arXiv subjects

Issei Yamamura

Publications and source records attributed to Issei Yamamura.

34 records · Page 2Linked to original sources

arXiv2012

Infrared and hard X-ray diagnostics of AGN identification from the Swift/BAT and AKARI all-sky surveys

We combine data from two all-sky surveys in order to study the connection between the infrared and hard X-ray (>10keV) properties for local active galactic nuclei (AGN). The Swift/Burst Alert Telescope all-sky survey provides an unbiased, flux-limited selection of hard X-ray detected AGN. Cross-correlating the 22-month hard X-ray survey with the AKARI all-sky survey, we studied 158 AGN detected by the AKARI instruments. We find a strong correlation for most AGN between the infrared (9, 18, and 90 micron) and hard X-ray (14-195 keV) luminosities, and quantify the correlation for various subsamples of AGN. Partial correlation analysis confirms the intrinsic correlation after removing the redshift contribution. The correlation for radio galaxies has a slope and normalization identical to that for Seyfert 1s, implying similar hard X-ray/infrared emission processes in both. In contrast, Compton-thick sources show a large deficit in the hard X-ray band, because high gas column densities diminish even their hard X-ray luminosities. We propose two photometric diagnostics for source classification: one is an X-ray luminosity vs. infrared color diagram, in which type 1 radio-loud AGN are well isolated from the others in the sample. The other uses the X-ray vs. infrared color as a useful redshift-independent indicator for identifying Compton-thick AGN. Importantly, Compton-thick AGN and starburst galaxies in composite systems can also be differentiated in this plane based upon their hard X-ray fluxes and dust temperatures. This diagram may be useful as a new indicator to classify objects in new and upcoming surveys such as WISE and NuSTAR.

astro-ph.HE↗

arXiv2012

Dramatic infrared variability of WISE J1810-3305: catching early dust ejection during the thermal pulse of an AGB star?

We present the discovery of a source with broadband infrared photometric characteristics similar to Sakurai's Object. WISE J180956.27-330500.2 (hereafter, J1810-3305) shows very red WISE colors, but a very blue 2MASS [K] vs. WISE [W1 (3.4 micron)] color. It was not visible during the IRAS era, but now has a 12 micron flux well above the IRAS point source catalog detection limit. There are also indications of variability in historical optical photographic plates, as well as in multi-epoch AKARI mid-infrared measurements. The broadband infrared spectral energy distribution, post-IRAS brightening and multiwavelength variability are all characteristics also shared by Sakurai's Object - a post asymptotic giant branch (post-AGB) star which underwent a late thermal pulse and recently ejected massive envelopes of dust that are currently expanding and cooling. Optical progenitor colors suggest that J1810-3305 may have been of late spectral class. Its dramatic infrared brightening, and the detection of a late-type optical counterpart are consistent with a scenario in which we have caught an extremely massive dust ejection event (in 1998 or shortly before) during the thermal pulse of an AGB star, thus providing a unique opportunity to observe stellar evolution in this phase. J1810-3305 is the only source in the entire WISE preliminary data release with similar infrared SED and variability, emphasizing the rarity of such sources. Confirmation of its nature is of great importance.

astro-ph.SR↗

arXiv2011

Akari Observations of Brown Dwarfs. II CO2 as Probe of Carbon and Oxygen Abundances in Brown Dwarfs

Recent observations with the infrared astronomical satellite AKARI have shown that the CO2 bands at 4.2 micron in three brown dwarfs are much stronger than expected from the unified cloudy model (UCM) based on recent solar C & O abundances. This result has been a puzzle, but we now find that this is simply an abundance effect: We show that these strong CO2 bands can be explained with the UCMs based on the classical C & O abundances (log Ac and log Ao), which are about 0.2 dex larger compared to the recent values. Since three other brown dwarfs could be well interpreted with the recent solar C & O abundances, we require at least two model sequences based on the different chemical compositions to interpret all the AKARI spectra. The reason for this is that the CO2 band is especially sensitive to C & O abundances, since the CO2 abundance depends approximately on AcAo^2 --- the cube of C & O abundances. For this reason, even low resolution spectra of very cool dwarfs, especially of CO2 cannot be understood unless a model with proper abundances is applied. For the same reason, CO2 is an excellent indicator of C & O abundances, and we can now estimate C & O abundances of brown dwarfs: Three out of six brown dwarfs observed with AKARI should have high C & O abundances similar to the classical solar values (e.g. logAc = 8.60 and logAo = 8.92), but the other three may have low C & O abundances similar to the recent solar values (e.g. logAc = 8.39 and logAo = 8.69). This result implies that three out of six brown dwarfs are highly metal rich relative to the Sun if the recent solar C & O abundances are correct.

