arXiv subjects

Yasushi Fukazawa

Publications and source records attributed to Yasushi Fukazawa.

At least 127 records · Page 7Linked to original sources

arXiv2016

Two Distinct-Absorption X-Ray Components from Type IIn Supernovae: Evidence for Asphericity in the Circumstellar Medium

We present multi-epoch X-ray spectral observations of three Type IIn supernovae (SNe) 2005kd, 2006jd, and 2010jl, acquired with Chandra, XMM-Newton, Suzaku, and Swift. Previous extensive X-ray studies of SN 2010jl have revealed that X-ray spectra are dominated by thermal emission, which likely arises from a hot plasma heated by a forward shock propagating into a massive circumstellar medium (CSM). Interestingly, an additional soft X-ray component was required to reproduce the spectra at a period of ~1-2 yr after the SN explosion. Although this component is likely associated with the SN, its origin remained an open question. We find a similar, additional soft X-ray component from the other two SNe IIn as well. Given this finding, we present a new interpretation for the origin of this component; it is thermal emission from a forward shock essentially identical to the hard X-ray component, but directly reaches us from a void of the dense CSM. Namely, the hard and soft components are responsible for the heavily- and moderately-absorbed components, respectively. The co-existence of the two components with distinct absorptions as well as the delayed emergence of the moderately-absorbed X-ray component would be evidence for asphericity of the CSM. We show that the X-ray spectral evolution can be qualitatively explained by considering a torus-like geometry for the dense CSM. Based on our X-ray spectral analyses, we estimate the radius of the torus-like CSM to be on the order of ~5 times 10^{16} cm.

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arXiv2016

The Athena X-ray Integral Field Unit (X-IFU)

The X-ray Integral Field Unit (X-IFU) on board the Advanced Telescope for High-ENergy Astrophysics (Athena) will provide spatially resolved high-resolution X-ray spectroscopy from 0.2 to 12 keV, with 5 arc second pixels over a field of view of 5 arc minute equivalent diameter and a spectral resolution of 2.5 eV up to 7 keV. In this paper, we first review the core scientific objectives of Athena, driving the main performance parameters of the X-IFU, namely the spectral resolution, the field of view, the effective area, the count rate capabilities, the instrumental background. We also illustrate the breakthrough potential of the X-IFU for some observatory science goals. Then we briefly describe the X-IFU design as defined at the time of the mission consolidation review concluded in May 2016, and report on its predicted performance. Finally, we discuss some options to improve the instrument performance while not increasing its complexity and resource demands (e.g. count rate capability, spectral resolution). The X-IFU will be provided by an international consortium led by France, The Netherlands and Italy, with further ESA member state contributions from Belgium, Finland, Germany, Poland, Spain, Switzerland and two international partners from the United States and Japan.

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arXiv2016

X-ray variability of GeV gamma-ray emitting radio galaxy NGC 1275

We analyzed Suzaku/XIS data of 2006--2015 observations of a gamma-ray emitting radio galaxy NGC 1275, and brightening of the nucleus in the X-ray band was found in 2013--2015, correlating with GeV Gamma-ray brightening. This is the first evidence of variability with correlation between GeV gamma-ray and X-ray for NGC 1275. We also analyzed Swift/XRT data of NGC 1275, and found that X-ray was flaring by a factor of $\sim$5 in several days in 2006, 2010, and 2013. The X-ray spectrum during the flare was featureless and somewhat steeper with a photon index of $\sim$2 against $\sim$1.7 in the normal state, indicating that a synchrotron component became brighter. A large Xray to GeV gamma-ray flux ratio in the flare could be explained by the shock-in-jet scenario. On the other hand, a long-term gradual brightening of radio, X-ray, and GeV gamma-ray with a larger gamma-ray amplitude could be origin of other than internal shocks, and then we discuss some possibilities.

