arXiv subjects

N. Isobe

Publications and source records attributed to N. Isobe.

At least 19 recordsLinked to original sources

arXiv2018

SPICA - a large cryogenic infrared space telescope Unveiling the obscured Universe

Measurements in the infrared wavelength domain allow us to assess directly the physical state and energy balance of cool matter in space, thus enabling the detailed study of the various processes that govern the formation and early evolution of stars and planetary systems in galaxies over cosmic time. Previous infrared missions, from IRAS to Herschel, have revealed a great deal about the obscured Universe, but sensitivity has been limited because up to now it has not been possible to fly a telescope that is both large and cold. SPICA is a mission concept aimed at taking the next step in mid- and far-infrared observational capability by combining a large and cold telescope with instruments employing state-of-the-art ultra-sensitive detectors. The mission concept foresees a 2.5-meter diameter telescope cooled to below 8 K. With cooling provided by mechanical coolers instead of depending on a limited cryogen supply, the mission lifetime can extend significantly beyond the required three years. SPICA offers instrumentation with spectral resolving powers ranging from R ~50 through 11000 in the 17-230 $\mu$m domain as well as R~28.000 spectroscopy between 12 and 18 $\mu$m. Additionally SPICA will provide efficient 30-37 $\mu$m broad band mapping, and polarimetric imaging in the 100-350 $\mu$m range. SPICA will provide unprecedented spectroscopic sensitivity of ~5 x $10^{-20}$ W/m$^2$ (5$\sigma$/1hr) - at least two orders of magnitude improvement over what has been attained to date. With this exceptional leap in performance, new domains in infrared astronomy will become accessible, allowing us, for example, to unravel definitively galaxy evolution and metal production over cosmic time, to study dust formation and evolution from very early epochs onwards, and to trace the formation history of planetary systems.

astro-ph.IM↗

arXiv2017

The Near-Infrared CO Absorption Band as a Probe to the Innermost Part of an AGN Obscuring Material

We performed a systematic analysis of the 4.67 $\mu$m CO ro-vibrational absorption band toward nearby active galactic nuclei (AGNs) and analyzed the absorption profiles of ten nearby galaxies collected from the AKARI and Spitzer spectroscopic observations that show the CO absorption feature by fitting a plane-parallel local thermal equilibrium gas model. We found that CO gas is warm (200--500 K) and has a large column density ($N_\mathrm{H}\gtrsim10^{23}~\mathrm{cm^{-2}}$). The heating of the gas is not explicable by either UV heating or shock heating because these processes cannot represent the large column densities of the warm gas. Instead, X-ray photons from the nuclei, which can produce large columns of warm gas with up to $N_\mathrm{H}\sim10^{24}~\mathrm{cm^{-2}}$, are the most convincing power source. The hydrogen column density estimated from the CO band is smaller than that inferred from X-ray observations. These results indicate that the region probed by the near-infrared CO absorption is in the vicinity of the nuclei and is located outside the X-ray emitting region. Furthermore, the covering factors close to unity required by the observed deep absorption profiles suggest that the probed region is close to the continuum source, which can be designated as the inner rim of the obscuring material around the AGN.

astro-ph.GA↗

arXiv2017

Development of a 32-channel ASIC for an X-ray APD Detector onboard the ISS

We report on the design and performance of a mixed-signal application specific integrated circuit (ASIC) dedicated to avalanche photodiodes (APDs) in order to detect hard X-ray emissions in a wide energy band onboard the International Space Station. To realize wide-band detection from 20 keV to 1 MeV, we use Ce:GAGG scintillators, each coupled to an APD, with low-noise front-end electronics capable of achieving a minimum energy detection threshold of 20 keV. The developed ASIC has the ability to read out 32-channel APD signals using 0.35 $\mu$m CMOS technology, and an analog amplifier at the input stage is designed to suppress the capacitive noise primarily arising from the large detector capacitance of the APDs. The ASIC achieves a performance of 2099 e$^{-}$ + 1.5 e$^{-}$/pF at root mean square (RMS) with a wide 300 fC dynamic range. Coupling a reverse-type APD with a Ce:GAGG scintillator, we obtain an energy resolution of 6.7% (FWHM) at 662 keV and a minimum detectable energy of 20 keV at room temperature (20 $^{\circ}$C). Furthermore, we examine the radiation tolerance for space applications by using a 90 MeV proton beam, confirming that the ASIC is free of single-event effects and can operate properly without serious degradation in analog and digital processing.