astro-ph.SR↗

arXiv2011

Detection of the detached dust shell of U Antliae at mid-infrared wavelengths with AKARI/IRC

We report mid-infrared (MIR) imaging observations of the carbon star U Ant made with the Infrared Camera (IRC) on board AKARI. Subtraction of the artifacts and extended PSF of the central star reveals the detached dust shell around the carbon star at MIR wavelengths (15 and 24μm) for the first time. The observed radial brightness profiles of the MIR emission are well explained by two shells at 43" and 50" from the central star detected in optical scattered light observations. Combining Herschel/PACS, AKARI/FIS, and AKARI/IRC data, we obtain the infrared spectral energy distribution (SED) of the thermal emission from the detached shell of U Ant in a wide infrared spectral range of 15- 160 μm. Thermal emission of amorphous carbon grains with a single temperature cannot account for the observed SED from 15 to 160 μm: it underestimates the emission at 15 μm. Alternatively, the observed SED is fitted by the model that amorphous carbon grains in the two shells have different temperatures of 60 and 104 K, which allocates most dust mass in the shell at 50". This supports previous suggestion that the 43" shell is gas-rich and the 50" one is dust-rich. We suggest a possibility that the segregation of the gas and dust resulting from the drift motion of submicron-sized dust grains relative to the gas and that the hot dust component associated with the gas-rich shell is composed of very small grains that are strongly coupled with the gas.

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arXiv2010

Calibration of the AKARI Far-Infrared Imaging Fourier Transform Spectrometer

The Far-Infrared Surveyor (FIS) onboard the AKARI satellite has a spectroscopic capability provided by a Fourier transform spectrometer (FIS-FTS). FIS-FTS is the first space-borne imaging FTS dedicated to far-infrared astronomical observations. We describe the calibration process of the FIS-FTS and discuss its accuracy and reliability. The calibration is based on the observational data of bright astronomical sources as well as two instrumental sources. We have compared the FIS-FTS spectra with the spectra obtained from the Long Wavelength Spectrometer (LWS) of the Infrared Space Observatory (ISO) having a similar spectral coverage. The present calibration method accurately reproduces the spectra of several solar system objects having a reliable spectral model. Under this condition the relative uncertainty of the calibration of the continuum is estimated to be $\pm$ 15% for SW, $\pm$ 10% for 70-85 cm^(-1) of LW, and $\pm$ 20% for 60-70 cm^(-1) of LW; and the absolute uncertainty is estimated to be +35/-55% for SW, +35/-55% for 70-85 cm^(-1) of LW, and +40/-60% for 60-70 cm^(-1) of LW. These values are confirmed by comparison with theoretical models and previous observations by the ISO/LWS.

astro-ph.IM↗

arXiv2010

AKARI Observations of Brown Dwarfs I.: CO and CO_2 Bands in the Near-Infrared Spectra

