in the Galactic black holes GX 339-4 and XTE J1650-500. We discuss these findings in the context of ionized Fe absorption lines found in the spectra of other Galactic sources, and connections to warm absorbers, winds, and jets in other accreting systems.

X‐ray reflection in the narrow‐line Seyfert 1 galaxy 1H 0707–495

Abstract: We apply a reflection-dominated model to the second XMM-Newton observation of the narrow-line Seyfert 1 galaxy 1H 0707-495. As in the first XMM-Newton observation, a sharp spectral drop is detected with energy that has shifted from 7 to 7.5 keV in two years. The drop is interpreted in terms of relativistically blurred ionized reflection from the accretion disc, while the energy shift can be accounted for by changes in the ionization state and, more importantly, emissivity profile on the disc. A flatter emissivity profile during the second higher flux observation reduces gravitational redshift effects, therefore shifting the edge to higher energy. Remarkably, ionized disc reflection and the associated power-law continuum provide a reasonable description of the broad-band spectrum, including the soft excess. Considering both observations, the spectral variability in 1H 0707-495 appears to be a result of the interplay between these two spectral components. The reflection component in the second observation is significantly less variable than the power law. Changes of the emissivity profile, spectral shape and variability properties (such as the rms spectrum) within the two observations are all consistent with a recently proposed model in which relativistic effects in the very inner regions of the nucleus play a major role.

Chandra/High energy transmission grating spectrometer spectroscopy of the galactic black hole GX 339-4: a relativistic iron emission line and evidence for a Seyfert-like warm absorber

Abstract: We observed the Galactic black hole GX 339-4 with the Chandra High Energy Transmission Grating Spectrometer (HETGS) for 75 ks during the decline of its 2002-2003 outburst. The sensitivity of this observation provides an unprecedented glimpse of a Galactic black hole at about a tenth of the luminosity of the outburst peak. The continuum spectrum is well described by a model consisting of multicolor disk blackbody (kT~=0.6 keV) and power-law (Gamma~=2.5) components. X-ray reflection models yield improved fits. A strong, relativistic Fe Kalpha emission line is revealed, indicating that the inner disk extends to the innermost stable circular orbit. The breadth of the line is sufficient to suggest that GX 339-4 may harbor a black hole with significant angular momentum. Absorption lines from H- and He-like O and He-like Ne and Mg are detected, as well as lines that are likely due to Ne II and Ne III. The measured line properties make it difficult to associate the absorption with the coronal phase of the interstellar medium. A scenario wherein the absorption lines are due to an intrinsic AGN-like warm-absorber geometry-perhaps produced by a disk wind in an extended disk-dominated state-may be more viable. We compare our results to Chandra observations of the Galactic black hole candidate XTE J1650-500 and discuss our findings in terms of prominent models for Galactic black hole accretion flows and connections to supermassive black holes.

X-ray reflection in the NLS1 1H 0707-495

Abstract: We apply a reflection-dominated model to the second XMM-Newton observation of the Narrow Line Seyfert 1 galaxy 1H 0707-495. As in the first XMM-Newton observation a sharp spectral drop is detected with energy that has shifted from 7 keV to 7.5 keV in two years. The drop is interpreted in terms of relativistically blurred ionised reflection from the accretion disc, while the energy shift can be accounted for by changes in the ionisation state and, more importantly, emissivity profile on the disc. A flatter emissivity profile during the second higher flux observation reduces gravitational redshift effects, therefore shifting the edge to higher energy. Remarkably, ionised disc reflection and the associated power law continuum provide a reasonable description of the broadband spectrum, including the soft excess. Considering both observations, the spectral variability in 1H 0707-495 appears to be due to the interplay between these two spectral components. The reflection component in the second observation is significantly less variable than the power law. Changes of the emissivity profile, spectral shape and variability properties (such as the rms spectrum) within the two observations are all consistent with a recently proposed model in which relativistic effects in the very inner regions of the nucleus play a major role.