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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

X-ray and Optical Correlation of Type I Seyfert NGC 3516 Studied with Suzaku and Japanese Ground-Based Telescopes

From 2013 April to 2014 April, we performed an X-ray and optical simultaneous monitoring of the type 1.5 Seyfert galaxy NGC 3516. It employed Suzaku, and 5 Japanese ground-based telescopes, the Pirka, Kiso Schmidt, Nayuta, MITSuME, and the Kanata telescopes. The Suzaku observations were conducted seven times with various intervals ranging from days, weeks, to months, with an exposure of $\sim50$ ksec each. The optical $B$-band observations not only covered those of Suzaku almost simultaneously, but also followed the source as frequently as possible. As a result, NGC 3516 was found in its faint phase with the 2-10 keV flux of $0.21-2.70 \times 10^{-11}$ erg s$^{-1}$ cm$^{-2}$. The 2-45 keV X-ray spectra were composed of a dominant variable hard power-law continuum with a photon index of $\sim1.7$, and a non-relativistic reflection component with a prominent Fe-K$α$ emission line. Producing the $B$-band light curve by differential image photometry, we found that the $B$-band flux changed by $\sim2.7 \times 10^{-11}$ erg s$^{-1}$ cm$^{-2}$, which is comparable to the X-ray variation, and detected a significant flux correlation between the hard power-law component in X-rays and the $B$-band radiation, for the first time in NGC 3516. By examining their correlation, we found that the X-ray flux preceded that of $B$ band by $2.0^{+0.7}_{-0.6}$ days ($1σ$ error). Although this result supports the X-ray reprocessing model, the derived lag is too large to be explained by the standard view which assumes a "lamppost"-type X-ray illuminator located near a standard accretion disk. Our results are better explained by assuming a hot accretion flow and a truncated disk.

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arXiv2016

Fe-K Line Time variability and Ni abundance of Distant Reflector in Seyfert Galaxies

We have performed systematic studies of narrow Fe-K line (6.4 keV) flux variability and Ni-K line intensity for Seyfert galaxies, using {\em Suzaku} and {\em XMM-Newton} archival data. Significant Fe-K line variability of several tens percents was detected for a pair of observations separated by 1000--2000 days (Cen A, IC 4329 A, NGC 3516, and NGC 4151) and 158 days (NGC 3516). These timescales are larger by a factor of 10--100 than the inner radius of torus, consistent with the view that X-ray reflection by torus is a main origin of a narrow Fe-K line. Ni-K line was detected with $>2σ$ level for the Circinus galaxy, Cen A, MRK 3, NGC 4388, and NGC 4151. A mean and variance of Ni-K$α$ to Fe-K$α$ line intensity ratio are 0.066 and 0.026, respectively. Comparing this with the Monte-Carlo simulation of reflection, Ni to Fe abundance ratio is 1.9$\pm$0.8 solar. We discuss the results and possibility of Ni abundance enhancement.

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arXiv2016

X-ray Spectral Model of Reprocessing by Smooth and Clumpy Molecular Tori in Active Galactic Nuclei with the MONACO framework

We construct an X-ray spectral model of reprocessing by a torus in an active galactic nucleus (AGN) with a Monte Carlo simulation framework MONACO. Two torus geometries of smooth and clumpy cases are considered and compared. In order to reproduce a Compton shoulder accurately, MONACO includes not only free electron scattering but also bound electron scattering. Raman and Reyleigh scattering are also treated, and scattering cross sections dependent on chemical states of hydrogen and helium are included. Doppler broadening by turbulence velocity can be implemented. Our model gives consistent results with other available models, such as MYTorus, except for differences due to different physical parameters and assumptions. We studied the dependence on torus parameters for Compton shoulder, and found that a intensity ratio of Compton shoulder to line core mainly depends on the column density, inclination angle, and metal abundance. For instance, an increase of metal abundance makes the Compton shoulder relatively weak. Also, shape of Compton shoulder depends on the column density. Furthermore, these dependences become different between smooth and clumpy cases. Then, we discuss the possibility of ASTRO-H SXS spectroscopy of Compton shoulder in AGN reflection spectra.

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arXiv2015

The first demonstration of the concept of "narrow-FOV Si/CdTe semiconductor Compton camera"

The Soft Gamma-ray Detector (SGD), to be deployed onboard the {\it ASTRO-H} satellite, has been developed to provide the highest sensitivity observations of celestial sources in the energy band of 60-600~keV by employing a detector concept which uses a Compton camera whose field-of-view is restricted by a BGO shield to a few degree (narrow-FOV Compton camera). In this concept, the background from outside the FOV can be heavily suppressed by constraining the incident direction of the gamma ray reconstructed by the Compton camera to be consistent with the narrow FOV. We, for the first time, demonstrate the validity of the concept using background data taken during the thermal vacuum test and the low-temperature environment test of the flight model of SGD on ground. We show that the measured background level is suppressed to less than 10\% by combining the event rejection using the anti-coincidence trigger of the active BGO shield and by using Compton event reconstruction techniques. More than 75\% of the signals from the field-of-view are retained against the background rejection, which clearly demonstrates the improvement of signal-to-noise ratio. The estimated effective area of 22.8~cm$^2$ meets the mission requirement even though not all of the operational parameters of the instrument have been fully optimized yet.

