arXiv subjects

Mihoko Yukita

Publications and source records attributed to Mihoko Yukita.

At least 19 recordsLinked to original sources

arXiv2026

The First Insights into an Ultraluminous X-ray Pulsar with XRISM: Phase-Resolved High-Resolution Spectroscopy of the Fe K-shell Band of M82 X-2

During the performance verification phase, XRISM observed the M82 galaxy for a net exposure of 207.7 ks, with the ultraluminous X-ray pulsar (ULXP) X-2 included in the field of view. A pulsation search identified a candidate signal with a period close to the previously known value, 1.38727 s, at a significance of $3.15σ$ based on Monte Carlo simulations. Using this candidate period, phase-resolved spectral analysis with the high spectral resolution of Resolve was performed. The spectra suggest that, if the candidate pulsation is real, the Fe K$α$ emission line in the pulse peak phase has a larger width ($36^{+60}_{-13}$ eV) than that in the remaining phase at a significance exceeding $3σ$. This suggests that at least a fraction of the Fe K$α$ emission is associated with the ULXP system. The observed width corresponds to a velocity dispersion of $(1.7^{+2.8}_{-0.6})\times10^3$ km s$^{-1}$, which is too large to be explained by motions in the companion star atmosphere. The rise time of the pulsation constrains the line-emitting region to be smaller than $6.3\times10^4$ km, suggesting an origin in the accretion flow. This work demonstrates the capability of XRISM Resolve for pulsation-resolved high-resolution spectroscopy of ULX pulsars.

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arXiv2025

Verification of the Timing System for the X-ray Imaging and Spectroscopy Mission in the GPS Unsynchronized Mode

We report the results from the ground and on-orbit verifications of the XRISM timing system when the satellite clock is not synchronized to the GPS time. In this case, the time is determined by a free-run quartz oscillator of the clock, whose frequency changes depending on its temperature. In the thermal vacuum test performed in 2022, we obtained the GPS unsynchronized mode data and the temperature-versus-clock frequency trend. Comparing the time values calculated from the data and the true GPS times when the data were obtained, we confirmed that the requirement (within a 350 $μ$s error in the absolute time, accounting for both the spacecraft bus system and the ground system) was satisfied in the temperature conditions of the thermal vacuum test. We also simulated the variation of the timing accuracy in the on-orbit temperature conditions using the Hitomi on-orbit temperature data and found that the error remained within the requirement over $\sim 3 \times 10^{5}$ s. The on-orbit tests were conducted in 2023 September and October as part of the bus system checkout. The temperature versus clock frequency trend remained unchanged from that obtained in the thermal vacuum test and the observed time drift was consistent with that expected from the trend.

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arXiv2025

XRISM Reveals a Remnant Torus in the Low-Luminosity AGN M81*

Up to 40% of galaxies in the local universe host a low-luminosity active galactic nucleus (LLAGN), making it vital to understand this mode of black hole accretion. However, the presence or absence of Seyfert-like geometries - an accretion disk close to the black hole, an optical broad line region (BLR), and a molecular torus - remains uncertain owing to the low flux levels of sources within this class. Herein, we present an analysis of a XRISM/Resolve spectrum of M81*, the LLAGN in the heart of the nearby spiral galaxy M81. A weak, neutral Fe K emission line is detected and resolved into K$_{α,1}$ and K$_{α,2}$ components. It shows a negligible velocity shift, and weak broadening (FWHM$=460^{+260}_{-160}~{\rm km}~{\rm s}^{-1}$) that corresponds to an inner emission radius of ${\rm r} \geq 2.7\times 10^{4}~GM/c^{2}$ for likely inclinations. The Fe K$_α$ line likely traces a torus. The upper limit on additional splitting of the Fe K$_α$ line components translates to a limit on the local magnetic field of ${\rm B} \leq 3.5\times 10^{8}$ Gauss, assuming Zeeman splitting. The spectra also reveal ionized plasma(s) through He-like Fe XXV and H-like Fe XXVI emission lines. These can be fit equally well assuming photoionization and collisional excitation. The H-like Fe XXVI line is better described when a second component is included with a red-shift of ${\rm v} = 1600~{\rm km}~{\rm s}^{-1}$, but this addition is of marginal statistical significance. We discuss these results in the context of radiatively inefficient accretion flow models, magnetically arrested disks, and possible links to the Fermi bubbles in the Milky Way.