Evidence of Black Hole Spin in GX 339?4: XMM-Newton/EPIC-pn and RXTE Spectroscopy of the Very High State

Abstract: We have analyzed spectra of the Galactic black hole GX 339-4 obtained through simultaneous 76 ks XMM-Newton/EPIC-pn and 10 ks Rossi X-Ray Timing Explorer observations during a bright phase of its 2002-2003 outburst. An extremely skewed, relativistic Fe Kalpha emission line and ionized disk reflection spectrum are revealed in these spectra. Self-consistent models for the Fe Kalpha emission-line profile and disk reflection spectrum rule out an inner disk radius compatible with a Schwarzschild black hole at more than the 8 sigma level of confidence. The best-fit inner disk radius of (2-3)rg suggests that GX 339-4 harbors a black hole with a>=0.8-0.9 (where rg=GM/c2 and a=cJ/GM2, and assuming that reflection in the plunging region is relatively small). This confirms indications for black hole spin based on a Chandra spectrum obtained later in the outburst. The emission line and reflection spectrum also rule out a standard power-law disk emissivity in GX 339-4 a broken power-law form with enhanced emissivity inside ~6rg gives improved fits at more than the 8 sigma level of confidence. The extreme red wing of the line and the steep emissivity require a centrally concentrated source of hard X-rays that can strongly illuminate the inner disk. Hard X-ray emission from the base of a jet-enhanced by gravitational light-bending effects-could create the concentrated hard X-ray emission; this process may be related to magnetic connections between the black hole and the inner disk. We discuss these results within the context of recent results from analyses of XTE J1650-500 and MCG -6-30-15, and of models for the inner accretion flow environment around black holes.

Flux and energy modulation of redshifted iron emission in NGC 3516: implications for the black hole mass

Abstract: We report on the tentative detection of the modulation of a transient, redshifted Fe K emission feature in the X-ray spectrum of the Seyfert galaxy NGC 3516. The detection of the spectral feature at 6.1 keV, in addition to a stable 6.4-keV line, has been reported previously. We find, on reanalysing the XMM—Newton data, that the feature varies systematically in flux at intervals of 25 ks. The peak moves in energy between 5.7 and 6.5 keV. The spectral evolution of the feature agrees with Fe K emission arising from a spot on the accretion disc, illuminated by a corotating flare located at a radius of (7—16) rg, modulated by Doppler and gravitational effects as the flare orbits around the black hole. Combining the orbital time-scale and the location of the orbiting flare, the mass of the black hole is estimated to be (1—5) ◊ 10 7 M , which is in good agreement with values obtained from other techniques. Ke yw ords: line: profiles — relativity — galaxies: active — galaxies: individual: NGC 3516 — X-rays: galaxies.

Flux and energy modulation of redshifted iron emission in NGC3516: implications for the black hole mass

Abstract: We report the tentative detection of the modulation of a transient, redshifted Fe Kalpha emission feature in the X-ray spectrum of the Seyfert galaxy NGC3516. The detection of the spectral feature at 6.1 keV, in addition to a stable 6.4 keV line, has been reported previously. We find on re-analysing the XMM-Newton data that the feature varies systematically in flux at intervals of 25 ks. The peak moves in energy between 5.7 keV and 6.5 keV. The spectral evolution of the feature agrees with Fe K emission arising from a spot on the accretion disc, illuminated by a corotating flare located at a radius of (7-16) rg, modulated by Doppler and gravitational effects as the flare orbits around the black hole. Combining the orbital timescale and the location of the orbiting flare, the mass of the black hole is estimated to be (1-5)e+7 Msun, which is in good agreement with values obtained from other techniques.

