arXiv subjects

Keitaro Takahashi

Publications and source records attributed to Keitaro Takahashi.

At least 73 records · Page 4Linked to original sources

arXiv2017

Multitracer technique for galaxy bispectrum - An application to constraints on non-local primordial non-Gaussianities -

We explore the use of galaxy bispectra with multitracer technique as a possible probe of primordial non-Gaussianities. We forecast future constraints on non-linearity parameters, $f_{\rm NL}^{\rm eq}$ and $f_{\rm NL}^{\rm orth}$, which respectively characterize the equilateral- and orthogonal-types primordial bispectra, and show that the multitracer analysis would be effective with reducing the cosmic-variance noise if the number density of galaxies is high enough. We find that the measurement of galaxy bispectrum by future galaxy surveys can reach the constraints on the non-local type primordial non-Gaussianities to the level severer than current one which has been obtained by cosmic microwave background observations.

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arXiv2017

Cosmic Magnetism in Centimeter and Meter Wavelength Radio Astronomy

Magnetic field is ubiquitous in the Universe and it plays essential roles in various astrophysical phenomena, yet its real origin and evolution are poorly known. This article reviews current understanding of magnetic fields in the interstellar medium, the Milky Way Galaxy, external galaxies, active galactic nuclei, clusters of galaxies, and the cosmic web. Particularly, the review concentrates on the achievements that have been provided by centimeter and meter wavelength radio observations. The article also introduces various methods to analyze linear polarization data, including synchrotron radiation, Faraday rotation, depolarization, and Faraday tomography.

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arXiv2017

Constraining the Contribution of Galaxies and Active Galactic Nuclei to Cosmic Reionization

Understanding the detailed process of cosmic reionization is one of the remaining problems in astrophysics and cosmology. Here we construct a model of cosmic reionization that includes contribution from high-$z$ galaxies and active galactic nuclei (AGNs), and calculate the reionization and thermal histories with the model. To keep the model to be general and realistic, we vary the escape fraction of ionizing photons, $f_{\rm esc}$, and the faint-end slope of the AGN luminosity function at high redshifts, $α_{\rm hz}$, within the constraints from the observed cosmic star formation history and the observed bright-end UV luminosity functions at $z \leq 6$. Additionally, we model the spectral energy distribution (SED) of AGNs which depends on the Eddington ratio and the black-hole mass. By comparing the computed reionization histories with the observed \ion{H}{i} fractions and the optical depth for Thomson scattering from Planck, we find that $α_{\rm hz} > -1.5$ and $f_{\rm esc} < 0.15$ are favored when we employ the bright-end luminosity function obtained by Giallongo et al. Our result suggests that an AGN-dominated model with the abundance of faint-AGNs being as large as the estimate by Giallongo et al. is allowed only if the contribution from high-z galaxies is almost negligible, while a galaxy-dominant model is also allowed. We also find that the shape of SED has a significant impact on the thermal history. Therefore it is expected that measurements of the thermal state of the IGM provide useful information on properties of ionizing sources.

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arXiv2017

Study of Vertical Magnetic Field in Face-on Galaxies using Faraday Tomography

Faraday tomography allows astronomers to probe the distribution of magnetic field along the line of sight (LOS), but that can be achieved only after Faraday spectrum is interpreted. However, the interpretation is not straightforward, mainly because Faraday spectrum is complicated due to turbulent magnetic field; it ruins the one-to-one relation between the Faraday depth and the physical depth, and appears as many small-scale features in Faraday spectrum. In this paper, employing "simple toy models" for the magnetic field, we describe numerically as well as analytically the characteristic properties of Faraday spectrum. We show that Faraday spectrum along "multiple loss" can be used to extract the global properties of magnetic field. Specifically, considering face-on spiral galaxies and modeling turbulent magnetic field as a random field with single coherence length, we numerically calculate Faraday spectrum along a number of LOSs and its shape-characterizing parameters, that is, the moments. When multiple LOSs cover a region of $\gtrsim (10\ {\rm coherence\ length)^2}$, the shape of Faraday spectrum becomes smooth and the shape-characterizing parameters are well specified. With the Faraday spectrum constructed as a sum of Gaussian functions with different means and variances, we analytically show that the parameters are expressed in terms of the regular and turbulent components of LOS magnetic field and the coherence length. We also consider the turbulent magnetic field modeled with power-law spectrum, and study how the magnetic field is revealed in Faraday spectrum. Our work suggests a way toward obtaining the information of magnetic field from Faraday tomography study.

