arXiv subjects

Naoki Isobe

Publications and source records attributed to Naoki Isobe.

At least 19 recordsLinked to original sources

arXiv2026

Far-infrared synchrotron properties of the inner lobes of the radio galaxy Centaurus A revealed with the Herschel observatory

Diffuse far-infrared synchrotron emission filling the northern inner lobe of the radio galaxy Centaurus A is investigated with the Spectral and Photometric Imaging Receiver onboard the Herschel observatory at its three photometric bands. The far-infrared flux density spatially integrated over the lobe is measured as $S_{\rm \nu} = 1.63 \pm 0.05$ Jy at the wavelength of $500$ $\mu$m (the frequency of $600$ GHz). A comparison between the far-infrared spectral index derived with Herschel ($\alpha = 1.32 \pm 0.19$) and the radio index ($\alpha = 0.66 \pm 0.04$) suggests a spectral break between these frequency ranges. The change of the spectral index through the break is indicated to be consistent with that of the standard cooling break ($\Delta \alpha = 0.5$) predicted for particle acceleration under the continuous energy injection condition. A broken power-law model incorporating the standard cooling break yields the break frequency as $\nu_{\rm b} = 218 \pm 83$ GHz. From the measured cooling break frequency, the magnetic field of the northern inner lobe is evaluated as $B \gtrsim 100$ $\mu$G. It is quantitatively estimated that the adiabatic cooling puts only a minor impact on the derived magnetic field. This magnetic field is higher than that under the minimum-energy condition by more than a factor of $5$. In addition, the derived magnetic field of the lobe is suggested to be at least by a factor of $4$ stronger than that of the inner-jet region implied in the previous very-high-energy gamma-ray study. Even if the line-of-sight orientation of the lobe is considered in its possible extreme case, the magnetic field is found to be reduced only by a factor of 2, and the above arguments about the strong magnetic field basically holds. The science impact of this result is discussed from the viewpoints of jet energetics, and of ultra-high energy cosmic rays.

astro-ph.HE↗

arXiv2026

Multiphase Gas Structure in the Circumnuclear Region of NGC 5506 Observed with ALMA

We present a study of the multiphase gas structure and kinematics of the circumnuclear disk (CND) of NGC 5506, a nearby edge-on Seyfert galaxy, at a spatial resolution of $\sim20$ pc. Observations of [C I](1-0), CO(3-2), and HCO$^{+}$(4-3) obtained with the Atacama Large Millimeter/submillimeter Array reveal the CND dominated by rotational motion on scales of several hundred parsecs. No significant differences in geometrical thickness or velocity structure are found between [C I](1-0) and CO(3-2) across the CND, whereas HCO$^{+}$(4-3) emission is more concentrated toward the disk plane. The ratio of velocity dispersion to rotational velocity, a proxy for disk scale height-to-radius ratio, is high ($\gtrsim0.9$) in the central region ($\lesssim30$ pc) for both [C I](1-0) and CO(3-2), indicating geometrically thick structures in both tracers. Regions where the [C I](1-0)/CO(3-2) ratio exceeds the CND average are spatially correlated with the [O III]$\lambda$5007 bicone observed with the Hubble Space Telescope, suggesting that CO is preferentially dissociated by the AGN-driven biconical ionized outflow. The observed CND scale height and velocity dispersions traced by [C I](1-0) and CO(3-2) are consistent with a model in which supernova-driven turbulence provides the vertical support for the CND.

astro-ph.GA↗

arXiv2025

Far-infrared probing with PRIMA into particle acceleration associated with relativistic jets from active galactic nuclei

It is presented that the Probe far-Infrared Mission for Astrophysics (PRIMA) has a high potential to study particle acceleration phenomena associated with jets emanating from active galactic nuclei. A special focus is put on hot spots of radio galaxies because they are widely regarded as the jet-terminal shock where particles are accelerated via the diffusive shock acceleration. To investigate the particle acceleration condition in the hot spots, it is of prime importance to evaluate their magnetic field strength. As a useful indicator of the magnetic field, we propose to adopt a synchrotron spectral feature called the cooling break, of which the frequency is determined by the mutual balance between the synchrotron radiative cooling and the adiabatic one. Referring to the standard physical parameter of the hot spots, the cooling break is expected to reside in or slightly below the far-infrared range covered with PRIMA. The feasibility of the PRIMA observations to measure the far-infrared flux density and to constrain their cooling break frequency is discussed for nearby well-studied hot spots. An affordable observational strategy with PRIMA is described. A possible application of the method to lobes of radio galaxies is also briefly discussed.