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arXiv2015

The Si/CdTe semiconductor Compton camera of the ASTRO-H Soft Gamma-ray Detector (SGD)

The Soft Gamma-ray Detector (SGD) is one of the instrument payloads onboard ASTRO-H, and will cover a wide energy band (60--600 keV) at a background level 10 times better than instruments currently in orbit. The SGD achieves low background by combining a Compton camera scheme with a narrow field-of-view active shield. The Compton camera in the SGD is realized as a hybrid semiconductor detector system which consists of silicon and cadmium telluride (CdTe) sensors. The design of the SGD Compton camera has been finalized and the final prototype, which has the same configuration as the flight model, has been fabricated for performance evaluation. The Compton camera has overall dimensions of 12 cm x 12 cm x 12 cm, consisting of 32 layers of Si pixel sensors and 8 layers of CdTe pixel sensors surrounded by 2 layers of CdTe pixel sensors. The detection efficiency of the Compton camera reaches about 15% and 3% for 100 keV and 511 keV gamma rays, respectively. The pixel pitch of the Si and CdTe sensors is 3.2 mm, and the signals from all 13312 pixels are processed by 208 ASICs developed for the SGD. Good energy resolution is afforded by semiconductor sensors and low noise ASICs, and the obtained energy resolutions with the prototype Si and CdTe pixel sensors are 1.0--2.0 keV (FWHM) at 60 keV and 1.6--2.5 keV (FWHM) at 122 keV, respectively. This results in good background rejection capability due to better constraints on Compton kinematics. Compton camera energy resolutions achieved with the final prototype are 6.3 keV (FWHM) at 356 keV and 10.5 keV (FWHM) at 662 keV, respectively, which satisfy the instrument requirements for the SGD Compton camera (better than 2%). Moreover, a low intrinsic background has been confirmed by the background measurement with the final prototype.

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arXiv2015

Kanata optical and X-ray monitoring of Gamma-ray emitting Narrow-Line Seyfert 1 and Radio galaxies

Broadband spectrum of AGN consists of multiple components such as jet emission and accretion disk emission. Temporal correlation study is useful to understand emission components and their physical origins. We have performed optical monitoring using Kanata telescope for 4 radio galaxies and 6 radio-loud Narrow-Line Seyfert 1 (RL-NLSy1): 2 gamma-ray-loud RL-NLSy1s, 1H 0323+342 and PMN J0948+0022, and 4 gamma-ray-quiet RL-NLSy1s. From these results, it is suggested that RL-NLSy1s show a disk-dominant phase and a jet-dominant phase in the optical band, but it is not well correlated with brightness.

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arXiv2015

Suzaku X-Ray Monitoring of Gamma-Ray-Emitting Radio Galaxy, NGC 1275

NGC 1275 is a gamma-ray-emitting radio galaxy at the center of the Perseus cluster. Its multi-wavelength spectrum is similar to that of blazers, and thus a jet-origin of gamma-ray emissions is believed. In the optical and X-ray region, NGC 1275 also shows a bright core, but their origin has not been understood, since a disk emission is not ruled out. In fact, NGC 1275 exhibits optical broad emission lines and a X-ray Fe-K line, which are typical for Seyfert galaxies. In our precious studies of NGC 1275 with Suzaku/XIS, no X-ray time variability was found from 2006 to 2011, regardless of moderate gamma-ray variability observed by {it Fermi}-LAT~\cite{Yamazaki}. We have continued monitoring observations of NGC 1275 with Suzaku/XIS. In 2013-2014, MeV/GeV gams-ray flux of NGC 1275 gradually increased and reached the maximum at the beginning of 2014. Correlated with this recent gamma-ray activity, we found that X-ray flux also increased, and this is the first evidence of X-ray variability of NGC 1275. Following these results, we discuss the emission component during the time variability, but we cannot decide the origin of X-ray variability correlating with gamma-ray. Therefore, for future observation, it is important to observe NGC 1275 by using Fermi gamma-ray, XMM-Newton, NuStar, ASTRO-H X-ray, CTA TeV gamma-ray and Kanata optical telescope.

