arXiv subjects

Masahiro Tsujimoto

Publications and source records attributed to Masahiro Tsujimoto.

At least 37 records · Page 2Linked to original sources

arXiv2024

JASMINE: Near-Infrared Astrometry and Time Series Photometry Science

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

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arXiv2023

Spectral Modeling of the Supersoft X-ray Source CAL87 based on Radiative Transfer Codes

Super Soft X-ray Sources (SSS) are white dwarf (WD) binaries that radiate almost entirely below $\sim$1~keV. Their X-ray spectra are often complex when viewed with the X-ray grating spectrometers, where numerous emission and absorption features are intermingled and hard to separate. The absorption features are mostly from the WD atmosphere, for which radiative transfer models have been constructed. The emission features are from the corona surrounding the WD atmosphere, in which incident emission from the WD surface is reprocessed. Modeling the corona requires different solvers and assumptions for the radiative transfer, which is yet to be achieved. We chose CAL87, a SSS in the Large Magellanic Cloud, which exhibits emission-dominated spectra from the corona as the WD atmosphere emission is assumed to be completely blocked by the accretion disk. We constructed a radiative transfer model for the corona using the two radiative transfer codes; xstar for a one-dimensional two-stream solver and MONACO for a three-dimensional Monte Carlo solver. We identified their differences and limitations in comparison to the spectra taken with the Reflection Grating Spectrometer onboard the XMM-Newton satellite. We finally obtained a sufficiently good spectral model of CAL87 based on the radiative transfer of the corona plus an additional collisionally ionized plasma. In the coming X-ray microcalorimeter era, it will be required to interpret spectra based on radiative transfer in a wider range of sources than what is presented here.

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arXiv2023

X-ray spectral variations of Circinus X-1 observed with NICER throughout an entire orbital cycle

Circinus X-1 (Cir X-1) is a neutron star binary with an elliptical orbit of 16.6~days. The source is unique for its extreme youth, providing a key to understanding early binary evolution. However, its X-ray variability is too complex to reach a clear interpretation. We conducted the first high cadence (every 4 hours on average) observations covering one entire orbit using the NICER X-ray telescope. The X-ray flux behavior can be divided into stable, dip, and flaring phases. The X-ray spectra in all phases can be described by a common model consisting of a partially covered disk black body emission and the line features from a highly-ionized photo-ionized plasma. The spectral change over the orbit is attributable to rapid changes of the partial covering medium in the line-of-sight and gradual changes of the disk black body emission. Emission lines of the H-like and He-like Mg, Si, S, and Fe are detected, most prominently in the dip phase. The Fe emission lines change to absorption in the course of the transition from the dip phase to the flaring phase. The estimated ionization degree indicates no significant changes, suggesting that the photo-ionized plasma is stable over the orbit. We propose a simple model in which the disk black body emission is partially blocked by such a local medium in the line-of-sight that has spatial structures depending on the azimuth of the accretion disk. Emission lines upon the continuum emission are from the photo-ionized plasma located outside of the blocking material.

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arXiv2023

Ground test results of the electromagnetic interference for the x-ray microcalorimeter onboard XRISM

Electromagnetic interference (EMI) for low-temperature detectors is a serious concern in many missions. We investigate the EMI caused by the spacecraft components to the x-ray microcalorimeter of the Resolve instrument onboard the X-Ray Imaging and Spectroscopy Mission (XRISM), which is currently under development by an international collaboration and is planned to be launched in 2023. We focus on the EMI from (a) the low-frequency magnetic field generated by the magnetic torquers (MTQ) used for the spacecraft attitude control and (b) the radio-frequency (RF) electromagnetic field generated by the S and X band antennas used for communication between the spacecraft and the ground stations. We executed a series of ground tests both at the instrument and spacecraft levels using the flight-model hardware in 2021-2022 in a JAXA facility in Tsukuba. We also conducted electromagnetic simulations partially using the Fugaku high-performance computing facility. The MTQs were found to couple with the microcalorimeter, which we speculate through pick-ups of low-frequency magnetic field and further capacitive coupling. There is no evidence that the resultant energy resolution degradation is beyond the current allocation of noise budget. The RF communication system was found to leave no significant effect. We present the result of the tests and simulation in this article.

