ASCA Observations Research Articles

「あすか」の見た活動的銀河核と宇宙X線背景放射( :X線天文衛星「あすか」)

「あすか」の見た活動的銀河核と宇宙X線背景放射( :X線天文衛星「あすか」)

VARIABLE X-RAY ABSORPTION TOWARD THE GRAVITATIONALLY-LENSED BLAZAR PKS 1830-211

We present X-ray spectral analysis of five Chandra and XMM-Newton observations of the gravitationally-lensed blazar PKS 1830-211 from 2000 to 2004. We show that the X-ray absorption toward PKS 1830-211 is a variable, and the variable absorption is most likely to be intrinsic with amplitudes of ~2 × 1022–30 × 1022 cm−2 depending on whether or not the absorber is partially covering the X-ray source. Our results confirm the variable absorption observed previously, although interpreted differently, in a sequence of ASCA observations. This large variation in the absorption column density can be interpreted as outflows from the central engine in the polar direction, consistent with recent numerical models of inflow/outflows in active galaxy nuclei. In addition, it could possibly be caused by the interaction between the blazar jet and its environment, or the variation from the geometric configuration of the jet. While the spectra can also be fitted with a variable absorption at the lens redshift, we show that this model is unlikely. We also rule out the simple microlensing interpretation of variability which was previously suggested.

Discovery of multiple Lorentzian components in the X-ray timing properties of the Narrow Line Seyfert 1 Ark 564

We present a power spectral analysis of a 100-ks XMM-Newton observation of the Narrow Line Seyfert 1 galaxy Ark 564. When combined with earlier RXTE and ASCA observations, these data produce a power spectrum covering seven decades of frequency which is well described by a power law with two very clear breaks. This shape is unlike the power spectra of almost all other active galactic nuclei (AGN) observed so far, which have only one detected break, and resemble Galactic binary systems in the soft state. The power spectrum can also be well described by the sum of two Lorentzian-shaped components, the one at higher frequencies having a hard spectrum, similar to those seen in Galactic binary systems. Previously we have demonstrated that the lag of the hard-band variations relative to the soft band in Ark 564 is dependent on variability time-scale, as seen in Galactic binary sources. Here we show that the time-scale dependence of the lags can be described well using the same two-Lorentzian model which describes the power spectrum, assuming that each Lorentzian component has a distinct time lag. Thus all X-ray timing evidence points strongly to two discrete, localized, regions as the origin of most of the variability. Similar behaviour is seen in Galactic X-ray binary systems in most states other than the soft state, i.e. in the low-hard and intermediate/very high states. Given the very high accretion rate of Ark 564 the closest analogy is with the very high (intermediate) state rather than the low-hard state. We therefore strengthen the comparison between AGN and Galactic binary sources beyond previous studies by extending it to the previously poorly studied very high accretion rate regime.

Statistics of X-ray observables for the cooling-core and non-cooling core galaxy clusters

We present a statistical study of the occurrence and effects of the cooling cores in the clusters of galaxies in a flux-limited sample, HIFLUGCS, based on ROSAT and ASCA observations. About 49% of the clusters in this sample have a significant, classically-calculated cooling-flow, mass-deposition rate. The upper envelope of the derived mass-deposition rate is roughly proportional to the cluster mass, and the fraction of cooling core clusters is found to decrease with it. The cooling core clusters are found to have smaller core radii than non-cooling core clusters, while some non-cooling core clusters have high beta values (> 0.8). In the relation of the X-ray luminosity vs. the temperature and the mass, the cooling core clusters show a significantly higher normalization. A systematic correlation analysis, also involving relations of the gas mass and the total infrared luminosity, indicates that this bias is shown to be mostly due to an enhanced X-ray luminosity for cooling core clusters, while the other parameters, like temperature, mass, and gas mass may be less affected by the occurrence of a cooling core. These results may be explained by at least some of the non-cooling core clusters being in dynamically young states compared with cooling core clusters, and they may turn into cooling core clusters in a later evolutionary stage.