Near-infrared medium-resolution spectra of seven bright brown dwarfs are presented. The spectra were obtained with the Infrared Camera (IRC) on board the infrared astronomical satellite AKARI, covering 2.5--5.0 um with a spectral resolution of approximately 120. The spectral types of the objects range from L5 to T8, and enable us to study the spectral evolution of brown dwarfs. The observed spectra are in general consistent with the predictions from the previous observations and photospheric models. We find that the CO fundamental band around 4.6 um is clearly seen even in the T8 dwarf 2MASS J041519-0935, confirming the presence of non-equilibrium chemical state in the atmosphere. We also identify the CO_2 fundamental stretching-mode band at 4.2 um for the first time in the spectra of late-L and T-type brown dwarfs. We analyze the observed spectra by comparing with the predicted ones based on the Unified Cloudy Model (UCM). Although overall spectral energy distributions (SEDs) can be reasonably fitted with the UCM, observed CO and CO_2 bands in late-L and T-dwarfs are unexpectedly stronger than the model predictions assuming local thermodynamical equilibrium (LTE). We examine the vertical mixing model and find that this model explains the CO band at least partly in the T-dwarfs 2MASS J041519-0935 and 2MASS J055919-1404. The CO fundamental band also shows excess absorption against the predicted one in the L9 dwarf SDSS J083008+4828. Since CO is already highly abundant in the upper photospheres of late-L dwarfs, the extra CO by vertical mixing has little effect on the CO band strengths, and the vertical mixing model cannot be applied to this L-dwarf. A more serious problem is that the significant enhancement of the CO_2 4.2 um band in both the late-L and T dwarfs cannot be explained at all by the vertical mixing model. The enhancement of the CO_2 band remains puzzling.

astro-ph.SR↗

arXiv2010

The AKARI/IRC Mid-Infrared All-Sky Survey

Context : AKARI is the first Japanese astronomical satellite dedicated to infrar ed astronomy. One of the main purposes of AKARI is the all-sky survey performed with six infrared bands between 9 and 200um during the period from 2006 May 6 to 2007 August 28. In this paper, we present the mid-infrared part (9um and 18um b ands) of the survey carried out with one of the on-board instruments, the Infrar ed Camera (IRC). Aims : We present unprecedented observational results of the 9 and 18um AKARI al l-sky survey and detail the operation and data processing leading to the point s ource detection and measurements. Methods : The raw data are processed to produce small images for every scan and point sources candidates, above the 5-sigma noise level per single scan, are der ived. The celestial coordinates and fluxes of the events are determined statisti cally and the reliability of their detections is secured through multiple detect ions of the same source within milli-seconds, hours, and months from each other. Results : The sky coverage is more than 90% for both bands. A total of 877,091 s ources (851,189 for 9um, 195,893 for 18um) are confirmed and included in the cur rent release of the point source catalogue. The detection limit for point source s is 50mJy and 90mJy for the 9um and 18um bands, respectively. The position accu racy is estimated to be better than 2". Uncertainties in the in-flight absolute flux calibration are estimated to be 3% for the 9um band and 4% for the 18um ban d. The coordinates and fluxes of detected sources in this survey are also compar ed with those of the IRAS survey and found to be statistically consistent.

astro-ph.IM↗

arXiv2009

Mass Loss History of the AGB star, R Cas

We report here on the discovery of an extended far-infrared shell around the AGB star, R Cassiopeia, made by AKARI and Spitzer. The extended, cold circumstellar shell of R Cas spans nearly 3 arcmin and is probably shaped by interaction with the interstellar medium. This report is one of several studies of well-resolved mass loss histories of AGB stars under AKARI and Spitzer observing programs labeled "Excavating Mass Loss History in Extended Dust Shells of Evolved Stars (MLHES)".

astro-ph.GA↗

arXiv2009

3-D Dynamics of Interactions between Stellar Winds and the Interstellar Medium as Seen by AKARI and Spitzer

Recent far-infrared mapping of mass-losing stars by the AKARI Infrared Astronomy Satellite and Spitzer Space Telescope have suggested that far-infrared bow shock structures are probably ubiquitous around these mass-losing stars, especially when these stars have high proper motion. Higher spatial resolution data of such far-infrared bow shocks now allow detailed fitting to yield the orientation of the bow shock cone with respect to the heliocentric space motion vector of the central star, using the analytical solution for these bow shocks under the assumption of momentum conservation across a physically thin interface between the stellar winds and interstellar medium (ISM). This fitting analysis of the observed bow shock structure would enable determination of the ambient ISM flow vector, founding a new technique to probe the 3-D ISM dynamics that are local to these interacting systems. In this review, we will demonstrate this new technique for three particular cases, Betelgeuse, R Hydrae, and R Cassiopeiae.