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arXiv2025

Development of the Timing System for the X-Ray Imaging and Spectroscopy Mission

This paper describes the development, design, ground verification, and in-orbit verification, performance measurement, and calibration of the timing system for the X-Ray Imaging and Spectroscopy Mission (XRISM). The scientific goals of the mission require an absolute timing accuracy of 1.0~ms. All components of the timing system were designed and verified to be within the timing error budgets, which were assigned by component to meet the requirements. After the launch of XRISM, the timing capability of the ground-tuned timing system was verified using the millisecond pulsar PSR~B1937+21 during the commissioning period, and the timing jitter of the bus and the ground component were found to be below $15~μ$s compared to the NICER (Neutron star Interior Composition ExploreR) profile. During the performance verification and calibration period, simultaneous observations of the Crab pulsar by XRISM, NuSTAR (Nuclear Spectroscopic Telescope Array), and NICER were made to measure the absolute timing offset of the system, showing that the arrival time of the main pulse with XRISM was aligned with that of NICER and NuSTAR to within $200~μ$s. In conclusion, the absolute timing accuracy of the bus and the ground component of the XRISM timing system meets the timing error budget of $500~μ$s.

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arXiv2024

A NuSTAR Census of the X-ray Binary Population of the M31 Disk

Using hard (E>10 keV) X-ray observations with NuSTAR, we are able to differentiate between accretion states, and thus compact object types, of neutron stars and black holes in X-ray binaries (XRBs) in M31, our nearest Milky Way-type neighbor. Using ten moderate-depth (20-50 ks) observations of the disk of M31 covering a total of ~0.45 deg$^{2}$, we detect 20 sources at 2$σ$ in the 4-25 keV band pass, 14 of which we consider to be XRB candidates. This complements an existing deeper (100-400 ks) survey covering ~0.2 deg$^{2}$ of the bulge and the northeastern disk. We make tentative classifications of 9 of these sources with the use of diagnostic color-intensity and color-color diagrams, which separate sources into various neutron star and black hole regimes, identifying 3 black holes and 6 neutron stars. In addition, we create X-ray luminosity functions for both the full (4-25 keV) and hard (12-25 keV) band, as well as sub-populations of the full band based on compact object type and association with globular clusters. Our best fit globular cluster XLF is shallower than the field XLF, and preliminary BH and NS XLFs suggest a difference in shape based on compact object type. We find that the cumulative disk XLFs in the full and hard band are best fit by power laws with indices of 1.32 and 1.28 respectively. This is consistent with models of the Milky Way XLF from Grimm et al. (2002), Voss & Ajello (2010), and Doroshenko et al. (2014).

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arXiv2023

The High Energy X-ray Probe: Resolved X-ray Populations in Extragalactic Environments

We construct simulated galaxy data sets based on the High Energy X-ray Probe (HEX-P) mission concept to demonstrate the significant advances in galaxy science that will be yielded by the HEX-P observatory. The combination of high spatial resolution imaging ($<$20 arcsec FWHM), broad spectral coverage (0.2-80 keV), and sensitivity superior to current facilities (e.g., XMM-Newton and NuSTAR) will enable HEX-P to detect hard (4-25 keV) X-ray emission from resolved point-source populations within $\sim$800 galaxies and integrated emission from $\sim$6000 galaxies out to 100 Mpc. These galaxies cover wide ranges of galaxy types (e.g., normal, starburst, and passive galaxies) and properties (e.g., metallicities and star-formation histories). In such galaxies, HEX-P will: (1) provide unique information about X-ray binary populations, including accretor demographics (black hole and neutron stars), distributions of accretion states and state transition cadences; (2) place order-of-magnitude more stringent constraints on inverse Compton emission associated with particle acceleration in starburst environments; and (3) put into clear context the contributions from X-ray emitting populations to both ionizing the surrounding interstellar medium in low-metallicity galaxies and heating the intergalactic medium in the $z > 8$ Universe.