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arXiv2017

Gravitational waves from an SMBH binary in M87

In this paper, we study gravitational-wave (GW) emission from a hypothetical supermassive black-hole (SMBH) binary at the center of M87. The existence of a SMBH other than that usually identified with the central AGN is a possible explanation for the observed displacement ($\sim O(1)~{\rm pc}$) between the AGN and the galactic centroid, and it is reasonable to assume consid- ering the evolution of SMBHs through galaxy mergers. Because the period of the binary and the resulting GWs is much longer than the observational time span, we calculate the variation of the GW amplitude, rather than the amplitude itself. We investigate the dependence on the orbital elements and the second BH mass taking the observational constraints into account. The frequency of the GWs is too low to be detected with the conventional pulsar timing array and we propose a new method to detect such low-frequency GWs with the distribution func- tion of pulsar spin-down rates. Although the GWs from a SMBH binary which explains the observed displacement is extremely hard to be detected even with the new method, GWs are still a useful way to probe the M87 center.

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arXiv2017

Anisotropies in the gravitational wave background as a probe of the cosmic string network

Pulsar timing arrays are one of the powerful tools to test the existence of cosmic strings through searching for the gravitational wave background. The amplitude of the background connects to information on cosmic strings such as the tension and string network properties. In addition, one may be able to extract more information on properties of cosmic strings by measuring anisotropies in the gravitational wave (GW) background. In this paper, we provide estimates of the level of anisotropy expected in the GW background generated by cusps on cosmic strings. We find that the anisotropy level strongly depends on the initial loop size $α$, and thus we may be able to put constraint on $α$ by measuring the anisotropy of the GW background. We also find that certain regions of the parameter space can be probed by shifting the observation frequency of GWs.

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arXiv2017

Constraining the EoR model parameters with the 21cm bispectrum

We perform a Fisher analysis to estimate the expected constraints on the Epoch of Reionization (EoR) model parameters (i.e., minimum virial temperature, the ionizing efficiency and the mean free path of ionizing photons) taking into account the thermal noise of the exisitting telescopes, MWA and LOFAR. We consider how the inclusion of the 21cm bispectrum improves the constraints compared to using the power spectrum alone. Assuming that we perfectly remove the foreground, we found that the bispectrum, which is calculated by the 21cmFAST code, can constrain the EoR model parameters more tightly than the power spectrum since the bispectrum is more sensitive to the EoR model parameters than the power spectrum. We also found that degeneracy between the EoR model parameters can be broken by combining the bispectrum with the power spectrum.

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arXiv2016

Cosmology with the Square Kilometre Array by SKA-Japan

In the past several decades, the standard cosmological model has been established and its parameters have been measured to a high precision, while there are still many of the fundamental questions in cosmology; such as the physics in the very early Universe, the origin of the cosmic acceleration and the nature of the dark matter. The future world's largest radio telescope, Square Kilometre Array (SKA), will be able to open the new frontier of cosmology and will be one of the most powerful tools for cosmology in the next decade. The cosmological surveys conducted by the SKA would have the potential not only to answer these fundamental questions but also deliver the precision cosmology. In this article we briefly review the role of the SKA from the view point of the modern cosmology. The cosmology science led by the SKA-Japan Consortium (SKA-JP) Cosmology Science Working Group is also discussed.

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arXiv2016

Studying topological structure in the epoch of reionization with 3D-Minkowski functionals of 21cm line fluctuations

The brightness temperature of the redshifted 21cm line brings rich information on the Inter Galactic Medium (IGM) from the Cosmic Dawn and Epoch of Reionization (EoR). While the power spectrum is a useful tool to statistically investigate the 21cm signal, the 21cm brightness temperature field is highly non-Gaussian, and the power spectrum is inadequate to characterize the non-Gaussianity. The Minkowski Functionals (MFs) are promising tools to extract non-gaussian features of the 21cm signal and give topological information such as morphology of ionized bubbles. In this work, we study the 21cm line signal in detail with MFs. To promote understanding of basic features of the 21cm signal, we calculate the MFs of not only the hydrogen neutral fraction but the matter density and spin temperature, which contribute to the brightness temperature fluctuations. We find that the structure of the brightness temperature depends mainly on the ionized fraction and the spin temperature at late and early stages of the EoR, respectively. Further, we investigate the redshift evolution of the MFs at $7 < z < 20$. Then, we find that, after the onset of reionization, the MFs reflect mainly the ionized bubble property. In addition, the MFs are sensitive to model parameters which are related to the topology of ionized bubbles and we consider the possibility of constraining the parameters by the future 21cm signal observations.

