arXiv subjects

Hajime Inoue

Publications and source records attributed to Hajime Inoue.

At least 37 records · Page 2Linked to original sources

arXiv2017

Hitomi Observation of Radio Galaxy NGC 1275: The First X-ray Microcalorimeter Spectroscopy of Fe-Kα Line Emission from an Active Galactic Nucleus

The origin of the narrow Fe-Kα fluorescence line at 6.4 keV from active galactic nuclei has long been under debate; some of the possible sites are the outer accretion disk, the broad line region, a molecular torus, or interstellar/intracluster media. In February-March 2016, we performed the first X-ray microcalorimeter spectroscopy with the Soft X-ray Spectrometer (SXS) onboard the Hitomi satellite of the Fanaroff-Riley type I radio galaxy NGC 1275 at the center of the Perseus cluster of galaxies. With the high energy resolution of ~5 eV at 6 keV achieved by Hitomi/SXS, we detected the Fe-Kα line with ~5.4 σ significance. The velocity width is constrained to be 500-1600 km s$^{-1}$ (FWHM for Gaussian models) at 90% confidence. The SXS also constrains the continuum level from the NGC 1275 nucleus up to ~20 keV, giving an equivalent width ~20 eV of the 6.4 keV line. Because the velocity width is narrower than that of broad Hα line of ~2750 km s$^{-1}$, we can exclude a large contribution to the line flux from the accretion disk and the broad line region. Furthermore, we performed pixel map analyses on the Hitomi/SXS data and image analyses on the Chandra archival data, and revealed that the Fe-Kα line comes from a region within ~1.6 kpc from the NGC 1275 core, where an active galactic nucleus emission dominates, rather than that from intracluster media. Therefore, we suggest that the source of the Fe-Kα line from NGC 1275 is likely a low-covering fraction molecular torus or a rotating molecular disk which probably extends from a pc to hundreds pc scale in the active galactic nucleus system.

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arXiv2017

Atmospheric gas dynamics in the Perseus cluster observed with Hitomi

Extending the earlier measurements reported in Hitomi collaboration (2016, Nature, 535, 117), we examine the atmospheric gas motions within the central 100~kpc of the Perseus cluster using observations obtained with the Hitomi satellite. After correcting for the point spread function of the telescope and using optically thin emission lines, we find that the line-of-sight velocity dispersion of the hot gas is remarkably low and mostly uniform. The velocity dispersion reaches maxima of approximately 200~km~s$^{-1}$ toward the central active galactic nucleus (AGN) and toward the AGN inflated north-western `ghost' bubble. Elsewhere within the observed region, the velocity dispersion appears constant around 100~km~s$^{-1}$. We also detect a velocity gradient with a 100~km~s$^{-1}$ amplitude across the cluster core, consistent with large-scale sloshing of the core gas. If the observed gas motions are isotropic, the kinetic pressure support is less than 10\% of the thermal pressure support in the cluster core. The well-resolved optically thin emission lines have Gaussian shapes, indicating that the turbulent driving scale is likely below 100~kpc, which is consistent with the size of the AGN jet inflated bubbles. We also report the first measurement of the ion temperature in the intracluster medium, which we find to be consistent with the electron temperature. In addition, we present a new measurement of the redshift to the brightest cluster galaxy NGC~1275.

