arXiv subjects

Hajime Kawahara

Publications and source records attributed to Hajime Kawahara.

At least 73 records · Page 4Linked to original sources

arXiv2014

Spectroscopic Coronagraphy for Planetary Radial Velocimetry of Exoplanets

We propose the application of coronagraphic techniques to the spectroscopic direct detection of exoplanets via the Doppler shift of planetary molecular lines. Even for an unresolved close-in planetary system, we show that the combination of a visible nuller and an extreme adaptive optics system can reduce the photon noise of a main star and increase the total signal-to-noise ratio (S/N) of the molecular absorption of the exoplanetary atmosphere: it works as a spectroscopic coronagraph. Assuming a 30 m telescope, we demonstrate the benefit of these high-contrast instruments for nearby close-in planets that mimic 55 Cnc b ($0.6 λ/D$ of the angular separation in the K band). We find that the tip-tilt error is the most crucial factor; however, low-order speckles also contribute to the noise. Assuming relatively conservative estimates for future wavefront control techniques, the spectroscopic coronagraph can increase the contrast to $ \sim 50-130 $ times and enable us to obtain $\sim 3-6 $ times larger S/N for warm Jupiters and Neptunes at 10 pc those without it. If the tip-tilt error can be reduced to $\lesssim 0.3$ mas (rms), it gains $\sim 10-30$ times larger S/N and enables us to detect warm super-Earths with an extremely large telescope. This paper demonstrates the concept of spectroscopic coronagraphy for future spectroscopic direct detection. Further studies of the selection of coronagraphs and tip-tilt sensors will extend the range of application of the spectroscopic direct detection beyond the photon collecting area limit.

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arXiv2013

Exploring Hot Gas at Junctions of Galaxy Filaments with Suzaku

We performed five pointing observations with Suzaku to search for hot gases associated with the junctions of galaxy filaments where no significant diffuse X-ray sources were detected so far. We discovered X-ray sources successfully in all five regions and analyzed two bright sources in each field. Spectral analysis indicates that three sources originate from X-ray diffuse halos associated with optically bright galaxies or groups of galaxies with kT~0.6-0.8 keV. Other three sources are possibly group- and cluster-scale X-ray halos with temperatures of ~1 keV and ~4 keV, respectively while the others are compact object origins such as AGNs. All the three observed intracluster media within the junctions of the galaxy filaments previously found are involved in ongoing mergers. Thus, we demonstrate that deep X-ray observations at the filament junctions identified by galaxy surveys are a powerful mean to explore growing halos in a hierarchical structure undetected so far.

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arXiv2013

Starspots - Transit Depth Relation of the Evaporating Planet Candidate KIC 12557548b

Violent variation of transit depths and an ingress-egress asymmetry of the transit light curve discovered in KIC 12557548 have been interpreted as evidences of a catastrophic evaporation of atmosphere with dust (M_p gtrsim 1 M_oplus/Gyr) from a close-in small planet. To explore what drives the anomalous atmospheric escape, we perform time-series analysis of the transit depth variation of Kepler archival data for ~ 3.5 yr. We find a ~ 30% periodic variation of the transit depth with P1 = 22.83 pm 0.21 days, which is within the error of the rotation period of the host star estimated using the light curve modulation, Prot = 22.91 pm 0.24 days. We interpret the results as evidence that the atmospheric escape of KIC 12557548b correlates with stellar activity. We consider possible scenarios that account for both the mass loss rate and the correlation with stellar activity. X-ray and ultraviolet (XUV)-driven evaporation is possible if one accepts a relatively high XUV flux and a high efficiency for converting the input energy to the kinetic energy of the atmosphere. Star-planet magnetic interaction is another possible scenario though huge uncertainty remains for the mass loss rate.

