Showing papers by "A. C. Fabian published in 2003"

X-Ray Spectroscopic Evidence for Intermediate-Mass Black Holes: Cool Accretion Disks in Two Ultraluminous X-Ray Sources

Abstract: We have analyzed an XMM-Newton observation of the nearby spiral galaxy NGC 1313, which contains two ultraluminous X-ray (ULX) sources. We measure isotropic luminosities of LX = 2.0 × 1040 ergs s-1 and LX = 6.6 × 1039 ergs s-1 for NGC 1313 X-1 and X-2 (0.2-10.0 keV, assuming a distance of 3.7 Mpc). The spectra statistically require soft and hard spectral components to describe the continuum emission; some prior studies of ULX sources have claimed cool soft components with lower statistics. The improvement over several single-component models exceeds the 8 σ level of confidence for X-1; the improvement for X-2 is significant at the 3 σ level. The soft components in these ULX spectra are well fitted by multicolor disk blackbody models with color temperatures of kT 150 eV. This temperature differs markedly from those commonly measured in the spectra of stellar mass (10 M☉) black holes in their brightest states (kT 1 keV). It is expected that the temperature of an accretion disk orbiting a black hole should decrease with increasing black hole mass. If the soft components we measure are due to emission from the inner region of an accretion disk, and disks extend close to the innermost stable circular orbit at the accretion rates being probed, the low color temperatures may be interpreted as spectroscopic evidence of black holes with intermediate masses: MBH 103 M☉. Simple Eddington scaling arguments suggest a minimum mass of MBH ~ 102 M☉. NGC 1313 X-1 and X-2 are found in optical nebulae, which may indicate that anisotropic emission geometries are unlikely to account for the fluxes observed.

The relationship between the optical Hα filaments and the X‐ray emission in the core of the Perseus cluster

Abstract: NGC 1275 in the centre of the Perseus cluster of galaxies, Abell 426, is surrounded by a spectacular filamentary Hα nebula. Deep Chandra X-ray imaging has revealed that the brighter outer filaments are also detected in soft X-rays. This can be due to conduction and mixing of the cold gas in the filaments with the hot, dense intracluster medium. We show the correspondence of the filaments in both wavebands and draw attention to the relationship of two prominent curved north-west filaments to an outer, buoyant radio bubble seen as a hole in the X-ray image. There is a strong resemblance in the shape of the hole and the disposition of the filaments to the behaviour of a large air bubble rising in water. If this is a correct analogy, then the flow is laminar and the intracluster gas around this radio source is not turbulent. We obtain a limit on the viscosity of this gas.

The relationship between the optical Halpha filaments and the X-ray emission in the core of the Perseus cluster

Abstract: NGC 1275 in the centre of the Perseus cluster of galaxies, Abell 426, is surrounded by a spectacular filamentary Halpha nebula. Deep Chandra X-ray imaging has revealed that the brighter outer filaments are also detected in soft X-rays. This can be due to conduction and mixing of the cold gas in the filaments with the hot, dense intracluster medium. We show the correspondence of the filaments in both wavebands and draw attention to the relationship of two prominent curved NW filaments to an outer, buoyant radio bubble seen as a hole in the X-ray image. There is a strong resemblance in the shape of the hole and the disposition of the filaments to the behaviour of a large air bubble rising in water. If this is a correct analogy, then the flow is laminar and the intracluster gas around this radio source is not turbulent. We obtain a limit on the viscosity of this gas.

Chandra/HETGS Spectroscopy of the Galactic Black Hole GX 339-4: A Relativistic Iron 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 ksec 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 K-alpha 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-like and He-like O, and He-like Ne and Mg are detected, as well as lines which 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.

Evidence for 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 ksec XMM-Newton/EPIC-pn and 10 ksec RXTE observations during a bright phase of its 2002-2003 outburst. An extremely skewed, relativistic Fe K-alpha emission line and ionized disk reflection spectrum are revealed in these spectra. Self-consistent models for the Fe K-alpha 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 r_g suggests that GX 339-4 harbors a black hole with a ~ 0.8-0.9 (where r_g = GM/c^2 and a=cJ/GM^2, 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 ~6 r_{g} gives improved fits at more than the 8 sigma level of confidence. The extreme red wing of the line and steep emissivity require a centrally--concentrated source of hard X-rays which 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 models for the inner accretion flow environment around black holes.