Effects of Downscattering on the Continuum and Line Spectra in a Powerful Wind Environment: Monte Carlo Simulations, Analytical Results, and Data Analysis

In an earlier paper, the general formulation and results for photon reprocessing (downscattering) that included recoil and Comptonization effects due to divergence of the flow were presented. In a second paper we showed the Monte Carlo (MC) simulated continuum and line spectra. We also provided an analytical description of the simulated continuum spectra using the diffusion approximation. We have simulated the propagation of monochromatic and continuum photons in a bulk outflow from a compact object. Electron scattering of the photons within the expanding flow leads to a decrease of their energy, which is of first order in V/c (where V is the outflow velocity). The downscattering effect of first order in V/c in the diverging flow is explained by semianalytical calculations and confirmed by MC simulations. We conclude that redshifted lines and downscattering bumps are intrinsic properties of the powerful outflows for which the Thomson optical depth is greater than 1. We fitted our model line profiles to the observations using four free parameters, β = V/c, the optical depth of the wind τ, the wind temperature kTe, and the original line photon energy E0. We show how the primary spectrum emitted close to the black hole is modified by reprocessing in the warm wind. In the framework of our wind model, the fluorescent iron line Kα is formed in the partly ionized wind as a result of illumination by central source continuum photons. The demonstrated application of our outflow model to the XMM-Newton observations of MCG -6-30-15 and to the ASCA observations of GRO J1655-40 points out a potential powerful spectral diagnostic for probes of the outflow-central object connection in Galactic and extragalactic black hole sources.

On the X-ray, optical emission line and black hole mass properties of local Seyfert galaxies

We investigate the relation between X-ray nuclear emission, optical emission line luminosities and black hole masses for a sample of 47 Seyfert galaxies. The sample, which has been selected from the Palomar optical spectroscopic survey of nearby galaxies (Ho et al. 1997a, ApJS, 112, 315), covers a wide range of nuclear powers, from L2 10keV # 10 43 erg/s down to very low luminosities (L2 10keV # 10 38 erg/s). Best available data from Chandra, XMM-Newton and, in a few cases, ASCA observations have been considered. Thanks to the good spatial resolution available from these observations and a proper modeling of the various spectral components, it has been possible to obtain accurate nuclear X-ray luminosities not contaminated by o! -nuclear sources and/or di! use emission. X-ray luminosities have then been corrected taking into account the likely candidate Compton thick sources, which are a high fraction (>30%) among type 2 Seyferts in our sample. The main result of this study is that we confirm strong linear correlations between 2-10 keV, (OIII)! 5007, H luminosities which show the same slope as quasars and luminous Seyfert galaxies, independent of the level of nuclear activity displayed. Moreover, despite the wide range of Eddington ratios (L/LEdd) tested here (six orders of magnitude, from 0.1 down to # 10 7 ), no correlation is found between the X-ray or optical emission line luminosities and the black hole mass. Our results suggest that Seyfert nuclei in our sample are consistent with being a scaled-down version of more luminous AGN.

The white dwarf in AE Aqr brakes harder

Taking advantage of the very precise de Jager et al. optical white dwarf orbit and spin ephemerides; ASCA, XMM-Newton and Chandra X-ray observations spread over 10yr; and a cumulative 27-yr baseline, we have found that in recent years the white dwarf in AE Aqr is spinning down at a rate that is slightly faster than predicted by the de Jager et al. spin ephemeris. At the present time, the observed period evolution is consistent with either a cubic term in the spin ephemeris with P = 3.46(56) x 10 -19 d -1 , which is inconsistent in sign and magnitude with magnetic dipole radiation losses, or an additional quadratic term with P = 2.0(1.0) x 10 -15 d d -1 , which is consistent with a modest increase in the accretion torques spinning down the white dwarf. Regular monitoring, in the optical, ultraviolet and/or X-rays, is required to track the evolution of the spin period of the white dwarf in AE Aqr.

