arXiv subjects

Aya Kubota

Publications and source records attributed to Aya Kubota.

At least 73 records · Page 4Linked to original sources

arXiv2008

Suzaku Results on Cygnus X-1 in the Low/Hard State

The black-hole binary Cygnus X-1 was observed for 17 ks with the Suzaku X-ray observatory in 2005 October, while it was in a low/hard state with a 0.7-300 keV luminosity of 4.6 x 10^37 erg/s. The XIS and HXD spectra, spanning 0.7-400 keV, were reproduced successfully incorporating a cool accretion disk and a hot Comptonizing corona. The corona is characterized by an electron temperature of ~100 keV, and two optical depths of ~0.4 and ~1.5 which account for the harder and softer continua, respectively. The disk has the innermost temperature of ~0.2 keV, and is thought to protrude half way into the corona. The disk not only provides seed photons to the Compton cloud, but also produces a soft spectral excess, a mild reflection hump, and a weakly broadened iron line. A comparison with the Suzaku data on GRO J1655-40 reveals several interesting spectral differences, which can mostly be attributed to inclination effects assuming that the disk has a flat geometry while the corona is grossly spherical. An intensity-sorted spectroscopy indicates that the continuum becomes less Comptonized when the source flares up on times scales of 1-200 s, while the underlying disk remains unchanged.

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arXiv2007

Discovery of a bright transient ultraluminous X-ray source Suzaku J1305-4931 in NGC 4945

This paper reports the discovery of a bright X-ray transient source, Suzaku J1305-4913, in the south-west arm of the nearby Seyfert II galaxy NGC 4945. It was detected at a 0.5 -- 10 keV flux of $2.2 \times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$ during the Suzaku observation conducted on 2006 January 15 -- 17, but was undetectable in a shorter observation on 2005 August 22 --23, with an upper limit of $1.7 \times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$ (90% confidence level). At a distance of 3.7 Mpc, the bolometric luminosity of the source becomes $L_{\rm bol} = 4.4 \times 10^{39} α$ erg s$^{-1}$, where $α= (\cos 60^\circ / \cos i)$ and $i$ is the disk inclination. Therefore, the source is classified into so-called ultraluminous X-ray sources (ULXs). The time-averaged X-ray spectrum of the source is described by a multi-color disk model, with the innermost accretion disk temperature of $T_{\rm in} = 1.69_{-0.05}^{+0.06}$ keV. During the 2006 January observation, it varied by a factor of 2 in intensity, following a clear correlation of $L_{\rm bol} \propto T_{\rm in}^4$. It is inferred that the innermost disk radius $R_{\rm in}$ stayed constant at $R_{\rm in} = 79_{-3.9}^{+4.0} α^{1/2}$ km, suggesting the presence of a standard accretion disk. Relating $R_{\rm in}$ with the last stable orbit around a non-rotating black hole yields a rather low black hole mass, $\sim 9 α^{1/2}$ solar masses, which would imply that the source is shining at a luminosity of $\sim3 α^{1/2} $ times the Eddington limit. These results can be better interpreted by invoking sub-Eddington emission from a rapidly spinning black hole with a mass of 20 -- 130 solar masses.

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arXiv2007

Modelling the behaviour of accretion flows in X-ray binaries

(abridged) We review how the recent increase in X-ray and radio data from black hole and neutron star binaries can be merged together with theoretical advances to give a coherent picture of the physics of the accretion flow in strong gravity. Both long term X-ray light curves, X-ray spectra, the rapid X-ray variability and the radio jet behaviour are consistent with a model where a standard outer accretion disc is truncated at low luminosities, being replaced by a hot, inner flow which also acts as the launching site of the jet. Decreasing the disc truncation radius leads to softer spectra, as well as higher frequencies (including QPO's) in the power spectra, and a faster jet. The collapse of the hot flow when the disc reaches the last stable orbit triggers the dramatic decrease in radio flux, as well as giving a qualitative (and often quantitative) explanation for the major hard--soft spectral transition seen in black holes and neutron stars. After collapse of the hot inner flow, the spectrum in black hole systems can be dominated by the disc emission. Its behaviour is consistent with the existence of a last stable orbit, and such data can be used to estimate the black hole spin. These systems can also show very different spectra at these high luminosities, in which the disc spectrum is strongly distorted by Comptonization. The structure of the accretion flow becomes increasingly uncertain as the luminosity approaches (and exceeds) the Eddington luminosity, though there is growing evidence that winds play an important role. We stress that these high Eddington fraction flows are key to understanding many disparate and currently very active fields such as ULX, Narrow Line Seyfert 1's, and the growth of the first black holes in the Early Universe.