Long-term spectral changes in the partial-covering candidate NLS1 1H 0707-495

Abstract: We compare two XMM-Newton observations of the Narrow-Line Seyfert 1 galaxy 1H 0707-495 separated by two years, and discuss the results in terms of the partial-covering phenomenon. The second longer observation once again displays a sharp (< 200 eV) spectral drop above 7 keV; however, in contrast to the first observation, the edge depth and energy have changed significantly. In addition to changes in the edge parameters, the high-energy spectrum appears steeper. The changes in the high-energy continuum can be adequately explained in terms of a partial-covering absorber outflowing from the central region. The low-energy spectrum also shows significant long-term spectral variability, including: a substantial increase in the disc temperature; detection of a ~0.9 keV emission feature; and the presence of warm absorption that was also detected during the ASCA mission, but not seen during the first XMM-Newton observation. The large increase in disc temperature, and more modest rise in luminosity, can be understood if we consider the slim-disc model for 1H 0707-495. In addition, the higher disc luminosity could be the driving force behind the outflow scenario and the re-appearance of a warm medium during the second XMM-Newton observation.

Long-term spectral changes in the partial-covering candidate narrow-line Seyfert 1 galaxy 1H 0707¿495

Abstract: We compare two XMM-Newton observations of the Narrow-Line Seyfert 1 galaxy 1H 0707–495 separated by two years, and discuss the results in terms of the partialcovering phenomenon. The second longer observation once again displays a sharp (< 200 eV) spectral drop above 7 keV; however, in comparison to the first observation, the edge depth and energy have changed significantly. In addition to changes in the edge parameters, the high-energy spectrum appears steeper. The changes in the highenergy continuum can be adequately explained in terms of a partial-covering absorber outflowing from the central region. The low-energy spectrum also shows significant long-term spectral variability, including: a substantial increase in the disc temperature; detection of a � 0.9 keV emission feature; and the presence of warm absorption that was also detected during the ASCA mission, but not seen during the first XMMNewton observation. The large increase in disc temperature, and more modest rise in luminosity, can be understood if we consider the slim-disc model for 1H 0707–495. In addition, the higher disc luminosity could be the driving force behind the outflow scenario and the re-appearance of a warm medium during the second XMM-Newton observation.

INTEGRAL/SPI Limits on Electron-Positron Annihilation Radiation from the Galactic Plane

Abstract: The center of our Galaxy is a known strong source of electron-positron 511-keV annihilation radiation. Thus far, however, there have been no reliable detections of annihilation radiation outside of the central radian of our Galaxy. One of the primary objectives of the INTEGRAL (INTErnational Gamma-RAy Astrophysics Laboratory) mission, launched in Oct. 2002, is the detailed study of this radiation. The Spectrometer on INTEGRAL (SPI) is a high resolution coded-aperture gamma-ray telescope with an unprecedented combination of sensitivity, angular resolution and energy resolution. We report results from the first 10 months of observation. During this period a significant fraction of the observing time was spent in or near the Galactic Plane. No positive annihilation flux was detected outside of the central region (|l| > 40 deg) of our Galaxy. In this paper we describe the observations and data analysis methods and give limits on the 511-keV flux.

Powerful, obscured active galactic nuclei among X-ray hard, optically dim serendipitous Chandra sources

Abstract: We present a small sample of Chandra X-ray sources selected from the fields of ACIS observations which probe fluxes around the break in the hard band source counts. The targets of these fields include nine nearby galaxy clusters, one distant cluster and two powerful radio galaxy fields. The follow-up of this serendipitous sample was biased towards X-ray hard and optically dim sources mostly not seen on the Digitized Sky Survey; for these, we present X-ray fluxes, optical and near-infrared photometry leading to 51 photometric redshifts in all and 18 independently measured spectroscopic redshifts. Few sources are associated with the target fields themselves. 56 of 58 sources imaged in the K-band are detected at K 20 with K median = 18, and of these, 38 have hard X-ray count ratios and 24 of these are significantly hard with most of the counts emerging about 2 keV. We find that almost all are active galactic nuclei (AGN) hosted in massive early-type host galaxies with a photometric redshift distribution peaking at z ∼ 1. Two type 2 quasars with intrinsic X-ray luminosity L 10 45 erg s −1 ,F e Kα emission lines and absorbing column density N H > 10 23 cm −2 ‐ and N H 10 24 cm −2 in one case ‐ are discussed in detail; the sample contains at least 12 potential type 2 quasars in all. We discuss various detection strategies for type 2 quasars and calculate their inferred space density. This combines and extends a number of results from subsamples already published by us.