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arXiv2017

Measurements of resonant scattering in the Perseus cluster core with Hitomi SXS

Thanks to its high spectral resolution (~5 eV at 6 keV), the Soft X-ray Spectrometer (SXS) on board Hitomi enables us to measure the detailed structure of spatially resolved emission lines from highly ionized ions in galaxy clusters for the first time. In this series of papers, using the SXS we have measured the velocities of gas motions, metallicities and the multi-temperature structure of the gas in the core of the Perseus cluster. Here, we show that when inferring physical properties from line emissivities in systems like Perseus, the resonant scattering (RS) effect should be taken into account. In the Hitomi waveband, RS mostly affects the FeXXV He$α$ line ($w$) - the strongest line in the spectrum. The flux measured by Hitomi in this line is suppressed by a factor ~1.3 in the inner ~30 kpc, compared to predictions for an optically thin plasma; the suppression decreases with the distance from the center. The $w$ line also appears slightly broader than other lines from the same ion. The observed distortions of the $w$ line flux, shape and distance dependence are all consistent with the expected effect of the resonant scattering in the Perseus core. By measuring the ratio of fluxes in optically thick ($w$) and thin (FeXXV forbidden, He$β$, Ly$α$) lines, and comparing these ratios with predictions from Monte Carlo radiative transfer simulations, the velocities of gas motions have been obtained. The results are consistent with the direct measurements of gas velocities from line broadening described elsewhere in this series, although the systematic and statistical uncertainties remain significant. Further improvements in the predictions of line emissivities in plasma models, and deeper observations with future X-ray missions will enable RS measurements to provide powerful constraints on the amplitude and anisotropy of clusters gas motions.

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arXiv2017

Hitomi X-ray studies of Giant Radio Pulses from the Crab pulsar

To search for giant X-ray pulses correlated with the giant radio pulses (GRPs) from the Crab pulsar, we performed a simultaneous observation of the Crab pulsar with the X-ray satellite Hitomi in the 2 -- 300 keV band and the Kashima NICT radio observatory in the 1.4 -- 1.7 GHz band with a net exposure of about 2 ks on 25 March 2016, just before the loss of the Hitomi mission.The timing performance of the Hitomi instruments was confirmed to meet the timing requirement and about 1,000 and 100 GRPs were simultaneously observed at the main and inter-pulse phases, respectively, and we found no apparent correlation between the giant radio pulses and the X-ray emission in either the main or inter-pulse phases.All variations are within the 2 sigma fluctuations of the X-ray fluxes at the pulse peaks, and the 3 sigma upper limits of variations of main- or inter- pulse GRPs are 22\% or 80\% of the peak flux in a 0.20 phase width, respectively, in the 2 -- 300 keV band.The values become 25\% or 110\% for main or inter-pulse GRPs, respectively, when the phase width is restricted into the 0.03 phase.Among the upper limits from the Hitomi satellite, those in the 4.5-10 keV and the 70-300 keV are obtained for the first time, and those in other bands are consistent with previous reports.Numerically, the upper limits of main- and inter-pulse GRPs in the 0.20 phase width are about (2.4 and 9.3) $\times 10^{-11}$ erg cm$^{-2}$, respectively. No significant variability in pulse profiles implies that the GRPs originated from a local place within the magnetosphere and the number of photon-emitting particles temporally increases.However, the results do not statistically rule out variations correlated with the GRPs, because the possible X-ray enhancement may appear due to a $>0.02$\% brightening of the pulse-peak flux under such conditions.

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arXiv2017

Search for Thermal X-ray Features from the Crab nebula with Hitomi Soft X-ray Spectrometer

The Crab nebula originated from a core-collapse supernova (SN) explosion observed in 1054 A.D. When viewed as a supernova remnant (SNR), it has an anomalously low observed ejecta mass and kinetic energy for an Fe-core collapse SN. Intensive searches were made for a massive shell that solves this discrepancy, but none has been detected. An alternative idea is that the SN1054 is an electron-capture (EC) explosion with a lower explosion energy by an order of magnitude than Fe-core collapse SNe. In the X-rays, imaging searches were performed for the plasma emission from the shell in the Crab outskirts to set a stringent upper limit to the X-ray emitting mass. However, the extreme brightness of the source hampers access to its vicinity. We thus employed spectroscopic technique using the X-ray micro-calorimeter onboard the Hitomi satellite. By exploiting its superb energy resolution, we set an upper limit for emission or absorption features from yet undetected thermal plasma in the 2-12 keV range. We also re-evaluated the existing Chandra and XMM-Newton data. By assembling these results, a new upper limit was obtained for the X-ray plasma mass of <~ 1Mo for a wide range of assumed shell radius, size, and plasma temperature both in and out of the collisional equilibrium. To compare with the observation, we further performed hydrodynamic simulations of the Crab SNR for two SN models (Fe-core versus EC) under two SN environments (uniform ISM versus progenitor wind). We found that the observed mass limit can be compatible with both SN models if the SN environment has a low density of <~ 0.03 cm-3 (Fe core) or <~ 0.1 cm-3 (EC) for the uniform density, or a progenitor wind density somewhat less than that provided by a mass loss rate of 10-5 Mo yr-1 at 20 km s-1 for the wind environment.