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arXiv2013

Probing the extreme planetary atmosphere of WASP-12b

We report near-infrared measurements of the terminator region transmission spectrum and dayside emission spectrum of the exoplanet WASP-12b obtained using the HST WFC3 instrument. The disk-average dayside brightness temperature averages about 2900 K, peaking to 3200 K around 1.46 microns. We modeled a range of atmospheric cases for both the emission and transmission spectrum and confirm the recent finding by Crossfield et al. (2012b) that there is no evidence for C/O >1 in the atmosphere of WASP-12b. Assuming a physically plausible atmosphere, we find evidence that the presence of a number of molecules is consistent with the data, but the justification for inclusion of these opacity sources based on the Bayesian Information Criterion (BIC) is marginal. We also find the near-infrared primary eclipse light curve is consistent with small amounts of prolate distortion. As part of the calibration effort for these data, we conducted a detailed study of instrument systematics using 65 orbits of WFC3-IR grims observations. The instrument systematics are dominated by detector-related affects, which vary significantly depending on the detector readout mode. The 256x256 subarray observations of WASP 12 produced spectral measurements within 15% of the photon-noise limit using a simple calibration approach. Residual systematics are estimated to be less than 70 parts per million.

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arXiv2013

Validity of Hydrostatic Equilibrium in Galaxy Clusters from Cosmological Hydrodynamical Simulations

We examine the validity of the hydrostatic equilibrium (HSE) assumption for galaxy clusters using one of the highest-resolution cosmological hydrodynamical simulations. We define and evaluate several effective mass terms corresponding to the Euler equations of the gas dynamics, and quantify the degree of the validity of HSE in terms of the mass estimate. We find that the mass estimated under the HSE assumption (the HSE mass) deviates from the true mass by up to ~ 30 %. This level of departure from HSE is consistent with the previous claims, but our physical interpretation is rather different. We demonstrate that the inertial term in the Euler equations makes a negligible contribution to the total mass, and the overall gravity of the cluster is balanced by the thermal gas pressure gradient and the gas acceleration term. Indeed the deviation from the HSE mass is well explained by the acceleration term at almost all radii. We also clarify the confusion of previous work due to the inappropriate application of the Jeans equations in considering the validity of HSE from the gas dynamics extracted from cosmological hydrodynamical simulations.

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arXiv2012

The ASTRO-H X-ray Observatory

The joint JAXA/NASA ASTRO-H mission is the sixth in a series of highly successful X-ray missions initiated by the Institute of Space and Astronautical Science (ISAS). ASTRO-H will investigate the physics of the high-energy universe via a suite of four instruments, covering a very wide energy range, from 0.3 keV to 600 keV. These instruments include a high-resolution, high-throughput spectrometer sensitive over 0.3-2 keV with high spectral resolution of Delta E < 7 eV, enabled by a micro-calorimeter array located in the focal plane of thin-foil X-ray optics; hard X-ray imaging spectrometers covering 5-80 keV, located in the focal plane of multilayer-coated, focusing hard X-ray mirrors; a wide-field imaging spectrometer sensitive over 0.4-12 keV, with an X-ray CCD camera in the focal plane of a soft X-ray telescope; and a non-focusing Compton-camera type soft gamma-ray detector, sensitive in the 40-600 keV band. The simultaneous broad bandpass, coupled with high spectral resolution, will enable the pursuit of a wide variety of important science themes.

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arXiv2012

The Spin Effect on Planetary Radial Velocimetry of Exoplanets

We consider the effect of planetary spin on the planetary radial velocity (PRV) in dayside spectra of exoplanets. To understand the spin effect qualitatively, we derive an analytic formula of the intensity-weighted radial velocity from planetary surface on the following assumptions: 1) constant and solid rotation without precession, 2) stable and uniform distribution of molecules/atoms, 3) emission models from dayside hemisphere, and 4) a circular orbit. On these assumptions, we find that the curve of the PRV is distorted by the planetary spin and this anomaly is characterized by spin radial velocity at equator and a projected angle on a celestial plane between the spin axis and the axis of orbital motion λ_p in a manner analogous to the Rossiter-McLaughlin effect. The latter can constrain the planetary obliquity. Creating mock PRV data with 3 km/s accuracy, we demonstrate how λ_p and the spin radial velocity at equator are estimated. We find that the stringent constraint of eccentricity is crucial to detect the spin effect. Though our formula is still qualitative, we conclude that the PRV in the dayside spectra will be a powerful means for constraining the planetary spin.