XMM-Newton discovery of the X-ray transient XMMU J181227.8−181234 in the Galactic plane

We report the discovery of an X-ray transient, observed in outburst with XMM-Newton on March 20, 2003, and with position 18h12m27.8s, -18 12 34 (J2000, approximate positional error 2). No known source is present at this position and the source was not detected during published ROSAT or ASCA observations of that region. However, the source may be associated with 1H1812-182 detected by HEAO 1, although the error bars on the HEAO 1 position are very large and the two sources could also be unrelated. Therefore, we name the source XMMU J181227.8-181234. Initially, the source was not detected using the All-Sky Monitor (ASM) on-board the Rossi X-ray Timing Explorer, however, reprocessing of the ASM data shows that the source was in fact detected and it was active for about 50 days. The X-ray spectrum of this transient is fitted equally well by an absorbed power-law (with a spectral index of 2.5) or multi-colour disk blackbody model (with kT ~ 2 keV), where we find that the source is highly absorbed. We detect an unabsorbed 0.5-10 keV flux in the range (2-5)E-9 ergs cm^{-2} s^{-1}, which at a distance of 8 kpc corresponds to a 0.5-10 keV luminosity of (1-4)E37 ergs s^{-1}. No pulsations were detected by timing analysis. A colour-colour diagram from ASM data of different accreting objects suggests that the transient is a high-mass X-ray binary, as is also suggested by the high absorption compared to the average interstellar value in the direction of the source. However, the power-law spectral index is far more typical of a low-mass X-ray binary. Thus, we are unable to conclusively identify the nature of the transient. We also report on three sources first detected by the ASCA Galactic Plane Survey that are close to this transient.

Spectral Statistics and Local Luminosity Function of a Complete Hard X-Ray Sample of the Brightest Active Galactic Nuclei

We have measured the X-ray spectral properties of a complete flux-limited sample of bright active galactic nuclei (AGNs) from the HEAO-1 all-sky catalogs to investigate their statistics and provide greater constraints on the bright end of the hard X-ray luminosity function (HXLF) and the AGN population synthesis model of the X-ray background. Spectral studies using data from ASCA, XMM-Newton, and/or BeppoSAX observations have been made for almost all AGNs in this sample. The spectral measurements enable us to construct the neutral absorbing column density (log NH) distribution and separate HXLFs for absorbed (log NH[cm-2] > 21.5) and unabsorbed AGNs in the local universe. Our results show evidence of a difference in the shapes of the HXLFs of absorbed and unabsorbed AGNs in that the absorbed AGN HXLF drops more rapidly at higher luminosities than that of unabsorbed AGNs, similar to what has been previously reported. In the LX-NH plot we find no AGNs in the high-luminosity, high-intrinsic-absorption regime (log LX[ergs s-1] > 44.5 and log NH[cm-2] > 21.5) in our sample, in which we expect approximately five AGNs if we assume that the absorbed and unabsorbed AGNs have identical HXLF shapes. We also find that the fluxes observed with ASCA or XMM-Newton are smaller than that observed with HEAO-1 by a factor of 0.29 on average, which is expected for reobservation of sources with a factor of ~2.5 variability amplitude scale.

ChandraObservation of Abell 2065: An Unequal Mass Merger?

We present an analysis of a 41 ks Chandra ACIS-I observation of the merging cluster Abell 2065. Previous ROSAT and ASCA observations provided evidence for an ongoing merger and suggested that there were two surviving cooling cores, which were associated with the two cD galaxies at the center of the cluster. The Chandra observation reveals only one X-ray surface brightness peak. Its gas is cool and displaced slightly from the more luminous, southern cD galaxy. Combined with the surface brightness profile, this suggests that this cool material has formed a cold front. In the higher spatial resolution Chandra image, the second feature to the north is not associated with the second cD; rather, it appears to be a trial of gas behind the main cD. We argue that only one of the two cooling cores has survived the merger. The survival of the cool core constrains the merger kinematics, placing an upper limit of 1900 km s-1 on the relative merger velocity. A surface brightness discontinuity, found at ~140 kpc from the southern cD, has a Mach number of 1.66; the nature of the discontinuity (shock or cold front) is not clear fron the data. We argue that Abell 2065 is an unequal mass merger. The more massive southern cluster has driven a shock into the ICM of the infalling northern cluster, which has disrupted the cool core of the latter. We estimate that core crossing occurred a few hundred Myr ago, probably for the first time.

