arXiv subjects

Kazutaka Yamaoka

Publications and source records attributed to Kazutaka Yamaoka.

At least 19 recordsLinked to original sources

arXiv2026

Jet Forecast: Timing of Discrete Jet Ejections of Stellar-Mass Black Holes Anticipated via X-ray Properties

The radio -- soft $γ$-ray data of the Galactic black hole binary XTE J1859+226 showed that discrete jets are launched when the inner radius of the accretion disk ($R_{\rm in}$) rapidly approaches the innermost stable circular orbit (ISCO). Given the correlations among the X-ray fractional variability amplitude (rms), $R_{\rm in}$, and the X-ray hardness, previous phenomenological findings related to jet ejections (``jet line'' and sharp drops of rms) also indicate that the rapid shrinkage of $R_{\rm in}$ down to the ISCO turns on the jet formation. In order to make clear how these properties appear in other objects, we investigated the X-ray and radio data of XTE J1550-564, GX 339-4, and MAXI J1820+070. We found that all the four sources show drops of $R_{\rm in}$ and rms 1-10 days ahead of a jet activity, and exhibit no huge jumps of the accretion rates prior to jet ejections. XTE J1859 and MAXI J1820 show the jet activities under sub-Eddington accretion. Models for the jet production that require an enhancement of $\dot{M}$ or a super-Eddington accretion rate are unlikely. Another behavior common to multiple sources is the curved shape of the rms--$R_{\rm in}$ correlation. Therefore, monitoring of $R_{\rm in}$ seems to be suitable to predict the timing of jet ejections. When both $R_{\rm in}$ and rms continuously decrease and when $R_{\rm in}$ is about to reach the ISCO, alerts can be circulated as a ``jet forecast'' for triggering Target-of-Opportunity observations.

astro-ph.HE↗

arXiv2026

The line modulations of H-like Fe, Ca, Ar, and S observed with $XRISM$/Resolve in Cyg X-3

Cygnus X-3, hosting a Wolf-Rayet (WR) star whose dense wind produces various spectral lines due to photoionization by X-rays from a compact object, provides an ideal laboratory for studying wind dynamics and density structure. We measured the orbital modulations of the Fe, Ca, Ar, and S Ly$α$ lines observed with the X-ray microcalorimeter (Resolve) onboard the $XRISM$, taking account of both emission and absorption lines of the Ly$α$ complexes. The modulations of Doppler shifts of the Fe, Ca, Ar, and S Ly$α$ lines showed amplitudes of 500 km s$^{-1}$ and phase offsets of 0.04, 0.09, 0.11, and 0.17, respectively, in units of an orbital period (4.8 hours) relative to the orbital motion of the compact object. This result indicated that H-like Fe most closely follows the compact object's motion. The line widths ranged from 400 to 1000 km s$^{-1}$. The intensities of both emission and absorption lines reached their minima around orbital phase 0.0 and their maxima around phase 0.5. The absorption peaks, however, did not align exactly with phase 0.5, suggesting the inhomogeneous structures such as an accretion wake and/or a bow shock. We compared the observed modulations with calculations based on a stellar wind model, accelerated by ultraviolet radiation from the WR star. One of the calculations qualitatively reproduced the observed trend that H-like Fe ions were concentrated near the compact object, whereas H-like S was distributed across the binary system, with H-like Ca and Ar showing intermediate spatial distributions. From this comparison, we estimated that a mass-loss rate of the stellar wind was approximately $5 \times 10^{-6}$-$1 \times 10^{-5}\ M_{\odot}\ {\rm yr}^{-1}$.

astro-ph.HE↗

arXiv2026

XRISM High-Resolution X-ray Spectroscopy of Cygnus X-1 -- Orbital and Short-Term Variability of Iron Absorption

