arXiv subjects

K. Iwasawa

Publications and source records attributed to K. Iwasawa.

At least 145 records · Page 8Linked to original sources

arXiv2001

The intracluster iron distribution around 4C+55.16

We report on the metal distribution in the intracluster medium around the radio galaxy 4C+55.16 (z=0.24) observed with the Chandra X-ray Observatory. The radial metallicity profile shows a dramatic change at 10 arcsec (~50 kpc) in radius from half solar to twice solar at inner radii. Also found was a plume-like feature located at ~3 arcsec to the south-west of the centre of the galaxy, which is mostly accounted for by a strong enhancement of iron L emission. The X-ray spectrum of the plume is characterized by the metal abundance pattern of Type Ia supernovae (SNeIa), i.e., large ratios of Fe to alpha-elements, with the iron metallicity being unusually high at 7.9(+24/-5.0) solar (90 per cent error). How the plume has been formed is not entirely clear. The inhomogeneous iron distribution suggested in this cluster provides important clues to understanding the metal enrichment process of the cluster medium.

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arXiv2001

Revealing the Dusty Warm Absorber in MCG--6-30-15 with the Chandra HETG

We present detailed evidence for a warm absorber in the Seyfert 1 galaxy MCG--6-30-15 and dispute earlier claims for relativistic O line emission. The HETG spectra show numerous narrow, unresolved (FWHM < 200 km/s) absorption lines from a wide range of ionization states of N, O, Mg, Ne, Si, S, Ar, and Fe. The O VII edge and 1s^2--1snp resonance line series to n=9 are clearly detected at rest in the AGN frame. We attribute previous reports of an apparently highly redshifted O VII edge to the 1s^2--1snp (n > 5) O VII resonance lines, and a neutral Fe L absorption complex. The shape of the Fe L feature is nearly identical to that seen in the spectra of several X-ray binaries, and in laboratory data. The implied dust column density agrees with that obtained from reddening studies, and gives the first direct X-ray evidence for dust embedded in a warm absorber. The O VIII resonance lines and weak edge are also detected, and the spectral rollover below 2 keV is explained by the superposition of numerous absorption lines and edges. We identify, for the first time, a KLL resonance in the O VI photoabsorption cross section, giving a measure of the O VI column density. The O VII (f) emission detected at the systemic velocity implies a covering fraction of ~5% (depending on the observed vs. time-averaged ionizing flux). Our observations show that a dusty warm absorber model is not only adequate to explain all the spectral features > 0.48 keV (< 26 Å) the data REQUIRE it. This contradicts the interpretation of Branduardi-Raymont et al. (2001) that this spectral region is dominated by highly relativistic line emission from the vicinity of the black hole.

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arXiv2001

Nuclear obscuration in the high-ionization Seyfert 2 galaxy Tol 0109-383

We report the BeppoSAX detection of a hard X-ray excess in the X-ray spectrum of the classical high-ionization Seyfert 2 galaxy Tol0109-383. The X-ray emission of this source observed below 7 keV is dominated by reflection from both cold and ionized gas, as seen in the ASCA data. The excess hard X-ray emission is presumably due to the central source absorbed by an optically thick obscuring torus with N(H)~2e24 cm-2. The strong cold X-ray reflection, if it is produced at the inner surface of the torus, is consistent with the picture where much of the inner nucleus of Tol0109-383 is exposed to direct view, as indicated by optical and infrared properties. However, the X-ray absorption must occur at small radii in order to hide the central X-ray source but leave the optical high-ionization emission line region unobscured. This may also be the case for objects like the Seyfert 1 galaxy Mrk231.

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arXiv2001

A hard X-ray constraint on the presence of an AGN in the ultra-luminous infrared galaxy Arp220

We present X-ray results on the ultraluminous infrared galaxy Arp220 obtained with BeppoSAX. The X-ray emission up to 10 keV is detected. No significant signal is detected with the PDS detector in the higher energy band. The 2-10 keV emission has a hard spectrum (photon-index of 1.7), similar to M82, and a luminosity of 1e41 erg/s. A population of X-ray binaries may be a major source of this X-ray emission. The upper limit of an iron K line equivalent width at 6.4 keV is ~600 eV. This observation imposes so far the tightest constraint on an active nucleus if present in Arp220. We find that a column density of X-ray absorption must exceed 10**25 cm-2 for an obscured active nucleus to be significant in the energetics, and the covering factor of the absorption should be almost unity. The underluminous soft X-ray starburst emission may need a good explanation, if the bolometric luminosity is primarily powered by a starburst.

