X-ray CCDs Research Articles

Mapping the Imprints of Stellar and Active Galactic Nucleus Feedback in the Circumgalactic Medium with X-Ray Microcalorimeters

The Astro2020 Decadal Survey has identified the mapping of the circumgalactic medium (CGM; the gaseous plasma around galaxies) as a key objective. We explore the prospects for characterizing the CGM in and around nearby galaxy halos with a future large-grasp X-ray microcalorimeter. We create realistic mock observations from hydrodynamical simulations (EAGLE, IllustrisTNG, and Simba) that demonstrate a wide range of potential measurements, which will address the open questions in galaxy formation and evolution. By including all background and foreground components in our mock observations, we show why it is impossible to perform these measurements with current instruments, such as X-ray CCDs, and why only microcalorimeters will allow us to distinguish the faint CGM emission from the bright Milky Way (MW) foreground emission lines. We find that individual halos of MW mass can, on average and depending on star formation rate, be traced out to large radii, around R 500, and for larger galaxies even out to R 200, using prominent emission lines, such as O vii, or O viii. Furthermore, we show that emission-line ratios for individual halos can reveal the radial temperature structure. Substructure measurements show that it will be possible to relate azimuthal variations to the feedback mode of the galaxy. We demonstrate the ability to construct temperature, velocity, and abundance ratio maps from spectral fitting for individual galaxy halos, which reveal rotation features, active galactic nucleus outbursts, and enrichment.

High-speed readout system of X-ray CMOS image sensor for time domain astronomy

We developed an FPGA-based high-speed readout system for a complementary metal-oxide-semiconductor (CMOS) image sensor to observe soft X-ray transients in future satellite missions, such as HiZ-GUNDAM. Our previous research revealed that the CMOS image sensor has low-energy X-ray detection capability (0.4–4 keV) and strong radiation tolerance, which satisfies the requirements of the HiZ-GUNDAM mission. However, CMOS sensors typically have small pixel sizes (e.g., ∼10 µm), resulting in large volumes of image data. GSENSE400BSI has 2048×2048 pixels, producing 6 Mbyte per frame. These large volumes of observed raw image data cannot be stored in a satellite bus system with a limited storage size. Therefore, only X-ray photon events must be extracted from the raw image data. Furthermore, the readout time of CMOS image sensors is approximately ten times faster than that of typical X-ray CCDs, requiring faster event extraction on a timescale of ∼0.1 s. To address these issues, we have developed an FPGA-based image signal processing system capable of high-speed X-ray event extraction onboard without storing raw image data. The developed compact system enabled mounting on a CubeSat mission, facilitating early in-orbit operation demonstration. Here, we present the design and results of the performance evaluation tests of the proposed FPGA-based readout system. Utilizing X-ray irradiation experiments, the results of the X-ray event extraction with the onboard and offline processing methods were consistent, validating the functionality of the proposed system.

Simulation-based spectral analysis of X-ray CCD data affected by photon pile-up

Abstract We have developed a simulation-based method of spectral analysis for pile-up-affected data of X-ray CCDs without any loss of photon statistics. As effects of the photon pile-up appear as complicated nonlinear detector responses, we employ a detailed simulation to calculate the important processes in an X-ray observation including physical interactions, detector signal generation, detector readout, and a series of data reduction processes. This simulation naturally reproduces X-ray-like and background-like events as results of X-ray photon merging in a single pixel or in a chunk of adjacent pixels, allowing us to construct a nonlinear spectral analysis framework that can treat pile-up-affected observation data. For validation, we have performed data analysis of Suzaku X-ray Imaging Spectrometer (XIS) observations using this framework with various parameters of the detector simulation, all of which are optimized for that instrument. We present three cases of different pile-up degrees: PKS 2155−304 (negligible pile-up), Aquila X-1 (moderate pile-up), and the Crab Nebula (strong pile-up); we show that the nonlinear analysis method produces results consistent with a conventional linear analysis for the negligible pile-up condition, and accurately corrects well-known pile-up effects such as spectral hardening and flux decrease for the pile-up cases. These corrected results are consistent with those obtained by a widely used core-exclusion method or by other observatories with much higher timing resolutions (without pile-up). Our framework is applicable to any types of CCDs used for X-ray astronomy, including future missions such as X-ray Imaging and Spectroscopy Mission (XRISM), by appropriate optimization of the simulation parameters.

Corrections to “Design and Characterizations of the Radiation-Hardened XCR4C ASIC for X-Ray CCDs for Space Astronomical Applications” [Jun 20 1175-1184

In the above article <xref ref-type="bibr" rid="ref1">[1]</xref>, on p. 1181, Fig. 7 appears the same as Fig. 6, which is incorrect. Instead, Fig. 7 should appear as below.