The X-ray nebula around the Seyfert 2 galaxy NGC4388

Abstract: We report on X-ray emission from the Seyfert 2 galaxy NGC4388 observed with the Chandra X-ray Observatory. A hard X-ray peak is found at the position of the active nucleus suggested by optical and radio observations. Extended soft X-ray emission correlates well with the ionization cone found in optical line emission. A large soft X-ray extension is found up to 16 kpc to the north of the galaxy. Photoionized gas with low ionization parameters (xi<3) appears to be the likely explanation of this emission. The same ionized gas clouds could be responsible for the optical [OIII] emission. Fe K line emission from cold material is found to be extended by a few kpc.

A deep Chandra observation of the cluster environment of the z= 1.786 radio galaxy 3C 294

Abstract: We report the results from a 200-ks Chandra observation of the z = 1.786 radio galaxy 3C 294 and its cluster environment, increasing by ten-fold our earlier observation. The diffuse emission, extending about 100 kpc around the nucleus, has a roughly hourglass shape in the north-south (N-S) direction with surprisingly sharp edges to the N and S. The spectrum of the diffuse emission is well fitted by either a thermal model of temperature 3.5 keV and abundance <0.9 Z O . (2a), or a power law with photon index 2.3. If the emission is due to hot gas then the sharp edges mean that it is probably not in hydrostatic equilibrium. Much of the emission is plausibly due to inverse Compton scattering of the cosmic microwave background (CMB) by non-thermal electrons produced earlier by the radio source. The required relativistic electrons would be of much lower energy and older than those responsible for the present radio lobes. This could account for the lack of detailed spatial correspondence between the X-rays and the radio emission, the axis of which is at a position angle of about 45°. Hot gas would still be required to confine the relativistic plasma; the situation could parallel that of the radio bubbles seen as holes in nearby clusters, except that in 3C 294 the bubbles are bright in X-rays owing to the extreme power in the source and the sixty-fold increase in the energy density of the CMB. The X-ray spectrum of the radio nucleus is hard, showing a reflection spectrum and iron line. The source is therefore an obscured radio-loud quasar.

The X‐ray nebula around the type 2 Seyfert galaxy NGC 4388

Abstract: We report on X-ray emission from the type 2 Seyfert galaxy NGC 4388 observed with the Chandra X-ray Observatory. A hard X-ray peak is found at the position of the active nucleus suggested by optical and radio observations. Extended soft X-ray emission correlates well with the ionization cone found in optical-line emission. A large soft X-ray extension is found up to 16 kpc (and possibly 30 kpc) to the north of the galaxy. Photoionized gas with low ionization parameters (log ξ ≤ 0.4) appears to be the likely explanation for this emission. The same ionized gas clouds could be responsible for the optical [O III] emission. Fe Ka line emission from cold material is found to be extended by a few kpc.

A deep Chandra observation of the cluster environment of the z=1.786 radio galaxy 3C294

Abstract: We report the results from a 200 ks Chandra observation of the z=1.786 radio galaxy 3C294 and its cluster environment, increasing by tenfold our earlier observation. The diffuse emission, extending about 100 kpc around the nucleus, has a roughly hourglass shape in the N-S direction with surprisingly sharp edges to the N and S. The spectrum of the diffuse emission is well fitted by either a thermal model of temperature 3.5 keV and abundance <0.9 solar (2-sigma), or a power-law with photon index 2.3. If the emission is due to hot gas then the sharp edges mean that it is probably not in hydrostatic equilibrium. Much of the emission is plausibly due to inverse Compton scattering of the Cosmic Microwave Background (CMB) by nonthermal electrons produced earlier by the radio source. The required relativistic electrons would be of much lower energy and older than those responsible for the present radio lobes. This could account for the lack of detailed spatial correspondence between the X-rays and the radio emission, the axis of which is at a position angle of about 45 deg. Hot gas would still be required to confine the relativistic plasma; the situation could parallel that of the radio bubbles seen as holes in nearby clusters, except that in 3C294 the bubbles are bright in X-rays owing to the extreme power in the source and the sixty fold increase in the energy density of the CMB. The X-ray spectrum of the radio nucleus is hard, showing a reflection spectrum and iron line. The source is therefore an obscured radio-loud quasar.

