arXiv subjects

Hiroki Akamatsu

Publications and source records attributed to Hiroki Akamatsu.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic Reconnection in Galaxy Clusters

Galaxy clusters contain an intra-cluster medium (ICM) with temperatures of tens of millions of Kelvin. Cosmological structure formation simulations show that this diffuse gas is heated not only by adiabatic gravitational compression but also by shock waves and turbulence generated during mergers of galaxy groups and clusters. These processes are expected to produce magnetic fields and cosmic rays, observed through synchrotron polarization. One structure formed during cluster evolution is the cold front, a contact discontinuity created when colder gas moves transonically through hotter gas. Using MeerKAT, GMRT, and ATCA, we recently discovered radio emission along cold fronts in two galaxy clusters, with spectra indicating re-acceleration at the discontinuity. This presents a new puzzle because the standard mechanism in galaxy clusters, Fermi acceleration, is not naturally expected there. We propose magnetic reconnection as the re-acceleration mechanism. Compression and stretching of magnetized plasma at the discontinuity can generate current sheets that trigger reconnection, as also suggested by simulations. With AA*, we will probe broadband radio spectra at high spatial resolution to constrain where re-acceleration occurs. Polarization measurements will reveal magnetic-field structures and clarify the conditions required for magnetic reconnection.

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arXiv2025

Three-Dimensional Polarimetric Structure of Jets from Radio Galaxy MRC 0600-399 in A3376

MRC 0600-399, in the merging galaxy cluster Abell 3376, is one of the unique Head-Tail radio galaxies having abruptly bending structures, the origin of which is still under discussion. A previous study suggested that the interaction between the jets and the magnetic fields on the cold front has produced such an outstanding feature; however, it is still not known how the jet propagates the long distance keeping the sharp shape. In this study, we performed a polarization analysis of MRC 0600-399 and a nearby radio source using MeerKAT L-band data. To interpret the Faraday structure, we constructed the Pseudo-3D visualization map of the reconstructed Faraday dispersion function 3D cube. MRC 0600-399 has a complicated Faraday depth structure, and some of the Faraday depth features are in good agreement with the 2D structure on the celestial sphere. For the northern jet of MRC 0600-399, the Faraday depth values show a clear gap at the tip region. Based on the polarization analysis and comparison with the other observation, we concluded that the synchrotron radiation from the tip region of the northern jet originates from the magnetic field on the cold front.

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arXiv2025

PSZ2 G181.06+48.47 I: X-ray exploration of a low-mass cluster with exceptionally-distant radio relics

Relics are diffuse, highly-polarized radio sources that trace merger-driven shocks at the periphery of merging galaxy clusters. The LOFAR survey recently discovered a rare example of double relics in the low-mass cluster PSZ2 G181.06+48.47. Through a detailed exploration of new Chandra and XMM-Newton observations, we reveal that PSZ2 G181.06+48.47 has a lower mass ($M_{500,X}=2.32^{+0.29}_{-0.25}\times10^{14}$ M$_{\odot}$) than previously thought. Despite its cool global temperature of $kT_{500}=3.62^{+0.15}_{-0.07}$ keV, PSZ2 G181.06+48.47 is one of the most disturbed clusters in the Planck sample, with a complex morphological and thermodynamic structure. We discover a set of three discontinuities within <500 kpc of the cluster center, and, from a surface brightness analysis, place $5σ$ upper limits of $M_{NE}<1.43$ and $M_{SW}<1.57$ for any shock associated with the relic locations. We also revise established scaling relations for double radio-relics by adding 12 new systems not included in previous work. The PSZ2 G181.06+48.47 relics have the widest separation (scaled for $r_{500}$) of all known double-relic systems. The exceptional distance from the cluster center ($>r_{200}$), indicates the relics may be associated with shocks in the ``run-away" phase. We propose that this late-stage, post-apocenter merger is captured as the two subclusters with a mass ratio of 1.2-1.4 fall back into each other. The outer relic shocks were likely produced at the first core passage, while the inner discontinuities are associated with the second infall.