astro-ph.HE↗

arXiv2024

Systematic Study of the Inner Structure of Molecular Tori in Nearby U/LIRGs using Velocity Decomposition of CO Rovibrational Absorption Lines

Determining the inner structure of the molecular torus around an active galactic nucleus is essential for understanding its formation mechanism. However, spatially resolving the torus is difficult because of its small size. To probe the clump conditions in the torus, we therefore perform the systematic velocity-decomposition analyses of the gaseous CO rovibrational absorption lines ($v=0\to 1,\Delta J=\pm 1$) at $\lambda\sim 4.67 \mathrm{\mu{m}}$ observed toward four (ultra)luminous infrared galaxies using the high-resolution ($R\sim 5000\text{--}10000$) spectroscopy from the Subaru Telescope. We find that each transition has two to five distinct velocity components with different line-of-sight (LOS) velocities ($V_\mathrm{LOS}\sim -240\text{--}+100\mathrm{km\,s^{-1}}$) and dispersions ($\sigma_V\sim 15\text{--}190\mathrm{km\,s^{-1}}$); i.e., the components (a), (b), ..., beginning with the broadest one in each target, indicating that the tori have clumpy structures. By assuming a hydrostatic disk ($\sigma_V\propto R_\mathrm{rot}^{-0.5}$), we find that the tori have dynamic inner structures, with the innermost component (a) outflowing with velocity $|V_\mathrm{LOS}|\sim 160\text{--}240\mathrm{km\,s^{-1}}$, and the outer components (b) and (c) outflowing more slowly or infalling with $|V_\mathrm{LOS}|\lesssim 100\mathrm{km\,s^{-1}}$. In addition, we find that the innermost component (a) can be attributed to collisionally excited hot ($\gtrsim 530$K) and dense ($n_\mathrm{H_2}\gtrsim 10^6\mathrm{cm^{-3}}$) clumps, based on the level populations. Conversely, the outer component (b) can be attributed to cold ($\sim 30\text{--}140$K) clumps radiatively excited by a far-infrared-to-submillimeter background with a brightness temperature higher than $\sim 20\text{--}400$K. These observational results demonstrate the clumpy and dynamic structure of tori in the presence of background radiation.

astro-ph.GA↗

arXiv2024

JASMINE image simulator for high-precision astrometry and photometry

JASMINE is a Japanese planned space mission that aims to reveal the formation history of our Galaxy and discover habitable exoEarths. For these objectives, the JASMINE satellite performs high-precision astrometric observations of the Galactic bulge and high-precision transit monitoring of M-dwarfs in the near-infrared (1.0-1.6 microns in wavelength). For feasibility studies, we develop an image simulation software named JASMINE-imagesim, which produces realistic observation images. This software takes into account various factors such as the optical point spread function (PSF), telescope jitter caused by the satellite's attitude control error (ACE), detector flat patterns, exposure timing differences between detector pixels, and various noise factors. As an example, we report a simulation for the feasibility study of astrometric observations using JASMINE-imagesim. The simulation confirms that the required position measurement accuracy of 4 mas for a single exposure of 12.5-mag objects is achievable if the telescope pointing jitter uniformly dilutes the PSF across all stars in the field of view. On the other hand, the simulation also demonstrates that the combination of realistic pointing jitter and exposure timing differences in the detector can significantly degrade accuracy and prevent achieving the requirement. This means that certain countermeasures against this issue must be developed. This result implies that this kind of simulation is important for mission planning and advanced developments to realize more realistic simulations help us to identify critical issues and also devise effective solutions.