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arXiv2001

The blazar GB1428+4217: a warm absorber at z=4.72?

BeppoSAX observations of the high redshift (z=4.72) blazar GB 1428+4217 confirm the presence of a complex soft X-ray spectrum first seen with the ROSAT PSPC. Flattening below a rest frame energy of 5 keV can be accounted for by absorption from an equivalent column density of (cold) gas with N_H ~ 8 x 10^22 cm^-2. Below 2 keV a (variable) excess of a factor ~ 20 above the extrapolated absorbed spectrum is also detected. These findings are consistent with and extend to higher redshifts the correlation between increasing soft X-ray flattening and increasing z, previously pointed out for large samples of radio-loud quasars. We propose that such features, including X-ray absorption, soft excess emission as well as absorption in the optical spectra, can be satisfactorily accounted for by the presence of a highly-ionized nuclear absorber with column N_H ~ 10^23 cm^-2, with properties possibly related to the conditions in the nuclear regions of the host galaxy. High energy X-ray emission consistent with the extrapolation of the medium energy spectrum is detected up to ~ 300 keV (rest frame).

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arXiv2000

PMN J0525-3343: soft X-ray spectral flattening in a blazar at z=4.4

We report optical, radio and X-ray observations of a new distant blazar, PMN J0525-3343, at a redshift of 4.4. The X-ray spectrum measured from ASCA and BeppoSAX flattens below a few keV, in a manner similar to the spectra of two other z>4 blazars, GB 1428+4217 (z=4.72) reported by Boller et al and RXJ 1028.6-0844 (z=4.28) by Yuan et al. The spectrum is well fitted by a power-law continuum which is either absorbed or breaks at a few keV. An intrinsic column density corresponding to 2 x 10^23 H-atoms cm-2 at solar abundance is required by the absorption model. This is however a million times greater than the neutral hydrogen, or dust, column density implied by the optical spectrum, which covers the rest-frame UV emission of the blazar nucleus. We discuss the problems raised and suggest that, unless there is intrinsic flattening in the spectral distribution of the particles/seed photons producing X-rays via inverse Compton scattering, the most plausible solution is a warm absorber close to the active nucleus.

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arXiv2000

Chandra imaging of the X-ray core of Abell 1795

We report the discovery of a 40 arcsec long X-ray filament in the core of the cluster of galaxies A1795. The feature coincides with an Halpha+NII filament found by Cowie et al in the early 1980s and resolved into at least 2 U-band filaments by McNamara et al in the mid 1990s. The (emission-weighted) temperature of the X-ray emitting gas along the filament is 2.5-3 keV, as revealed by X-ray colour ratios. The deprojected temperature will be less. A detailed temperature map of the core of the cluster presented. The cD galaxy at the head of the filament is probably moving through or oscillating in the cluster core. The radiative cooling time of the X-ray emitting gas in the filament is about 3x10^8 yr which is similar to the age of the filament obtained from its length and velocity. This suggests that the filament is produced by cooling of the gas from the intracluster medium. The filament, much of which is well separated from the body of the cD galaxy and its radio source, is potentially of great importance in helping to understand the energy and ionization source of the optical nebulosity common in cooling flows.

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arXiv2000

Chandra detection of reflected X-ray emission from the type 2 QSO in IRAS 09104+4109

We present X-ray imaging spectroscopy of the extremely luminous infrared galaxy IRAS 09104+4109 (z=0.442) obtained with the Chandra X-ray Observatory. With the arcsec resolution of Chandra, an unresolved source at the nucleus is separated from the surrounding cluster emission. A strong iron K line at 6.4 keV on a very hard continuum is detected from the nuclear source, rendering IRAS 09104+4109 the most distant reflection-dominated X-ray source known. Combined with the BeppoSAX detection of the excess hard X-ray emission, it provides further strong support to the presence of a hidden X-ray source of quasar luminosity in this infrared galaxy. Also seen is a faint linear structure to the North, which coincides with the main radio jet. An X-ray deficit in the corresponding region suggests an interaction between the cluster medium and the jet driven by the active nucleus.