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arXiv2017

Hitomi constraints on the 3.5 keV line in the Perseus galaxy cluster

High-resolution X-ray spectroscopy with Hitomi was expected to resolve the origin of the faint unidentified E=3.5 keV emission line reported in several low-resolution studies of various massive systems, such as galaxies and clusters, including the Perseus cluster. We have analyzed the Hitomi first-light observation of the Perseus cluster. The emission line expected for Perseus based on the XMM-Newton signal from the large cluster sample under the dark matter decay scenario is too faint to be detectable in the Hitomi data. However, the previously reported 3.5 keV flux from Perseus was anomalously high compared to the sample-based prediction. We find no unidentified line at the reported high flux level. Taking into account the XMM measurement uncertainties for this region, the inconsistency with Hitomi is at a 99% significance for a broad dark-matter line and at 99.7% for a narrow line from the gas. We do not find anomalously high fluxes of the nearby faint K line or the Ar satellite line that were proposed as explanations for the earlier 3.5 keV detections. We do find a hint of a broad excess near the energies of high-n transitions of Sxvi (E=3.44 keV rest-frame) -- a possible signature of charge exchange in the molecular nebula and another proposed explanation for the unidentified line. While its energy is consistent with XMM pn detections, it is unlikely to explain the MOS signal. A confirmation of this interesting feature has to wait for a more sensitive observation with a future calorimeter experiment.

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arXiv2016

The Quiescent Intracluster Medium in the Core of the Perseus Cluster

Clusters of galaxies are the most massive gravitationally-bound objects in the Universe and are still forming. They are thus important probes of cosmological parameters and a host of astrophysical processes. Knowledge of the dynamics of the pervasive hot gas, which dominates in mass over stars in a cluster, is a crucial missing ingredient. It can enable new insights into mechanical energy injection by the central supermassive black hole and the use of hydrostatic equilibrium for the determination of cluster masses. X-rays from the core of the Perseus cluster are emitted by the 50 million K diffuse hot plasma filling its gravitational potential well. The Active Galactic Nucleus of the central galaxy NGC1275 is pumping jetted energy into the surrounding intracluster medium, creating buoyant bubbles filled with relativistic plasma. These likely induce motions in the intracluster medium and heat the inner gas preventing runaway radiative cooling; a process known as Active Galactic Nucleus Feedback. Here we report on Hitomi X-ray observations of the Perseus cluster core, which reveal a remarkably quiescent atmosphere where the gas has a line-of-sight velocity dispersion of 164+/-10 km/s in a region 30-60 kpc from the central nucleus. A gradient in the line-of-sight velocity of 150+/-70 km/s is found across the 60 kpc image of the cluster core. Turbulent pressure support in the gas is 4% or less of the thermodynamic pressure, with large scale shear at most doubling that estimate. We infer that total cluster masses determined from hydrostatic equilibrium in the central regions need little correction for turbulent pressure.

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arXiv2016

Origin of the Broad Iron Line Feature and the Soft X-ray Variation in Seyfert Galaxies

Many Seyfert galaxies are known to exhibit significant X-ray spectral variations and seemingly broad iron K-emission line features. In this paper, we show that the "variable partial covering model", which has been successfully proposed for MCG-6.30-15 (Miyakawa, Ebisawa & Inoue 2012) and 1H0707.495 (Mizumoto, Ebisawa & Sameshima 2014), can also explain the spectral variations in 2-10 keV as well as the broad iron line features in 20 other Seyfert galaxies observed with Suzaku. In this model, the absorbed spectral component through the optically-thick absorbing clouds has a significant iron K-edge, which primarily accounts for the observed seemingly broad iron line feature. Fluctuation of the absorbing clouds in the line of sight of the extended X-ray source results in variation of the partial covering fraction, which causes an anti-correlation between the direct (not covered) spectral component and the absorbed (covered) spectral component below ~10 keV. Observed spectral variation in 2-10 keV in a timescale of less than ~day is primarily explained by such variations of the partial covering fraction, while the intrinsic soft X-ray luminosity is hardly variable.