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arXiv2007

Low/Hard State Spectra of GRO J1655-40 Observed with Suzaku

The Galactic black-hole binary GRO J1655$-$40 was observed with Suzaku on 2005 September 22--23, for a net exposure of 35 ks with the X-ray Imaging Spectrometer (XIS) and 20 ks with the Hard X-ray Detector (HXD). The source was detected over a broad and continuous energy range of 0.7--300 keV, with an intensity of $\sim$50 mCrab at 20 keV. At a distance of 3.2 kpc, the 0.7--300 keV luminosity is $ \sim 5.1 \times 10^{36}$ erg s$^{-1}$ ($\sim 0.7$ % of the Eddington luminosity for a 6 $M_{\odot}$ black hole). The source was in a typical low/hard state, exhibiting a power-law shaped continuum with a photon index of $\sim 1.6$. During the observation, the source intensity gradually decreased by 25% at energies above $\sim 3$ keV, and by 35% below 2 keV. This, together with the soft X-ray spectra taken with the XIS, suggests the presence of an independent soft component that can be represented by emission from a cool ($\sim 0.2$ keV) disk. The hard X-ray spectra obtained with the HXD reveal a high-energy spectral cutoff, with an e-folding energy of $\sim 200$ keV. Since the spectral photon index above 10 keV is harder by $\sim 0.4$ than that observed in the softer energy band, and the e-folding energy is higher than those of typical reflection humps, the entire 0.7--300 keV spectrum cannot be reproduced by a single thermal Comptonization model, even considering reflection effects. Instead, the spectrum (except the soft excess) can be successfully explained by invoking two thermal-Comptonization components with different $y$-parameters. In contrast to the high/soft state spectra of this object in which narrow iron absorption lines are detected with equivalent widths of 60--100 eV, the present XIS spectra bear no such features beyond an upper-limit equivalent width of 25 eV.

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arXiv2006

Detailed spectral study of an ultra-luminous compact X-ray source M81 X-9 in the disk dominated state

We report on the results of detailed spectral studies of the ultra-luminous X-ray source (ULX), M81 X-9 (Holmberg IX X-1), made with XMM-Newton on 2001 April 22 and with ASCA on 1999 April 6. On both occasions, the source showed an unabsorbed 0.5-10 keV luminosity of ~2E40 erg/s (assuming a distance of 3.4Mpc) and a soft spectrum apparently represented by a multi-color disk model with an innermost disk temperature of 1.3-1.5 keV. Adding a power-law model further improved the fit. However, as previously reported, the high luminosity cannot be reconciled with the high disk temperature within a framework of the standard accretion disk radiating at a sub-Eddington luminosity. Therefore, we modified the multi-color disk model, and allowed the local disk temperature to scale as r^(-p) on the distance r from the black hole, with p being a free parameter. We then found that the XMM-Newton and the ASCA spectra can be both reproduced successfully with p~0.6 and the innermost disk temperature of 1.4-1.8 keV. These flatter temperature profiles suggest deviation from the standard Shakura-Sunyaev disk, and are consistent with predictions of a slim disk model.