ASCA observations of OAO 1657-415 and its dust-scattered X-ray halo

We report on two ASCA observations of the high-mass X-ray binary pulsar OAO 1657-415. A short observation near mid-eclipse caught the source in a low-intensity state, with a weak continuum and iron emission dominated by the 6.4-keV fluorescent line. A later, longer observation found the source in a high-intensity state and covered the uneclipsed through mid-eclipse phases. In the high-intensity state, the non-eclipse spectrum has an absorbed continuum component due to scattering by material near the pulsar and 80 per cent of the fluorescent iron emission comes from less than 19 lt-sec away from the pulsar. We find a dust-scattered X-ray halo whose intensity decays through the eclipse. We use this halo to estimate the distance to the source as 7.1 +/- 1.3 kpc.

FUSEObservation of the Narrow‐Line Seyfert 1 Galaxy RE 1034+39: Dependence of Broad Emission Line Strengths on the Shape of the Photoionizing Spectrum

We present an analysis from simultaneous FUSE, ASCA, and EUVE observations, as well as a reanalysis of archival HST spectra, from the extreme narrow-line Seyfert 1 Galaxy RE 1034+39 (KUG 1031+398). RE 1034+39 has an unusually hard spectral energy distribution (SED) that peaks in the soft X-rays. Its emission lines are unusual in that they can all be modeled as a Lorentzian centered at the rest wavelength with only a small range in velocity widths. In order to investigate whether the unusual SED influences the emission-line ratios and equivalent widths, we present three complementary types of photoionization analysis. The FUSE spectrum was particularly important because it includes the high-ionization line O VI. First, we use the photoionization code CLOUDY and the SED developed from the coordinated observations to confirm that the emission lines are consistent with observed hard SED. The best model parameters were an ionization parameter log U ≈ -2 and a hydrogen number density log nH = 9.75 (cm-2). Second, we present a locally optimally emitting cloud model. This model produced enhanced O VI as observed, but also yielded far too strong Mg II. Third, we develop a series of semiempirical SEDs, run CLOUDY models, and compare the results with the measured values using a figure of merit (FOM). The FOM minimum indicates SED and gas properties similar to those inferred from the one-zone model using the RE 1034+39 continuum. Furthermore, the FOM increases sharply toward softer continua, indicating that a hard SED is required by the data in the context of a one-zone model.

XMM-NewtonObservations of High-Redshift Quasars

We report on our XMM observations of the high redshift quasars BR 2237--0607 (z=4.558) and BR 0351--1034 (z=4.351), together with 14 other z>4 objects found in the XMM public archive. Contrary to former reports, we do not find high redshift radio-loud quasars to be more absorbed than their radio-quiet counterparts. We find that the optical to X-ray spectral index alpha-ox is correlated with the luminosity density at 2500 A, but does not show a correlation with redshift. The mean 2-10 keV power-law slope of the 9 high redshift radio-quiet quasars in our sample for which a spectral analysis can be performed is alpha-x1.23+-0.48, similar to alpha-x=1.19 found from the ASCA observations of low redshift Narrow-Line Seyfert 1 galaxies (NLS1s), and significantly different from alpha-x=0.78 found for low redshift Broad-Line Seyfert galaxies. While the optical/UV spectra of low to high redshift quasars look remarkably similar, we find a first indication of a difference in their X-ray spectrum. The steep X-ray spectral index suggests high Eddington ratios L/L_Edd. These observations give credence to the hypothesis of Mathur (2000) that NLS1s are low luminosity cousins of high redshift quasars, both likely to be in their early evolutionary stage.