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arXiv2016

Rapid Merger of Binary Primordial Black Holes: an Implication for GW150914

We propose a new scenario for the evolution of a binary of primordial black holes (PBHs). We consider a dynamical friction by ambient dark matter, scattering of dark matter particles with a highly eccentric orbit besides the standard two-body relaxation process to refill the loss cone, and interaction between the binary and a circumbinary disk, assuming that PBHs do not constitute the bulk of dark matter. Binary PBHs lose the energy and angular momentum by these processes, which could be sufficiently efficient for a typical configuration. Such a binary coalesces due to the gravitational wave emission in a time scale much shorter than the age of the universe. We estimate the density parameter of the resultant gravitational wave background. Astrophysical implication concerning the formation of intermediate-mass to supermassive black holes is also discussed.

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arXiv2016

Probing higher-order primordial non-Gaussianity with galaxy surveys

With a radio continuum galaxy survey by the Square Kilometre Array (SKA), a photometric galaxy survey by Euclid and their combination, we forecast future constraints on primordial non-Gaussianity. We focus on the potential impact of local-type higher-order nonlinear parameters on the parameter estimation and particularly the confirmation of the inflationary consistency inequality. Nonstandard inflationary models, such as multifield models, introduce the scale-dependent stochastic clustering of galaxies on large scales, which is a unique probe of mechanism for generating primordial density fluctuations. Our Fisher matrix analysis indicates that a deep and wide survey provided by SKA is more advantageous to constrain $τ_{\rm NL}$, while Euclid has a strong constraining power for $f_{\rm NL}$ due to the redshift information, suggesting that the joint analysis between them is quite essential to break the degeneracy between the nonlinear parameters. The combination of the full SKA and Euclid will achieve the precision level needed to confirm the consistency inequality even for $f_{\rm NL}\approx 1.5$ and $τ_{\rm NL}\approx 17$, though it is still hard for a single survey to confirm it when $f_{\rm NL}\lesssim 2.7$.

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arXiv2016

Performance test of RM CLEAN and its evaluation with chi-square value

RM CLEAN is a standard method to reconstruct the distribution of cosmic magnetic fields and polarized sources along the line of sight (LOS) from observed polarization spectrum. This method is similar to the CLEAN algorithm for aperture synthesis radio telescope images but it is rather unclear in what cases RM CLEAN works well. In this paper, we evaluate the performance of RM CLEAN by simulating spectro-polarimetric observations of two compact sources located in the same LOS, varying the relative initial polarization angle and Faraday depth systematically. Especially, we focus on if the two polarized sources can be resolved in the Faraday depth space and how well the source parameters can be estimated. We confirm the previous studies that two sources cannot be resolved when they are closely located in the Faraday depth space for specific values of the relative initial polarization angle. Further, we calculate the chi-square value for the fit between the mock data of polarization spectrum and the one from RM CLEAN. Then we find that the chi-squared value is not always significantly large even when RM CLEAN gives wrong results.

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arXiv2016

Expected constraints on models of the epoch of reionization with the variance and skewness in redshifted 21cm-line fluctuations

Redshifted 21cm-line signal from neutral hydrogens in the intergalactic medium (IGM) gives a direct probe of the epoch of reionization (EoR). In this paper, we investigate the potential of the variance and skewness of the probability distribution function of the 21cm brightness temperature for constraining EoR models. These statistical quantities are simple, easy to calculate from the observed visibility and thus suitable for the early exploration of the EoR with ongoing telescopes such as the Murchison Widefield Array (MWA) and LOw Frequency ARray (LOFAR). We show, by performing Fisher analysis, that the variance and skewness at $z=7-9$ are complementary to each other to constrain the EoR model parameters such as the minimum virial temperature of halos which host luminous objects, ionizing efficiency and mean free path of ionizing photons in the IGM. Quantitatively, the constraining power highly depends on the quality of the foreground subtraction and calibration. We give a best case estimate of the constraints on the parameters, neglecting the systematics other than the thermal noise.