Evidence for an intermediate-mass black hole and a multi-zone warm absorber in NGC 4395

Abstract: We report on the results of an analysis in the X-ray band of a recent long ASCA observation of NGC 4395, the most variable low-luminosity active galactic nucleus known. A relativistically broadened iron line at ∼6.4 keV is clearly resolved in the time-averaged spectrum, with an equivalent width of 310 + 7 0 - 9 0 eV. Time-resolved spectral analysis of the heavily absorbed soft X-ray band confirms the existence of a variable, multi-zone warm absorber in this source, as proposed in a previous analysis of a shorter ASCA observation. The light curve of the source is wildly variable on time-scales of hours or less, and a factor of nearly 10 change in count rate was recorded in a period of less than 2000 s. The long observation and variability of the source allowed the power density spectrum (PDS) to be constructed to an unprecedented level of detail. There is evidence for a break in the PDS from a slope of a ∼ 1 to a ∼ 1.8 at a frequency of around 3 x 10 - 4 Hz. The central black hole mass of NGC 4395 is estimated to be approximately 10 4 -10 5 M O . using the break in the PDS, a result consistent with previous analyses using optical and kinematical techniques.

Powerful obscured AGN among X-ray hard, optically-dim serendipitous Chandra sources

Abstract: We present a small sample of Chandra X-ray sources selected from the flelds of ACIS observations which probe ∞uxes around the break in the hard band source counts. The targets of these flelds include 9 nearby galaxy clusters, 1 distant cluster and 2 powerful radio galaxy flelds. The follow-up of this serendipitous sample was biased towards Xray hard and optically-dim sources mostly not seen on the Digitized Sky Survey; for these, we present X-ray ∞uxes, 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 flelds themselves. Fifty-six of 58 sources imaged in the K-band are detected at K »20 with Kmedian=18, and of these, 38 have hard X-ray count ratios and 24 of these are signiflcantly hard with most of the counts emerging about 2 keV. We flnd that almost all are 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 i1 , Fe Kfi emission lines and absorbing column density NH>10 23 cm i2 { and NH »10 24 cm i2 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.

The X-ray variability of the Narrow-Line Seyfert 1 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, NLS1 IRAS 13224-3809. IRAS 13224-3809 continues to display the extremely variable behavior that was previously observed with ROSAT and ASCA; however, 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 minute segments we found almost all of the segments to be variable at > 3 sigma. 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 (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 to the soft component. We also detect a delay between the 0.3-1.5 keV count rate and the 0.8-1.5 keV to 0.3-0.8 keV hardness ratio, implying flux induced spectral variability. Such delays and asymmetries in the cross correlation functions could be suggesting reprocessing of soft and hard photons. In general, much of the timing behavior can be attributed to erratic eclipsing behavior associated with the partial covering phenomenon, in addition to intrinsic variability in the source. The variability behavior of IRAS 13224-3809 suggests a complicated combination of effects which we have started to disentangle with this present analysis.

IZw1 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 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.

Chandra reveals X-rays along the radio axis in the quasar 3C 9 at z = 2.012

Abstract: A Chandra observation of the radio-loud quasar 3C 9 at redshift z = 2.012 has revealed extended X-ray emission coincident with the radio structure. Of particular interest is the appearance of Xray emission on both sides of the nucleus, which argues against the X-ray emission being highly beamed. We present the properties and discuss possible scenarios for the X-ray emission, which is likely to be due to either non-thermal inverse Compton emission from relativistic electrons with small bulk Lorentz factor acting most probably on the cosmic microwave background but could involve infrared synchrotron photons or thermal emission from shocked cold gas surrounding the quasar. The thermal possibility implies a high mass for the cold gas unless it is highly clumped.

Iron Kα line profiles and the inner boundary condition of accretion flows

Abstract: Recent X-ray observations have shown evidence for exceptionally broad and skewed iron Ka emission lines from several accreting black hole systems. The lines are assumed to be due to fluorescence of the accretion disc illuminated by a surrounding corona and require a steep emissivity profile increasing into the innermost radius. This appears to question both standard accretion disc theory and the zero-torque assumption for the inner boundary condition, both of which predict a much less extreme profile. Instead it argues that a torque may be present due to magnetic coupling with matter in the plunging region or even to the spinning black hole itself. Discussion so far has centred on the torque acting on the disc. However, the crucial determinant of the iron line profile is the radial variation of the power radiated in the corona. Here we study the effects of different inner boundary conditions on the coronal emissivity and on the profiles of the observable Fe Ka lines. We argue that in the extreme case where a prominent highly redshifted component of the iron line is detected, requiring a steep emissivity profile in the innermost part and a flatter one outside, energy from the gas plunging into the black hole is being fed directly to the corona.