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arXiv2006

Suzaku discovery of iron absorption lines in outburst spectra of the X-ray transient 4U 1630-472

We present the results of six Suzaku observations of the recurrent black hole transient 4U1630-472 during its decline from outburst from February 8 to March 23 in 2006. All observations show the typical high/soft state spectral shape in the 2-50keV band, roughly described by an optically thick disk spectrum in the soft energy band plus a weak power-law tail that becomes dominant only above \~20keV. The disk temperature decreases from 1.4keV to 1.2keV as the flux decreases by a factor 2, consistent with a constant radius as expected for disk-dominated spectra. All the observations reveal significant absorption lines from highly ionized (H-like and He-like) iron Ka at 7.0keV and 6.7keV. The brightest datasets also show significant but weaker absorption structures between 7.8keV and 8.2keV, which we identify as a blend of iron Kb and nickel Ka absorption lines. The energies of these absorption lines suggest a blue shift with an outflow velocity of ~1000km/s.. The H-like iron Ka equivalent width remains approximately constant at ~30 eV over all the observations, while that of the He-like Ka line increases from 7 eV to 20 eV. Thus the ionization state of the material decreases, as expected from the decline in flux. We fit the profiles with Voigt functions (curve of growth) to derive absorbing columns, then use these together with detailed photo-ionization calculations to derive physical parameters of the absorbing material. The data then constrain the velocity dispersion of the absorber to 200-2000 km/s, and the size of the plasma as ~1E10 cm assuming a source distance of 10 kpc.

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arXiv2006

An optically thick inner corona geometry for the Very High State Galactic Black Hole XTE J1550-564

(truncated version) The X-ray spectra of Galactic binary systems in the very high state show both strong disk emission and a strong, steep tail to high energies. We use simultaneous optical-ASCA-RXTE data from the black hole transient XTE J1550-564 as a specific example, and show that these have disc spectra which are significantly lower in temperature than those seen from the same source at the same luminosity when in the disc dominated state. If these give a true picture of the disc then either the disc emissivity has reduced, and/or the disc truncates above the last stable orbit. However, it is often assumed that the tail is produced by Compton scattering, in which case its shape in these spectra requires that the Comptonising region is marginally optically thick (tau~2-3), and covers a large fraction of the inner disc. This will distort our view of the disc. We build a theoretical model of a Comptonising corona over an inner disc, and fit this to the data, but find that it still requires a large increase in inner disc radius for a standard disc emissivity. Instead it seems more probable that the disc emissivity changes in the presence of the corona. We implement the specific inner disc-corona coupling model of Svensson & Zdziarski (1994) and show that this can explain the low temperature/high luminosity disc emission seen in the very high state with only a small increase in radius of the disc. While this inferred disc truncation is probably not significant given the model uncertainties, it is consistent with the low frequency QPO and gives continuity of properties with the low/hard state spectra.

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arXiv2005

Three Spectral States of the Disk X-Ray Emission of the Black-Hole Candidate 4U 1630-47

We studied a time history of X-ray spectral states of a black-hole candidate, 4U 1630-47, utilizing data from a number of monitoring observations with the Rossi X-Ray Timing Explorer over 1996--2004. These observations covered five outbursts of 4U 1630-47, and most of the data recorded typical features of the high/soft states. We found that the spectra in the high/soft states can be further classified into three states. The first spectral state is explained by a concept of the standard accretion disk picture. The second state appears in the so-called very high state, where a dominant hard component is seen and the disk radius apparently becomes too small. These phenomena are explained by the effect of inverse Compton scattering of disk photons, as shown by Kubota, Makishima, & Ebisawa (2001, ApJ, 560, L147) for GRO J1655-40. The third state is characterized in such a way that the disk luminosity varies in proportion to $T_{\rm in}^2$, rather than $T_{\rm in}^4$, where $T_{\rm in}$ is the inner-disk temperature. This state is suggested to be an optically-thick and advection-dominated slim disk, as suggested by Kubota & Makishima (2004, ApJ, 601, 428) for XTE J1550-564. The second and third states appear, with good reproducibility, when $T_{\rm in}$ and the total X-ray luminosity are higher than 1.2 keV and $\sim2.5\times10^{38}(D/10\quad{\rm kpc})^2łeft[\cosθ/(1/\sqrt{3})]^{-1}$ erg s$^{-1}$, respectively, where $D$ is the distance to the object and $θ$ is the inclination angle to the disk. The present results suggest that these three spectral states commonly appear among black-hole binaries under high accretion rates.