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arXiv2024

Development of the Low Frequency Telescope focal plane detector arrays for LiteBIRD

LiteBIRD, a forthcoming JAXA mission, aims to accurately study the microwave sky within the 40-400 GHz frequency range divided into 15 distinct nominal bands. The primary objective is to constrain the CMB inflationary signal, specifically the primordial B-modes. LiteBIRD targets the CMB B-mode signal on large angular scales, where the primordial inflationary signal is expected to dominate, with the goal of reaching a tensor-to-scalar ratio sensitivity of $σ_r\sim0.001$. LiteBIRD frequency bands will be split among three telescopes, with some overlap between telescopes for better control of systematic effects. Here we report on the development status of the detector arrays for the Low Frequency Telescope (LFT), which spans the 34-161 GHz range, with 12 bands subdivided between four types of trichroic pixels consisting of lenslet-coupled sinuous antennas. The signal from the antenna is bandpass filtered and sensed by AlMn Transition-Edge Sensors (TES). We provide an update on the status of the design and development of LiteBIRD's LFT LF1 (40-60-78 GHz), LF2 (50-68-89 GHz) pixels. We discuss design choices motivated by LiteBIRD scientific goals. In particular we focus on the details of the optimization of the design parameters of the sinuous antenna, on-chip bandpass filters, cross-under and impedance transformers and all the RF components that define the LF1 and LF2 pixel detection chain. We present this work in the context of the technical challenges and physical constraints imposed by the finite size of the instrument.

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arXiv2024

A Method of Measuring TES Complex ETF Response in Frequency-domain Multiplexed Readout by Single Sideband Power Modulation

The digital frequency domain multiplexing (DfMux) technique is widely used for astrophysical instruments with large detector arrays. Detailed detector characterization is required for instrument calibration and systematics control. We conduct the TES complex electrothermal-feedback (ETF) response measurement with the DfMux readout system as follows. By injecting a single sideband signal, we induce modulation in TES power dissipation over a frequency range encompassing the detector response. The modulated current signal induced by TES heating effect is measured, allowing for the ETF response characterization of the detector. With the injection of an upper sideband, the TES readout current shows both an upper and a lower sideband. We model the upper and lower sideband complex ETF response and verify the model by fitting to experimental data. The model not only can fit for certain physical parameters of the detector, such as loop gain, temperature sensitivity, current sensitivity, and time constant, but also enables us to estimate the systematic effect introduced by the multiplexed readout. The method is therefore useful for in-situ detector calibration and for estimating systematic effects during astronomical telescope observations, such as those performed by the upcoming LiteBIRD satellite.

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arXiv2023

Simulations of two-temperature jets in galaxy clusters: II. X-ray property of forward shock

Forward shocks by radio jets, driven into the intracluster medium, are one of the indicators that can be used to evaluate the power of the jet. Meanwhile high-angular-resolution X-ray observations show the Mach numbers of powerful radio jets are smaller compared to that of theoretical and numerical studies, $\mathcal{M_{\rm obs}} < 2$. Our aim is to systematically investigate various factors, such as projection effects and temperature non-equilibration between protons and electrons, that influence the Mach number estimate in a powerful jet. Using two-temperature magnetohydrodynamic simulation data for the Cygnus A radio jets, whose Mach number is approximately 6, we construct mock X-ray maps of simulated jets from various viewing angles. Further, we evaluate the shock Mach number from density/temperature jump using the same method of X-ray observations. Our results demonstrate that measurements from density jump significantly underestimate the Mach numbers, $\mathcal{M} < 2$, around the jet head at a low viewing angle, $\lessapprox 50^{\circ}$. The observed post-shock temperature is strongly reduced by the projection effect, as our jet is in the cluster center where the gas density is high. On the other hand, the temperature jump is almost unity, even if thermal electrons are in instant equilibration with protons. Upon comparison, we find that shock property of our model at viewing angle of $<$ $55^{\circ}$ is in a good agreement with that of Cygnus A observations.