astro-ph.GA↗

arXiv2009

Calibration and Performance of the AKARI Far-Infrared Surveyor (FIS) -- Slow-Scan Observation Mode for Point Sources

We present the characterization and calibration of the Slow-Scan observation mode of the Far-Infrared Surveyor (FIS) onboard the AKARI satellite. The FIS, one of the two focal-plane instruments on AKARI, has four photometric bands between 50--180 um with two types of Ge:Ga array detectors. In addition to the All-Sky Survey, FIS has also taken detailed far-infrared images of selected targets by using the Slow-Scan mode. The sensitivity of the Slow-Scan mode is one to two orders of magnitude better than that of the All-Sky Survey, because the exposure time on a targeted source is much longer. The point spread functions (PSFs) were obtained by observing several bright point-like objects such as asteroids, stars, and galaxies. The derived full widths at the half maximum (FWHMs) are ~30'' for the two shorter wavelength bands and ~40'' for the two longer wavelength bands, being consistent with those expected by the optical simulation, although a certain amount of excess is seen in the tails of the PSFs. The flux calibration has been performed by the observations of well-established photometric calibration standards (asteroids and stars) in a wide range of fluxes. After establishing the method of aperture photometry, the photometric accuracy for point-sources is better than +-15% in all of the bands expect for the longest wavelength.

astro-ph.IM↗

arXiv2008

AKARI/FIS Mapping of the ISM-Wind Bow Shock around Alpha Ori

We present 10' x 50' scan maps around an M supergiant Alpha Ori at 65, 90, 140 and 160 microns obtained with the AKARI Infrared Astronomy Satellite. Higher spatial resolution data with the exact analytic solution permit us to fit the de-projected shape of the stellar wind bow shock around Alpha Ori to have the stand-off distance of 4.8', position angle of 55 degrees and inclination angle of 56 degrees. The shape of the bow shock suggests that the peculiar velocity of Alpha Ori with respect to the local medium is v_* = 40 (n_H)^(-1/2), where n_H is the hydrogen nucleus density at Alpha Ori. We find that the local medium is of n_H = 1.5 to 1.9 cm^(-3) and the velocity of the local flow is at 11 km s^(-1) by using the most recent astrometric solutions for Alpha Ori under the assumption that the local medium is moving away from the Orion OB 1 association. AKARI images may also reveal a vortex ring due to instabilities on the surface of the bow shock as demonstrated by numerical models. This research exemplifies the potential of AKARI All-Sky data as well as follow-up observations with Herschel Space Telescope and Stratospheric Observatory for Infrared Astronomy for this avenue of research in revealing the nature of interaction between the stellar wind and interstellar medium.

astro-ph↗

arXiv2008

Timeline analysis and wavelet multiscale analysis of the AKARI All-Sky Survey at 90 micron

We present a careful analysis of the point source detection limit of the AKARI All-Sky Survey in the WIDE-S 90 $μ$m band near the North Ecliptic Pole (NEP). Timeline Analysis is used to detect IRAS sources and then a conversion factor is derived to transform the peak timeline signal to the interpolated 90 $μ$m flux of a source. Combined with a robust noise measurement, the point source flux detection limit at S/N $>5$ for a single detector row is $1.1\pm0.1$ Jy which corresponds to a point source detection limit of the survey of $\sim$0.4 Jy. Wavelet transform offers a multiscale representation of the Time Series Data (TSD). We calculate the continuous wavelet transform of the TSD and then search for significant wavelet coefficients considered as potential source detections. To discriminate real sources from spurious or moving objects, only sources with confirmation are selected. In our multiscale analysis, IRAS sources selected above $4σ$ can be identified as the only real sources at the Point Source Scales. We also investigate the correlation between the non-IRAS sources detected in Timeline Analysis and cirrus emission using wavelet transform and contour plots of wavelet power spectrum. It is shown that the non-IRAS sources are most likely to be caused by excessive noise over a large range of spatial scales rather than real extended structures such as cirrus clouds.