astro-ph.IM↗

arXiv2023

JASMINE: Near-Infrared Astrometry and Time Series Photometry Science

Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE) is a planned M-class science space mission by the Institute of Space and Astronautical Science, the Japan Aerospace Exploration Agency. JASMINE has two main science goals. One is the Galactic archaeology with Galactic Center Survey, which aims to reveal the Milky Way's central core structure and formation history from Gaia-level (~25 $\mu$as) astrometry in the Near-Infrared (NIR) Hw-band (1.0-1.6 $\mu$m). The other is the Exoplanet Survey, which aims to discover transiting Earth-like exoplanets in the habitable zone from NIR time-series photometry of M dwarfs when the Galactic center is not accessible. We introduce the mission, review many science objectives, and present the instrument concept. JASMINE will be the first dedicated NIR astrometry space mission and provide precise astrometric information of the stars in the Galactic center, taking advantage of the significantly lower extinction in the NIR. The precise astrometry is obtained by taking many short-exposure images. Hence, the JASMINE Galactic center survey data will be valuable for studies of exoplanet transits, asteroseismology, variable stars and microlensing studies, including discovery of (intermediate mass) black holes. We highlight a swath of such potential science, and also describe synergies with other missions.

astro-ph.IM↗

arXiv2023

ALMA ACA detection of submillimeter emission associated with the west hot spot of the radio galaxy Pictor A

In order to investigate the far-infrared excess detected from the west hot spot of the radio galaxy Pictor A with the Herschel observatory, a submillimeter photometry is performed with the Atacama Compact Array (ACA) of the Atacama Large Millimeter/submillimeter Array at Band 8 with the reference frequency of 405 GHz. A submillimeter source is discovered at the radio peak of the hot spot. Because the 405 GHz flux density of the source, $80.7\pm3.1$ mJy, agrees with the extrapolation of the synchrotron radio spectrum, the far-infrared excess is suggested to exhibit no major contribution at the ACA band. In contrast, by subtracting the power-law spectrum tightly constrained by the radio and ACA data, the significance of the excess in the Herschel band is well confirmed. No diffuse submillimeter emission is detected within the ACA field of view, and thus, the excess is ascribed to the west hot spot itself. In comparison to the previous estimate based on the Herschel data, the relative contribution of the far-infrared excess is reduced by a factor of $\sim 1.5$. The spectrum of the excess below the far-infrared band is determined to be harder than that of the diffusive shock acceleration. This strengthens the previous interpretation that the excess originates via the magnetic turbulence in the substructures within the hot spot. The ACA data are utilized to evaluate the magnetic field strength of the excess and of diffuse radio structure associated to the hot spot.

astro-ph.HE↗

arXiv2023

Warm Molecular Gas in the Central Parsecs of the Buried Nucleus of NGC 4418 Traced with the Fundamental CO Ro-vibrational Absorptions

We investigated the inner buried nucleus of a nearby luminous infrared galaxy NGC 4418 using high-resolution spectroscopy of fundamental carbon monoxide (CO) ro-vibrational absorptions around $4.67 \mu$m for the first time. This method allowed us to examine the physical and kinematical properties in the hot inner region of this nucleus. We detected a series of both very deep (partly saturated) $^{12}$CO and moderately deep (optically thin) $^{13}$CO absorption lines and inferred a large column density ($N_\mathrm{H2}=(5\pm3)\times10^{23}$ cm$^{-2}$ in front of the $5 \mu$m photosphere) of warm ($T_\mathrm{ex}\simeq170$ K) molecular gas by assuming an isothermal plane-parallel slab illuminated by a compact background MIR-emitting source. We modeled that the warm CO absorber almost covers the central heating source and that it is an inner layer around the $5 \mu$m photosphere (at $r=$several pc) of a compact shroud of gas and dust ($d\sim100$ pc). The width of the absorption lines ($110$ km s$^{-1}$) and their small deviation from the systemic velocity ($<10$ km s$^{-1}$) are consistent with a warm and turbulent layer with little bulk motion in the radial direction.

astro-ph.GA↗

arXiv2022

Probing dynamics and thermal properties inside molecular tori with CO rovibrational absorption lines