astro-ph.IM↗

arXiv2017

Feedback and feeding in the context of galaxy evolution with SPICA: direct characterization of molecular outflows and inflows

A far-infrared observatory such as the {\it SPace Infrared telescope for Cosmology and Astrophysics} ({\it SPICA}), with its unprecedented spectroscopic sensitivity, would unveil the role of feedback in galaxy evolution during the last $\sim10$ Gyr of the Universe ($z=1.5-2$), through the use of far- and mid-infrared molecular and ionic fine structure lines that trace outflowing and infalling gas. Outflowing gas is identified in the far-infrared through P-Cygni line shapes and absorption blueshifted wings in molecular lines with high dipolar moments, and through emission line wings of fine-structure lines of ionized gas. We quantify the detectability of galaxy-scale massive molecular and ionized outflows as a function of redshift in AGN-dominated, starburst-dominated, and main-sequence galaxies, explore the detectability of metal-rich inflows in the local Universe, and describe the most significant synergies with other current and future observatories that will measure feedback in galaxies via complementary tracers at other wavelengths.

astro-ph.GA↗

arXiv2017

Galaxy evolution studies with the SPace IR telescope for Cosmology and Astrophysics (SPICA): the power of IR spectroscopy

IR spectroscopy in the range 12-230 micron with the SPace IR telescope for Cosmology and Astrophysics (SPICA) will reveal the physical processes that govern the formation and evolution of galaxies and black holes through cosmic time, bridging the gap between the James Webb Space Telescope (JWST) and the new generation of Extremely Large Telescopes (ELTs) at shorter wavelengths and the Atacama Large Millimeter Array (ALMA) at longer wavelengths. SPICA, with its 2.5-m telescope actively-cooled to below 8K, will obtain the first spectroscopic determination, in the mid-IR rest-frame, of both the star-formation rate and black hole accretion rate histories of galaxies, reaching lookback times of 12 Gyr, for large statistically significant samples. Densities, temperatures, radiation fields and gas-phase metallicities will be measured in dust-obscured galaxies and active galactic nuclei (AGN), sampling a large range in mass and luminosity, from faint local dwarf galaxies to luminous quasars in the distant Universe. AGN and starburst feedback and feeding mechanisms in distant galaxies will be uncovered through detailed measurements of molecular and atomic line profiles. SPICA's large-area deep spectrophotometric surveys will provide mid-IR spectra and continuum fluxes for unbiased samples of tens of thousands of galaxies, out to redshifts of z~6. Furthermore, SPICA spectroscopy will uncover the most luminous galaxies in the first few hundred million years of the Universe, through their characteristic dust and molecular hydrogen features.

astro-ph.GA↗

arXiv2016

Revised Wavelength and Spectral Response Calibrations for AKARI Near-Infrared Grism Spectroscopy: Cryogenic Phase

We perform revised spectral calibrations for the AKARI near-infrared grism to quantitatively correct for the effect of the wavelength-dependent refractive index. The near-infrared grism covering the wavelength range of 2.5--5.0 micron with a spectral resolving power of 120 at 3.6 micron, is found to be contaminated by second-order light at wavelengths longer than 4.9 micron which is especially serious for red objects. First, we present the wavelength calibration considering the refractive index of the grism as a function of the wavelength for the first time. We find that the previous solution is positively shifted by up to 0.01 micron compared with the revised wavelengths at 2.5--5.0 micron. In addition, we demonstrate that second-order contamination occurs even with a perfect order-sorting filter owing to the wavelength dependence of the refractive index. Second, the spectral responses of the system from the first- and second-order light are simultaneously obtained from two types of standard objects with different colors. The response from the second-order light suggests leakage of the order-sorting filter below 2.5 micron. The relations between the output of the detector and the intensities of the first- and second-order light are formalized by a matrix equation that combines the two orders. The removal of the contaminating second-order light can be achieved by solving the matrix equation. The new calibration extends the available spectral coverage of the grism mode from 4.9 micron up to 5.0 micron. The revision can be used to study spectral features falling in these extended wavelengths, e.g., the carbon monoxide fundamental ro-vibrational absorption within nearby active galactic nuclei.