Unveiling the nature of the highly obscured active galactic nucleus in NGC 5643 with XMM–Newton

Abstract: We present results from an XMM-Newton observation of the nearby Seyfert 2 galaxy NGC 5643. The nucleus exhibits a very flat X-ray continuum above 2 keV, together with a prominent Kα fluorescent iron line. This indicates heavy obscuration. We measure an absorbing column density N H in the range 6-10 x 10 23 cm -2 , either directly covering the nuclear emission, or covering its Compton reflection. In the latter case, we might be observing a rather unusual geometry for the absorber, whereby reflection from the inner far side of a torus is in turn obscured by its near side outer atmosphere. The nuclear emission might be then either covered by a Compton-thick absorber, or undergoing a transient state of low activity. A second source (christened 'X-1' in this paper) at the outskirts of the NGC 5643 optical surface outshines the nucleus in X-rays. If belonging to NGC 5643, it is the third brightest (L X ∼ 4 x 10 40 erg s -1 ) known ultraluminous X-ray source. Comparison with past large aperture spectra of NGC 5643 unveils dramatic X-ray spectral changes above 1 keV. We interpret these as due to variability of the active nucleus and of source X-1 intrinsic X-ray powers by factors of ≥ 10 and 5, respectively.

High-redshift quasars and the supermassive black hole mass budget: constraints on quasar formation models

Abstract: We investigate the constraints on models of supermassive black hole (SMBH) and quasar formation obtainable from two recent observational developments: the discovery of luminous quasars at z ∼ 6, and estimates of the local mass density of SMBHs. If ∼90 per cent of this mass was accreted at redshifts z ≤ 3, as suggested by the observed quasar luminosity functions, these joint constraints pose a challenge for models, which must account for the observed luminous quasar population at z ∼ 6 within a very limited 'mass budget'. We investigate a class of models based within the hierarchical structure formation scenario, in which major mergers lead to black hole formation and fuelling, and the resulting quasars shine at their Eddington-limited rate until their fuel is exhausted. We show that the simplest such model, in which a constant fraction of the gas within the halo is accreted in each major merger, cannot satisfy both constraints simultaneously. When this model is normalized to reproduce the number density of luminous quasars at z ∼ 6, the mass budget is grossly exceeded owing to an overabundance of lower-mass SMBHs. We explore a range of modifications to the simple model designed to overcome this problem. We show that both constraints can be satisfied if the gas accretion fraction scales as a function of the halo virial velocity. Similar scalings have been proposed in order to reproduce the local M○-σ relation. Successful models can also be constructed by restricting the formation of seed black holes to redshifts above z crit ∼ 11.5 or to haloes above a velocity threshold υ crit ∼ 55 km s -1 , or assuming that only a fraction of major mergers result in formation of a seed SMBH. We also briefly discuss the issue of trying to assume a 'universal M○-σ relation' within the framework of simple Press-Schechter models, and further show that a fixed universal relation between SMBH mass and host halo mass is unlikely.

The X-ray variability of the narrow-line type 1 Seyfert galaxy IRAS 13224-3809 from an XMM-Newton observation

Abstract: We report on the XMM‐Newton timing properties of the most X-ray variable, radio-quiet, narrow-line type 1 Seyfert galaxy IRAS 13224‐3809. IRAS 13224‐3809 continues to display the extremely variable behaviour that was previously observed with ROSAT and ASCA ;h owever, no giant, rapid flaring events are observed. We detect variations by a factor as high as ∼8 during the 64-ks observation and the variability is persistent throughout the light curve. Dividing the light curve into 9-min segments, we found almost all of the segments to be variable at >3σ . When the time-averaged cross-correlation function is calculated for the 0.3‐0.8 keV band with the 3‐10 keV band, the cross-correlation profile is skewed, indicating a possible smearing of the signal to longer times (the soft band leading the hard). A correlation between count rate and hardness ratio is detected in four energy bands. In three cases, the correlation is consistent with spectral hardening at lower count rates, which can be explained in terms of a partial-covering model. The other band displays the reverse effect, showing spectral hardening at higher count rates. We can explain this trend as a more variable power-law component compared with the soft component. We also detect a delay between the 0.3‐1.5 keV count rate and the 0.8‐1.5 to 0.3‐0.8 keV hardness ratio, implying flux-induced spectral variability. Such delays and asymmetries in the cross-correlation functions could suggest reprocessing of soft and hard photons. In general, much of the timing behaviour can be attributed to erratic eclipsing behaviour associated with the partial covering phenomenon, in addition to intrinsic variability in the source. The variability behaviour of IRAS 13224‐3809 suggests a complicated combination of effects which we have started to disentangle with this present analysis.