Simultaneous XMM-Newton and ESO VLT observations of supernova 1995N: probing the wind-ejecta interaction

We present the results of the first XMM-Newton observation of the interacting Type IIn supernova 1995N, performed in 2003 July. We find that the 0.2–10.0 keV unabsorbed flux dropped to a value of ≃1.8 × 10−13 erg cm−2 s−1, almost one order of magnitude lower than that of a previous ASCA observation of 1998 January. From all the available X-ray measurements, an interesting scenario emerges where the X-ray light emission may be produced by two-phase (clumpy/smooth) circumstellar material. The X-ray spectral analysis shows statistically significant evidence for the presence of two distinct components, which can be modelled with emission from optically thin, thermal plasmas at different temperatures. The exponent of the ejecta density distribution inferred from these temperatures is n≃ 6.4. From the fluxes of the two spectral components, we derive an estimate of the mass-loss rate of the supernova progenitor, , at the upper end of the interval exhibited by red supergiants. Coordinated optical and infrared observations allow us to reconstruct the simultaneous infrared to X-ray flux distribution of SN 1995N. We find that, at ∼9 yr after explosion, the direct X-ray thermal emission due to the wind-ejecta interaction is ∼5 times larger than the total reprocessed infrared/optical flux.

Low-energy absorption towards the ultra-compact X-ray binary 4U 1850-087 located in the globular cluster NGC 6712

We report the results of two XMM-Newton observations of the ultra-compact low-mass X-ray binary 4U 1850-087 located in the galactic globular cluster NGC 6712. A broad emission feature at 0.7 keV was detected in an earlier ASCA observation and explained as the result of an unusual Ne/O abundance ratio in the absorbing material local to the source. We find no evidence for this feature and derive Ne/O ratios in the range 0.14–0.21, consistent with that of the interstellar medium. During the second observation, when the source was ~10% more luminous, there is some evidence for a slightly higher Ne/O ratio and additional absorption. Changes in the Ne/O abundance ratio have been detected from another ultra-compact binary, 4U 1543-624. We propose that these changes result from an X-ray–induced wind which is evaporated from an O and Ne rich degenerate donor. As the source X-ray intensity increases so does the amount of evaporation and hence the column densities and abundance ratio of Ne and O.

On the Temperature Profile of Radiatively Efficient Geometrically Thin Disks in Black Hole Binaries with theASCAGIS

X-ray spectra of black hole binaries in the standard high/soft state were studied comprehensively using ASCA GIS data and, partially, RXTE PCA data. A mathematical disk model was applied to several black hole binaries to see whether the observed accretion disk temperature profile was consistent with that expected from the standard accretion disk model. This model is called a p-free disk and assumes that the spectrum is composed of multitemperature blackbody emission, where the local temperature T(r) at radius r is given by T(r) ∝ r-p, with p being a positive free parameter. The standard disk, where the gravitational energy of the accreted matter is effectively released as blackbody radiation, roughly requires p ≈ 3/4, while a small deviation is expected depending on the inner boundary conditions, general relativistic effects, and disk vertical structures. Our sample objects included LMC X-1, LMC X-3, XTE J2012+381, and GRO J1655-40. During the ASCA observations, these black hole binaries showed characteristics of the standard high/soft state. Under the standard modeling of high-state black hole binaries, the sources show disk inner temperatures of 0.76-1.17 keV, a disk fraction of the total 0.7-10 keV flux of 54%-98%, and absorption columns of (0.7-12) × 1021 cm-2. The best-fit values of p were found in the range 0.6-0.8, and the standard value of p = 3/4 was accepted for all the sources. The obtained values of p are also compared with those expected for the standard accretion disk in the Schwarzschild metric using the so-called GRAD model. The observed p-values were indeed found to be consistent with those expected from the standard accretion disk in the Schwarzschild metric.

X-ray orbital modulations in intermediate polars

We present an analysis of 30 archival ASCA and RXTE X-ray observations of 16 intermediate polars to investigate the nature of their orbital modulation. We show that X-ray orbital modulation is widespread amongst these systems, but not ubiquitous as indicated by previous studies that included fewer objects. Only seven of the sixteen systems show a clearly statistically significant modulation depth whose amplitude decreases with increasing X-ray energy. Interpreting this as due to photoelectric absorption in material at the edge of an accretion disc would imply that such modulations are visible for all system inclination angles in excess of 60 degrees. However, it is also apparent that the presence of an X-ray orbital modulation can appear and disappear on a timescale of ∼years or months in an individual system. This may be evidence for the presence of a precessing, tilted accretion disc, as inferred in some low mass X-ray binaries.