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arXiv2000

Evidence for Ionised Accretion Discs in Five Narrow-Line Seyfert 1 Galaxies

(Abridged) We present the results of fitting ASCA spectra of six narrow-line Seyfert 1 (NLS1) galaxies with the ionised reflection models of Ross & Fabian (1993). We find that five of the galaxies (TONS 180, PKS 0558-504, Ark 564, Mrk 335 and PG 1244+026) are well fit by the ionised disc model, and these are often better fits than the alternative models considered. The sixth galaxy, NGC 4051, has additional spectral complexity that cannot be well described by a simple ionised disc model or any of the other alternative models. All six of the NLS1s we considered show evidence for a broad Fe Kalpha line, but none of the line centroids are consistent with emission from highly ionised Fe. This is most likely due to the line being redshifted because of relativistic effects. We note that sources with larger ionisation parameters tend to have larger Fe Kalpha EWs. We interpret this as evidence that ionised Fe features are making their presence felt in the spectra. Since most of our sources have steep spectra, highly ionised features are predicted even by the new variable density reflection models. The one source we analyse with a photon index less than two has the lowest ionisation parameter in our sample. We conclude that our result is the strongest evidence yet that NLS1 might have ionised accretion discs. This result gives further weight to the hypothesis that these objects contain rapidly accreting black holes.

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arXiv2000

The X-ray Spectrum and Light Curve of Supernova 1995N

We report on multi-epoch X-ray observations of the Type IIn (narrow emission line) supernova SN 1995N with the ROSAT and ASCA satellites. The January 1998 ASCA X-ray spectrum is well fitted by a thermal bremsstrahlung (kT~10 keV, N_H~6e20 cm^-2) or power-law (alpha~1.7, N_H~1e21 cm^-2) model. The X-ray light curve shows evidence for significant flux evolution between August 1996 and January 1998: the count rate from the source decreased by 30% between our August 1996 and August 1997 ROSAT observations, and the X-ray luminosity most likely increased by a factor of ~2 between our August 1997 ROSAT and January 1998 ASCA observations, although evolution of the spectral shape over this interval is not ruled out. The high X-ray luminosity, L_X~1e41 erg/sec, places SN 1995N in a small group of Type IIn supernovae with strong circumstellar interaction, and the evolving X-ray luminosity suggests that the circumstellar medium is distributed inhomogeneously.

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arXiv2000

Chandra imaging of the complex X-ray core of the Perseus cluster

We report subarcsec-resolution X-ray imaging of the core of the Perseus cluster around the galaxy NGC 1275 with the Chandra X-ray Observatory. The ROSAT-discovered holes associated with the radio lobes have X-ray bright rims which are cooler than the surrounding gas and not due to shocks. The holes themselves may contain some hotter gas. We map strong photoelectric absorption across the Northern lobe and rim due to a small infalling irregular galaxy, known as the high velocity system. Two outer holes, one of which was previously known, are identified with recently found spurs of low-frequency radio emission. The spiral appearance of the X-ray cooler gas and the outer optical parts of NGC 1275 may be due to angular momentum in the cooling flow.

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arXiv2000

The X-ray variability of the Seyfert~1 galaxy MCG-6-30-15 from long ASCA and RXTE observations