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arXiv2015

Origin of the X-ray Broad Iron Spectral Feature in GRS 1915+105

The X-ray spectrum of GRS 1915+105 is known to have a ``broad iron spectral feature'' in the spectral hard state. Similar spectral features are often observed in Active Galactic Nuclei (AGNs) and other black-hole binaries (BHBs), and several models have been proposed for explaining it. In order to distinguish spectral models, time variation provides an important key. In AGNs, variation amplitude has been found to drop significantly at the iron K-energy band at timescales of ~10 ks. If spectral variations of black-holes are normalized by their masses, the spectral variations of BHBs at timescales of sub-seconds should exhibit similar characteristics to those of AGNs. In this paper, we investigated spectral variations of GRS 1915+105 at timescales down to ~10 ms. This was made possible for the first time with the Suzaku XIS Parallel-sum clocking (P-sum) mode, which has the CCD energy-resolution as well as a time-resolution of 7.8 ms. Consequently, we found that the variation amplitude of GRS 1915+105 does not drop at the iron K-energy band at any timescales from 0.06 s to 63000 s, and that the entire X-ray flux and the iron feature are independently variable at timescales of hours. These are naturally understood in the framework of the ``partial covering'' model, in which variation timescales of the continuum flux and partial absorbers are independent. The difference of energy dependence of the variation amplitude between AGN and BHB is presumably due to different mechanisms of the outflow winds, i.e., the partial absorbers are due to UV-line driven winds (AGN) or thermally-driven winds (BHB).

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arXiv2015

Variability of the X-ray Broad Iron Spectral Features in Active Galactic Nuclei and Black-hole Binaries

The "broad iron spectral features" are often seen in X-ray spectra of Active Galactic Nuclei (AGN) and black-hole binaries (BHB). These features may be explained either by the "relativistic disc reflection" scenario or the "partial covering" scenario: It is hardly possible to determine which model is valid from time-averaged spectral analysis. Thus, X-ray spectral variability has been investigated to constrain spectral models. To that end, it is crucial to study iron structure of BHBs in detail at short time-scales, which is, for the first time, made possible with the Parallel-sum clocking (P-sum) mode of XIS detectors on board Suzaku. This observational mode has a time-resolution of 7.8~ms as well as a CCD energy-resolution. We have carried out systematic calibration of the P-sum mode, and investigated spectral variability of the BHB GRS 1915+105. Consequently, we found that the spectral variability of GRS 1915+105 does not show iron features at sub-seconds. This is totally different from variability of AGN such as 1H0707--495, where the variation amplitude significantly drops at the iron K-energy band. This difference can be naturally explained in the framework of the "partial covering" scenario.

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arXiv2012

A Variable Partial Covering Model for the Seyfert 1 Galaxy MCG-6-30-15

We propose a simple spectral model for the Seyfert 1 Galaxy MCG-6-30-15 that can explain most of the 1 - 40 keV spectral variation by change of the partial covering fraction, similar to the one proposed by Miller et al. (2008). Our spectral model is composed of three continuum components; (1) a direct power-law component, (2) a heavily absorbed power-law component by mildly ionized intervening matter, and (3) a cold disk reflection component far from the black hole with moderate solid-angle (Ω/2π \approx 0.3) accompanying a narrow fluorescent iron line. The first two components are affected by the surrounding highly ionized thin absorber with N_H \approx 10^{23.4}cm-2 and log ξ \approx 3.4. The heavy absorber in the second component is fragmented into many clouds, each of which is composed of radial zones with different ionization states and column densities, the main body (N_H \approx 10^24.2cm-2, log ξ \approx 1.6), the envelope (N_H \approx 10^22.1cm-2, log ξ \approx 1.9) and presumably a completely opaque core. These parameters of the ionized absorbers, as well as the intrinsic spectral shape of the X-ray source, are unchanged at all. The central X-ray source is moderately extended, and its luminosity is not significantly variable. The observed flux and spectral variations are mostly explained by variation of the geometrical partial covering fraction of the central source from 0 (uncovered) to \sim0.63 by the intervening ionized clouds in the line of sight. The ionized iron K-edge of the heavily absorbed component explains most of the seemingly broad line-like feature, a well-known spectral characteristic of MCG-6-30-15. The direct component and the absorbed component anti-correlate, cancelling their variations each other, so that the fractional spectral variation becomes the minimum at the iron energy band; another observational characteristic of MCG-6-30-15 is thus explained.