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arXiv2023

Ground test results of the micro-vibration interference for the x-ray microcalorimeter onboard XRISM

Resolve is a payload hosting an X-ray microcalorimeter detector operated at 50 mK in the X-Ray Imaging and Spectroscopy Mission (XRISM). It is currently under development as part of an international collaboration and is planned to be launched in 2023. A primary technical concern is the micro-vibration interference in the sensitive microcalorimeter detector caused by the spacecraft bus components. We conducted a series of verification tests in 2021-2022 on the ground, the results of which are reported here. We defined the micro-vibration interface between the spacecraft and the Resolve instrument. In the instrument-level test, the flight-model hardware was tested against the interface level by injecting it with micro-vibrations and evaluating the instrument response using the 50 mK stage temperature stability, ADR magnet current consumption rate, and detector noise spectra. We found strong responses when injecting micro-vibration at about 200, 380, and 610 Hz. In the former two cases, the beat between the injected frequency and cryocooler frequency harmonics were observed in the detector noise spectra. In the spacecraft-level test, the acceleration and instrument responses were measured with and without suspension of the entire spacecraft. The reaction wheels (RWs) and inertial reference units (IRUs), two major sources of micro-vibration among the bus components, were operated. In conclusion, the observed responses of Resolve are within the acceptable levels in the nominal operational range of the RWs and IRUs. There is no evidence that the resultant energy resolution degradation is beyond the current allocation of noise budget.

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arXiv2023

X-ray time lag evaluation of MAXI J1820+070 with a differential cross-correlation analysis

MAXI J1820$+$070 is a transient black hole binary (BHB) discovered on 2018 March 11. The unprecedented rich statistics brought by the NICER X-ray telescope allows detailed timing analysis up to $\sim$1~kHz uncompromised by the photon shot noise. To estimate the time lags, the Fourier analysis was applied, which led to two different conclusions for the system configuration; one supporting the lamp-post configuration with a stable accretion disk extending close to the innermost stable circular orbit and the other supporting the truncated accretion disk contracting with time. Using the same data set, we present the results based on the cross-correlation function (CCF). The CCF is calculated between two different X-ray bands and one side is subtracted from the other side, which we call the differential CCF (dCCF). The soft and hard lags respectively of $\sim$0.03 and 3~s are clearly identified without being diluted by the spectral mixture, demonstrating the effectiveness of the dCCF analysis. The evolution of these lags is tracked, along with spectral changes for the first 120~days since the discovery. Both the dCCF and spectral fitting results are interpreted that the soft lag is a reverberation lag between the Comptonized emission and the soft excess emission and that the hard lag is between the disk black body emission and the Comptonized emission. The evolution of these lags is in line with the picture of the truncated disk contracting with time.

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arXiv2022

X-ray and optical spectroscopic study of a gamma Cassiopeiae analog source pi Aquarii

Gamma Cas analog sources are a subset of Be stars that emit intense and hard X-ray emission. Two competing ideas for their X-ray production mechanism are (a) the magnetic activities of the Be star and its disk and (b) the accretion from the Be star to an unidentified compact object. Among such sources, Pi Aqr plays a pivotal role as it is one of the only two spectroscopic binaries observed for many orbital cycles and one of the three sources with X-ray brightness sufficient for detailed X-ray spectroscopy. Bjorkman et al. (2002) estimated the secondary mass > 2.0 Mo with optical spectroscopy, which would argue against the compact object being a white dwarf (WD). However, their dynamical mass solution is inconsistent with an evolutionary solution and their radial velocity measurement is inconsistent with later work by Naze et al. (2019). We revisit this issue by adding a new data set with the NuSTAR X-ray observatory and the HIDES echelle spectrograph. We found that the radial velocity amplitude is consistent with Naze et al. (2019), which is only a half of that claimed by Bjorkman et al. (2002). Fixing the radial velocity amplitude of the primary, the secondary mass is estimated as < 1.4 Mo over an assumed range of the primary mass and the inclination angle. We further constrained the inclination angle and the secondary mass independently by fitting the X-ray spectra with a non-magnetic or magnetic accreting WD model under the assumption that the secondary is indeed a WD. The two results match well. We thus argue that the possibility of the secondary being a WD should not be excluded for pi Aqr.