We present the first high-resolution spectroscopy of the black hole high-mass X-ray binary Cygnus X-1 with XRISM, including orbital-phase-resolved analyses and tentative evidence of short-term variability in the Fe-K band on second timescales. Using data from the Performance Verification phase in April 2024, we analyzed spectral variability across orbital phases with the Resolve microcalorimeter and the Xtend CCD imager. The unprecedented resolution of Resolve reveals variability in highly ionized Fe absorption lines. The absorption features show orbital-phase-dependent variability in column density, ionization state, and blueshifted velocity, suggesting structural variations in the focused stellar wind along the line of sight. We also find indications of subtle broadening of the neutral Fe emission profile. In addition, intensity-sorted spectroscopy during dip phases suggests possible variability on timescales of a few seconds in the absorption features, consistent with cooler, denser and lower-ionized gas clumps. Although the statistical significance is limited, these results hint that the stellar wind and the X-rays from the accretion disk around the black hole may interact on timescales as short as a few seconds. These XRISM results constrain wind-fed accretion in Cyg X-1 and highlight Resolve's capability to probe plasma environments in high-mass X-ray binaries.

astro-ph.HE↗

arXiv2026

XRISM High-Resolution X-ray Spectroscopy of Cygnus X-1 -- highly ionized Iron absorption structures

We present the first high-resolution X-ray spectral analysis of Cygnus X-1 using XRISM. The observation was carried out from April 7 to 10, 2024, covering the orbital phase range 0.65--0.17 during its low/hard state. Taking advantage of the exceptional energy resolution of the Resolve instrument, we examined highly ionized iron absorption lines and characterized the ionization states, column densities, and line-of-sight velocities of the absorbing plasma. Spectral analysis revealed an ionization parameter of approximately 3, column densities of a few times 10^21 cm^-2, and a blueshifted velocity of approximately 100 km s^-1. The observation was divided into two phases: before and after orbital phase phi_orb = 0.9, corresponding to non-dipping and dipping intervals. While only weak absorption features were present before phi_orb = 0.9, strong absorption by He-like and H-like Fe appeared during the dipping phase. We measured equivalent widths of 2.3 eV, 0.4 eV, and 1.2 eV for He-like Fe K-alpha, and H-like Ly-alpha1 and Ly-alpha2, respectively, demonstrating the capability of XRISM Resolve to securely detect narrow absorption features of only a few eV. These measurements trace the motion of the absorbing material and offer insight into the kinematics and spatial distribution of the wind in the vicinity of the black hole. These findings enhance our understanding of wind-fed accretion in Cygnus X-1 and highlight the importance of continued high-resolution X-ray observations to further constrain the physical properties of winds and accretion flows in high-mass X-ray binaries.

astro-ph.HE↗

arXiv2025

XRISM spectroscopy on orbital modulation of Fe Ly$α$ lines in Cygnus X-3

To understand physical processes such as mass transfer and binary evolution in X-ray binaries, the orbital parameters of the system are fundamental and crucial information. Cygnus X-3 is a high-mass X-ray binary composed of a compact object of unknown nature and a Wolf-Rayet star, which is of great interest in the context of wind-fed mass accretion and binary evolution. Here we present XRISM/Resolve high-resolution spectroscopy focusing on the Fe Ly$α$ lines in its hypersoft state. We perform an orbital phase-resolved spectral analysis of the lines to study the orbital modulation of the emission and absorption lines. It is found that the emission lines reflect the orbital motion of the compact object whose estimated velocity amplitude is $430^{~~+150}_{~~-140}~~\mathrm{km\,s^{~-1}}$, while the absorption lines show a variation that can be interpreted as originating from the stellar wind. We discuss possible mass ranges for the binary components using the mass function with the estimated value of the velocity amplitude in this work, combined with the relation between the mass loss rate and the orbital period derivative and the empirical mass and mass loss rate relation for Galactic Wolf-Rayet stars. They are constrained to be $(1.3\text{-}5.1)\,M_\odot$ and $(9.3\text{-}12)\,M_\odot$ for the assumed inclination angle of $i = 25$ deg, which becomes more relaxed to $(1.3\text{-}24)\,M_\odot$ and $(9.3\text{-}16)\,M_\odot$ for $i = 35$ deg, respectively. Thus, it remains unclear whether the system harbors a black hole or a neutron star.