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arXiv2023

Modeling the High-Energy Ionizing Output from Simple Stellar and X-ray Binary Populations

We present a methodology for modeling the joint ionizing impact due to a "simple X-ray population" (SXP) and its corresponding simple stellar population (SSP), where "simple" refers to a single age and metallicity population. We construct composite spectral energy distributions (SEDs) including contributions from ultra-luminous X-ray sources (ULXs) and stars, with physically meaningful and consistent consideration of the relative contributions of each component as a function of instantaneous burst age and stellar metallicity. These composite SEDs are used as input for photoionization modeling with Cloudy, from which we produce a grid for the time- and metallicity-dependent nebular emission from these composite populations. We make the results from the photoionization simulations publicly available. We find that the addition of the SXP prolongs the high-energy ionizing output from the population, and correspondingly increases the intensity of nebular lines such as He II $λ$1640,4686, [Ne V] $λ$3426,14.3$μ$m, and [O IV] 25.9$μ$m by factors of at least two relative to models without an SXP spectral component. This effect is most pronounced for instantaneous bursts of star formation on timescales $>$ 10 Myr and at low metallicities ($\sim$ 0.1 $Z_{\odot}$), due to the imposed time- and metallicity-dependent behavior of the SXP relative to the SSP. We propose nebular emission line diagnostics accessible with JWST suitable for inferring the presence of a composite SXP + SSP, and discuss how the ionization signatures compare to models for sources such as intermediate mass black holes.

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arXiv2023

Overview of the Advanced X-ray Imaging Satellite (AXIS)

The Advanced X-ray Imaging Satellite (AXIS) is a Probe-class concept that will build on the legacy of the Chandra X-ray Observatory by providing low-background, arcsecond-resolution imaging in the 0.3-10 keV band across a 450 arcminute$^2$ field of view, with an order of magnitude improvement in sensitivity. AXIS utilizes breakthroughs in the construction of lightweight segmented X-ray optics using single-crystal silicon, and developments in the fabrication of large-format, small-pixel, high readout rate CCD detectors with good spectral resolution, allowing a robust and cost-effective design. Further, AXIS will be responsive to target-of-opportunity alerts and, with onboard transient detection, will be a powerful facility for studying the time-varying X-ray universe, following on from the legacy of the Neil Gehrels (Swift) X-ray observatory that revolutionized studies of the transient X-ray Universe. In this paper, we present an overview of AXIS, highlighting the prime science objectives driving the AXIS concept and how the observatory design will achieve these objectives.

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arXiv2023

A broadband X-ray imaging spectroscopy in the 2030s: the FORCE mission

In this multi-messenger astronomy era, all the observational probes are improving their sensitivities and overall performance. The Focusing on Relativistic universe and Cosmic Evolution (FORCE) mission, the product of a JAXA/NASA collaboration, will reach a 10 times higher sensitivity in the hard X-ray band ($E >$ 10~keV) in comparison with any previous hard X-ray missions, and provide simultaneous soft X-ray coverage. FORCE aims to be launched in the early 2030s, providing a perfect hard X-ray complement to the ESA flagship mission Athena. FORCE will be the most powerful X-ray probe for discovering obscured/hidden black holes and studying high energy particle acceleration in our Universe and will address how relativistic processes in the universe are realized and how these affect cosmic evolution. FORCE, which will operate over 1--79 keV, is equipped with two identical pairs of supermirrors and wideband X-ray imagers. The mirror and imager are connected by a high mechanical stiffness extensible optical bench with alignment monitor systems with a focal length of 12~m. A light-weight silicon mirror with multi-layer coating realizes a high angular resolution of $<15''$ in half-power diameter in the broad bandpass. The imager is a hybrid of a brand-new SOI-CMOS silicon-pixel detector and a CdTe detector responsible for the softer and harder energy bands, respectively. FORCE will play an essential role in the multi-messenger astronomy in the 2030s with its broadband X-ray sensitivity.