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arXiv2023

Line Emission Mapper (LEM): Probing the physics of cosmic ecosystems

The Line Emission Mapper (LEM) is an X-ray Probe for the 2030s that will answer the outstanding questions of the Universe's structure formation. It will also provide transformative new observing capabilities for every area of astrophysics, and to heliophysics and planetary physics as well. LEM's main goal is a comprehensive look at the physics of galaxy formation, including stellar and black-hole feedback and flows of baryonic matter into and out of galaxies. These processes are best studied in X-rays, and emission-line mapping is the pressing need in this area. LEM will use a large microcalorimeter array/IFU, covering a 30x30' field with 10" angular resolution, to map the soft X-ray line emission from objects that constitute galactic ecosystems. These include supernova remnants, star-forming regions, superbubbles, galactic outflows (such as the Fermi/eROSITA bubbles in the Milky Way and their analogs in other galaxies), the Circumgalactic Medium in the Milky Way and other galaxies, and the Intergalactic Medium at the outskirts and beyond the confines of galaxies and clusters. LEM's 1-2 eV spectral resolution in the 0.2-2 keV band will make it possible to disentangle the faintest emission lines in those objects from the bright Milky Way foreground, providing groundbreaking measurements of the physics of these plasmas, from temperatures, densities, chemical composition to gas dynamics. While LEM's main focus is on galaxy formation, it will provide transformative capability for all classes of astrophysical objects, from the Earth's magnetosphere, planets and comets to the interstellar medium and X-ray binaries in nearby galaxies, AGN, and cooling gas in galaxy clusters. In addition to pointed observations, LEM will perform a shallow all-sky survey that will dramatically expand the discovery space.

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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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arXiv2023

Simulation and Measurement of Out-of-Band Resonances for the FDM Readout of a TES Bolometer

With applications in cosmology, infrared astronomy and CMB survey, frequency-division multiplexing (FDM) proved to be a viable readout for transition-edge sensors (TES). We investigate the occurrence of out-of-band resonances (OBR) which could constrain the bandwidth of the FDM readout of TES bolometers. The study includes SPICE modeling of the entire setup including the cryogenic harness, LC filters, Superconducting Quantum Interference Device (SQUID) and room-temperature amplifier. Simulation results show that the long harness (for flight model) could cause multiple reflections that generate repetitive spikes in the spectrum. Peaks of the OBR are mainly due to the parasitic capacitances at the input of SQUID. Implementing a low-pass RC circuit (snubber) at the input of the SQUID dampened the OBR. As a result, the first peak only appears around 20 MHz which is a safe margin for the 1 MHz-3.8 MHz FDM in use in the prototype readout. Using a spectrum analyzer and broadband LNAs,we also measured the OBR for the prototype FDM readout in the lab up to 500 MHz. The measurement was conducted at temperatures of 50 mK and 4 K and for various biasing of the DC SQUID. It turns out that OBR are more intense at 50 mK and are caused by the harness impedance mismatch rather than the SQUID. Simulation codes and supporting materials are available at https://github.com/githubamin/LT-Spice-Simulation-of-FDM-readout.

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arXiv2022

Diffuse radio source candidate in CIZA J1358.9-4750

We report on results of our upgraded Giant Metrewave Radio Telescope (uGMRT) observations for an early-stage merging galaxy cluster, CIZA J1358.9-4750 (CIZA1359), in Band-3 (300--500 MHz). We achieved the image dynamic range of $\sim 38,000$ using the direction dependent calibration and found a candidate of diffuse radio emission at 4~$σ_{rms}$ significance. The flux density of the candidate at 400~MHz, $24.04 \pm 2.48$~mJy, is significantly positive compared to noise, where its radio power, $2.40 \times 10^{24}$~W~Hz$^{-1}$, is consistent with those of typical diffuse radio sources of galaxy clusters. The candidate is associated with a part of the X-ray shock front at which the Mach number reaches its maximum value of $\mathcal{M}\sim 1.7$. The spectral index ($F_ν\propto ν^α$) of the candidate, $α= - 1.22 \pm 0.33$, is in agreement with an expected value derived from the standard diffusive shock acceleration (DSA) model. But such a low Mach number with a short acceleration time would require seed cosmic-rays supplied from active galactic nucleus (AGN) activities of member galaxies, as suggested in some other clusters. Indeed, we found seven AGN candidates inside the diffuse source candidate. Assuming the energy equipartition between magnetic fields and cosmic-rays, the magnetic field strength of the candidate was estimated to be $2.1~μ$G. We also find head-tail galaxies and radio phoenixes or fossils near the CIZA1359.

