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Keitaro Takahashi

Publications and source records attributed to Keitaro Takahashi.

At least 55 records · Page 3Linked to original sources
arXiv2020

Detectability of CMB Weak Lensing and HI Cross Correlation and constraints on cosmological parameters

Neutral hydrogen (H$_{\rm I}$) intensity mapping is capable of measuring redshift evolution of H$_{\rm I}$ density parameter $Ω_{\rm H_{\rm I}}$, which is an important parameter to understand structure formation in the post-reionization epoch. Future H$_{\rm I}$ observation with Square Kilometre Array (SKA) can significantly improve constraints on the parameter. However, the observation of H$_{\rm I}$ suffers from the contamination from extremely bright foreground emissions, and it is necessary to consider a signal validation method complementary to the measurement of H$_{\rm I}$ auto power spectrum. In this work, we propose to take a cross correlation between a 21cm-line intensity map and a convergence map reconstructed from observation of the cosmic microwave background by Planck and estimate expected ideal constraints on cosmological parametersby ignoring the foreground contamination. We find that the SKA1-mid operated in single-dish mode has a sufficient capability to detect the cross correlation on the large scales. Further, by Fisher analysis assuming a constant linear bias of H$_{\rm I}$, $b$, and SKA1-mid observation of 1,000 hours for each of Band 1 and 2, we show that $Ω_{\rm H_{\rm I}} b$ can be constrained with a precision of $6-13\%$ at a wide range of redshifts of $0.0<z<2.0$, if we fix other cosmological parameters (density parameter of cold dark matter, $Ω_{\rm c} h^{2}$, spectral index of primordial fluctuations, $n_s$, and Hubble constant, $H_0$) using values from the Planck observations. On the other hand, small-scale measurements with interferometer mode of SKA1-mid will not have a significant impact on constraining the parameters with 1,000 hours of observation time for each of Band 1 and 2, due to the limited resolution of the CMB lensing of Planck.

astro-ph.CO↗
arXiv2020

Constraints on Primordial Power Spectrum from Galaxy Luminosity Functions

We derive constraints on primordial power spectrum, for the first time, from galaxy UV luminosity functions (LFs) at high redshifts. Since the galaxy LFs reflect an underlying halo mass function which depends on primordial fluctuations, one can constrain primordial power spectrum, particularly on small scales. We perform a Markov Chain Monte Carlo analysis by varying parameters for primordial power spectrum as well as those describing astrophysics. We adopt the UV LFs derived from Hubble Frontier Fields data at $z = 6 -10$, which enable us to probe primordial fluctuations on the scales of $k \sim 10 - 10^3~{\rm Mpc}^{-1}$. Our analysis also clarifies how the assumption on cosmology such as primordial power spectrum affects the determination of astrophysical parameters.

astro-ph.CO↗
arXiv2020

The Impact of Tandem Redundant/Sky-Based Calibration in MWA Phase II Data Analysis

Precise instrumental calibration is of crucial importance to 21-cm cosmology experiments. The Murchison Widefield Array's (MWA) Phase II compact configuration offers us opportunities for both redundant calibration and sky-based calibration algorithms; using the two in tandem is a potential approach to mitigate calibration errors caused by inaccurate sky models. The MWA Epoch of Reionization (EoR) experiment targets three patches of the sky (dubbed EoR0, EoR1, and EoR2) with deep observations. Previous work in \cite{Li_2018} and \cite{Wenyang_2019} studied the effect of tandem calibration on the EoR0 field and found that it yielded no significant improvement in the power spectrum over sky-based calibration alone. In this work, we apply similar techniques to the EoR1 field and find a distinct result: the improvements in the power spectrum from tandem calibration are significant. To understand this result, we analyze both the calibration solutions themselves and the effects on the power spectrum over three nights of EoR1 observations. We conclude that the presence of the bright radio galaxy Fornax A in EoR1 degrades the performance of sky-based calibration, which in turn enables redundant calibration to have a larger impact. These results suggest that redundant calibration can indeed mitigate some level of model-incompleteness error.