We present an analysis of the long RXTE observation of the Seyfert~1 galaxy MCG-6-30-15, taken in July 1997. Our results show that the behaviour is complicated. We find clear evidence from colour ratios and direct spectral fitting that changes to the intrinsic photon index are taking place. Spectral hardening is evident during periods of diminished intensity; in particular, a general trend for harder spectra is seen in the period following the hardest RXTE flare. Flux-correlated studies further show that the 3-10 keV photon index steepens while that in the 10-20 keV band, flattens with flux. The largest changes come from the spectral index below 10keV; however, changes in the intrinsic power law slope, and reflection both contribute in varying degrees to the overall spectral variability. We find that the iron line flux is consistent with being constant over large time intervals on the order of days (although the ASCA and RXTE spectra show that $F_{Kα}$ changes on shorter time intervals of order < 10ks), and equivalent width which anticorrelates with the continuum flux, and reflection fraction. Flux-correlated studies point at possible ionization signatures, while detailed spectral analysis of short time intervals surrounding flare events hint tentatively at observed spectral responses to the flare. We present a simple model for partial ionization where the bulk of the variability comes from within 6r_g. Temporal analysis further provides evidence for possible time (< 1000s) and phase (phi~0.6 rad) lags. Finally, we report an apparent break in the power density spectrum (~ 4-5 x 10^{-6}Hz) and a possible 33 hr period. Estimates for the mass of the black hole in MCG-6-30-15 are discussed in the context of spectral and temporal findings.

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arXiv2000

X-ray absorption and rapid variability of the dwarf Seyfert nucleus of NGC4395

We report the detection of an absorbed central X-ray source and its strong, rapid variability in NGC4395, the least luminous Seyfert nucleus known. The X-ray source exhibits a number of flares with factors of 3-4 flux changes during a half day ASCA observation. Such X-ray variability is in constrast to the behaviour of other low luminosity active galaxies. It provides further support for an accreting black hole model rather than an extreme stellar process in accounting for the nuclear activity of NGC4395. The soft X-ray emission below 3 keV is strongly attenuated by absorption. The energy spectrum in this absorption band shows a dramatic change in response to the variation in continuum luminosity. A variable warm absorber appears to be an explanation for the spectral change. The absorption-corrected 2-10 keV luminosity is 4e39 erg/s for a source distance of 2.6 Mpc, and at 1 keV is one order of magnitude above previous ROSAT estimates. Our X-ray results infer the nuclear source of NGC4395 to be a scaled-down version of higher luminosity Seyfert nuclei, with an intermediate mass (10^4-10^5 Msun) black hole, unlike the nearby low luminosity active galaxies in which underfed massive black holes are suspected to reside.

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arXiv2000

The X-ray spectra of Compton-thick Seyfert 2 galaxies as seen by BeppoSAX

Results from BeppoSAX observations of Compton--thick Seyfert 2 galaxies are summarized and reviewed, and their general properties derived and discussed. In five out of the seven observed sources, the nucleus is directly visible at high X-ray energies, where the photons penetrate absorbers with column densities in the range 1.1-4.3x10^{24} cm-2 (in the other two sources, NGC 1068 and NGC 7674, the nucleus is instead totally obscured at all energies, implying even larger column densities). In most sources there is unambiguous evidence of a reflection component from optically thick, cold matter, while in two (or maybe four) cases there is also evidence of reflection from ionized matter. For the sources with a measured X-ray luminosity, a comparison with the infrared luminosity is made; while in two cases (the Circinus galaxy and NGC 4945) the IR emission appears to be dominated by starburst activity, in the other three sources (NGC 6240, Mrk 3 and TOL 0109-383) it is likely to be dominated by reprocessing of the UV and X-ray photons emitted by an AGN.

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arXiv2000

Broad iron lines in Active Galactic Nuclei

An intrinsically narrow line emitted by an accretion disk around a black hole appears broadened and skewed as a result of the Doppler effect and gravitational redshift. The fluorescent iron line in the X-ray band at 6.4-6.9keV is the strongest such line and is seen in the X-ray spectrum of many active galactic nuclei and, in particular, Seyfert galaxies. It is an important diagnostic with which to study the geometry and other properties of the accretion flow very close to the central black hole. The broad iron line indicates the presence of a standard thin accretion disk in those objects, often seen at low inclination. The broad iron line has opened up strong gravitational effects around black holes to observational study with wide-reaching consequences for both astrophysics and physics.