Unveiling the nature of the highly obscured AGN in NGC5643 with XMM-Newton

Abstract: We present results from an XMM-Newton observation of the nearby Seyfert 2 galaxy NGC5643. The nucleus exhibits a very flat X-ray continuum above 2 keV, together with a prominent K-alpha fluorescent iron line. This indicates heavy obscuration. We measure an absorbing column density N_H in the range 6-10 x 10^{23} atoms/cm/cm, either directly covering the nuclear emission, or covering its Compton-reflection. In the latter case, we might be observing a rather unusual geometry for the absorber, whereby reflection from the inner far side of a torus is in turn obscured by its near side outer atmosphere. The nuclear emission might be then either covered by a Compton-thick absorber, or undergoing a transient state of low activity. A second source (christened "X-1" in this paper) at the outskirts of NGC5643 optical surface outshines the nucleus in X-rays. If belonging to NGC5643, it is the third brightest (L_X ~ 4 x 10^{40} erg/s) known Ultra Luminous X-ray source. Comparison with past large aperture spectra of NGC 5643 unveils dramatic X-ray spectral changes above 1 keV. We interpret them as due to variability of the active nucleus and of source X-1 intrinsic X-ray powers by a factor >10 and 5, respectively.

An improved approach to measuring H0 using X-ray and SZ observations of galaxy clusters

Abstract: We present an improved method for predicting the Sunyaev–Zeldovich (SZ) effect in galaxy clusters from spatially resolved, spectroscopic X-ray data. Using the deprojected electron density and temperature profiles measured within a fraction of the virial radius, and assuming a Navarro–Frenk–White mass model, we show how the pressure profile of the X-ray gas can be extrapolated to large radii, allowing the Comptonization parameter profile for the cluster to be predicted precisely. We apply our method to Chandra observations of three X-ray-luminous, dynamically relaxed clusters with published SZ data: RX J1347.5–1145, Abell 1835 and Abell 478. Combining the predicted and observed SZ signals, we determine improved estimates for the Hubble constant from each cluster and obtain a weighted mean of H0= 69 ± 8 km s−1 Mpc−1 for a cosmology with Ωm= 0.3 and ΩΛ= 0.7. This result is in good agreement with independent findings from the Hubble Key Project and the combination of cosmic microwave background and galaxy cluster data.

Abundance constraints and direct redshift measurement of the diffuse x-ray emission from a distant cluster of galaxies

Abstract: We report on the XMM-Newton (XMM) observation of RX J1053.7+5735, one of the most distant X-ray selected clusters of galaxies, which also shows an unusual double-lobed X-ray morphology, indicative of a possible equal-mass cluster merger. The cluster was discovered during the ROSAT deep pointings in the direction of the Lockman Hole. All XMM Lockman Hole observations (PV, AO-1 & AO-2 phases) with the European Photon Imaging Camera (EPIC) were combined for the analysis, totaling exposure times ∼648 ks, 738 ks, and 758 ks for pn, MOS1, and MOS2, respectively. With this "deep" dataset, we could detect the Fe K line and obtain a strong constraint on cluster metallicity, which is difficult to achieve for clusters at z > 1. The best-fit abundance is 0.46 +0.11 −0.07 times the solar value. The Fe line emission also allows us to directly estimate the redshift of diff use gas, with av aluez = 1.14 +0.01 −0.01 . This is one of the first clusters whose X-ray redshift is directly measured prior to the secure knowledge of cluster redshift by optical/NIR spectroscopy. We could also estimate the X-ray redshift separately for each of the two lobes in the double-lobed structure, and the result is consistent with the two lobes being part of one cluster system at the same redshift. Comparison with other metallicity measurements of nearby and distant clusters shows that there is little evolution in the ICM metallicity from z ∼ 1t o the present.