astro-ph.IM↗
arXiv2020

Faraday dispersion function of disk galaxies with axisymmetric global magnetic fields I

Faraday tomography is a novel method to probe 3-dimensional structure of magnetic fields of polarized radio sources. In this paper, we investigate Faraday dispersion function (FDF) of disk galaxies extending a simple analytic model of galactic magnetic fields developed in Ideguchi et al. (2017). The model consists of axisymmetric coherent fields and turbulent fields and we consider the effects of inclination, relative amplitude of coherent and turbulent magnetic fields and pitch angle of coherent fields. Our simple model makes it easy to obtain physical interpretation of FDFs and helps understanding observational results. We find FDFs have two peaks when galaxies are observed with non zero inclination. The gap and relative height of two peaks are dependent on the inclination angle and pitch angle but are not affected by relative amplitude of coherent and turbulent magnetic fields so much. These findings give us an important caution that two peaks in observed FDFs do not necessarily imply the presence of two separate radio sources within a beam.

astro-ph.GA↗
arXiv2020

Microlensed Radio Emission from Exoplanets

In this paper, we investigate the detectability of radio emission from exoplanets, especially hot Jupiters, which are magnified by gravitational microlensing. Because hot Jupiters have orbital periods much shorter than the characteristic timescale of microlensing, the magnification curve has a unique wavy feature depending on the orbital parameters. This feature is useful to identify radio emission from exoplanets and, in addition to magnification, makes it easier to detect exoplanets directly. We also estimate the expected event rate red of the detectable level of microlensed planetary radio emissions, assuming the LOFAR and the first phase of the Square Kilometre Array.

astro-ph.EP↗
arXiv2020

Detectability of 21cm signal during the Epoch of Reionization with 21cm-Lyman-$α$ emitter cross-correlation. III. Model dependence

Detecting $\rm H_I$ 21cm line in the intergalactic medium (IGM) during the Epoch of Reionization (EoR) suffers from foreground contamination such as Galactic synchrotron and extragalactic radio sources. Cross-correlation between the 21cm line and Lyman-$α$ emitter (LAE) galaxies is a powerful tool to identify the 21cm signal since the 21cm line emission has correlation with LAEs while the LAEs are statistically independent of the foregrounds. So far, the detectability of 21cm-LAE cross-power spectrum has been investigated with simple LAE models where the observed Ly$α$ luminosity is proportional to the dark matter halo mass. However, the previous models were inconsistent with the latest observational data of LAEs obtained with Subaru/Hyper Suprime-Cam (HSC). Here, we revisit the detectability of 21cm-LAE cross-power spectrum adopting a state-of-the-art LAE model consistent with all Subaru/HSC observations such as the Ly$α$ luminosity function, LAE angular auto-correlation, and the LAE fractions in the continuum selected galaxies. We find that resultant cross-power spectrum with the updated LAE model is reduced at small scales ($k\sim 1\ \rm Mpc^{-1}$) compared to the simple models, while the amplitudes at large scales ($k \lesssim 0.2 \ \rm Mpc^{-1}$) are not affected so much. We conclude that the large-scale signal would be detectable with Square Kilometre Array (SKA) and HSC LAE cross-correlation but detecting the small scale signal would require an extended HSC LAE survey with an area of $\sim 75\ \rm deg^2$ or 3000 hrs observation time of 21cm line with SKA.

astro-ph.CO↗
arXiv2020

Probing Small Scale Primordial Power Spectrum with 21cm Line Global Signal

We argue that the global signal of neutral hydrogen 21cm line can be a powerful probe of primordial power spectrum on small scales. Since the amplitude of small scale primordial fluctuations is important to determine the early structure formation and the timing when the sources of Lyman $α$ photons are produced, they in turn affect the neutral hydrogen 21cm line signal. We show that the information of the position of the absorption trough can severely constrain the small scale amplitude of primordial fluctuations once astrophysical parameters relevant to the 21cm line signal are fixed. We also discuss how the uncertainties of astrophysical parameters affect the constraints.