Chandra’s Tryst with SN 1995N

We present the spectroscopic and imaging analysis of a type IIn supernova SN 1995N observed with the Chandra X-ray observatory on 2004 March 27. We compare the spectrum obtained from our Chandra observation with that of the previous observation with ASCA in 1998. We find the presence of Neon lines in the Chandra spectrum that were not reported in the ASCA observation. We see no evidence of Iron in both epochs. The observed absorption column depth indicates an extra component over and above the galactic absorption component and is possibly due to a cool dense shell between the reverse-shock and the contact discontinuity in the ejecta. The ASCA and the ROSAT observations suggested a non-linear behavior of the X-ray light curve. However, with the higher spatial resolution and sensitivity of Chandra, we separate out many nearby sources in the supernova field-of-view that had additionally contributed to the supernova flux due to the large Point Spread Function of the ASCA. Taking out the contribution of those nearby sources, we find that the light curves are consistent with a linear decline profile. We consider the light curve in the high energy band separately. We discuss our results in the context of models of nucleosynthesis and the interaction of the shock waves with the circumstellar medium in core collapse supernovae.

X‐Ray Luminosity and Absorption Column Fluctuations in the H2O Maser Galaxy NGC 4258 from Weeks to Years

We report monitoring of the 0.3-10 keV spectrum of NGC4258 with XMM over 1.5 years.We als o report reprocessing of an overlapping series of archival Chandra observations. By including earlier ASCA and SAX observations, we present a new, nine-year time series of models fit to the X-ray spectrum of NGC4258. Over the nine years, the photoelectric absorbing column (~10^23 cm^-2) did not vary detectably, except for a ~40% drop between two ASCA epochs separated by 3 years and a ~60% rise between two XMM epochs separated by just 5 months. In contrast, factor of 2-3 changes are seen in absorbed flux on the timescale of years. These are uncorrelated with changes in absorbing column and indicative of central engine variability. The most rapid change in luminosity (5-10 keV) that we detect is ~30% over 19 days. The warped disk, a known source of H2O maser emission in NGC4258, is believed to cross the line of sight to the central engine. We propose that the variations in absorbing column arise from inhomogeneities sweeping across the line of sight in the rotating disk at the radius where the disk crosses the line of sight. We estimate that the inhomogeneities are ~10^15 cm in size at the crossing radius of 0.29 pc, slightly smaller than the expected scale height of the disk. This result thus provides strong evidence that the warped accretion disk is the absorber. This is the first direct confirmation that obscuration in type-2 AGN may, in some cases, arise in thin, warped accretion disks, rather than in geometrically thick tori. We do not detect Fe Kalpha line emission in any of our XMM spectra. We do not observe evidence of absorption lines in the XMM or reprocessed Chandra data.

Extreme X-Ray Behavior of the Low-Luminosity Active Nucleus in NGC 4395

We present the results of a 17 ks Chandra observation of the nearby dwarf spiral galaxy NGC 4395, focusing on the X-ray properties of the moderate-mass black hole that resides in its nucleus. Chandra affords the first high-quality, broadband X-ray detection of this object that is free of contamination from nearby sources in the field. We find that the nuclear X-ray emission is unresolved in the Chandra image and confirm the rapid, large-amplitude X-ray variability reported in previous studies. The spectrum of the nuclear source shows evidence for absorption by an ionized medium. There is also evidence for spectral variability over the course of the Chandra observation, although contrary to prior reports, it appears to be uncorrelated with fluctuations in the hard X-ray count rate. It is possible that the short-term spectral variability results from column density changes in the ionized absorber. By far the most unusual high-energy property of NGC 4395 is the shape of its spectrum above 1 keV. The Chandra data indicate a power-law photon index of Gamma \approx 0.6, which is much flatter than the Gamma \approx 1.8 X-ray spectra typical of active galactic nuclei and the slope of the nuclear X-ray spectrum measured from an earlier ASCA observation of NGC 4395. This extreme flatness and dramatic long-term variability of the X-ray spectrum are unprecedented among active galactic nuclei. A variety of possibilities for the origin of the flat continuum slope are considered; none provides a fully satisfactory explanation.