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arXiv2009

Spectral Variation of the Seyfert 1 Galaxy MCG-6-30-15 observed with Suzaku

We have investigated spectral variation of the Seyfert 1 galaxy MCG-6-30-15 observed with Suzaku in January 2006 for three separate periods spreading over fourteen days. We found that the time-averaged continuum energy spectrum between 1 keV and 40 keV can be approximated with a spectral model composed of the direct power-law component, its reflection component, two warm absorbers with different ionization states, and neutral absorption. We have taken two approaches to study its spectral variation at various timescales: The first approach is to make intensity-sliced spectra and study correlation between the intensity and spectral shape. The second approach is to study spectral changes between the intervals when the source flux is above ("bright state") and below ("faint state") the average for fixed time-intervals. In both approaches, we found a clear correlation between the intensity in the 6 -- 10 keV band and the spectral ratio of 0.5 -- 3.0 keV/6.0-- 10 keV. Such a spectral variation requires change of the apparent slope of the direct component, whereas the shape and intensity of the reflection component being invariable. The observed apparent spectral change is explained by variation of the ionization degree of one of the two warm absorbers due to intrinsic source luminosity variation. Current results suggest that the warm absorber has a critical role to explain the observed continuum spectral shape and variation of MCG-6-30-15, which is essential to constrain parameters of the putatively broadened iron line emission feature.

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arXiv2009

Origin of Thermal and Non-Thermal Hard X-ray Emission from the Galactic Center

We analyse new results of Chandra and Suzaku which found a flux of hard X-ray emission from the compact region around Sgr A$^\ast$ (r ~ 100 pc). We suppose that this emission is generated by accretion processes onto the central supermassive blackhole when an unbounded part of captured stars obtains an additional momentum. As a result a flux of subrelativistic protons is generated near the Galactic center which heats the background plasma up to temperatures about 6-10 keV and produces by inverse bremsstrahlung a flux of non-thermal X-ray emission in the energy range above 10 keV.

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arXiv2009

Particle Propagation in the Galactic Center and Spatial Distribution of Non-Thermal X-rays

We showed that if the non-thermal emission from the Galactic center in the range 14-40 keV is due to inverse bremsstrahlung emission of subrelativistic protons, their interactions with hot and cold fractions of the interstellar medium are equally important. Our estimation show that about 30% of the total non-thermal flux from the GC in the range 14-40 keV is generated in regions of cold gas while the rest is produced by proton interaction with hot plasma. From the spatial distribution of 6.7 keV iron line we concluded the spatial distribution of hot plasma is strongly non-uniform that should be taken into account in analysis of protons propagation in the GC. From the Suzaku data we got independent estimates for the diffusion coefficient of subrelativistic protons in the GC, which was in the range $ 10^{26} - 10^{27}$ cm$^2$s$^{-1}$

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arXiv2009

Line and Continuum Emission from the Galactic Center. III. Origin of 6.4 keV Line Emission from Molecular Clouds in the Galactic Center

We analyze the 6.4 keV line and continuum emission from the molecular cloud Sgr B2 and the source HESS J1745-303, which is supposed to be a complex of molecular gas. From the HESS results it follows that Sgr A is a source of high energy protons, which penetrate into molecular clouds producing there a TeV gamma-ray flux. We present arguments that Sgr A may also produce a flux of subrelativistic protons which generate the 6.4 keV line and bremsstrahlung continuum emission from the clouds.