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arXiv2022

Elevated Hot Gas and High-Mass X-ray Binary Emission in Low Metallicity Galaxies: Implications for Nebular Ionization and Intergalactic Medium Heating in the Early Universe

High-energy emission associated with star formation has been proposed as a significant source of interstellar medium (ISM) ionization in low-metallicity starbursts and an important contributor to the heating of the intergalactic medium (IGM) in the high-redshift ($z > 8$) Universe. Using Chandra observations of a sample of 30 galaxies at $D \approx$~200--450 Mpc that have high specific star-formation rates of 3--9 Gyr$^{-1}$ and metallicities near $Z \approx 0.3 Z_\odot$, we provide new measurements of the average 0.5--8 keV spectral shape and normalization per unit star-formation rate (SFR). We model the sample-combined X-ray spectrum as a combination of hot gas and high-mass X-ray binary (HMXB) populations and constrain their relative contributions. We derive scaling relations of $\log L_{\rm 0.5-8 keV}^{\rm HMXB}$/SFR $= 40.19 \pm 0.06$ and $\log L_{\rm 0.5-2 keV}^{\rm gas}$/SFR $= 39.58^{+0.17}_{-0.28}$; significantly elevated compared to local relations. The HMXB scaling is also somewhat higher than $L_{\rm 0.5-8 keV}^{\rm HMXB}$-SFR-$Z$ relations presented in the literature, potentially due to our galaxies having relatively low HMXB obscuration and young and X-ray luminous stellar populations. The elevation of the hot gas scaling relation is at the level expected for diminished attenuation due to a reduction of metals; however, we cannot conclude that an $L_{\rm 0.5-2 keV}^{\rm gas}$-SFR-$Z$ relation is driven solely by changes in ISM metal content. Finally, we present SFR-scaled spectral models (both emergent and intrinsic) that span the X-ray--to--IR band, providing new benchmarks for studies of the impact of ISM ionization and IGM heating in the early Universe.

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arXiv2022

A Chandra Virgo cluster survey of spiral galaxies. I. Introduction to the survey and a new ULX sample

We present an analysis of the ultraluminous X-ray source (ULX) population in 75 Virgo cluster late-type galaxies, including all those with a star formation rate >~ 1 M_{sun}/yr and a representative sample of the less star-forming ones. This study is based on 110 observations obtained over 20 years with the Chandra X-ray Observatory Advanced Camera for Imaging Spectroscopy. As part of a Large Chandra Program, new observations were obtained for 52 of these 75 galaxies. The data are complete to a sensitivity of about 10^{39} erg/s, with a typical detection limit of about 3 x 10^{38} erg/s for the majority of the sources. The catalogue contains about 80 ULXs (0.3-10 keV luminosity >10^{39} erg/s), and provides their location, observed flux, de-absorbed luminosity, and (for the 25 most luminous ones) simple X-ray spectral properties. We discuss the ULX luminosity function in relation to the mass and star formation rate of the sample galaxies. We show that recent models of low-mass plus high-mass X-ray binary populations (scaling with stellar mass and star formation rate, respectively) are mostly consistent with our observational results. We tentatively identify the most luminous X-ray source in the sample (a source in IC 3322A with L_{X} ~ 6 x 10^{40} erg/s) as a recent supernova or its young remnant. The properties of the sample galaxies (morphologies, stellar masses, star formation rates, total X-ray luminosities from their point-source population) are also summarised.

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arXiv2020

A 60-kpc Galactic Wind Cone in NGC 3079

Galactic winds are associated with intense star formation and AGNs. Depending on their formation mechanism and velocity they may remove a significant fraction of gas from their host galaxies, thus suppressing star formation, enriching the intergalactic medium, and shaping the circumgalactic gas. However, the long-term evolution of these winds remains mostly unknown. We report the detection of a wind from NGC 3079 to at least 60 kpc from the galaxy. We detect the wind in FUV line emission to 60 kpc (as inferred from the broad FUV filter in GALEX) and in X-rays to at least 30~kpc. The morphology, luminosities, temperatures, and densities indicate that the emission comes from shocked material, and the O/Fe ratio implies that the X-ray emitting gas is enriched by Type II supernovae. If so, the speed inferred from simple shock models is about 500 km/s, which is sufficient to escape the galaxy. However, the inferred kinetic energy in the wind from visible components is substantially smaller than canonical hot superwind models.