astro-ph.IM↗

arXiv2015

Probing Precise Location of Radio Core in TeV Blazar Mrk 501 with VERA at 43 GHz

We investigate the position of the radio core in a blazar by multi-epoch astrometric observations at 43 GHz. Using the VLBI Exploration of Radio Astrometry (VERA), we have conducted four adjacent observations in February 2011 and another four in October 2011, and succeeded in measuring the position of the radio core in the TeV blazar Mrk 501 relative to a distant compact quasar NRAO 512. During our observations, we find that (1) there is no positional change within ~0.2 mas or ~2.0 pc de-projected with $\pm1σ$ error for the weighted-mean phase-referenced positions of Mrk 501 core relative to NRAO 512 over four adjacent days, and (2) there is an indication of position change for 3C 345 core relative to NRAO 512. By applying our results to the standard internal shock model for blazars, we constrain the bulk Lorenz factors of the ejecta.

astro-ph.HE↗

arXiv2012

Possible Detection of Apparent Superluminal inward motion in Markarian 421 after the Giant X-ray flare in February, 2010

We report on the VLBI follow-up observations using the Japanese VLBI Network (JVN) array at 22 GHz for the largest X-ray flare of TeV blazar Mrk 421 that occurred in mid-February, 2010. The total of five epochs of observations were performed at intervals of about 20 days between March 7 and May 31, 2010. No new-born component associated with the flare was seen directly in the total intensity images obtained by our multi-epoch VLBI observations. However, one jet component located at ~1 mas north-west from the core was able to be identified, and its proper motion can be measured as -1.66+/-0.46 mas yr^-1, which corresponds to an apparent velocity of -3.48+/-0.97 c. Here, this negative velocity indicates that the jet component was apparently moving toward the core. As the most plausible explanation, we discuss that the apparent negative velocity was possibly caused by the ejection of a new component, which could not be resolved with our observations. In this case, the obtained Doppler factor of the new component is around 10 to 20, which is consistent with the ones typically estimated by model fittings of spectral energy distribution for this source.

astro-ph.HE↗

arXiv2011

Diffraction-limited Subaru imaging of M82: sharp mid-infrared view of the starburst core

We present new imaging at 12.81 and 11.7 microns of the central ~40"x30" (~0.7x0.5 kpc) of the starburst galaxy M82. The observations were carried out with the COMICS mid-infrared (mid-IR) imager on the 8.2m Subaru telescope, and are diffraction-limited at an angular resolution of <0".4. The images show extensive diffuse structures, including a 7"-long linear chimney-like feature and another resembling the edges of a ruptured bubble. This is the clearest view to date of the base of the kpc-scale dusty wind known in this galaxy. These structures do not extrapolate to a single central point, implying multiple ejection sites for the dust. In general, the distribution of dust probed in the mid-IR anticorrelates with the locations of massive star clusters that appear in the near-infrared. The 10-21 micron mid-IR emission, spatially-integrated over the field of view, may be represented by hot dust with temperature of ~160 K. Most discrete sources are found to have extended morphologies. Several radio HII regions are identified for the first time in the mid-IR. The only potential radio supernova remnant to have a mid-IR counterpart is a source which has previously also been suggested to be a weak active galactic nucleus. This source has an X-ray counterpart in Chandra data which appears prominently above 3 keV and is best described as a hot (~2.6 keV) absorbed thermal plasma with a 6.7 keV Fe K emission line, in addition to a weaker and cooler thermal component. The mid-IR detection is consistent with the presence of strong [NeII]12.81um line emission. The broad-band source properties are complex, but the X-ray spectra do not support the active galactic nucleus hypothesis. We discuss possible interpretations regarding the nature of this source.

astro-ph.GA↗

arXiv2010

Evidence of non-thermal X-ray emission from radio lobes of Cygnus A

Using deep Chandra ACIS observation data for Cygnus A, we report evidence of non-thermal X-ray emission from radio lobes surrounded by a rich intra-cluster medium (ICM). The diffuse X-ray emission, which are associated with the eastern and western radio lobes, were observed in a 0.7--7 keV Chandra$ ACIS image. The lobe spectra are reproduced with not only a single-temperature Mekal model, such as that of the surrounding ICM component, but also an additional power-law (PL) model. The X-ray flux densities of PL components for the eastern and western lobes at 1 keV are derived as 77.7^{+28.9}_{-31.9} nJy and 52.4^{+42.9}_{-42.4} nJy, respectively, and the photon indices are 1.69^{+0.07}_{-0.13} and 1.84^{+2.90}_{-0.12}, respectively. The non-thermal component is considered to be produced via the inverse Compton (IC) process, as is often seen in the X-ray emission from radio lobes. From a re-analysis of radio observation data, the multiwavelength spectra strongly suggest that the seed photon source of the IC X-rays includes both cosmic microwave background radiation and synchrotron radiation from the lobes. The derived parameters indicate significant dominance of the electron energy density over the magnetic field energy density in the Cygnus A lobes under the rich ICM environment.