astro-ph.CO↗
arXiv2020

Artificial neural networks for selection of pulsar candidates from the radio continuum surveys

Pulsar search with time-domain observation is very computationally expensive and data volume will be enormous with the next generation telescopes such as the Square Kilometre Array. We apply artificial neural networks (ANNs), a machine learning method, for efficient selection of pulsar candidates from radio continuum surveys, which are much cheaper than time-domain observation. With observed quantities such as radio fluxes, sky position and compactness as inputs, our ANNs output the "score" that indicates the degree of likeliness of an object to be a pulsar. We demonstrate ANNs based on existing survey data by the TIFR GMRT Sky Survey (TGSS) and the NRAO VLA Sky Survey (NVSS) and test their performance. Precision, which is the ratio of the number of pulsars classified correctly as pulsars to that of any objects classified as pulsars, is about 96$\%$. Finally, we apply the trained ANNs to unidentified radio sources and our fiducial ANN with five inputs (the galactic longitude and latitude, the TGSS and NVSS fluxes and compactness) generates 2,436 pulsar candidates from 456,866 unidentified radio sources. These candidates need to be confirmed if they are truly pulsars by time-domain observations. More information such as polarization will narrow the candidates down further.

astro-ph.IM↗
arXiv2019

Constraints on ultra-low-frequency gravitational waves with statistics of pulsar spin-down rates

We probe ultra-low-frequency gravitational waves (GWs) with statistics of spin-down rates of milli-second pulsars (MSPs) by a method proposed in our prevous work (Yonemaru et al. 2016). The considered frequency range is $10^{-12}{\rm Hz} \lesssim f_{\rm GW} \lesssim 10^{-10}$Hz, which cannot be accessed by the conventional pulsar timing array. The effect of such low-frequency GWs appears as a bias to spin-down rates which has a quadrupole pattern in the sky. We use the skewness of the spin-down rate distribution and the number of MSPs with negative spin-down rates to search for the bias induced by GWs. Applying this method to 149 MSPs selected from the ATNF pulsar catalog, we derive upper bounds on the time derivative of the GW amplitudes of $\dot{h} < 6.2 \times 10^{-18}~{\rm sec}^{-1}$ and $\dot{h} < 8.1 \times 10^{-18}~{\rm sec}^{-1}$ in the directions of the Galactic Center and M87, respectively. Approximating the GW amplitude as $\dot{h} \sim 2 πf_{\rm GW} h$, the bounds translate into $h < 3 \times 10^{-9}$ and $h < 4 \times 10^{-9}$, respectively, for $f_{\rm GW} = 1/(100~{\rm yr})$. Finally, we give the implications to possible super-massive black hole binaries at these sites.

astro-ph.HE↗
arXiv2019

Detailed study of detection method for ultra-low-frequency gravitational waves with pulsar spin-down rate statistics

A new detection method for gravitational waves (GWs) with ultra-low frequencies ($f_{\rm GW} \lesssim 10^{-10}~{\rm Hz}$), which is much lower than the range of pulsar timing arrays (PTAs), was proposed in Yonemaru et al. (2016). This method utilizes the statistical properties of spin-down rates of milli-second pulsars (MSPs) and the sensitivity was evaluated in Yonemaru et al. (2018). There, some simplifying assumptions, such as neglect of the "pulsar term" and spatially uniform distribution of MSPs, were adopted and the sensitivity on the time derivative of GW amplitude was estimated to be $10^{-19}~{\rm s}^{-1}$ independent of the direction, polarization and frequency of GWs. In this paper, extending the previous analysis, realistic simulations are performed to evaluate the sensitivity more reasonably. We adopt a model of 3-dimensional pulsar distribution in our Galaxy and take the pulsar term into account. As a result, we obtain expected sensitivity as a function of the direction, polarization and frequency of GWs. The dependence on GW frequency is particularly significant and the sensitivity becomes worse by a few orders for $< 10^{-12}~{\rm Hz}$ compared to the previous estimates.