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arXiv2012

Systematic X-ray Analysis of Radio Relic Clusters with SUZAKU

We perform a systematic X-ray analysis of six giant radio relics in four clusters of galaxies using the Suzaku satellite. The sample includes CIZA 2242.8-5301, Zwcl 2341.1-0000, the South-East part of Abell 3667 and previously published results of the North-West part of Abell 3667 and Abell 3376. Especially we first observed the narrow (50 kpc) relic of CIZA 2242.8-5301 by Suzaku satellite, which enable us to reduce the projection effect. We report X-ray detections of shocks at the position of the relics in CIZA2242.8-5301 and Abell 3667 SE. At the position of the two relics in ZWCL2341.1-0000, we do not detect shocks. From the spectroscopic temperature profiles across the relic, we find that the temperature profiles exhibit significant jumps across the relics for CIZA 2242.8-5301, Abell 3376, Abell 3667NW, and Abell 3667SE. We estimated the Mach number from the X-ray temperature or pressure profile using the Rankine-Hugoniot jump condition and compared it with the Mach number derived from the radio spectral index. The resulting Mach numbers (M=1.5-3) are almost consistent with each other, while the Mach number of CIZA2242 derived from the X-ray data tends to be lower than that of the radio observation. These results indicate that the giant radio relics in merging clusters are related to the shock structure, as suggested by previous studies of individual clusters.

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arXiv2012

Mapping Earth Analogs from Photometric Variability: Spin-Orbit Tomography for Planets in Inclined Orbits

Aiming at obtaining detailed information of surface environment of Earth analogs, Kawahara & Fujii (2011) proposed an inversion technique of annual scattered light curves named the spin-orbit tomography (SOT), which enables one to sketch a two-dimensional albedo map from annual variation of the disk-integrated scattered light, and demonstrated the method with a planet in a face-on orbit. We extend it to be applicable to general geometric configurations, including low-obliquity planets like the Earth in inclined orbits. We simulate light curves of the Earth in an inclined orbit in three photometric bands (0.4-0.5um, 0.6-0.7um, and 0.8-0.9um) and show that the distribution of clouds, snow, and continents is retrieved with the aid of the SOT. We also demonstrate the SOT by applying it to an upright Earth, a tidally locked Earth, and Earth analogs with ancient continental configurations. The inversion is model independent in the sense that we do not assume specific albedo models when mapping the surface, and hence applicable in principle to any kind of inhomogeneity. This method can potentially serve as a unique tool to investigate the exohabitats/exoclimes of Earth analogs.

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arXiv2012

Can Ground-based Telescopes Detect The Oxygen 1.27 Micron Absorption Feature as a Biomarker in Exoplanets ?

The oxygen absorption line imprinted in the scattered light from the Earth-like planets has been considered the most promising metabolic biomarker of the exo-life. We examine the feasibility of the detection of the 1.27 micron oxygen band from habitable exoplanets, in particular, around late- type stars observed with a future instrument on a 30 m class ground-based telescope. We analyzed the night airglow around 1.27 micron with IRCS/echelle spectrometer on Subaru and found that the strong telluric emission from atmospheric oxygen molecules declines by an order of magnitude by midnight. By compiling nearby star catalogs combined with the sky background model, we estimate the detectability of the oxygen absorption band from an Earth twin, if it exists, around nearby stars. We find that the most dominant source of photon noise for the oxygen 1.27 micron band detection comes from the night airglow if the contribution of the stellar PSF halo is suppressed enough to detect the planet. We conclude that the future detectors for which the detection contrast is limited by photon noise can detect the oxygen 1.27 micron absorption band of the Earth twins for ~50 candidates of the late type star. This paper demonstrates the importance of deploying small inner working angle efficient coronagraph and extreme adaptive optics on extremely large telescopes, and clearly shows that doing so will enable study of potentially habitable planets.

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arXiv2011

Mapping Clouds and Terrain of Earth-like Planets from Photometric Variability: Demonstration with Planets in Face-on Orbits

We develop an inversion technique of annual scattered light curves to sketch a two-dimensional albedo map of exoplanets in face-on orbits. As a test-bed for future observations of extrasolar terrestrial planets, we apply this mapping technique to simulated light curves of a mock Earth-twin at a distance of 10 pc in a face-on circular orbit. A primary feature in recovered albedo maps traces the annual mean distribution of clouds. To extract information of other surface types, we attempt to reduce the cloud signal by taking difference of two bands. We find that the inversion of reflectivity difference between 0.8-0.9 and 0.4-0.5 micron bands roughly recover the continental distribution, except for high latitude regions persistently covered with clouds and snow. The inversion of the reflectivity difference across the red edge (0.8-0.9 and 0.6-0.7 micron) emphasizes the vegetation features near the equator. The planetary obliquity and equinox can be estimated simultaneously with the mapping under the presence of clouds. We conclude that the photometric variability of the scattered light will be a powerful means for exploring the habitat of a second Earth.