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arXiv2022

Design of the on-board data compression for the bolometer data of LiteBIRD

LiteBIRD is a space-borne experiment dedicated to detecting large-scale $B$-mode anisotropies in the linear polarization of the Cosmic Microwave Background (CMB) predicted by the theory of inflation. It is planned to be launched in the late 2020s to the second Lagrange point (L2) of the Sun-Earth system. LiteBIRD will map the sky in 15 frequency bands. In comparison to $\it{Planck}$ HFI, the previous low-temperature bolometer-based satellite for CMB observations, the number of detector has increased by two orders of magnitude, up to $\sim$5000 detectors in total. The data rate is 19 Hz from each detector. The bandpass to the ground is limited to 10 Mbps using the X-band for a few hours per day. These require the data to be compressed by more than 50 %. The exact value depends on how much information entropy is contained in the real data. We have thus evaluated the compression by simulating the time-ordered data of polarization sensitive bolometers. The foreground emission, detector noise, cosmic ray glitches, leakage from the CMB intensity to polarization, etc. are simulated. We investigated several algorithms and demonstrated that the required compression ratio can be achieved by some of them. We describe the details of this evaluation and propose algorithms that can be employed in the on-board digital electronics of LiteBIRD.

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arXiv2021

Deep Near-infrared Imaging Observation of the Faint X-ray Point Sources Constituting the Galactic Bulge X-ray Emission

Presence of the apparently extended hard (2-10 keV) X-ray emission along the Galactic plane has been known since the early 1980s. With a deep X-ray exposure using the Chandra X-ray Observatory of a slightly off-plane region in the Galactic bulge, most of the extended emission was resolved into faint discrete X-ray sources in the Fe K band (Revnivtsev et al.,2009). The major constituents of these sources have long been considered to be X-ray active stars and magnetic cataclysmic variables (CVs). However, recent works including our NIR imaging and spectroscopic studies (Morihana et al.,2013, 2016) argue that other populations should be more dominant. To investigate this further, we conducted a much deeper NIR imaging observation at the center of the Chandra's exposure field. We have used the MOIRCS on the Subaru telescope, reaching the limiting magnitude of ~18 mag in the J, H, and Ks bands in this crowded region, and identified ~50 % of the X-ray sources with NIR candidate counterparts. We classified the X-ray sources into three groups (A, B, and C) based on their positions in the X-ray color-color diagram and characterized them based on the X-ray and NIR features. We argue that the major populations of the Group A and C sources are, respectively, CVs (binaries containing magnetic or non-magnetic white dwarfs with high accretion rates) and X-ray active stars. The major population of the Group B sources is presumably WD binaries with low mass accretion rates. The Fe K equivalent width in the composite X-ray spectrum of the Group B sources is the largest among the three and comparable to that of the Galactic bulge X-ray emission. This leads us to speculate that there are numerous WD binaries with low mass accretion rates, which are not recognized as CVs, but are the major contributor of the apparently extended X-ray emission.

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arXiv2021

Simulations of systematic effects arising from cosmic rays in the LiteBIRD space telescope, and effects on the measurements of CMB $B$-modes

Systematic effects arising from cosmic rays have been shown to be a significant threat to space telescopes using high-sensitivity bolometers. The LiteBIRD space mission aims to measure the polarised Cosmic Microwave Background with unprecedented sensitivity, but its positioning in space will also render it susceptible to cosmic ray effects. We present an end-to-end simulator for evaluating the expected scale of cosmic ray effect on the LiteBIRD space mission, which we demonstrate on a subset of detectors on the 166 GHz band of the Low Frequency Telescope. The simulator couples the expected proton flux at L2 with a model of the thermal response of the LFT focal plane and the electrothermal response of its superconducting detectors, producing time-ordered data which is projected into simulated sky maps and subsequent angular power spectra.

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arXiv2021

X-ray transmission calibration of the gate valve for the X-ray astronomy satellite XRISM

\textit{Resolve} onboard the X-ray satellite XRISM is a cryogenic instrument with an X-ray microcalorimeter in a Dewar. A lid partially transparent to X-rays (called gate valve, or GV) is installed at the top of the Dewar along the optical axis. Because observations will be made through the GV for the first few months, the X-ray transmission calibration of the GV is crucial for initial scientific outcomes. We present the results of our ground calibration campaign of the GV, which is composed of a Be window and a stainless steel mesh. For the stainless steel mesh, we measured its transmission using the X-ray beamline at ISAS. For the Be window, we used synchrotron facilities to measure the transmission and modeled the data with (i) photoelectric absorption and incoherent scattering of Be, (ii) photoelectric absorption of contaminants, and (iii) coherent scattering of Be changing at specific energies. We discuss the physical interpretation of the transmission discontinuity caused by the Bragg diffraction in poly-crystal Be, which we incorporated into our transmission phenomenological model. We present the X-ray diffraction measurement on the sample to support our interpretation. The measurements and the constructed model meet the calibration requirements of the GV. We also performed a spectral fitting of the Crab nebula observed with Hitomi SXS and confirmed improvements of the model parameters.