astro-ph.HE↗
arXiv2018

Faraday Tomography with Sparse Modeling

Faraday tomography (or rotation measure synthesis) is a procedure to convert linear polarization spectra into the Faraday dispersion function, which provides us with unique information of magneto-ionic media along the line of sight. Mathematical formulation of Faraday tomography is similar to polarimetric imaging of radio interferometry, where many new methods have been actively developed and shown to outperform the standard CLEAN approaches. In this paper, we propose a sparse reconstruction technique to Faraday tomography. This technique is being developed for interferometric imaging and utilizes computationally less expensive convex regularization functions such as $\ell_1$-norm and total variation (TV) or total squared variation (TSV). The proposed technique solves a convex optimization, and therefore its solution is determined uniquely regardless of the initial condition for given regularization parameters that can be optimized by data themselves. Using a physically-motivated model of turbulent galactic magnetized plasma, we demonstrate that the proposed technique outperforms RM-CLEAN and provides higher-fidelity reconstruction. The proposed technique would be a powerful tool in broadband polarimetry with the Square Kilometre Array (SKA) and its precursors.

astro-ph.IM↗
arXiv2018

Optimum Frequency of Faraday Tomography to Explore the Inter-Galactic Magnetic Field in Filaments of Galaxies

Faraday tomography is thought to be a powerful tool to explore cosmic magnetic field. Broadband radio polarimetric data is essential to ensure the quality of Faraday tomography, but such data is not easy to obtain because of radio frequency interferences (RFIs). In this paper, we investigate optimum frequency coverage of Faraday tomography so as to explore Faraday rotation measure (RM) due to the intergalactic magnetic field (IGMF) in filaments of galaxies. We adopt a simple model of the IGMF and estimate confidence intervals of the model parameters using the Fisher information matrix. We find that meaningful constraints for RM due to the IGMF are available with data at multiple narrowbands which are scattered over the ultra-high frequency (UHF, 300 MHz - 3000 MHz). The optimum frequency depends on the Faraday thickness of the Milky Way foreground. These results are obtained for a wide brightness range of the background source including fast radio bursts (FRBs). We discuss the relation between the polarized-intensity spectrum and the optimum frequency.

astro-ph.IM↗
arXiv2018

Impact of EDGES 21cm Global Signal on Primordial Power Spectrum

We investigate the impact of the recent observation of the 21cm global signal by EDGES on primordial power spectrum, particularly focusing on the running parameters $α_s$ and $ β_s$ which characterize the detailed scale dependence of the primordial spectrum. When primordial power spectrum is enhanced/suppressed on small scales, the structure formation proceeds faster/slower and changes the abundance of small size halos, which affects the sources of Lyman $α$ radiation at high redshifts to alter the position of the absorption line. Recent observation of EDGES detected the 21cm absorption line at $z=17.2$ and this result also indicates that the brightness temperature is consistent with zero for $z \lesssim 14$ and $z \gtrsim 22$, which can exclude a scenario giving the absorption line at such redshifts. We argue that the bound on the running parameters can be obtained by requiring that the absorption line should not exceed observational bounds at such redshift ranges and found that some parameter space of $α_s$ and $β_s$ allowed by Planck may be disfavored for some values of astrophysical parameters.

astro-ph.CO↗
arXiv2018

Detectability of 21cm-signal during the Epoch of Reionization with 21cm-Lyman-$α$ emitter cross-correlation. I