The hard X-ray spectrum of the Seyfert galaxy IRAS 18325-5926: cool corona, reflection from an ionized disk and variable iron K emission

Abstract: We report our analysis of 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 (photon-index of ~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 disk with significant relativistic blurring. The iron K-alpha emission feature is then mainly due to FeXXV. The recent seven observations shows 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 line strength may be determined in a more complex way.

Deep ISOPHOT far-infrared imaging of M 86

Abstract: Deep far-infrared (FIR) imaging data obtained with ISOPHOT at 60 µm, 90 µm, 150 µm, and 180 µm reveal a complex FIR morphology of the Virgo cluster elliptical M 86 (NGC4406). A close pair of sources is associated with the centre of the M 86 galaxy and optically discovered dust streamers. Both components have a cold FIR spectrum from the thermal emission of dust with a temperature of ≈18K. A compact off-center source lying ≈3.5 � north-west is likely an unrelated background source. The brightest FIR source long-wards of 90 µm consists of a compact central core surrounded by an extended halo. It lies close to the nearby spiral galaxy NGC4402 at the position of the strong northern depression in X-ray brightness. It represents the first detection of an intergalactic dust cloud in the intergalactic region between galaxies. Additionally, three much fainter FIR sources are detected in the field. The ISOPHOT data do not support the earlier suggestion based on IRAS and X-ray data that the unresolved north-western component is cold dust removed from M 86 by ram pressure stripping. The observational evidence suggests that at least for the overall FIR morphology, gravitational interactions between M 86 and neighboring galaxies are much more important than ram stripping of dust. Moreover, the X-ray morphology appears to be partially affected by dusty foreground absorbers. Since M 86 is not showing signs of ongoing stripping of interstellar dust by the intra-cluster medium, a clear case for this dynamical interaction has yet to be found.

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

Abstract: The iron Kα 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 Kα line. This paper compares predictions of iron Kα flux generated by impacting electrons and protons to that from photoionization. Nonthermal 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 Kα flux from particle impact is found to be less than 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 102-104 times more energy flux than radiation for particle impact to be a significant producer of Fe Kα 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 Kα 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.

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

Abstract: 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.

X-ray-extreme-ultraviolet simultaneous observations of ngc 5548: fast extreme-ultraviolet dip without x-ray variation

Abstract: The bright Seyfert 1 galaxy NGC 5548 was monitored in X-rays by ASCA for 9 days and in EUV by EUVE for 12 days continuously in 1996. The periods of X-ray observations were really simultaneously covered by EUV observations. The 0.1-10 keV combined continuum of X-ray and EUV is well presented by a simple power law with a photon index Γ = 1.78 plus the Wien part of a disk blackbody with kT = 17 eV. On long timescales of days, the variability of the EUV flux is in good accordance with that of the X-rays. Furthermore, there is a clear positive correlation between Γ and the 2-10 keV flux. These results are consistent with the scenario that the X-rays are produced by inverse Compton scattering of soft photons in hot electron clouds. On the other hand, focusing on the short-term variability, we found that the EUV flux shows 20,000 s variability with 40% amplitude with neither X-ray flux nor spectral changes when the source is at its maximum flux state. This seems to conflict with the ordinary framework of the inverse Compton model. To explain such fast variability of the EUV flux accompanied by no X-ray change, we suggest that the EUV emission consists of two components; relatively stable disk emission and a variable component that is dominant at EUV flares.

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

Abstract: We summarise the primary results from a 320 ks observation of the bright Seyfert 1 galaxy MCG-6-30-15 with XMM-Newton and Beppo-SAX.

What can be learnt from extragalactic X‐ray surveys?

Abstract: Extragalactic X-ray surveys reveal many Active Galactic Nuclei (AGN) as well as clusters and groups of galaxies. Many of the AGN are obscured, leading to the hard spectrum of the X-ray Background. Constraints from the absorptioncorrected X-ray Background spectrum and the local mass density of black holes on the radiative efficiency and growth of accreting black holes are discussed. The use of clusters for the determination of cosmological parameters, and the energy and metallicity loss by galaxies and AGN, is noted.