astro-ph.CO↗

arXiv2009

Multiwavelength observation from radio through very-high-energy Gamma-ray of OJ 287 during the 12-year cycle flare in 2007

We performed simultaneous multiwavelength observations of OJ 287 with the Nobeyama Millimeter Array for radio, the KANATA telescope and the KVA telescope for optical, the Suzaku satellite for X-ray and the MAGIC telescope for very high energy (VHE) gamma-ray in 2007. The observations were conducted for a quiescent state in April and in a flaring state in November-December. We clearly observed increase of fluxes from radio to X-ray bands during the flaring state while MAGIC could not detect significant VHE gamma-ray emission from the source. We could derive an upper limit (95% confidence level) of 1.7% of the Crab Nebula flux above 150 GeV from about 41.2 hours of the MAGIC observation. A simple SSC model suggests that the observed flaring activity could be caused by evolutions in the distribution of the electron population rather than changes of the magnetic field strength or Doppler beaming factor in the jet.

astro-ph.CO↗

arXiv2008

The estimate of kinetic powers of jets in FRII radio galaxies: existence of invisible components?

We investigate the total kinetic powers (L_j) and ages (t_age) of powerful jets in FR II radio galaxies by comparison of the dynamical model of expanding cocoons with observations. We select four FR II radio sources (Cygnus A, 3C 223, 3C 284, and 3C 219), for which the mass-density profiles of intracluster medium (ICM) are known in the literature. It is found that large fractions > 0.02 - 0.7 of the Eddington luminosity (L_Edd) are carried away as a kinetic power of jet. The upper limit of estimated 2 L_j / L_Edd are larger than unity (< 10) for some sources, suggesting a possibility of super-Eddington mass accretions. As a consequence of the large powers, we also find that the total energy stored in the cocoon (E_c) exceeds the energy derived from the minimum energy condition for the energy of radiating non-thermal electrons and magnetic fields (E_min): 4< E_c /E_min <310. This implies that most of the energy in cocoon is carried by invisible components such as thermal leptons (electron and positron) and/or protons.

astro-ph↗

arXiv2007

Large Kinetic Power in FRII Radio Jets

We investigate the total kinetic powers (L_{j}) and ages (t_{age}) of powerful jets of four FR II radio sources (Cygnus A, 3C 223, 3C 284, and 3C 219) by the detail comparison of the dynamical model of expanding cocoons with observed ones. It is found that these sources have quite large kinetic powers with the ratio of L_{j} to the Eddington luminosity (L_{Edd}) resides in $0.02 <L_{j}/L_{Edd} <10$. Reflecting the large kinetic powers, we also find that the total energy stored in the cocoon (E_{c}) exceed the energy derived from the minimum energy condition (E_{min}): $2< E_{c}/E_{min} <160$. This implies that a large amount of kinetic power is carried by invisible components such as thermal leptons (electron and positron) and/or protons.

astro-ph↗

arXiv2007

The Suzaku Observation of the Nucleus of the Radio-Loud Active Galaxy Centaurus A: Constraints on Abundances of the Accreting Material

A Suzaku observation of the nucleus of the radio-loud AGN Centaurus A in 2005 has yielded a broadband spectrum spanning 0.3 to 250 keV. The net exposure times after screening were: 70 ks per X-ray Imaging Spectrometer (XIS) camera, 60.8 ks for the Hard X-ray Detector (HXD) PIN, and 17.1 ks for the HXD-GSO. The hard X-rays are fit by two power-laws of the same slope, absorbed by columns of 1.5 and 7 * 10^{23} cm^{-2} respectively. The spectrum is consistent with previous suggestions that the power-law components are X-ray emission from the sub-pc VLBI jet and from Bondi accretion at the core, but it is also consistent with a partial covering interpretation. The soft band is dominated by thermal emission from the diffuse plasma and is fit well by a two-temperature VAPEC model, plus a third power-law component to account for scattered nuclear emission, jet emission, and emission from X-ray Binaries and other point sources. Narrow fluorescent emission lines from Fe, Si, S, Ar, Ca and Ni are detected. The Fe K alpha line width yields a 200 light-day lower limit on the distance from the black hole to the line-emitting gas. Fe, Ca, and S K-shell absorption edges are detected. Elemental abundances are constrained via absorption edge depths and strengths of the fluorescent and diffuse plasma emission lines. The high metallicity ([Fe/H]=+0.1) of the circumnuclear material suggests that it could not have originated in the relatively metal-poor outer halo unless enrichment by local star formation has occurred. Relative abundances are consistent with enrichment from Type II and Ia supernovae.