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arXiv2005

Observational studies of stellar black hole binaries and ULXs

We outline a framework for understanding the X-ray spectra of high mass accretion rate stellar black holes based on X-ray data from RXTE and ASCA. Three spectral regimes can be separated out by the behaviour of the observed disk luminosity and temperature. The well established "standard regime" is seen when the disk dominates the spectrum, where only a small fraction of the luminosity is emitted in the power law tail. These spectra generally satisfy the standard relation expected for thermal emission from a constant area, namely that the disk bolometric luminosity, Ldisk, is proportional to its maximum temperature, Tin^4. However, at higher luminosities this starts to change to Tin^2. This "apparently standard regime" is still dominated by the disk emission, but this difference luminosity-temperature relation and subtle changes in spectral shape may show that another cooling process is required in addition to radiative cooling. At intermediate luminosities there is an anomalous regime (or weak very high state) where the disk temperature and luminosity are less clearly related. These spectra are characterized by the presence of a much stronger comptonized tail indicating high energy electrons. When observed disk emission is corrected for the the effects of comptonisation then these points lie back on the standard relation. The growth of this comptonising corona is also clearly linked to the quasi-periodic oscillations, as these are observed preferentially in the anomalous regime. This presented picture was found to explain the spectral behavior of both black hole binaries in our Galaxy and LMC. Spectral evolution of several bright ULXs observed with ASCA were also successfully explained in the same picture.

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arXiv2005

Spectral Evolution of a Luminous Compact X-Ray Source in NGC 253 with Chandra and XMM-Newton

Spectral studies of a luminous X-ray source, NGC 253 X21, are presented based on archival Chandra and XMM-Newton data. The Chandra observation on 1999 December 16 detected the source at a bolometric luminosity of 0.3 x 10^39 erg/s (assuming isotropic emission), while an XMM-Newton observation on 2000 June 3 revealed a short-term source variation in the range of (0.6--1.3) x 10^39 erg/s. All spectra from these observations were successfully modeled by emission from an optically thick accretion disk. The average inner disk radius was kept constant at 63 x (cos 60/cos i)^(1/2) km, where i is the disk inclination, and did not vary significantly, while the disk inner temperature changed in the range of 0.9--1.4 keV. Assuming that this object is an accreting Schwarzschild black hole, and that the disk inner radius coincides with its last stable orbit, the mass of the black hole is estimated to be ~ 7 M_sun. The disk luminosity corresponds to (30--120) x (cos 60/cos i) % of the Eddington limit of this black hole. Therefore, this luminous X-ray source, NGC 253 X21, is understood consistently to be an accreting stellar mass black hole in the standard disk state.

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arXiv2005

On the temperature profile of radiatively efficient geometrically thin disks in black hole binaries with the ASCA GIS

X-ray spectra of black hole binaries(BHBs) in the high/soft state were studied comprehensively by using ASCA GIS data, and partially RXTE PCA data. A mathematical disk model was applied to several BHBs to see if the observed accretion disk temperature profile was consistent with that expected from the standard accretion disk model. This model is called p-free disk, and assumes that the spectrum is composed of multi-temperature blackbody emission of which 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 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 BHBs showed characteristics of the standard high/soft state. Under the standard modeling of high-state black hole binaries, the sources show Tin=0.76-1.17keV, the disk fraction to the total 0.7-10keV flux of 54-98%, and Nh=(0.7--12)E21/cm^2. The best-fit p-values were found in 0.6-0.8, and the standard value of p=3/4 was accepted for all the sources.The obtained p-values are also compared with those expected for the standard accretion disk in the Schwarzschild metric, and they were consistent with those expected from the standard accretion disk.