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arXiv2022

The NuSTAR and Chandra view of CL 0217+70 and Its Tell-Tale Radio Halo

Mergers of galaxy clusters are the most energetic events in the universe, driving shock and cold fronts, generating turbulence, and accelerating particles that create radio halos and relics. The galaxy cluster CL 0217+70 is a remarkable late stage merger, with a double peripheral radio relic and a giant radio halo. A Chandra study detects surface brightness edges that correspond to radio features within the halo. In this work, we present a study of this cluster with NuSTAR and Chandra data using spectro-imaging methods. The global temperature is found to be kT = 9.1 keV. We set an upper limit for the IC flux of ~2.7x10^(-12) erg s^(-1) cm^(-2), and a lower limit to the magnetic field of 0.08 microG. Our local IC search revealed a possibility that IC emission may have a significant contribution at the outskirts of a radio halo emission and on/near shock regions within ~0.6 r500 of clusters. We detected a "hot spot" feature in our temperature map coincident a surface brightness edge, but our investigation on its origin is inconclusive. If the "hot spot" is the downstream of a shock, we set a lower limit of kT > 21 keV to the plasma, that corresponds to M~2. We found three shock fronts within 0.5 r500. Multiple weak shocks within the cluster center hint at an ongoing merger activity and continued feeding of the giant radio halo. CL 0217+70 is the only example hosting these secondary shocks in multiple form.

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arXiv2022

DESHIMA 2.0: development of an integrated superconducting spectrometer for science-grade astronomical observations

Integrated superconducting spectrometer (ISS) technology will enable ultra-wideband, integral-field spectroscopy for (sub)millimeter-wave astronomy, in particular, for uncovering the dust-obscured cosmic star formation and galaxy evolution over cosmic time. Here we present the development of DESHIMA 2.0, an ISS for ultra-wideband spectroscopy toward high-redshift galaxies. DESHIMA 2.0 is designed to observe the 220-440 GHz band in a single shot, corresponding to a redshift range of $z$=3.3-7.6 for the ionized carbon emission ([C II] 158 $μ$m). The first-light experiment of DESHIMA 1.0, using the 332-377 GHz band, has shown an excellent agreement among the on-sky measurements, the lab measurements, and the design. As a successor to DESHIMA 1.0, we plan the commissioning and the scientific observation campaign of DESHIMA 2.0 on the ASTE 10-m telescope in 2023. Ongoing upgrades for the full octave-bandwidth system include the wideband 347-channel chip design and the wideband quasi-optical system. For efficient measurements, we also develop the observation strategy using the mechanical fast sky-position chopper and the sky-noise removal technique based on a novel data-scientific approach. In the paper, we show the recent status of the upgrades and the plans for the scientific observation campaign.

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arXiv2022

Frequency domain multiplexing readout for large arrays of transition-edge sensors

We report our most recent progress and demonstration of a frequency domain multiplexing (FDM) readout technology for transition-edge sensor (TES) arrays, both of which we have been developing, in the framework of the X-IFU instrument on board the future Athena X-ray telescope. Using Ti/Au TES micro-calorimeters, high-Q LC filters and analog/digital electronics developed at SRON and low-noise two-stage SQUID amplifiers from VTT Finland, we demonstrated feasibility of our FDM readout technology, with the simultaneous readout of 37 pixels with an energy resolution at of 2.23 eV at an energy of 6 keV. We finally outline our plans for further scaling up and improving our technology.

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arXiv2022

Performance of the SRON Ti/Au Transition Edge Sensor X-ray Calorimeters

In the early 2030s, ESAs new X-ray observatory, Athena, is scheduled to be launched. It will carry two main instruments, one of which is the X-ray Integral Field Unit (X-IFU), an X-ray imaging spectrometer, which will consist of an array of several thousand transition-edge sensors (TESs) with a proposed energy resolution of 2.5 eV for photon energies up to 7 keV. At SRON we develop the backup TES array based on Ti/Au bilayers with a transition temperature just below 100 mK. In this contribution we will give a broad overview of the properties and capabilities of these state-of-the-art detectors. Over the years we have fabricated and studied a large number of detectors with various geometries, providing us with a good understanding of how to precisely control the properties of our detectors. We are able to accurately vary the most important detector properties, such as the normal resistance, thermal conductance and critical temperature. This allows us to finely tune our detectors to meet the demands of various applications. The detectors have demonstrated excellent energy resolutions of below 1.8 eV for 5.9 keV X-rays. By tuning the properties of the devices, they can be optimally matched to various read-out schemes using both AC and DC biasing. The next step is to increase the size of our TES arrays from our current kilo-pixel arrays towards the full-sized array for X-IFU.