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arXiv2000

Testing the connection between the X-ray and submillimetre source populations using Chandra

The powerful combination of the Chandra X-ray telescope, the SCUBA submillimetre-wave camera and the gravitational lensing effect of the massive galaxy clusters A2390 and A1835 has been used to place stringent X-ray flux limits on six faint submillimetre SCUBA sources and deep submillimetre limits on three Chandra sources which lie in fields common to both instruments. One further source is marginally detected in both the X-ray and submillimetre bands. For all the SCUBA sources our results are consistent with starburst-dominated emission. For two objects, including SMMJ14011+0252 at z=2.55, the constraints are strong enough that they can only host powerful active galactic nuclei if they are both Compton-thick and any scattered X-ray flux is weak or itself absorbed. The lensing amplification for the sources is in the range 1.5-7, assuming that they lie at z>1. The brightest detected X-ray source has a faint extended optical counterpart (I~22) with colours consistent with a galaxy at z~1. The X-ray spectrum of this galaxy is hard, implying strong intrinsic absorption with a column density of about 1e23 cm-2 and an intrinsic (unabsorbed) 2-10 keV luminosity of 3e44 erg/s. This source is therefore a Type-II quasar. The weakest detected X-ray sources are not detected in HST images down to I~26.

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arXiv1999

Zw 1718.1-0108: a highly disturbed galaxy cluster at low Galactic latitude

We report the discovery of highly distorted X-ray emission associated with the nearby cluster Zw1718.1-0108, one of whose dominant members is the powerful radio galaxy 3C353. This cluster has been missed by previous X-ray cluster surveys because of its low Galactic latitude (b = 19.5deg), despite its brightness in the hard X-ray band (2-10 keV flux of 1.2E-11 erg/s/cm2). Our optical CCD image of the central part of the cluster reveals many member galaxies which are dimmed substantially by heavy Galactic extinction. We have measured redshifts of three bright galaxies near the X-ray emission peak and they are all found to be around z=0.028. The ASCA GIS and ROSAT HRI images show three aligned X-ray clumps embedded in low surface-brightness X-ray emission extended by about 30 arcmin. The averaged temperature measured with ASCA is kT=4.3+/-0.2 keV, which appears to be hot for the bolometric luminosity 1.1E+44 erg/s, when compared to the temperature-luminosity correlation of galaxy clusters. The irregular X-ray morphology and evidence for a non-uniform temperature distribution suggest that the system is undergoing a merger of substructures. Since the sizes and luminosities of the individual clumps are consistent with those of galaxy groups, Zw1718.1-0108 is interpreted as an on-going merger of galaxy groups in a dark matter halo forming a cluster of galaxies and thus is in a transition phase of cluster formation.

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arXiv1999

New X-ray Constraints on Starburst and Seyfert Activity in the Barred Spiral Galaxy NGC 1672

The nearby barred spiral galaxy NGC 1672 shows dramatic starburst activity and may also host a Seyfert 2 nucleus. We present new X-ray observations that set constraints on starburst and Seyfert activity in NGC 1672. Two ROSAT HRI exposures, taken in 1992 and 1997, are used to investigate long-term variability of the known X-ray sources and to search for new sources of X-ray emission. We find large-amplitude (about 69%) variability from X-3, one of the off-nuclear sources located near an end of the galactic bar. X-3 has a peak observed 0.2-2.0 keV luminosity of about 2.5 x 10^{39} erg/s, and it is probably a luminous X-ray binary or young supernova remnant. We do not observe variability of the nuclear source X-1 or the strong off-nuclear source X-2. Our analyses also reveal two new off-nuclear sources, one of which is associated with a bright region along a spiral arm, and we find evidence for large-scale diffuse X-ray emission throughout part of the disk of NGC 1672. Furthermore, we use ASCA data taken in 1995 to constrain the hard X-ray properties of NGC 1672. While the nuclear source X-1 is the dominant soft X-ray source in NGC 1672, we find that the bulk of the 2-10 keV and 5-10 keV emission is spatially coincident with the off-nuclear source X-3, giving it an apparent 0.2-8 keV luminosity of about 6 x 10^{39} erg/s. A power-law plus Raymond-Smith model provides an acceptable fit to the full-band ASCA spectra. We do not find any evidence for a luminous but absorbed nuclear X-ray source. If there is a luminous Seyfert 2 nucleus in NGC 1672, it must be obscured by a `Compton-thick' torus with a column density of > 2 x 10^{24} cm^{-2}.

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