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arXiv2011

Colors of a Second Earth II: Effects of Clouds on Photometric Characterization of Earth-like Exoplanets

As a test-bed for future investigations of directly imaged terrestrial exoplanets, we present the recovery of the surface components of the Earth from multi-band diurnal light curves obtained with the EPOXI spacecraft. We find that the presence and longitudinal distribution of ocean, soil and vegetation are reasonably well reproduced by fitting the observed color variations with a simplified model composed of a priori known albedo spectra of ocean, soil, vegetation, snow and clouds. The effect of atmosphere, including clouds, on light scattered from surface components is modeled using a radiative transfer code. The required noise levels for future observations of exoplanets are also determined. Our model-dependent approach allows us to infer the presence of major elements of the planet (in the case of the Earth, clouds and ocean) with observations having S/N $\gtrsim 10$ in most cases and with high confidence if S/N $\gtrsim 20$. In addition, S/N $\gtrsim 100$ enables us to detect the presence of components other than ocean and clouds in a fairly model-independent way. Degradation of our inversion procedure produced by cloud cover is also quantified. While cloud cover significantly dilutes the magnitude of color variations compared to the cloudless case, the pattern of color changes remains. Therefore, the possibility of investigating surface features through light curve fitting remains even for exoplanets with cloud cover similar to the Earth's.

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arXiv2011

Studying the Warm-Hot Intergalactic Medium in Emission

We assess the possibility to detect the warm-hot intergalactic medium (WHIM) in emission and to characterize its physical conditions and spatial distribution through spatially resolved X-ray spectroscopy, in the framework of the recently proposed DIOS, EDGE, Xenia, and ORIGIN missions, all of which are equipped with microcalorimeter-based detectors. For this purpose we analyze a large set of mock emission spectra, extracted from a cosmological hydrodynamical simulation. These mock X-ray spectra are searched for emission features showing both the OVII K alpha triplet and OVIII Ly alpha line, which constitute a typical signature of the warm hot gas. Our analysis shows that 1 Ms long exposures and energy resolution of 2.5 eV will allow us to detect about 400 such features per deg^2 with a significance >5 sigma and reveals that these emission systems are typically associated with density ~100 above the mean. The temperature can be estimated from the line ratio with a precision of ~20%. The combined effect of contamination from other lines, variation in the level of the continuum, and degradation of the energy resolution reduces these estimates. Yet, with an energy resolution of 7 eV and all these effects taken into account, one still expects about 160 detections per deg^2. These line systems are sufficient to trace the spatial distribution of the line-emitting gas, which constitute an additional information, independent from line statistics, to constrain the poorly known cosmic chemical enrichment history and the stellar feedback processes.

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arXiv2011

SUZAKU Observation of a New Merging Group of Galaxies at a Filamentary Junction

We report on a new merging group of galaxies, Suzaku J1552+2739 at z ~ 0.08 revealed by a SUZAKU observation. The group was found by observing a junction of galaxy filaments optically identified in the Sloan Digital Sky Survey spectroscopic data. Suzaku J1552+2739 exhibits an irregular morphology and presents several peaks in its X-ray image. A bright elliptical galaxy, observable in the central peak, allows the localization of the group at z = 0.083. We found a significant hot spot visible in the X-ray hardness map, close to the second peak. The spectroscopic temperature is T = 1.6+0.4-0.1 keV within R500 = 0.6 Mpc and T = 3 - 5 keV in the hot spot. We interpret those results as Suzaku J1552+2739 being located in the center of a major merging process. The observation of a galaxy group showing multiple X-ray peaks and a hot spot at the same time is rare and we believe in particular that the study of Suzaku J1552+2739 potentially presents a significant interest to better understand the dynamical and thermal evolution of the intragroup and intracluster medium, as well as its relation with surrounding environment.