astro-ph↗

arXiv2007

The Far-Infrared Surveyor (FIS) for AKARI

The Far-Infrared Surveyor (FIS) is one of two focal plane instruments on the AKARI satellite. FIS has four photometric bands at 65, 90, 140, and 160 um, and uses two kinds of array detectors. The FIS arrays and optics are designed to sweep the sky with high spatial resolution and redundancy. The actual scan width is more than eight arcmin, and the pixel pitch is matches the diffraction limit of the telescope. Derived point spread functions (PSFs) from observations of asteroids are similar to the optical model. Significant excesses, however, are clearly seen around tails of the PSFs, whose contributions are about 30% of the total power. All FIS functions are operating well in orbit, and its performance meets the laboratory characterizations, except for the two longer wavelength bands, which are not performing as well as characterized. Furthermore, the FIS has a spectroscopic capability using a Fourier transform spectrometer (FTS). Because the FTS takes advantage of the optics and detectors of the photometer, it can simultaneously make a spectral map. This paper summarizes the in-flight technical and operational performance of the FIS.

astro-ph↗

arXiv2007

The Far-Infrared Properties of Spatially Resolved AKARI Observations

We present the spatially resolved observations of IRAS sources from the Japanese infrared astronomy satellite AKARI All-Sky Survey during the performance verification (PV) phase of the mission. We extracted reliable point sources matched with IRAS point source catalogue. By comparing IRAS and AKARI fluxes, we found that the flux measurements of some IRAS sources could have been over or underestimated and affected by the local background rather than the global background. We also found possible candidates for new AKARI sources and confirmed that AKARI observations resolved IRAS sources into multiple sources. All-Sky Survey observations are expected to verify the accuracies of IRAS flux measurements and to find new extragalactic point sources.

astro-ph↗

arXiv2005

Very Large Telescope three micron spectra of dust-enshrouded red giants in the Large Magellanic Cloud

We present ESO/VLT spectra in the 2.9--4.1 micron range for a large sample of infrared stars in the Large Magellanic Cloud (LMC), selected on the basis of MSX and 2MASS colours to be extremely dust-enshrouded AGB star candidates. Out of 30 targets, 28 are positively identified as carbon stars, significantly adding to the known population of optically invisible carbon stars in the LMC. We also present spectra for six IR-bright stars in or near three clusters in the LMC, identifying four of them as carbon stars and two as oxygen-rich supergiants. We analyse the molecular bands of C2H2 at 3.1 and 3.8 micron, HCN at 3.57 micron, and sharp absorption features in the 3.70--3.78 micron region that we attribute to C2H2. There is evidence for a generally high abundance of C2H2 in LMC carbon stars, suggestive of high carbon-to-oxygen abundance ratios at the low metallicity in the LMC. The low initial metallicity is also likely to have resulted in less abundant HCN and CS. The sample of IR carbon stars exhibits a range in C2H2:HCN abundance ratio. We do not find strong correlations between the properties of the molecular atmosphere and circumstellar dust envelope, but the observed differences in the strengths and shapes of the absorption bands can be explained by differences in excitation temperature. High mass-loss rates and strong pulsation would then be seen to be associated with a large scale height of the molecular atmosphere.

astro-ph↗

arXiv2003

ASTRO-F/FIS Observing Simulation: Detection Limits for Point Sources

We describe the observing simulation software FISVI (FIS Virtual Instrument), which was developed for the Far-Infrared Surveyor (FIS) that will be on the Japanese infrared astronomy mission ASTRO-F. The FISVI has two purposes: one is to check the specifications and performances of the ASTRO-F/FIS as a whole; the other is to prepare input data sets for the data analysis softwares prior to launch. In the FISVI, special care was taken by introducing the "Compiled PSF (Point Spread Function)" to optimise inevitable, but time-consuming, convolution processes. With the Compiled PSF, we reduce the computation time by an order of magnitude. The photon and readout noises are included in the simulations. We estimate the detection limits for point sources from the simulation of virtual patches of the sky mostly consisting of distant galaxies. We studied the importance of source confusion for simple power-law models for N(>S), the number of sources brighter than S. We found that source confusion plays a dominant role in the detection limits only for models with rapid luminosity evolution for the galaxy counts, the evolution of which is suggested by recent observations.

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