astro-ph.HE↗

arXiv2025

In-orbit Performance of the Soft X-ray Imaging Telescope Xtend aboard XRISM

We present a summary of the in-orbit performance of the soft X-ray imaging telescope Xtend onboard the XRISM mission, based on in-flight observation data, including first-light celestial objects, calibration sources, and results from the cross-calibration campaign with other currently-operating X-ray observatories. XRISM/Xtend has a large field of view of $38.5'\times38.5'$, covering an energy range of 0.4--13 keV, as demonstrated by the first-light observation of the galaxy cluster Abell 2319. It also features an energy resolution of 170--180 eV at 6 keV, which meets the mission requirement and enables to resolve He-like and H-like Fe K$α$ lines. Throughout the observation during the performance verification phase, we confirm that two issues identified in SXI onboard the previous Hitomi mission -- light leakage and crosstalk events -- are addressed and suppressed in the case of Xtend. A joint cross-calibration observation of the bright quasar 3C273 results in an effective area measured to be $\sim420$ cm$^{2}$@1.5 keV and $\sim310$ cm$^{2}$@6.0 keV, which matches values obtained in ground tests. We also continuously monitor the health of Xtend by analyzing overclocking data, calibration source spectra, and day-Earth observations: the readout noise is stable and low, and contamination is negligible even one year after launch. A low background level compared to other major X-ray instruments onboard satellites, combined with the largest grasp ($Ω_{\rm eff}\sim60$ ${\rm cm^2~degree^2}$) of Xtend, will not only support Resolve analysis, but also enable significant scientific results on its own. This includes near future follow-up observations and transient searches in the context of time-domain and multi-messenger astrophysics.

astro-ph.IM↗

arXiv2025

Initial operations of the Soft X-ray Imager onboard XRISM

XRISM (X-Ray Imaging and Spectroscopy Mission) is an astronomical satellite with the capability of high-resolution spectroscopy with the X-ray microcalorimeter, Resolve, and wide field-of-view imaging with the CCD camera, Xtend. Xtend consists of the mirror assembly (XMA: X-ray Mirror Assembly) and detector (SXI: Soft X-ray Imager). The SXI is composed of CCDs, analog and digital electronics, and a mechanical cooler. After the successful launch on September 6th, 2023 (UT) and subsequent critical operations, the mission instruments were turned on and set up. The CCDs have been kept at the designed operating temperature of $-110^\circ$C after the electronics and cooling system were successfully set up. During the initial operation phase, which continued for more than a month after the critical operations, we verified the observation procedure, stability of the cooling system, all the observation options with different imaging areas and/or timing resolutions, and time-tagged and automated operations including those for South Atlantic Anomaly passages. We optimized the operation procedure and observation parameters including the cooler settings, imaging areas for the small window modes, and event selection algorithm. We summarize our policy and procedure of the initial operations for the SXI. We also report on a couple of issues we faced during the initial operations and lessons learned from them.

astro-ph.IM↗

arXiv2025

Soft X-ray Imager of the Xtend system onboard XRISM

The Soft X-ray Imager (SXI) is the X-ray charge-coupled device (CCD) camera for the soft X-ray imaging telescope Xtend installed on the X-ray Imaging and Spectroscopy Mission (XRISM), which was adopted as a recovery mission for the Hitomi X-ray satellite and was successfully launched on 2023 September 7 (JST). In order to maximize the science output of XRISM, we set the requirements for Xtend and find that the CCD set employed in the Hitomi/SXI or similar, i.e., a $2 \times 2$ array of back-illuminated CCDs with a $200~μ$m-thick depletion layer, would be practically best among available choices, when used in combination with the X-ray mirror assembly. We design the XRISM/SXI, based on the Hitomi/SXI, to have a wide field of view of $38' \times 38'$ in the $0.4-13$ keV energy range. We incorporated several significant improvements from the Hitomi/SXI into the CCD chip design to enhance the optical-light blocking capability and to increase the cosmic-ray tolerance, reducing the degradation of charge-transfer efficiency in orbit. By the time of the launch of XRISM, the imaging and spectroscopic capabilities of the SXI has been extensively studied in on-ground experiments with the full flight-model configuration or equivalent setups and confirmed to meet the requirements. The optical blocking capability, the cooling and temperature control performance, and the transmissivity and quantum efficiency to incident X-rays of the CCDs are also all confirmed to meet the requirements. Thus, we successfully complete the pre-flight development of the SXI for XRISM.