A recent hydrodynamic model, "radiation-driven fountain model" (Wada et al. 2016), presented a dynamical picture that active galactic nuclei (AGNs) tori sustain their geometrical thickness by gas circulation around AGNs, and previous papers confirmed that this picture is consistent with multi-wavelength observations of nearby Seyfert galaxies. Recent near-infrared observations implied that CO rovibrational absorption lines ($\Delta J=\pm1$, $v=0-1$, $\lambda \sim 4.7$ $\mathrm{\mu m}$) could probe physical properties of the inside tori. However, the origin of the CO absorption lines has been under debate. In this paper, we investigate the origin of the absorption lines and conditions for detecting them by performing line radiative transfer calculations based on the radiation-driven fountain model. We find that CO rovibrational absorption lines are detected at inclination angles $\theta_\mathrm{obs} = 50-80$ $^{\circ}$. At the inclination angle $\theta_\mathrm{obs} = 77$ $^{\circ}$, we observe multi-velocity components: inflow ($v_\mathrm{LOS}=30$ $\mathrm{kms^{-1}}$), systemic ($v_\mathrm{LOS}=0 \, \mathrm{kms^{-1}}$), and outflows ($v_\mathrm{LOS}=-75,\, -95,$ and $-105$ $\mathrm{kms^{-1}}$). The inflow and outflow components ($v_\mathrm{LOS}= 30$ and $-95$ $\mathrm{kms^{-1}}$) are collisionally excited at the excitation temperature of $186$ and $380$ K up to $J=12$ and $4$, respectively. The inflow and outflow components originate from the accreting gas on the equatorial plane at $1.5$ pc from the AGN center and the outflowing gas driven by AGN radiation pressure at $1.0$ pc, respectively. These results suggest that CO rovibrational absorption lines can provide us with the velocities and kinetic temperatures of the inflow and outflow in the inner a-few-pc regions of AGN tori, and the observations can probe the gas circulation inside the tori.

astro-ph.GA↗

arXiv2022

$\textit{Herschel}$ discovery of far-infrared emission from the hot spot D in the radio galaxy Cygnus A

The far infrared counterpart of hot spot D, the terminal hot spot of the eastern jet hosted by the radio galaxy Cygnus A, is detected with \textit{Herschel} Aperture photometery of the source performed in 5 photometric bands covering the wavelength range of $70\unicode{x2013}350$ $\mathrm{\mu m}$. After removing the contamination from another nearby hot spot, E, the far-infrared intensity of hot spot D is derived as $83\pm13$ and $269\pm66$ mJy at $160$ and $350$ $\mathrm{\mu m}$, respectively. Since the far-infrared spectrum of the object smoothly connects to the radio one, the far-infrared emission is attributed to the synchrotron radiation from the radio-emitting electron population. The radio-to-near-infrared spectrum is confirmed to exhibit a far-infrared break feature at the frequency of $\nu_\mathrm{br}=2.0^{+1.2}_{-0.8} \times10^{12}$ Hz. The change in energy index at the break ($\Delta\alpha=0.5$) is interpreted as the impact of radiative cooling on an electron distribution sustained by continuous injection from diffusive shock acceleration. By ascribing the derived break to this cooling break, the magnetic field, $B$, in the hot spot is determined as a function of its radius, $R$ within a uniform one-zone model combined with the strong relativistic shock condition. An independent $B$-$R$ constraint is obtained by assuming the X-ray spectrum is wholly due to synchrotron-self-Compton emission. By combining these conditions, the two parameters are tightly determined as $B=120\unicode{x2013}150$ $\mathrm{\mu G}$ and $R=1.3\unicode{x2013}1.6$ kpc. A further investigation into the two conditions indicates the observed X-ray flux is highly dominated by the synchrotron-self-Compton emission.

astro-ph.HE↗

arXiv2022

Extremely Buried Nucleus of IRAS 17208$-$0014 Observed at Sub-Millimeter and Near-Infrared Wavelengths

The ultraluminous infrared galaxy IRAS 17208$-$0014 is a late-stage merger that hosts a buried active galactic nucleus (AGN). To investigate its nuclear structure, we performed high spatial resolution ($\sim0.\!\!^{\prime\prime}04\sim32\,\mathrm{pc}$) Atacama Large Millimeter/submillimeter Array (ALMA) observations in Band 9 ($\sim$450\,\micron\ or $\sim$660\,GHz), along with near-infrared AKARI spectroscopy in 2.5--5.0\,\micron. The Band 9 dust continuum peaks at the AGN location, and toward this position CO($J$=6--5) and CS($J$=14--13) are detected in absorption. Comparison with non-local thermal equilibrium calculations indicates that, within the central beam ($r\sim20\,\mathrm{pc}$), there exists a concentrated component that is dense ($10^7\,\mathrm{cm}^{-2}$) and warm ($>$200\,K) and has a large column density ($N_\mathrm{H_2}>10^{23}\,\mathrm{cm}^{-2}$). The AKARI spectrum shows deep and broad CO ro-vibrational absorption at 4.67\,\micron. Its band profile is well reproduced with a similarly dense and large column but hotter ($\sim$1000\,K) gas. The region observed through absorption in the near-infrared is highly likely in the nuclear direction, as in the sub-millimeter, but with a narrower beam including a region closer to the nucleus. The central component is considered to possess a hot structure where vibrationally excited HCN emission originates. The most plausible heating source for the gas is X-rays from the AGN. The AKARI spectrum does not show other AGN signs in 2.5--4\,\micron, but this absence may be usual for AGNs buried in a hot mid-infrared core. Besides, based on our ALMA observations, we relate various nuclear structures of IRAS 17208$-$0014 that have been proposed in the literature.