Detection of the redshifted 21cm-line signal from neutral hydrogen in the intergalactic medium (IGM) during the Epoch of Reionization (EoR) is complicated by intense foregrounds such as galactic synchrotron and extragalactic radio galaxies. The 21cm-Lyman-$α$ emitter(LAE) cross-correlation is one of the tools available to reduce the foreground effects because the foreground emission from such radio sources is statistically independent of LAE distribution. LAE surveys during the EoR at redshifts $z=6.6$ and $7.3$ are ongoing by the Subaru Hyper Suprime-Cam (HSC). Additionally, Prime Focus Spectrograph (PFS) will provide precise redshift information of the LAEs discovered by the HSC survey. In this paper, we investigate the detectability of the 21cm signal with the 21cm-LAE cross-correlation by using our improved reionization simulations. We also focus on the error budget and evaluate it quantitatively in order to consider a strategy to improve the signal-to-noise ratio. In addition, we explore an expansion of the LAE survey to suggest optimal survey parameters and show a potential to measure a characteristic size of ionized bubbles via the turnover scale of the cross-power spectrum. As a result, we find that the Murchison Widefield Array (MWA) has ability to detect the cross-power spectrum signal on large scales by combining LAE Deep field survey of HSC. We also show that the sensitivity is improved dramatically at small scales by adding redshift information from the PFS measurements. The Square Kilometre Array (SKA) has a potential to measure the turnover scale with an accuracy of $6\times10^{-3}~{\rm Mpc^{-1}}$.

astro-ph.CO↗
arXiv2018

Sensitivity of new detection method for ultra-low frequency gravitational waves with pulsar spin-down rate statistics

A new detection method for ultra-low frequency gravitational waves (GWs) with a frequency much lower than the observational range of pulsar timing arrays (PTAs) was suggested in Yonemaru et al. (2016). In the PTA analysis, ultra-low frequency GWs ($\lesssim 10^{-10}$ Hz) which evolve just linearly during the observation time span are absorbed by the pulsar spin-down rates since both have the same effect on the pulse arrival time. Therefore, such GWs cannot be detected by the conventional method of PTAs. However, the bias on the observed spin-down rates depends on relative direction of a pulsar and GW source and shows a quadrupole pattern in the sky. Thus, if we divide the pulsars according to the position in the sky and see the difference in the statistics of the spin-down rates, ultra-low frequency GWs from a single source can be detected. In this paper, we evaluate the potential of this method by Monte-Carlo simulations and estimate the sensitivity, considering only the "Earth term" while the "pulsar term" acts like random noise for GW frequencies $10^{-13}-10^{-10}$ Hz. We find that with 3,000 milli-second pulsars, which are expected to be discovered by a future survey with the Square Kilometre Array, GWs with the derivative of amplitude of about $3 \times 10^{-19}~\rm{s}^{-1}$ can in principle be detected. Implications for possible supermassive binary black holes in Sgr${}^*$ and M87 are also given.

astro-ph.HE↗
arXiv2017

Performance test of QU-fitting in cosmic magnetism study

QU-fitting is a standard model-fitting method to reconstruct distribution of magnetic fields and polarized intensity along a line of sight (LOS) from an observed polarization spectrum. In this paper, we examine the performance of QU-fitting by simulating observations of two polarized sources located along the same LOS, varying the widths of the sources and the gap between them in Faraday depth space, systematically. Markov Chain Monte Carlo (MCMC) approach is used to obtain the best-fit parameters for a fitting model, and Akaike and Bayesian Information Criteria (AIC and BIC, respectively) are adopted to select the best model from four fitting models. We find that the combination of MCMC and AIC/BIC works fairly well in model selection and estimation of model parameters in the cases where two sources have relatively small widths and a larger gap in Faraday depth space. On the other hand, when two sources have large width in Faraday depth space, MCMC chain tends to be trapped in a local maximum so that AIC/BIC cannot select a correct model. We discuss the causes and the tendency of the failure of QU-fitting and suggest a way to improve it.

astro-ph.IM↗
arXiv2017

The variance of dispersion measure of high-redshift transient objects as a probe of ionized bubble size during reionization

The dispersion measure (DM) of high-redshift $(z \gtrsim 6)$ transient objects such as Fast Radio Bursts can be a powerful tool to probe the intergalactic medium during the Epoch of Reionization. In this paper, we study the variance of the DMs of objects with the same redshift as a potential probe of the size distribution of ionized bubbles. We calculate the DM variance with a simple model with randomly-distributed spherical bubbles. It is found that the DM variance reflects the characteristics of the probability distribution of the bubble size. We find the variance can be measured precisely enough to obtain the information on the typical size with a few hundred sources at a single redshift.

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