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arXiv2020

On the X-ray Spectral Energy Distributions of Star-Forming Galaxies: the 0.3-30 keV Spectrum of the Low-Metallicity Starburst Galaxy VV 114

Binary population synthesis combined with cosmological models suggest that X-ray emission from star-forming galaxies, consisting primarily of emission from X-ray binaries (XRBs) and the hot interstellar medium (ISM), could be an important, and perhaps dominant, source of heating of the intergalactic medium prior to the epoch of reionization. However, such models rely on empirical constraints for the X-ray spectral energy distributions (SEDs) of star-forming galaxies, which are currently lacking for low-metallicity galaxies. Using a combination of Chandra, XMM-Newton, and NuSTAR observations, we present new constraints on the 0.3-30 keV SED of the low-metallicity starburst galaxy VV 114, which is known to host several ultra-luminous X-ray sources (ULXs) with luminosities above 10$^{40}$ erg s$^{-1}$. We use an archival Chandra observation of VV 114 to constrain the contributions to the X-ray SED from the major X-ray emitting components of the galaxy, and newly acquired, nearly simultaneous XMM-Newton and NuSTAR observations to extend the spectral model derived from Chandra to cover the 0.3-30 keV range. Using our best-fit galaxy-wide spectral model, we derive the 0.3-30 keV SED of VV 114, which we find is dominated by emission from the XRB population, and in particular ULXs, at energies > 1.5 keV, and which we find to have an elevated galaxy-integrated X-ray luminosity per unit star formation rate relative to higher-metallicity star-forming galaxies. We discuss our results in terms of the effect of metallicity on XRB populations and the hot ISM, and the importance of X-ray emission from star-forming galaxies in the high redshift Universe.

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arXiv2020

X-ray Binary Luminosity Function Scaling Relations in Elliptical Galaxies: Evidence for Globular Cluster Seeding of Low-Mass X-ray Binaries in Galactic Fields

We investigate X-ray binary (XRB) luminosity function (XLF) scaling relations for Chandra detected populations of low-mass XRBs (LMXBs) within the footprints of 24 early-type galaxies. Our sample includes Chandra and HST observed galaxies at D < 25 Mpc that have estimates of the globular cluster (GC) specific frequency (SN) reported in the literature. As such, we are able to directly classify X-ray-detected sources as being either coincident with unrelated background/foreground objects, GCs, or sources that are within the fields of the galaxy targets. We model the GC and field LMXB population XLFs for all galaxies separately, and then construct global models characterizing how the LMXB XLFs vary with galaxy stellar mass and SN. We find that our field LMXB XLF models require a component that scales with SN, and has a shape consistent with that found for the GC LMXB XLF. We take this to indicate that GCs are "seeding" the galactic field LMXB population, through the ejection of GC-LMXBs and/or the diffusion of the GCs in the galactic fields themselves. However, we also find that an important LMXB XLF component is required for all galaxies that scales with stellar mass, implying that a substantial population of LMXBs are formed "in situ," which dominates the LMXB population emission for galaxies with SN < 2. For the first time, we provide a framework quantifying how directly-associated GC LMXBs, GC-seeded LMXBs, and in-situ LMXBs contribute to LMXB XLFs in the broader early-type galaxy population.

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arXiv2019

X-ray Binary Luminosity Function Scaling Relations for Local Galaxies Based on Subgalactic Modeling

We present new Chandra constraints on the X-ray luminosity functions (XLFs) of X-ray binary (XRB) populations, and their scaling relations, for a sample of 38 nearby galaxies (D = 3.4-29 Mpc). Our galaxy sample is drawn primarily from the Spitzer infrared nearby galaxy survey (SINGS), and contains a wealth of Chandra (5.8 Ms total) and multiwavelength data, allowing for star-formation rates (SFRs) and stellar masses (M*) to be measured on subgalactic scales. We divided the 2478 X-ray detected sources into 21 subsamples in bins of specific-SFR (sSFR = SFR/M*) and constructed XLFs. To model the XLF dependence on sSFR, we fit a global XLF model, containing contributions from high-mass XRBs (HMXBs), low-mass XRBs (LMXBs), and background sources from the cosmic X-ray background (CXB) that respectively scale with SFR, M*, and sky area. We find an HMXB XLF that is more complex in shape than previously reported and an LMXB XLF that likely varies with sSFR, potentially due to an age dependence. When applying our global model to XLF data for each individual galaxy, we discover a few galaxy XLFs that significantly deviate from our model beyond statistical scatter. Most notably, relatively low-metallicity galaxies have an excess of HMXBs above ~10^38 erg/s and elliptical galaxies that have relatively rich populations of globular clusters (GCs) show excesses of LMXBs compared to the global model. Additional modeling of how the XRB XLF depends on stellar age, metallicity, and GC specific frequency is required to sufficiently characterize the XLFs of galaxies.