astro-ph.GA↗

arXiv2021

Anomalous Hydrogen Recombination-Line Ratios in Ultraluminous Infrared Galaxies

We conducted systematic observations of the H I Br$\alpha$ (4.05 $\mu$m) and Br$\beta$ (2.63 $\mu$m) lines in 52 nearby ($z<0.3$) ultraluminous infrared galaxies (ULIRGs) with AKARI. Among 33 ULIRGs wherein the lines are detected, three galaxies show anomalous Br$\beta$/Br$\alpha$ line ratios ($\sim1.0$), which are significantly higher than those for case B (0.565). Our observations also show that ULIRGs have a tendency to exhibit higher Br$\beta$/Br$\alpha$ line ratios than those observed in Galactic H II regions. The high Br$\beta$/Br$\alpha$ line ratios cannot be explained by a combination of dust extinction and case B since dust extinction reduces the ratio. We explore possible causes for the high Br$\beta$/Br$\alpha$ line ratios and show that the observed ratios can be explained by a combination of an optically thick Br$\alpha$ line and an optically thin Br$\beta$ line. We simulated the H II regions in ULIRGs with the Cloudy code, and our results show that the high Br$\beta$/Br$\alpha$ line ratios can be explained by high-density conditions, wherein the Br$\alpha$ line becomes optically thick. To achieve a column density large enough to make the Br$\alpha$ line optically thick within a single H II region, the gas density must be as high as $n\sim10^8$ $\mathrm{cm}^{-3}$. We therefore propose an ensemble of H II regions, in each of which the Br$\alpha$ line is optically thick, to explain the high Br$\beta$/Br$\alpha$ line ratio.

astro-ph.GA↗

arXiv2021

Study of the Inner Structure of the Molecular Torus in IRAS 08572+3915 NW with Velocity Decomposition of CO Rovibrational Absorption Lines

Understanding the inner structure of the clumpy molecular torus surrounding the active galactic nucleus is essential in revealing the forming mechanism. However, spatially resolving the torus is difficult because of its size of a few parsecs. Thus, to probe the clump conditions in the torus, we performed the velocity decomposition of the CO rovibrational absorption lines ($\Delta{v}=0\to 1,\ \Delta{J}=\pm 1$) at $\lambda\sim 4.67\,\mathrm{\mu{m}}$ observed toward an ultraluminous infrared galaxy IRAS 08572+3915 NW with the high-resolution spectroscopy ($R\sim 10{,}000$) of Subaru Telescope. Consequently, we found that each transition had two outflowing components, i.e., (a) and (b), both at approximately $\sim -160\,\mathrm{km\,s^{-1}}$, but with broad and narrow widths, and an inflowing component, i.e., (c), at approximately $\sim +100\,\mathrm{km\,s^{-1}}$, which were attributed to the torus. The ratios of the velocity dispersions of each component lead to those of the rotating radii around the black hole of $R_\mathrm{rot,a}:R_\mathrm{rot,b}:R_\mathrm{rot,c}\approx 1:5:17$, indicating the torus where clumps are outflowing in the inner regions and inflowing in the outer regions if a hydrostatic disk with $\sigma_V\propto R_\mathrm{rot}^{-0.5}$ is assumed. Based on the kinetic temperature of components (a) and (b) of $\sim 720\,\mathrm{K}$ and $\sim 25\,\mathrm{K}$ estimated from the level population, the temperature gradient is $T_\mathrm{kin}\propto R_\mathrm{rot}^{-2.1}$. Magnetohydrodynamic models with large density fluctuations of two orders of magnitude or more are necessary to reproduce this gradient.