An improved approach to measuring H_0 using X-ray and SZ observations of galaxy clusters

Abstract: We present an improved method for predicting the Sunyaev-Zeldovich (SZ) effect in galaxy clusters from spatially-resolved, spectroscopic X-ray data. Using the deprojected electron density and temperature profiles measured within a fraction of the virial radius, and assuming a Navarro, Frenk & White (1995) mass model, we show how the pressure profile of the X-ray gas can be extrapolated to large radii, allowing the Comptonization parameter profile for the cluster to be predicted precisely. We apply our method to Chandra observations of three X-ray luminous, dynamically relaxed clusters with published SZ data: RX J1347.5-1145, Abell 1835 and Abell 478. Combining the predicted and observed SZ signals, we determine improved estimates for the Hubble constant from each cluster and obtain a weighted mean of H_0=69\pm8km/s/Mpc for a cosmology with Omega_m=0.3 and Omega_Lambda=0.7. This result is in good agreement with independent findings from the Hubble Key Project and the combination of cosmic microwave background and galaxy cluster data.

I Zw 1 observed with XMM-Newton. Low-energy spectral complexity, iron lines, and hard X-ray flares

Abstract: We present a 20 ks XMM-Newton observation of the prototypical Narrow-Line Seyfert 1 galaxy I Zw 1. 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 Kα 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 Kα line in addition to a blend of He- and H-like Fe Kα 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.

The hard X-ray spectrum of the Seyfert galaxy IRAS 18325–5926: reflection from an ionized disc and variable iron K emission

Abstract: We report our analysis of the X-ray spectra of the Seyfert galaxy IRAS 18325-5926 (= Fairall 49) obtained from various X-ray observatories prior to XMM-Newton, including new results from two RXTE and one BeppoSAX observations. A relatively steep continuum slope (Γ ≃ 2.2) in the 2-15 keV band is confirmed. The continuum spectrum observed with the BeppoSAX PDS shows a possible roll-over at energies above 30 keV, indicating a Comptonizing corona cooler than in other Seyfert nuclei. The X-ray spectrum above 2 keV is best explained with a model including reflection from a highly ionized disc with significant relativistic blurring. The iron Ka emission feature is then mainly due to Fe XXV. The seven recent observations show that the iron K emission flux appears to follow the continuum between the observations separated by a few months to years, although some exceptions suggest that the linestrength may be determined in a more complex way.

On the transmission- to reprocessing-dominated spectral state transitions in Seyfert 2 galaxies

Abstract: We present Chandra and XMM-Newton observations of a small sample (11 objects) of optically-selected Seyfert~2 galaxies, for which ASCA and BeppoSAX had suggested Compton-thick obscuration of the Active Nucleus (AGN). The main goal of this study is to estimate the rate of transitions between "transmission-" and "reprocessing-dominated" states. We discover one new transition in NGC4939, with a possible additional candidate in NGC5643. This indicates a typical occurrence rate of at least ~0.02/year. These transitions could be due to large changes of the obscuring gas column density, or to a transient dimming of the AGN activity, the latter scenario being supported by detailed analysis of the best studied events. Independently of the ultimate mechanism, comparison of the observed spectral dynamics with Monte-Carlo simulations demonstrates that the obscuring gas is largely inhomogeneous, with multiple absorbing components possibly spread through the whole range of distances from the nucleus between a fraction of parsecs up to several hundreds parsecs. As a by-product of this study, we report the first measurement ever of the column density covering the AGN in NGC3393 (N_H ~ 4.4x10^{24} atoms/cm/cm), and the discovery of soft X-ray extended emission, apparently aligned along the host galaxy main axis in NGC5005. The latter object hosts most likely an historically misclassified low-luminosity Compton-thin AGN.