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arXiv2022

Susceptibility study of TES micro-calorimeters for X-ray spectroscopy under FDM readout

We present a characterization of the sensitivity of TES X-ray micro-calorimeters to environmental conditions under frequency-domain multiplexing (FDM) readout. In the FDM scheme, each TES in a readout chain is in series with a LC band-pass filter and AC biased with an independent carrier at MHz range. Using TES arrays, cold readout circuitry and warm electronics fabricated at SRON and SQUIDs produced at VTT Finland, we characterize the sensitivity of the detectors to bias voltage, bath temperature and magnetic field. We compare our results with the requirements for the Athena X-IFU instrument, showing the compliance of the measured sensitivities. We find in particular that FDM is intrinsically insensitive to the magnetic field because of TES design and AC readout.

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arXiv2022

Mitigation of the Magnetic Field Susceptibility of Transition Edge Sensors using a Superconducting Groundplane

Transition edge sensor (TES) microcalorimeters and bolometers are used for a variety of applications. The sensors are based on the steep temperature-dependent resistance of the normal-to-superconducting transition, and are thus intrinsically sensitive to magnetic fields. Conventionally the detectors are shielded from stray magnetic fields using external magnetic shields. However, in particular for applications with strict limits on the available space and mass of an instrument, external magnetic shields might not be enough to obtain the required shielding factors or field homogeneity. Additionally, these shields are only effective for magnetic fields generated external to the TES array, and are ineffective to mitigate the impact of internally generated magnetic fields. Here we present an alternative shielding method based on a superconducting groundplane deposited directly on the backside of the silicon nitride membrane on which the TESs are located. We demonstrate that this local shielding for external magnetic fields has a shielding factor of at the least ~ 75, and is also effective at reducing internal self-induced magnetic fields, as demonstrated by measurements and simulation of the eddy current losses in our AC biased detectors. Measurements of 5.9 keV X-ray photons show that our shielded detectors have a high resilience to external magnetic fields, showing no degradation of the energy resolution or shifts of the energy scale calibration for fields of several microTesla, values higher than expected in typical real-world applications.

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arXiv2022

X-ray spectra of the Fe-L complex III: systematic uncertainties in the atomic data

There has been a growing request from the X-ray astronomy community for a quantitative estimate of systematic uncertainties originating from the atomic data used in plasma codes. Though there have been several studies looking into atomic data uncertainties using theoretical calculations, in general, there is no commonly accepted solution for this task. We present a new approach for estimating uncertainties in the line emissivities for the current models of collisional plasma, mainly based upon dedicated analysis of observed high resolution spectra of stellar coronae and galaxy clusters. We find that the systematic uncertainties of the observed lines consistently show anti-correlation with the model line fluxes, after properly accounting for the additional uncertainties from the ion concentration calculation. The strong lines in the spectra are in general better reproduced, indicating that the atomic data and modeling of the main transitions are more accurate than those for the minor ones. This underlying anti-correlation is found to be roughly independent on source properties, line positions, ion species, and the line formation processes. We further apply our method to the simulated XRISM and Athena observations of collisional plasma sources and discuss the impact of uncertainties on the interpretation of these spectra. The typical uncertainties are 1-2% on temperature and 3-20% on abundances of O, Ne, Fe, Mg, and Ni.

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arXiv2022

The NuSTAR, XMM-Newton, and Suzaku view of A3395 at the intercluster filament interface

Galaxy clusters are the largest virialized objects in the universe. Their merger dynamics and their interactions with the cosmic filaments that connect them are important for our understanding of the formation of large-scale structure. In addition, cosmic filaments are thought to possess the missing baryons in the universe. Studying the interaction of galaxy clusters and filaments therefore has the potential to unveil the the origin of the baryons and the physical processes that occur during merger stages of galaxy clusters. In this paper, we study the connection between A3395 and the intercluster filament with NuSTAR, XMM-Newton, and Suzaku data. Since the NuSTAR observation is moderately contaminated by scattered light, we present a novel technique developed for disentangling this background from the emission from the intracluster medium. We find that the interface of the cluster and the intercluster filament connecting A3395 and A3391 does not show any signs of heated plasma, as was previously thought. This interface has low temperature, high density, and low entropy, thus we suggest that the gas is cooling, being enhanced by the turbulent or tidal 'weather' driven during the early stage of the merger. Furthermore, our temperature results from the NuSTAR data are in agreement with those from XMM-Newton, and from joint NuSTAR and XMM-Newton analysis for a region with ~25% scattered light contamination within 1 sigma. We show that the temperature constraint of the intracluster medium is valid even when the data are contaminated up to ~25% for ~5 keV cluster emission.

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