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arXiv2021

Simulation of the cosmic ray effects for the LiteBIRD satellite observing the CMB B-mode polarization

The LiteBIRD satellite is planned to be launched by JAXA in the late 2020s. Its main purpose is to observe the large-scale B-mode polarization in the Cosmic Microwave Background (CMB) anticipated from the Inflation theory. LiteBIRD will observe the sky for three years at the second Lagrangian point (L2) of the Sun-Earth system. Planck was the predecessor for observing the CMB at L2, and the onboard High Frequency Instrument (HFI) suffered contamination by glitches caused by the cosmic-ray (CR) hits. We consider the CR hits can also be a serious source of the systematic uncertainty for LiteBIRD. Thus, we have started a comprehensive end-to-end simulation study to assess impact of the CR hits for the LiteBIRD detectors. Here, we describe procedures to make maps and power spectra from the simulated time-ordered data, and present initial results. Our initial estimate is that $C_l^{BB}$ by CR is $\sim 2 \times 10^{-6}~μ$K$_{\mathrm{CMB}}^{2}$ in a one-year observation with 12 detectors assuming that the noise is 1~aW/$\sqrt{\mathrm{Hz}}$ for the differential mode of two detectors constituting a polarization pair.

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arXiv2018

X-ray reverberation lags of the Fe-K line due to AGN disc winds

Short X-ray reverberation lags are seen across a broad Fe-K energy band in more than twenty active galactic nuclei (AGNs). This broad iron line feature in the lag spectrum is most significant in super-Eddington sources such as Ark 564 ($L/L_{\rm Edd}\sim 1$) and 1H 0707--495 ($L/L_{\rm Edd}\gtrsim 10$). The observed lag timescales correspond to very short distances of several $R_g/c$, so that they have been used to argue for extremely small `lamp-post' coronae close to the event horizon of rapidly spinning black holes. Here we show for the first time that these Fe-K short lags are more likely to arise from scattering in a highly-ionised wind, launched at $\sim 50\,R_g$, rotating and outflowing with a typical velocity of $0.2c$. We show that this model can simultaneously fit the time-averaged energy spectra and the short-timescale lag-energy spectra of both 1H 0707--495 and Ark 564. The Fe-K line in 1H 0707--495 has a strong P-Cygni-like profile, which requires that the wind solid angle is large and that our line of sight intercepts the wind. By contrast the lack of an absorption line in the energy spectrum of Ark 564 requires rather face-on geometry, while the weaker broad Fe-K emission in the energy and lag-energy spectra argue for a smaller solid angle of the wind. This is consistent with theoretical predictions that the winds get stronger when the sources are more super-Eddington, supporting the idea of AGN feedback via radiation pressure driven winds.

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arXiv2018

Suzaku and NuSTAR X-Ray Spectroscopy of gamma Cas and HD110432

Gamma Cas and its dozen analogs comprise a small but distinct class of X-ray sources. They are early Be-type stars with an exceptionally hard thermal X-ray emission. The X-ray production mechanism has been under intense debate. Two competing ideas are (i) the magnetic activities in the Be star and its disk and (ii) the mass accretion onto the unidentified white dwarf (WD). We adopt the latter as a working hypothesis and apply physical models developed to describe the X-ray spectra of classical WD binaries containing a late-type companion. Models of non-magnetic and magnetic accreting WDs were applied to gamma Cas and its brightest analog HD110432 using the Suzaku and NuSTAR data. The spectra were fitted by the two models, including the Fe I fluorescence and the Compton reflection in a consistent geometry. The derived physical parameters, such as the WD mass and mass accretion rate, are in a reasonable range in comparison to their classical WD binary counterparts. Additional pieces of evidence in the X-ray spectra (partial covering, Fe L lines, and Fe I fluorescence) were not conclusive enough to classify these two sources into a sub-class of accreting WD binaries. We discuss further observations, especially long-term temporal behaviors, which are important to elucidate the nature of these sources more if indeed they host accreting WDs.