astro-ph.IM↗

arXiv2024

X-ray Spectral and Timing Properties of the Black Hole Binary XTE J1859+226 and their Relation to Jets

We compiled the X-ray and soft gamma-ray observations of the Galactic black hole binary XTE J1859+226 in the 1999--2000 outburst from RXTE, ASCA, BeppoSAX and CGRO. Throughout systematic spectral analysis using a two-component model consisting of a multi-temperature accretion disk plus a fraction of its flux convolved with an empirical Comptonized powerlaw component, we found that the innermost radius ($r_{\rm in}$) and temperature (Tin) of the disk are very variable with time in the rising phase of soft X-ray flux where Type-A/B/C low-frequency quasi-periodic oscillations (QPOs) were found. After this phase, $r_{\rm in}$ remains constant at around 60 km assuming a distance of 8 kpc and an inclination angle of 67$^{\circ}$, and Tin smoothly decays with time. The constant $r_{\rm in}$ suggests a presence of the innermost stable circular orbit (ISCO), with $r_{\rm in}$ repeatedly moving closer and farther away from the ISCO in the rising phase. Both disk parameters are remarkably correlated with independently analyzed timing properties such as QPO frequency and rms variability. Type-A/B QPOs are seen only when $r_{\rm in}$ is close to the ISCO, while Type-C are seen when $r_{\rm in}$ is truncated and the frequency changes with a relation of $r^{-1.0}_{\rm in}$, supporting that Type-C QPOs occur at the inner edge of the truncated disk. Accurate determinations of the frequency--$r_{\rm in}$ relation for various objects should be a powerful tool to discriminate plausible Type-C QPO models. Furthermore, we suggest that jet ejection events may occur when $r_{\rm in}$ rapidly approaches to the ISCO, along with rapid changes of the disk flux, the rms variability and the hardness ratio. A rapid shrinkage of $r_{\rm in}$ down to the ISCO can be a useful index as a precursor of radio flares for triggering Target-of-Opportunity observations and would provide constraints on jet launching mechanisms.

astro-ph.HE↗

arXiv2024

Status of Xtend telescope onboard X-Ray Imaging and Spectroscopy Mission (XRISM)

Xtend is one of the two telescopes onboard the X-ray imaging and spectroscopy mission (XRISM), which was launched on September 7th, 2023. Xtend comprises the Soft X-ray Imager (SXI), an X-ray CCD camera, and the X-ray Mirror Assembly (XMA), a thin-foil-nested conically approximated Wolter-I optics. A large field of view of $38^{\prime}\times38^{\prime}$ over the energy range from 0.4 to 13 keV is realized by the combination of the SXI and XMA with a focal length of 5.6 m. The SXI employs four P-channel, back-illuminated type CCDs with a thick depletion layer of 200 $μ$m. The four CCD chips are arranged in a 2$\times$2 grid and cooled down to $-110$ $^{\circ}$C with a single-stage Stirling cooler. Before the launch of XRISM, we conducted a month-long spacecraft thermal vacuum test. The performance verification of the SXI was successfully carried out in a course of multiple thermal cycles of the spacecraft. About a month after the launch of XRISM, the SXI was carefully activated and the soundness of its functionality was checked by a step-by-step process. Commissioning observations followed the initial operation. We here present pre- and post-launch results verifying the Xtend performance. All the in-orbit performances are consistent with those measured on ground and satisfy the mission requirement. Extensive calibration studies are ongoing.

astro-ph.IM↗

arXiv2023

Xtend, the Soft X-ray Imaging Telescope for the X-ray Imaging and Spectroscopy Mission (XRISM)

Xtend is a soft X-ray imaging telescope developed for the X-Ray Imaging and Spectroscopy Mission (XRISM). XRISM is scheduled to be launched in the Japanese fiscal year 2022. Xtend consists of the Soft X-ray Imager (SXI), an X-ray CCD camera, and the X-ray Mirror Assembly (XMA), a thin-foil-nested conically approximated Wolter-I optics. The SXI uses the P-channel, back-illuminated type CCD with an imaging area size of 31 mm on a side. The four CCD chips are arranged in a 2$\times$2 grid and can be cooled down to $-120$ $^{\circ}$C with a single-stage Stirling cooler. The XMA nests thin aluminum foils coated with gold in a confocal way with an outer diameter of 45~cm. A pre-collimator is installed in front of the X-ray mirror for the reduction of the stray light. Combining the SXI and XMA with a focal length of 5.6m, a field of view of $38^{\prime}\times38^{\prime}$ over the energy range from 0.4 to 13 keV is realized. We have completed the fabrication of the flight model of both SXI and XMA. The performance verification has been successfully conducted in a series of sub-system level tests. We also carried out on-ground calibration measurements and the data analysis is ongoing.