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arXiv2010

The persistent cosmic web and its filamentary structure II: Illustrations

The recently introduced discrete persistent structure extractor (DisPerSE, Soubie 2010, paper I) is implemented on realistic 3D cosmological simulations and observed redshift catalogues (SDSS); it is found that DisPerSE traces equally well the observed filaments, walls, and voids in both cases. In either setting, filaments are shown to connect onto halos, outskirt walls, which circumvent voids. Indeed this algorithm operates directly on the particles without assuming anything about the distribution, and yields a natural (topologically motivated) self-consistent criterion for selecting the significance level of the identified structures. It is shown that this extraction is possible even for very sparsely sampled point processes, as a function of the persistence ratio. Hence astrophysicists should be in a position to trace and measure precisely the filaments, walls and voids from such samples and assess the confidence of the post-processed sets as a function of this threshold, which can be expressed relative to the expected amplitude of shot noise. In a cosmic framework, this criterion is comparable to friend of friend for the identifications of peaks, while it also identifies the connected filaments and walls, and quantitatively recovers the full set of topological invariants (Betti numbers) {\sl directly from the particles} as a function of the persistence threshold. This criterion is found to be sufficient even if one particle out of two is noise, when the persistence ratio is set to 3-sigma or more. The algorithm is also implemented on the SDSS catalogue and used to locat interesting configurations of the filamentary structure. In this context we carried the identification of an ``optically faint'' cluster at the intersection of filaments through the recent observation of its X-ray counterpart by SUZAKU. The corresponding filament catalogue will be made available online.

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arXiv2010

Impact of Chandra calibration uncertainties on galaxy cluster temperatures: application to the Hubble Constant

We perform a uniform, systematic analysis of a sample of 38 X-ray galaxy clusters with three different Chandra calibrations. The temperatures change systematically between calibrations. Cluster temperatures change on average by roughly ~6% for the smallest changes and roughly ~13% for the more extreme changes between calibrations. We explore the effects of the changing cluster spectral properties on Sunyaev-Zel'dovich effect (SZE) and X-ray determinations of the Hubble constant. The Hubble parameter changes by +10% and -13% between the current calibration and two previous Chandra calibrations, indicating that changes in the cluster temperature basically explain the entire change in H_0. Although this work focuses on the difference in spectral properties and resultant Hubble parameters between the calibrations, it is intriguing to note that the newer calibrations favor a lower value of the Hubble constant, H_0 ~ 60 km s-1 Mpc-1, typical of results from SZE/X-ray distances. Both galaxy clusters themselves and the details of the instruments must be known precisely to enable reliable precision cosmology with clusters, which will be feasible with combined efforts from ongoing observations and planned missions and observatories covering a wide range of wavelengths.

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arXiv2010

The axis ratio distribution of X-ray clusters observed by XMM-Newton

We derive the axis ratio distribution of X-ray clusters using the XMM-Newton catalogue (Snowden et al. 2008). By fitting the contour lines of the X-ray image by ellipses, we confirm the X-ray distribution is well approximated by the elliptic distribution with a constant axis ratio and direction. We construct a simple model describing the axis ratio of the X-ray gas assuming the hydrostatic equilibrium embedded in the triaxial dark matter halo model proposed by Jing & Suto (2002) and the hydrostatic equilibrium. We find that the observed probability density function of the axis ratio is consistent with this model prediction.

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arXiv2010

Global Mapping of Earth-like Exoplanets from Scattered Light Curves

Scattered lights from terrestrial exoplanets provide valuable information about the planetary surface. Applying the surface reconstruction method proposed by Fujii et al. (2010) to both diurnal and annual variations of the scattered light, we develop a reconstruction method of land distribution with both longitudinal and latitudinal resolutions. We find that one can recover a global map of an idealized Earth-like planet on the following assumptions: 1) cloudless, 2) a face-on circular orbit, 3) known surface types and their reflectance spectra 4) no atmospheric absorption, 5) known rotation rate 6) static map, and 7) no moon. Using the dependence of light curves on the planetary obliquity, we also show that the obliquity can be measured by adopting the chi-square minimization or the extended information criterion. We demonstrate a feasibility of our methodology by applying it to a multi-band photometry of a cloudless model Earth with future space missions such as the occulting ozone observatory (O3). We conclude that future space missions can estimate both the surface distribution and the obliquity at least for cloudless Earth-like planets within 5 pc.

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