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arXiv2018

X-ray short-time lags in the Fe-K energy band produced by scattering clouds in active galactic nuclei

X-rays illuminating the accretion disc in active galactic nuclei give rise to an iron K line and its associated reflection spectrum which are lagged behind the continuum variability by the light-travel time from the source to the disc. The measured lag timescales in the iron band can be as short as $\sim R_g/c$, where $R_g$ is the gravitational radius, which is often interpreted as evidence for a very small continuum source close to the event horizon of a rapidly spinning black hole. However, the short lags can also be produced by reflection from more distant material, because the primary photons with no time-delay dilute the time-lags caused by the reprocessed photons. We perform a Monte-Carlo simulation to calculate the dilution effect in the X-ray reverberation lags from a half-shell of neutral material placed at $100\,R_g$ from the central source. This gives lags of $\sim2\,R_g/c$, but the iron line is a distinctly narrow feature in the lag-energy plot, whereas the data often show a broader line. We show that both the short lag and the line broadening can be reproduced if the scattering material is outflowing at $\sim0.1c$. The velocity structure in the wind can also give shifts in the line profile in the lag-energy plot calculated at different frequencies. Hence we propose that the observed broad iron reverberation lags and shifts in profile as a function of frequency of variability can arise from a disc wind at fairly large distances from the X-ray source.

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arXiv2018

Can the relativistic light bending model explain X-ray spectral variations of Seyfert galaxies?

Many Seyfert galaxies are known to exhibit Fe-K broad emission line features in their X-ray energy spectra. The observed lines have three distinct features; (1) the line profiles are skewed and show significant low-energy tails, (2) the Fe-K band have low variability, which produces a broad and deep dip in the root-mean-square (rms) spectra, and (3) photons in this band have time lags behind those in the adjacent energy bands with amplitudes of several $R_g/c$, where $R_g$ is the gravitational radius. The "relativistic light bending model" is proposed to explain these observed features, where a compact X-ray source (lamp post) above an extreme Kerr black hole illuminates the innermost area of the accretion disc. In this paper, we critically examine the relativistic light bending model by computing the rms spectra and the lag features using a ray-tracing technique, when a lamp post moves vertically on the black hole spin axis. As a result, we found that the observed deep rms dip requires that the iron is extremely overabundant ($\gtrsim10$ solar), whereas the observed lag amplitude is consistent with the normal iron abundance. Furthermore, disappearance of the lag in the high-flux state requires a source height as high as $\sim40\,R_g$, which contradicts the relativistically broad emission line feature. Our simulations agree with the data that the reverberation feature moves to lower frequencies with larger source height, however, if this scenario is correct, the simulations predict detection of a clear Fe-K lag at low frequencies, which is not constrained in the data. Therefore, we conclude that the relativistic light bending model may not explain the characteristic Fe-K spectral variations in Seyfert galaxies.

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arXiv2018

In-flight Calibration of Hitomi Soft X-ray Spectrometer (3) Effective Area

We present the result of the in-flight calibration of the effective area of the Soft X-ray Spectrometer (SXS) onboard the Hitomi X-ray satellite using an observation of the Crab nebula. We corrected for the artifacts when observing high count rate sources with the X-ray microcalorimeter. We then constructed a spectrum in the 0.5-20 keV band, which we modeled with a single power-law continuum attenuated by an interstellar extinction. We evaluated the systematic uncertainty upon the spectral parameters by various calibration items. In the 2-12 keV band, the SXS result is consistent with the literature values in flux (2.20 $\pm$ 0.08) $\times$10$^{-8}$ erg s$^{-1}$ cm$^{-2}$ with a 1$σ$ statistical uncertainty) but is softer in the power-law index (2.19 $\pm$ 0.11). The discrepancy is attributable to the systematic uncertainty of about $+$6/$-$7% and $+$2/$-$5% respectively for the flux and the power-law index. The softer spectrum is affected primarily by the systematic uncertainty of the Dewar gate valve transmission and the event screening.

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arXiv2017

Inflight Calibration of the Hitomi Soft X-ray Spectrometer (2) Point Spread Function

We present results of inflight calibration of the point spread function (PSF) of the Soft X-ray Telescope (SXT-S) that focuses X-ray onto the pixel array of the Soft X-ray Spectrometer system (SXS). We make a full array image of a point-like source by extracting a pulsed component of the Crab nebula emission. Within the limited statistics afforded by an exposure time of only 6.9~ksec and the limited knowledge of the systematic uncetainties, we find that the raytracing model of 1'.2 half-power-diameter (HPD) is consistent with an image of the observed event distributions across pixels. The ratio between the Crab pulsar image and the raytracing shows scatter from pixel to pixel that is 40% or less in all except one pixel. The pixel-to-pixel ratio has a spread of 20%, on average, for the 15 edge pixels, with an averaged statistical error of 17% (1 sigma). In the central 16 pixels, the corresponding ratio is 15% with an error of 6%.

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