astro-ph↗

arXiv2006

Hard X-ray Detector (HXD) on Board Suzaku

The Hard X-ray Detector (HXD) on board Suzaku covers a wide energy range from 10 keV to 600 keV by combination of silicon PIN diodes and GSO scintillators. The HXD is designed to achieve an extremely low in-orbit back ground based on a combination of new techniques, including the concept of well-type active shield counter. With an effective area of 142 cm^2 at 20 keV and 273 cm2 at 150 keV, the background level at the sea level reached ~1x10^{-5} cts s^{-1} cm^{-2} keV^{-1} at 30 keV for the PI N diodes, and ~2x10^{-5} cts s^{-1} cm^{-2} keV^{-1} at 100 keV, and ~7x10^{-6} cts s^{-1} cm^{-2} keV^{-1} at 200 keV for the phoswich counter. Tight active shielding of the HXD results in a large array of guard counters surrounding the main detector parts. These anti-coincidence counters, made of ~4 cm thick BGO crystals, have a large effective area for sub-MeV to MeV gamma-rays. They work as an excellent gamma-ray burst monitor with limited angular resolution (~5 degree). The on-board signal-processing system and the data transmitted to the ground are also described.

astro-ph↗

arXiv2006

In-Orbit Performance of the Hard X-ray Detector on board Suzaku

The in-orbit performance and calibration of the Hard X-ray Detector (HXD) on board the X-ray astronomy satellite Suzaku are described. Its basic performances, including a wide energy bandpass of 10-600 keV, energy resolutions of ~4 keV (FWHM) at 40 keV and ~11% at 511 keV, and a high background rejection efficiency, have been confirmed by extensive in-orbit calibrations. The long-term gains of PIN-Si diodes have been stable within 1% for half a year, and those of scintillators have decreased by 5-20%. The residual non-X-ray background of the HXD is the lowest among past non-imaging hard X-ray instruments in energy ranges of 15-70 and 150-500 keV. We provide accurate calibrations of energy responses, angular responses, timing accuracy of the HXD, and relative normalizations to the X-ray CCD cameras using multiple observations of the Crab Nebula.

astro-ph↗

arXiv2006

Suzaku Observation of Two Ultraluminous X-Ray Sources in NGC 1313

Two ultraluminous X-ray sources (ULXs) in the nearby Sb galaxy NGC 1313, named X-1 and X-2, were observed with Suzaku on 2005 September 15. During the observation for a net exposure of 28~ks (but over a gross time span of 90~ks), both objects varied in intensity by about 50~%. The 0.4--10 keV X-ray luminosity of X-1 and X-2 was measured as $2.5 \times 10^{40}~{\rm erg~s^{-1}}$ and $5.8 \times 10^{39}~{\rm erg~s^{-1}}$, respectively, with the former the highest ever reported for this ULX. The spectrum of X-1 can be explained by a sum of a strong and variable power-law component with a high energy cutoff, and a stable multicolor blackbody with an innermost disk temperature of $\sim 0.2$ keV. These results suggest that X-1 was in a ``very high'' state, where the disk emission is strongly Comptonized. The absorber within NGC 1313 toward X-1 is suggested to have a subsolar oxygen abundance. The spectrum of X-2 is best represented, in its fainter phase, by a multicolor blackbody model with the innermost disk temperature of 1.2--1.3 keV, and becomes flatter as the source becomes brighter. Hence X-2 is interpreted to be in a slim-disk state. These results suggest that the two ULXs have black hole masses of a few tens to a few hundreds solar masses.

astro-ph↗

arXiv2006

Cyclotron resonance energies at a low X-ray luminosity: A0535+262 observed with Suzaku

The binary X-ray pulsar A0535+262 was observed with the Suzaku X-ray observatory, on 2005 September 14 for a net exposure of 22 ksec. The source was in a declining phase of a minor outburst, exhibiting 3--50 keV luminosity of about $3.7 \times 10^{35}$ ergs s$^{-1}$ at an assumed distance of 2 kpc. In spite of the very low source intensity (about 30 mCrab at 20 keV), its electron cyclotron resonance was detected clearly with the Suzaku Hard X-ray Detector, in absorption at about 45 keV. The resonance energy is found to be essentially the same as those measured when the source is almost two orders of magnitude more luminous. These results are compared with the luminosity-dependent changes in the cyclotron resonance energy, observed from 4U 0115+63 and X 0331+53.

astro-ph↗