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arXiv2015

An emergence of new polarized emission region in blazar Mrk 421 associated with X-ray flare

We report on long-term multi-wavelength monitoring of blazar Mrk~421 from 2010 to 2011. The source exhibited extreme X-ray flares in 2010. Our research group performed optical photopolarimetric follow-up observations using the Kanata telescope. In 2010, the variability in the X-ray band was significant, while the optical and ultraviolet (UV) flux decreased gradually. Polarization properties also exhibited unique variability in 2010, suggesting the presence of systematic component of polarization and magnetic field alignment for the emergence of a new polarized emission region. In contrast, in 2011 the variability in the X-ray band was smaller, and the variability in the optical and UV bands was larger, than in 2010. To explore the reasons for these differences, spectral fitting analysis was performed via simple synchrotron-self Compton modelling; the results revealed different behaviors in terms of spectral evolution between these periods, suggesting different variability mechanisms between 2010 and 2011. In 2010, the radiation was likely the result of energy injection into the emitting regions with an aligned magnetic field. In contrast, in 2011 the superposition of different emission regions may have contributed to the low degree of observed polarization. It also implies that high-energy electron which were not accelerated to ultra-relativistic velocities were injected in 2011.

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arXiv2014

The ASTRO-H X-ray Astronomy Satellite

The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions developed by the Institute of Space and Astronautical Science (ISAS), with a planned launch in 2015. The ASTRO-H mission is equipped with a suite of sensitive instruments with the highest energy resolution ever achieved at E > 3 keV and a wide energy range spanning four decades in energy from soft X-rays to gamma-rays. The simultaneous broad band pass, coupled with the high spectral resolution of Delta E < 7 eV of the micro-calorimeter, will enable a wide variety of important science themes to be pursued. ASTRO-H is expected to provide breakthrough results in scientific areas as diverse as the large-scale structure of the Universe and its evolution, the behavior of matter in the gravitational strong field regime, the physical conditions in sites of cosmic-ray acceleration, and the distribution of dark matter in galaxy clusters at different redshifts.

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arXiv2014

Suzaku Observations of gamma-Ray Bright Radio Galaxies: Origin of the X-ray Emission and Broad-Band Modelin

We performed a systematic X-ray study of eight nearby gamma-ray bright radio galaxies with Suzaku for understanding the origin of their X-ray emissions. The Suzaku spectra for five of those have been presented previously, while the remaining three (M87, PKS0625-354, and 3C78) are presented here for the first time. Based on the Fe-K line strength, X-ray variability, and X-ray power-law photon indices, and using additional information on the [O III] line emission, we argue for a jet origin of the observed X-ray emission in these three sources. We also analyzed five years of Fermi Large Area Telescope (LAT) GeV gamma-ray data on PKS0625-354 and 3C78 to understand these sources within the blazar picture. We found significant gamma-ray variability in the former object. Overall, we note that the Suzaku spectra for both PKS0625-354 and 3C78 are rather soft, while the LAT spectra are unusually hard when compared with other gamma-ray detected low-power (FR I) radio galaxies. We demonstrate that the constructed broad-band spectral energy distributions of PKS0625-354 and 3C78 are well described by a one-zone synchrotron/synchrotron self-Compton model. The results of the modeling indicate lower bulk Lorentz factors compared to those typically found in other BL Lac objects, but consistent with the values inferred from modeling other LAT-detected FR\,I radio galaxies. Interestingly, the modeling also implies very high peak ($\sim 10^{16}$ Hz) synchrotron frequencies in the two analyzed sources, contrary to previously-suggested scenarios for FR I/BL Lac unification. We discuss the implications of our findings in the context of the FR I/BL Lac unification schemes.

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arXiv2014

PoGOLino: a scintillator-based balloon-borne neutron detector

PoGOLino is a balloon-borne scintillator-based experiment developed to study the largely unexplored high altitude neutron environment at high geomagnetic latitudes. The instrument comprises two detectors that make use of LiCAF, a novel neutron sensitive scintillator, sandwiched by BGO crystals for background reduction. The experiment was launched on March 20th 2013 from the Esrange Space Centre, Northern Sweden (geomagnetic latitude of $65^\circ$), for a three hour flight during which the instrument took data up to an altitude of 30.9 km. The detector design and ground calibration results are presented together with the measurement results from the balloon flight.