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arXiv2005

X-Ray Spectra of the Narrow-Line Seyfert 1 Galaxy Ton S180 in Comparison with Galactic Black Holes

An analysis was made of 0.3-15 keV X-ray spectra of a Narrow-Line Seyfert 1 Galaxy, Ton S180, using archival data from ASCA, RXTE, and XMM-Newton. At energies above 2.5 keV, a power-law with a photon index of ~ 2.3 successfully and consistently reproduced the spectra from all of these observatories. Assuming this power-law component to extend toward lower energies, a soft excess, which is one of the most remarkable features of Narrow-Line Seyfert 1 Galaxies, is explained by another power-law multiplied by a thermal cutoff at ~ 0.4 keV. Some similarities have been observed between this object and Galactic black hole binaries in very high state, the latter being realized under high accretion rates. Attempts have been made to interpret the soft excess in terms of Comptonization of the disk photons by an electron cloud surrounding the accretion disk, like BHBs in a very high state.

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arXiv2004

The very high state accretion disk structure from the galactic black hole transient XTE J1550-564

The 1998 outburst of the bright Galactic black hole binary system XTE J1550-564 was used to constrain the accretion disc structure in the strongly Comptonised very high state spectra. These data show that the disc emission is not easily compatible with the constant area L ~ T^4 behaviour seen during the thermal dominated high/soft state and weakly comptonised very high state. Even after correcting for the effects of the scattering geometry, the disc temperature is always much lower than expected for its derived luminosity in the very high state. The simplest interpretation is that this indicates that the optically thick disc is truncated in the strongly Comptonised very high state, so trivially giving the observed continuity of properties between the low/hard and very high states of Galactic black holes.

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arXiv2003

The Three Spectral Regimes Found in the Stellar Black Hole XTE J1550--564 in its High/Soft State

The present paper describes the analysis of multiple RXTE/PCA data of the black hole binary with superluminal jet, XTE J1550-564, acquired during its 1999--2000 outburst. The X-ray spectra show features typical of the high/soft spectral state, and can approximately be described by an optically thick disk spectrum plus a power-law tail. Three distinct spectral regimes, named standard regime, anomalous regime, and apparently standard regime, have been found from the entire set of the observed spectra. When the X-ray luminosity is well below 6E38 erg/s (assuming a distance of 5 kpc), XTE J1550-564 resides in the standard regime, where the soft spectral component dominates the power-law component and the observed disk inner radius is kept constant. When the luminosity exceeds the critical luminosity, the apparently standard regime is realized, where luminosity of the optically thick disk rises less steeply with the temperature, and the spectral shape is moderately distorted from that of the standard accretion disk. In this regime, radial temperature gradient of the disk has been found to be flatter than that of the standard accretion disk. The results of the apparently standard regime are suggestive of a slim disk (e.g., Abramowicz et al. 1988, Watarai et al. 2000) which is a solution predicted under high mass accretion rate. In the intermediate anomalous regime, the spectrum becomes much harder, and the disk inner radius derived using a simple disk model spectrum apparently varies significantly with time. These properties can be explained as a result of significant thermal inverse Comptonization of the disk photons, as was found from GRO J1655-40 in its anomalous regime by Kubota, Makishima and Ebisawa (2001).

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arXiv2003

Accretion Disk Spectra of the Ultra-luminous X-ray Sources in Nearby Spiral Galaxies and Galactic Superluminal Jet Sources

Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies and Galactic superluminal jet sources share the common spectral characteristic that they have unusually high disk temperatures which cannot be explained in the framework of the standard optically thick accretion disk in the Schwarzschild metric. On the other hand, the standard accretion disk around the Kerr black hole might explain the observed high disk temperature, as the inner radius of the Kerr disk gets smaller and the disk temperature can be consequently higher. However, we point out that the observable Kerr disk spectra becomes significantly harder than Schwarzschild disk spectra only when the disk is highly inclined. This is because the emission from the innermost part of the accretion disk is Doppler-boosted for an edge-on Kerr disk, while hardly seen for a face-on disk. The Galactic superluminal jet sources are known to be highly inclined systems, thus their energy spectra may be explained with the standard Kerr disk with known black hole masses. For ULXs, on the other hand, the standard Kerr disk model seems implausible, since it is highly unlikely that their accretion disks are preferentially inclined, and, if edge-on Kerr disk model is applied, the black hole mass becomes unreasonably large (> 300 M_solar). Instead, the slim disk (advection dominated optically thick disk) model is likely to explain the observed super-Eddington luminosities, hard energy spectra, and spectral variations of ULXs. We suggest that ULXs are accreting black holes with a few tens of solar mass, which is not unexpected from the standard stellar evolution scenario, and that their X-ray emission is from the slim disk shining at super-Eddington luminosities.