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arXiv2008

Luminosity dependence of the electron temperature in the bright hard state of the black hole candidate GX 339--4

We have analyzed 200 Rossi X-ray Timing Explorer observations of the black hole candidate GX 339--4, all from the bright hard state periods between 1996 and 2005. Purpose of our study is to investigate the radiation mechanisms in the hard state of GX 339--4. The broadband 3--200 keV spectra were successfully modeled by a simple analytic model, power--law with an exponential cut-off modified with a smeared edge. The obtained energy cut-off ($E_{\rm{cut}}$) was distributed over 50--200 keV, and the photon index over 1.4--1.7. We found a clear anti-correlation ($E_{\rm{cut}} \propto L^{-0.70\pm0.06}$) between the X-ray luminosity ($L$) in 2--200 keV and $E_{\rm{cut}}$, when $L$ is larger than $7 \times 10^{37}$ erg s$^{-1}$ (assuming a distance of 8 kpc), while $E_{\rm{cut}}$ is roughly constant at around 200 keV when $L$ is smaller than $7 \times 10^{37}$ erg s$^{-1}$. This anti-correlation remained unchanged by adopting a more physical thermal Comptonization model, which resulted in the anti-correlation that can be expressed as $kT_{\rm{e}} \propto L^{-0.24\pm0.06}$. These anti-correlations can be quantitatively explained by a picture in which the energy-flow rate from protons to electrons balances with the inverse Compton cooling.

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arXiv2007

Clear anti-correlation between luminosity and high energy cutoff in the low/hard state of the black hole candidate GX339-4

We have analyzed the 171 RXTE data sets of the black hole candidate GX 339-4 in the low/hard state during its 1996--2005 outburst. All the broadband spectra were successfully modeled by a simple analytic model, power-law with an exponential cutoff. The obtained energy cutoff($E_{\rm{cut}}$) is distributed over 50--300 keV, and the photon index over 1.4-1.6. We found a clear correlation ($E_{\rm{cut}}$ is proportional to $L^{-0.75 \pm 0.04}$) between luminosity in 2-200 keV (L) and $E_{\rm{cut}}$ when L is larger than 5$\times 10^{37}$ erg $s^{-1}$, while $E_{\rm{cut}}$ is almost constant at 200 keV when L is smaller than 5$\times 10^{37}$ erg $s^{-1}$. This anti-correlation is unchanged by adopting the more physical model of thermal Comptonization by Sunyaev and Titarchuk, although a slightly different relation is obtained as the electron temperature is proportional to $L^{-0.23 \pm 0.02}$. These anti-correlations are qualitatively explained by a picture where the energy flow rate from protons to electrons balance with cooling due to inverse Compton scattering.

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arXiv2006

Revealing the High Energy Emission from the Obscured Seyfert Galaxy MCG -5-23-16 with Suzaku

We report on a 100 ks Suzaku observation of the bright, nearby (z=0.008486) Seyfert 1.9 galaxy MCG -5-23-16. The broad-band (0.4-100 keV) X-ray spectrum allows us to determine the nature of the high energy emission with little ambiguity. The X-ray continuum consists of a cutoff power-law of photon index $Γ=1.9$, absorbed through Compton-thin matter of column density $N_{\rm H}=1.6\times10^{22}$ cm$^{-2}$. A soft excess is observed below 1 keV and is likely a combination of emission from scattered continuum photons and distant photoionized gas. The iron K line profile is complex, showing narrow neutral iron K$α$ and K$β$ emission, as well as a broad line which can be modeled by a moderately inclined accretion disk. The line profile shows either the disk is truncated at a few tens of gravitational radii, or the disk emissivity profile is relatively flat. A strong Compton reflection component is detected above 10 keV, which is best modeled by a combination of reflection off distant matter and the accretion disk. The reflection component does not appear to vary. The overall picture is that this Seyfert 1.9 galaxy is viewed at moderate (50 degrees) inclination through Compton-thin matter at the edge of a Compton-thick torus covering $2π$ steradians, consistent with unified models.

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