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arXiv2014

Variable optical polarization during high state in gamma-ray loud narrow line Seyfert 1 galaxy 1H 0323+342

We present results of optical polarimetric and multi-band photometric observations for gamma-ray loud narrow-line Seyfert 1 galaxy 1H 0323+342. This object has been monitored by 1.5 m Kanata telescope since 2012 September but following a gamma-ray flux enhancement detected by Fermi-LAT on MJD 56483 (2013 July 10) dense follow-up was performed by ten 0.5-2.0 m telescopes in Japan over one week. The 2-year R_C-band light curve showed clear brightening corresponding to the gamma-ray flux increase and then decayed gradually. The high state as a whole lasted for ~20 days, during which we clearly detected optical polarization from this object. The polarization degree (PD) of the source increased from 0-1% in quiescence to ~3% at maximum and then declined to the quiescent level, with the duration of the enhancement of less than 10 days. The moderate PD around the peak allowed us to precisely measure the daily polarization angle (PA). As a result, we found that the daily PAs were almost constant and aligned to the jet axis, suggesting that the magnetic field direction at the emission region is transverse to the jet. This implies either a presence of helical/toroidal magnetic field or transverse magnetic field compressed by shock(s). We also found small-amplitude intra-night variability during the 2-hour continuous exposure on a single night. We discuss these findings based on the turbulent multi-zone model recently advocated by Marscher (2014). Optical to ultraviolet (UV) spectrum showed a rising shape in the higher frequency and the UV magnitude measured by Swift/UVOT was steady even during the flaring state, suggesting that thermal emission from accretion disk is dominant in that band.

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arXiv2014

Detailed Investigation of the Gamma-Ray Emission in the Vicinity of SNR W28 with FERMI-LAT

We present a detailed investigation of the $γ$-ray emission in the vicinity of the supernova remnant (SNR) W28 (G6.4$-$0.1) observed by the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope. We detected significant $γ$-ray emission spatially coincident with TeV sources HESS J1800$-$240A, B, and C, located outside the radio boundary of the SNR. Their spectra in the 2-100 GeV band are consistent with the extrapolation of the power-law spectra of the TeV sources. We also identified a new source of GeV emission, dubbed Source W, which lies outside the boundary of TeV sources and coincides with radio emission from the western part of W28. All of the GeV $γ$-ray sources overlap with molecular clouds in the velocity range from 0 to 20 km s$^{-1}$. Under the assumption that the $γ$-ray emission towards HESS J1800-240A, B, and C comes from $π^0$ decay due to the interaction between the molecular clouds and cosmic rays (CRs) escaping from W28, they can be naturally explained by a single model in which the CR diffusion coefficient is smaller than the theoretical expectation in the interstellar space. The total energy of the CRs escaping from W28 is constrained through the same modeling to be larger than $\sim$ 2 $\times$ 10$^{49}$ erg. The emission from Source W can also be explained with the same CR escape scenario.

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arXiv2014

Long-Lasting X-Ray Emission from Type IIb Supernova 2011dh and Mass-Loss History of The Yellow Supergiant Progenitor

Type IIb Supernova (SN) 2011dh, with conclusive detection of an unprecedented Yellow Supergiant (YSG) progenitor, provides an excellent opportunity to deepen our understanding on the massive star evolution in the final centuries toward the SN explosion. In this paper, we report on detection and analyses of thermal X-ray emission from SN IIb 2011dh at ~500 days after the explosion on Chandra archival data, providing a solidly derived mass loss rate of an YSG progenitor for the first time. We find that the circumstellar media (CSM) should be dense, more than that expected from a Wolf-Rayet (WR) star by one order of magnitude. The emission is powered by a reverse shock penetrating into an outer envelope, fully consistent with the YSG progenitor but not with a WR progenitor. The density distribution at the outermost ejecta is much steeper than that expected from a compact WR star, and this finding must be taken into account in modeling the early UV/optical emission from SNe IIb. The derived mass loss rate is 3 x 10^{-6} Msun/year for the mass loss velocity of ~20 km/s in the final ~1,300 years before the explosion. The derived mass loss properties are largely consistent with the standard wind mass loss expected for a giant star. This is not sufficient to be a main driver to expel nearly all the hydrogen envelope. Therefore, the binary interaction, with a huge mass transfer having taken place at >1,300 years before the explosion, is a likely scenario to produce the YSG progenitor.

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