astro-ph.IM↗

arXiv2022

Simultaneous Radio and Optical Polarimetry of GRB 191221B Afterglow

Gamma-ray bursts (GRBs) are the most luminous transients in the universe and are utilized as probes of early stars, gravitational wave counterparts, and collisionless shock physics. In spite of studies on polarimetry of GRBs in individual wavelengths that characterized intriguing properties of prompt emission and afterglow, no coordinated multi-wavelength measurements have yet been performed. Here, we report the first coordinated simultaneous polarimetry in the optical and radio bands for the afterglow associated with the typical long GRB 191221B. Our observations successfully caught the radio emission, which is not affected by synchrotron self-absorption, and show that the emission is depolarized in the radio band compared to the optical one. Our simultaneous polarization angle measurement and temporal polarization monitoring indicate the existence of cool electrons that increase the estimate of jet kinetic energy by a factor of $>$ 4 for this GRB afterglow. Further coordinated multi-wavelength polarimetric campaigns would improve our understanding of the total jet energies and magnetic field configurations in the emission regions of various types of GRBs, which are required to comprehend the mass scales of their progenitor systems and the physics of collisionless shocks.

astro-ph.HE↗

arXiv2021

Performance evaluation of GAGG(Ce)/LFS scintillator + MPPC array readout with ASIC

We constructed a gamma-ray detector by combining two types of scintillator array detectors with an MPPC array and evaluated the spectral performance by reading out the signals from the MPPC with a low-power integrated circuit (ASIC) manufactured by IDEAS in Norway. One of the two types of scintillators is a GAGG(Ce) (Ce-doped $ \rm{Gd_3Al_2Ga_3O_{12}}$) scintillator, and the other is an LFS scintillator. The scintillator array is 2.5 cm $\times$ 2.5 cm in size and is coated with $ \rm{BaSO_4}$-based white paint for GAGG(Ce) and an enhanced specular reflector (ESR) for LFS except for the side optically coupled to the MPPC. The spectra derived from the array are affected by the MPPC photon saturations and light leakage from the adjacent pixels, and we carefully corrected for both effects in our data analysis. The energy resolution of 662 keV at 20 $^\circ$C is 6.10$\pm$0.04\% for the GAGG(Ce) scintillator array and 8.57$\pm$0.15\% for the LFS scintillator array, this is equivalent to the typical energy resolution found in the references. The energy resolution depends on the temperature: the energy resolution improves as the temperature decreases. We found that the contribution of thermal noise from the MPPCs to the energy resolution is negligible within the range of --20 to 40 $^\circ$C, and the energy resolution is mainly determined by the light yield of the crystals.

physics.ins-det↗

arXiv2020

Origin of the in-orbit instrumental background of the Hard X-ray Imager onboard Hitomi

Understanding and reducing the in-orbit instrumental backgrounds are essential to achieving high sensitivity in hard X-ray astronomical observations. The observational data of the Hard X-ray Imager (HXI) on board the Hitomi satellite provides useful information on the background components, owing to its multi-layer configuration with different atomic numbers: the HXI consists of a stack of four layers of Si (Z = 14) detectors and one layer of CdTe (Z = 48, 52) detector surrounded by well-type BGO (Bi4Ge3O12) active shields. Based on the observational data, the backgrounds of top Si layer, the three underlying Si layers, and the CdTe layer are inferred to be dominated by different components, namely, low-energy electrons, albedo neutrons, and proton-induced radioactivation, respectively. Monte Carlo simulations of the in-orbit background of the HXI reproduce the observed background spectrum of each layer well, thereby verifying the above hypothesis quantitatively. In addition, we suggest the inclusion of an electron shield to reduce the background.