astro-ph.GA↗

arXiv2020

Herschel SPIRE discovery of far-infrared excess synchrotron emission from the west hot spot of the radio galaxy Pictor A

A far-infrared counterpart to the west hot spot of the radio galaxy Pictor A is discovered with the Spectral and Photometric Imaging REceiver (SPIRE) onboard Herschel. The color-corrected flux density of the source is measured as $70.0 \pm 9.9$ mJy at the wavelength of 350 $\mu$m. A close investigation into its radio-to-optical spectrum indicates that the mid-infrared excess over the radio synchrotron component, detected with WISE and Spitzer, significantly contributes to the far-infrared band. Thanks to the SPIRE data, it is revealed that the spectrum of the excess is described by a broken power-law model subjected to a high-energy cutoff. By applying the radiative cooling break under continuous energy injection ($\Delta \alpha = 0.5$), the broken power-law model supports an idea that the excess originates in 10-pc scale substructures within the hot spot. From the break frequency, $\nu_{\rm b} = 1.6_{-1.0}^{+3.0} \times 10^{12}$ Hz, the magnetic field was estimated as $B\simeq1$-$4$ mG. This is higher than the minimum-energy magnetic field of the substructures by a factor of $3$--$10$. Even if the origin of the excess is larger than $\sim 100$ pc, the magnetic field stronger than the minimum-energy field is confirmed. It is proposed that regions with a magnetic field locally boosted via plasma turbulence are observed as the substructures. The derived energy index below the break, $\alpha \sim 0.22$ (conservatively $<0.42$), is difficult to be attributed to the strong-shock acceleration ($\alpha = 0.5$). Stochastic acceleration and magnetic reconnection are considered as a plausible alternative mechanism.

astro-ph.HE↗

arXiv2019

A systematic study of ULIRGs using near-infrared absorption bands reveals a strong UV environment in their star-forming regions

We present a systematic study of the 3.0 um H2O ice and the 3.4 um aliphatic carbon absorption features toward 48 local ultraluminous infrared galaxies (ULIRGs) using spectra obtained by the AKARI Infrared Camera to investigate the UV environment in their star-forming regions. All the ULIRGs in our sample exhibit a ratio of optical depth of H2O ice to silicate dust (tau3.0/tau9.7) that is lower than that in the Taurus dark cloud. This implies that ULIRGs cannot be described as an ensemble of low-mass star-forming regions and that a significant amount of high-mass star-forming regions contribute to star-forming clouds in local ULIRGs. The results also show that the ratios of optical depth of aliphatic carbon to silicate dust, tau3.4/tau9.7, exhibit diverse values. We investigate two effects that can affect this ratio: the geometric temperature gradient (which increases the ratio) and the intense UV environment (which decreases it). The geometric temperature gradient is typically considered as a sign of active galactic nuclei (AGN). ULIRGs with AGN signs (optical classification, NIR color, and a PAH emission strength of 3.3 um) indeed tend to exhibit a large tau3.4/tau9.7 ratio. However, we find that the presence of buried AGN is not the only cause of the geometric temperature gradient, because the enhancement of the ratio is also evident in pure starburst-like ULIRGs without these AGN signs. Regarding the intense UV environment in star-forming regions, the correlation between the aliphatic carbon ratio and the ratio of the [C II] 158 um line luminosity to the far-infrared luminosity (L[CII]/LFIR), which represents the UV environment in photodissociation regions, implies that the intense UV environment causes the decrease of the aliphatic carbon ratio. We find that an intense UV environment (G/nH > 3) in star-forming regions is needed for the aliphatic carbon ratio to be suppressed.

astro-ph.GA↗

arXiv2018

The 7-year MAXI/GSC X-ray Source Catalog in the High Galactic-Latitude Sky (3MAXI)