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arXiv2019

Astro2020: Hot Drivers of Stellar Feedback from 10 to 10,000 pc

Stellar feedback -- stars regulating further star formation through the injection of energy and momentum into the interstellar medium -- operates through a complex set of processes that originate in star clusters but shape entire galaxies. A mature theory of stellar feedback is essential to a complete theory of star and galaxy formation, but the energy and momentum injected by hot gas into its surroundings remains unclear. With a next-generation X-ray observatory, breakthrough progress can be made through precision measurements of the temperature, density, velocity, and abundances of hot gas on scales of star clusters to galactic superwinds.

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arXiv2019

A ~60-day super-orbital period originating from the ultraluminous X-ray pulsar in M82

Ultraluminous X-ray (ULX) pulsars are a new class of object powered by apparent super-critical accretion onto magnetized neutron stars. Three sources in this class identified so far; M82 X-2, NGC 5907 ULX-1 and NGC 7793 P13, have been found to have two properties in common; $\sim1$-s spin periods, and for NGC 5907 ULX-1 and NGC 7793 P13 periodic X-ray flux modulations on timescales of $\sim60-80$ days. M82 X-2 resides in a crowded field that includes the ULX M82 X-1 separated from X-2 by 5", as well as other bright point sources. A 60-day modulation has been observed from the region but the origin has been difficult to identify; both M82 X-1 and X-2 have been suggested as the source. In this paper we present the analysis of a systematic monitoring campaign by Chandra, the only X-ray telescope capable of resolving the crowded field. From a simple Lomb-Scargle periodogram analysis and a more sophisticated Gaussian Process analysis we find that only X-2 exhibits a periodic signal around 60 days supporting previous claims that it is the origin. We also construct a phase-averaged flux profile of the modulations from higher cadence Swift/XRT data and find that the flux variations in the Chandra data are fully consistent with the flux profile. Since the orbit of the neutron star and its companion is known to be 2.5 days, the $\sim60$-day period must be super-orbital in origin. The flux of the modulations varies by a factor of $\sim$100 from minimum to maximum, with no evidence for spectral variations, making the origin difficult to explain.

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arXiv2018

NuSTAR Hard X-ray View of Low-luminosity Active Galactic Nuclei: High-energy Cutoff and Truncated Thin Disk

We report the analysis of simultaneous XMM-Newton+NuSTAR observations of two low-luminosity Active Galactic Nuclei (LLAGN), NGC 3998 and NGC 4579. We do not detect any significant variability in either source over the ~3 day length of the NuSTAR observations. The broad-band 0.5-60 keV spectrum of NGC 3998 is best fit with a cutoff power-law, while the one for NGC 4579 is best fit with a combination of a hot thermal plasma model, a power-law, and a blend of Gaussians to fit an Fe complex observed between 6 and 7 keV. Our main spectral results are the following: (1) neither source shows any reflection hump with a $3σ$ reflection fraction upper-limits $R<0.3$ and $R<0.18$ for NGC 3998 and NGC 4579, respectively; (2) the 6-7 keV line complex in NGC 4579 could either be fit with a narrow Fe K line at 6.4 keV and a moderately broad Fe XXV line, or 3 relatively narrow lines, which includes contribution from Fe XXVI; (3) NGC 4579 flux is 60% brighter than previously detected with XMM-Newton, accompanied by a hardening in the spectrum; (4) we measure a cutoff energy $E_{\rm cut}=107_{-18}^{+27}$ keV in NGC 3998, which represents the lowest and best constrained high-energy cutoff ever measured for an LLAGN; (5) NGC 3998 spectrum is consistent with a Comptonization model with either a sphere ($τ\approx3\pm1$) or slab ($τ\approx1.2\pm0.6$) geometry, corresponding to plasma temperatures between 20 and 150 keV. We discuss these results in the context of hard X-ray emission from bright AGN, other LLAGN, and hot accretion flow models.

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