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arXiv2002

Another Interpretation of the Power-law type Spectrum of an Ultraluminous Compact X-ray Source in IC 342

The ultraluminous compact X-ray sources (ULXs) generally show a curving spectrum in the 0.7--10 keV ASCA bandpass, which looks like a high temperature analogue of the disk dominated high/soft state spectra seen in Galactic black hole binaries (BHBs) at high mass accretion rates. Several ULXs have been seen to vary, and to make a transition at their lowest luminosity to a spectrum which looks more like a power law. These have been previously interpreted as the analogue of the power law dominated low/hard state in Galactic BHBs. However, the ULX luminosity at which the transition occurs must be at least 10--50 per cent of the Eddington limit assuming that their highest luminosity phase corresponds to the Eddington limit, while for the Galactic BHBs the high/soft--low/hard transition occurs at a few per cent of the Eddington limit. Here we show that the apparently power law spectrum in a ULX in IC342 can be equally well fit over the ASCA bandpass by a strongly Comptonised optically thick accretion disk with the maximum temperature of ~1 keV. Recent work on the Galactic BHBs has increasingly shown that such components are common at high mass accretion rates, and that this often characterises the very high (or anomalous) state. Thus we propose that the power law type ULX spectra are not to be identified with the low/hard state, but rather represent the Comptonisation dominated very high/anomalous state in the Galactic BHBs.

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arXiv2001

Accretion Disk Spectra of the Ultra-luminous Compact X-ray Sources in Nearby Galaxies and the Super-luminal Jet Sources

The Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies and the Galactic super-luminal jet sources share the common spectral characteristic that they have extremely high disk temperatures which cannot be explained in the framework of the standard accretion disk model in the Schwarzschild metric. We examine several possibilities to solve this ``too-hot'' disk problem. In particular, we have calculated an extreme Kerr disk model to fit the observed spectra. We found that the Kerr disk will become significantly harder compared to the Schwarzschild disk only when the disk is highly inclined. For super-luminal jet sources, which are known to be inclined systems, the Kerr disk model may work if we choose proper values for the black hole angular momentum. For the ULXs, however, the Kerr disk interpretation will be problematic, as it is highly unlikely that their accretion disks are preferentially inclined.

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arXiv2001

Excess Hard X-ray Emission from the Obscured Low Luminosity AGN In the Nearby Galaxy M 51 (NGC 5194)

We observed the nearby galaxy M~51 (NGC 5194) with BeppoSAX. The X-ray properties of the nucleus below 10 keV are almost the same as the ASCA results regarding the hard component and the neutral Fe K$α$ line, but the intensity is about half of the ASCA 1993 data. Beyond this, in the BeppoSAX PDS data, we detected a bright hard X-ray emission component which dominates above 10 keV. The 10 -- 100 keV flux and luminosity of this component are respectively $2\times10^{-11}$ erg s$^{-1}$ cm$^{-2}$ and $2\times10^{41}$ erg s$^{-1}$. These are about 10 times higher than the extrapolation from the soft X-ray band, and similar to the flux observed with Ginga, which found a bright power law component in 2 -- 20 keV band. Considering other wavelength properties and the X-ray luminosity, together with strong neutral Fe K line, the hard X-ray emission most likely arises from a low luminosity active nucleus, which is obscured with a column density of $\sim10^{24}$cm$^{-2}$. This suggests that hidden low luminosity AGNs may well be present in other nearby galaxies. We interpret the discrepancy between Ginga and other X-ray satellites to be due to a large variability of absorption column density toward the line of sight over several years, suggesting that the Compton thick absorption material may be present on a spatial scale of a parsec. Apart from the nucleus, several ultra-luminous off-nuclear X-ray sources detected in M~51 exhibit long-term time variability, suggesting the state transition similar to that observed in Galactic black hole candidates.

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