We present the third MAXI/GSC catalog in the high Galactic-latitude sky ($|b| > 10^\circ$) based on the 7-year data from 2009 August 13 to 2016 July 31, complementary to that in the low Galactic-latitude sky ($|b| < 10^\circ$; Hori et al. 2018). We compile 682 sources detected at significances of $s_{\rm D,4-10~keV} \geq 6.5$ in the 4--10 keV band. A two-dimensional image fit based on the Poisson likelihood algorithm ($C$-statistics) is adopted for the detections and constraints on their fluxes and positions. The 4--10 keV sensitivity reaches $\approx 0.48$ mCrab, or $\approx 5.9 \times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$, over the half of the survey area. Compared with the 37-month catalog (Hiroi et al. 2013), which adopted a threshold of $s_{\rm D,4-10~keV} \geq 7$, the source number increases by a factor of $\sim$1.4. The fluxes in the 3--4 keV and 10--20 keV bands are further estimated, and hardness ratios (HRs) are calculated using the 3--4 keV, 4--10 keV, 3--10 keV, and 10--20 keV band fluxes. We also make the 4--10 keV lightcurves in one year bins for all the sources and characterize their variabilities with an index based on a likelihood function and the excess variance. Possible counterparts are found from five major X-ray survey catalogs by Swift, Uhuru, RXTE, XMM-Newton, and ROSAT, and an X-ray galaxy-cluster catalog (MCXC). Our catalog provides the fluxes, positions, detection significances, HRs, one-year bin lightcurves, variability indices, and counterpart candidates.

astro-ph.HE↗

arXiv2017

Mid-infrared excess from the west hot spot of the radio galaxy Pictor A unveiled by WISE

Mid-infrared properties are reported of the west hot spot of the radio galaxy Pictor A with the Wide-field Infrared Survey Explorer (WISE). The mid-infrared counterpart to the hot spot, WISE J051926.26-454554.1, is listed in the AllWISE source catalog. The source was detected in all the four WISE photometric bands. A comparison between the WISE and radio images reinforces the physical association of the \wise\ source to the hot spot. The WISE flux density of the source was carefully evaluated. A close investigation of the multi-wavelength synchrotron spectral energy distribution from the object reveals a mid-infrared excess at the wavelength of $\lambda=22$ $\mu$m with a statistical significance of $4.8 \sigma$ over the simple power-law extrapolation from the synchrotron radio spectrum. The excess is reinforced by single and double cutoff power-law modeling of the radio-to-optical spectral energy distribution. The synchrotron cutoff frequency of the main and excess components was evaluated as $7.1 \times 10^{14}$ Hz and $5.5 \times 10^{13}$ Hz, respectively. From the cutoff frequency, the magnetic field of the emission region was constrained as a function of the region size. In order to interpret the excess component, an electron population different from the main one dominating the observed radio spectrum is necessary. The excess emission is proposed to originate in a sub structure within the hot spot, in which the magnetic field is by a factor of a few stronger than that in the minimum energy condition. The relation of the mid-infrared excess to the X-ray emission is briefly discussed.

astro-ph.HE↗

arXiv2016

Star Formation in Ultraluminous Infrared Galaxies Probed with AKARI Near-Infrared Spectroscopy

We conducted systematic observations of the HI Br-alpha line (4.05 micron) and the polycyclic aromatic hydrocarbon (PAH) feature (3.3 micron) in 50 nearby (z<0.3) ultraluminous infrared galaxies (ULIRGs) with AKARI. The Br-alpha line is predicted to be the brightest among the HI lines under high dust-extinction conditions (A_V>15 mag). The Br-alpha line traces ionizing photons from OB stars and so is used as an indicator of star formation on the assumption of the initial mass function. We detected the Br-alpha line in 33 ULIRGs. The luminosity of the line (L_BrA) correlates well with that of the 3.3 micron PAH emission (L_3.3). Thus we utilize L_3.3 as an indicator of star formation in fainter objects where the Br-alpha line is undetected. The mean L_BrA/L_IR ratio in LINERs/Seyferts is significantly lower than that in HII galaxies. This difference is reconfirmed with the L_3.3/L_IR ratio in the larger sample (46 galaxies). Using the ratios, we estimate that the contribution of starburst in LINERs/Seyferts is ~67%, and active galactic nuclei contribute to the remaining ~33%. However, comparing the number of ionizing photons, Q_BrA, derived from L_BrA with that, Q_IR, expected from star formation rate required to explain L_IR, we find that the mean Q_BrA/Q_IR ratio is only 55.5+/-7.5% even in HII galaxies which are thought to be energized by pure starburst. This deficit of ionizing photons traced by the Br-alpha line is significant even taking heavy dust extinction into consideration. We propose that dust within HII regions absorbs a significant fraction of ionizing photons.

astro-ph.GA↗