astro-ph.IM↗

arXiv2020

Experimental studies on the charge transfer inefficiency of CCD developed for the soft X-ray imaging telescope Xtend aboard the XRISM satellite

We present experimental studies on the charge transfer inefficiency (CTI) of charge-coupled device (CCD) developed for the soft X-ray imaging telescope, Xtend, aboard the XRISM satellite. The CCD is equipped with a charge injection (CI) capability, in which sacrificial charge is periodically injected to fill the charge traps. By evaluating the re-emission of the trapped charge observed behind the CI rows, we find that there are at least three trap populations with different time constants. The traps with the shortest time constant, which is equivalent to a transfer time of approximately one pixel, are mainly responsible for the trailing charge of an X-ray event seen in the following pixel. A comparison of the trailing charge in two clocking modes reveals that the CTI depends not only on the transfer time but also on the area, namely the imaging or storage area. We construct a new CTI model with taking into account with both transfer-time and area dependence. This model reproduces the data obtained in both clocking modes consistently. We also examine apparent flux dependence of the CTI observed without the CI technique. The higher incident X-ray flux is, the lower the CTI value becomes. It is due to a sacrificial charge effect by another X-ray photon. This effect is found to be negligible when the CI technique is used.

astro-ph.IM↗

arXiv2020

Optical Blocking Performance of CCDs Developed for the X-ray Astronomy Satellite XRISM

We have been developing P-channel Charge-Coupled Devices (CCDs) for the upcoming X-ray Astronomy Satellite XRISM, planned to be launched in 2021. While the basic design of the CCD camera (Soft X-ray Imager: SXI) is almost the same as that of the lost Hitomi (ASTRO-H) observatory, we are planning to reduce the "light leakages" that is one of the largest problems recognized in Hitomi data. We adopted a double-layer optical blocking layer on the XRISM CCDs and also added an extra aluminum layer on the backside of them. We develop a newly designed test sample CCD and irradiate it with optical light to evaluate the optical blocking performance. As a result, light leakages are effectively reduced compared with that of the Hitomi CCDs. We thus conclude that the issue is solved by the new design and that the XRISM CCDs satisfy the mission requirement for the SXI.

astro-ph.IM↗

arXiv2019

Radiation hardness of a p-channel notch CCD developed for the X-ray CCD camera onboard the XRISM satellite

We report the radiation hardness of a p-channel CCD developed for the X-ray CCD camera onboard the XRISM satellite. This CCD has basically the same characteristics as the one used in the previous Hitomi satellite, but newly employs a notch structure of potential for signal charges by increasing the implant concentration in the channel. The new device was exposed up to approximately $7.9 \times 10^{10} \mathrm{~protons~cm^{-2}}$ at 100 MeV. The charge transfer inefficiency was estimated as a function of proton fluence with an ${}^{55} \mathrm{Fe}$ source. A device without the notch structure was also examined for comparison. The result shows that the notch device has a significantly higher radiation hardness than those without the notch structure including the device adopted for Hitomi. This proves that the new CCD is radiation tolerant for space applications with a sufficient margin.

astro-ph.IM↗

arXiv2018

Discovery and state transitions of the new Galactic black hole candidate MAXI J1535-571

We report on the detection and subsequent X-ray monitoring of the new Galactic black hole candidate MAXI J1535-571 with the MAXI/GSC. Afterthe discovery on 2017 September 2 made independently with MAXI and the Swift/BAT, the source brightened gradually, and in a few weeks, reached the peak intensity of ~5 Crab, or ~1.6 x 10^{-7} erg cm^{-2} s^{-1} in terms of the 2--20 keV flux. On the initial outburst rise, the X-ray spectrum was described by a power-law model with a photon index of ~2, while after a hard-to-soft transition which occurred on September 18, the spectrum required a disk blackbody component in addition. At around the flux peak, the 2-8 keV and 15-50 keV light curves showed quasi-periodic and anti-correlated fluctuations with an amplitude of 10--20%, on a time scale of ~1-day. Based on these X-ray properties obtained with the MAXI/GSC, we discuss the evolution of the spectral state of this source, and give constraints on its system parameters.

astro-ph.HE↗