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arXiv2016

21cm-line bispectrum as method to probe Cosmic Dawn and Epoch of Reionization

Redshifted 21cm signal is a promising tool to investigate the state of intergalactic medium (IGM) in the Cosmic Dawn (CD) and Epoch of Reionization(EoR). In our previous work (Shimabukuro et al 2015), we studied the variance and skewness to give a clear interpretation of 21cm power spectrum and found that skewness is a good indicator of the epoch when X-ray heating becomes effective. Thus, the non-Gaussian feature of the spatial distribution of the 21cm signal is expected to be useful to investigate the astrophysical effects in the CD and EoR. In this paper, in order to investigate such a non-Gaussian feature in more detail, we focus on the bispectrum of the 21cm signal. It is expected that the 21cm brightness temperature bispectrum is produced by non-gaussianity due to the various astrophysical effects such as Wouthysen-Field (WF) effect, X-ray heating and reionization. We study the various properties of 21cm bispectrum such as scale dependence, shape dependence and redshift evolution. And also we study the contribution from each component of 21cm bispectrum. We find that the contribution from each component has characteristic scale-dependent feature, and it is expected that we could obtain more detailed information on the IGM in the CD and EoR by using the 21cm bispectrum in the future experiments, combined with the power spectrum and skewness.

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arXiv2016

Resolving 4-D Nature of Magnetism with Depolarization and Faraday Tomography: Japanese SKA Cosmic Magnetism Science

Magnetic fields play essential roles in various astronomical objects. Radio astronomy has revealed that magnetic fields are ubiquitous in our Universe. However, the real origin and evolution of magnetic fields is poorly proven. In order to advance our knowledge of cosmic magnetism in coming decades, the Square Kilometre Array (SKA) should have supreme sensitivity than ever before, which provides numerous observation points in the cosmic space. Furthermore, the SKA should be designed to facilitate wideband polarimetry so as to allow us to examine sightline structures of magnetic fields by means of depolarization and Faraday Tomography. The SKA will be able to drive cosmic magnetism of the interstellar medium, the Milky Way, galaxies, AGN, galaxy clusters, and potentially the cosmic web which may preserve information of the primeval Universe. The Japan SKA Consortium (SKA-JP) Magnetism Science Working Group (SWG) proposes the project "Resolving 4-D Nature of Magnetism with Depolarization and Faraday Tomography", which contains ten scientific use cases.

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arXiv2016

SKA-Japan Pulsar Science with the Square Kilometre Array

The Square Kilometre Array will revolutionize pulsar studies with its wide field-of-view, wide-band observation and high sensitivity, increasing the number of observable pulsars by more than an order of magnitude. Pulsars are of interest not only for the study of neutron stars themselves but for their usage as tools for probing fundamental physics such as general relativity, gravitational waves and nuclear interaction. In this article, we summarize the activity and interests of SKA-Japan Pulsar Science Working Group, focusing on an investigation of modified gravity theory with the supermassive black hole in the Galactic Centre, gravitational-wave detection from cosmic strings and binary supermassive black holes, a study of the physical state of plasma close to pulsars using giant radio pulses and determination of magnetic field structure of Galaxy with pulsar pairs.

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arXiv2016

Japanese Cosmic Dawn/Epoch of Reionization Science with the Square Kilometre Array

Cosmic reionization is known to be a major phase transition of the gas in the Universe. Since astronomical objects formed in the early Universe, such as the first stars, galaxies and black holes, are expected to have caused cosmic reionization, the formation history and properties of such objects are closely related to the reionization process. In spite of the importance of exploring reionization, our understandings regarding reionization is not sufficient yet. Square Kilometre Array (SKA) is a next-generation large telescope that will be operated in the next decade. Although several programs of next-generation telescopes are currently scheduled, the SKA will be the unique telescope with a potential to directly observe neutral hydrogen up to z~30, and provide us with valuable information on the Cosmic Dawn (CD) and the Epoch of Reionization (EoR). The early science with the SKA will start in a few years; it is thus the time for us to elaborate a strategy for CD/EoR Science with the SKA. The purpose of this document is to introduce Japanese scientific interests in the SKA project and to report results of our investigation.

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arXiv2015

X-raying extended emission and rapid decay of short gamma-ray bursts

Extended emission is a mystery in short gamma-ray bursts (SGRBs). By making time resolved spectral analyses of brightest nine events observed by ${\it Swift}$ XRT, we obviously classify the early X-ray emission of SGRBs into two types. One is the extended emission with exponentially rapid decay, which shows significant spectral softening during hundreds seconds since the SGRB trigger and is also detected by ${\it Swift}$-BAT. The other is a dim afterglow only showing power-law decay over $10^4$ s. The correlations between the temporal decay and spectral indices of the extended emissions are inconsistent with the $α$-$β$ correlation expected for the high-latitude curvature emission from a uniform jet. The observed too-rapid decay suggests the emission from a photosphere or a patchy surface, and manifests the stopping central engine via such as magnetic reconnection at the black hole.

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