Performance of a directly deposited optical blocking filter on x-ray CCDs: case study from the REgolith X-ray Imaging Spectrometer (REXIS) experiment

The REgolith X-ray Imaging Spectrometer (REXIS) is a soft x-ray spectrometer and the student collaboration instrument aboard NASA’s OSIRIS-REx asteroid sample return mission. REXIS utilizes MIT Lincoln Laboratory CCID-41 x-ray detectors coated with a directly deposited optical blocking filter (OBF) with a thickness of 320 nm. The aluminum coating, developed at MIT Lincoln Laboratory, is designed to block visible light from the detector, to maintain high sensitivity to soft x-rays in the presence of reflected sunlight from the surface of the target asteroid Bennu. The scientific objective for the REXIS instrument is to measure the stimulated x-ray flux fluoresced from Bennu to discern elemental abundances present on the asteroid’s surface. The coating technique applied for blocking visible light had not previously been used on the CCD-41s in an extended space flight mission. The performance of the OBF on the flight detectors was not characterized before and after environmental stress testing. Therefore, to mature the OBF to technology readiness level (TRL) 6, the flight spare detectors were tested while the instrument was on the way to the asteroid. The flight spare hardware underwent vibration and thermal environmental stress testing to test the durability and effectiveness of the OBF. This testing informed our expectations of the in-flight OBF once it reached the asteroid and helped mature the TRL level of this directly deposited OBF. We discuss the setup and results of those tests and address the performance of the flight OBF at the asteroid. We conclude that depositing an aluminum OBF onto the surface of a charge-coupled device is able to withstand stresses of launch and an extended life-mission in interplanetary space.

X-ray backlighter requirements for refraction-based electron density diagnostics through Talbot-Lau deflectometry.

Talbot-Lau x-ray interferometers can map electron density gradients in High Energy Density (HED) samples. In the deflectometer configuration, it can provide refraction, attenuation, elemental composition, and scatter information from a single image. X-ray backlighters in Talbot-Lau deflectometry must meet specific requirements regarding source size and x-ray spectra, amongst others, to accurately diagnose a wide range of HED experiments. 8 keV sources produced in the high-power laser and pulsed power environment were evaluated as x-ray backlighters for Talbot-Lau x-ray deflectometry. In high-power laser experiments, K-shell emission was produced by irradiating copper targets (500 × 500 × 12.5 μm3 foils, 20 μm diameter wire, and >10 μm diameter spheres) with 30 J, 8-30 ps laser pulses and a 25 μm copper wire with a 60 J, 10 ps laser pulse. In the pulsed power environment, single (2 × 40 μm) and double (4 × 25 μm) copper x-pinches were driven at ∼1 kA/ns. Moiré fringe formation was demonstrated for all x-ray sources explored, and detector performance was evaluated for x-ray films, x-ray CCDs, and imaging plates in context of spatial resolution, x-ray emission, and fringe contrast.

Astronomical imaging with the X-ray observatory Hitomi

We report an imaging capability of the Japan-led X-ray observatory Hitomi, formerly known as ASTRO-H. It carries four scientific instruments of Soft X-ray Imager (SXI: CCD camera), Hard X-ray Imager (HXI), Soft X-ray Spectrometer, and Soft Gamma-ray Detector, allowing us to perform a wide-band high-sensitive imaging spectroscopy. We highlight the specification and the performance we obtained with primarily regard to X-ray and soft gamma-ray imaging. Primary imaging instrument SXI utilizes four large-area X-ray CCDs positioned at the focal plane of Soft X-ray Telescope (SXT-I). Imaging area with a size of 62mm square makes a largest field of view (FoV) of 38′ square among the focal plane X-ray detectors, which enables us to observe extended objects such as clusters of galaxies and galactic supernova remnants with a single pointing in the soft X-ray band from 0.4 to 12keV. HXI employs the hybrid sensors consisting of four layers of double-sided silicon strip detectors and a single layer of cadmium telluride double-sided strip detector covering the energy band from 5 to 80keV. After the successful launch of Hitomi on February 17th, 2016 and the subsequent start up of all the instruments, imaging performance of both imagers are verified as expected from the ground calibration tests. The position of the active galactic nucleus of the central galaxy NGC1275 in the Perseus cluster is precisely seen by SXI, while the positional difference of line-of-sight velocity dispersion of the hot intracluster medium is measured inside by SXS. From the observation of Crab nebula, we obtain the on-pulse hard X-ray image with HXI as well as the time-averaged image in which the torus of the pulsar wind nebula can be seen.

Annealing bounds to prevent further Charge Transfer Inefficiency increase of the Chandra X-ray CCDs

After the front-illuminated CCDs on board the X-ray telescope Chandra were damaged by radiation after launch, it was decided to anneal them in an effort to remove the defects introduced by the irradiation. The annealing led to an unexpected increase of the Charge Transfer Inefficiency (CTI). The performance degradation is attributed to point defect interactions in the devices. Specifically, the annealing at 30°C activated the diffusion of the main interstitial defect in the device, the carbon interstitial, which led to its association with a substitutional impurity, ultimately resulting in a stable and electrically active defect state. Because the formation reaction of this carbon interstitial and substitutional impurity associate is diffusion limited, we recommend a higher upper bound for the annealing temperature and duration of any future CCD anneals, that of −50°C for one day or −60°C for a week, to prevent further CTI increase.

An hybrid detector GEM-ASIC for 2-D soft X-ray imaging for laser produced plasma and pulsed sources

The following paper presents a new 2-D detector (`GEMpix') in the soft X-ray range, having a wide dynamic range thanks to its intrisic gain, working in charge integration mode to be used for diagnosing laser produced plasma (LPP) or X-ray pulsed sources. It is a gas detector based on the Gas Electron Multiplier (GEM) technology with a quad-medipix chip as read-out electronics. In our prototype, the substitution of semiconductor material with a gas triple-GEM allows several advantages with respect to the detectors commonly used in LPP, as X-ray CCDs and Micro Channel Plates or Image Plates. In these experiments the configuration Time-over-Threshold (ToT) has been used, to measure the total charge released to the gas and collected by each pixel, integrated over the X-ray burst duration. Intensity response and spatial resolution has been measured first in laboratory for calibration, as function of the voltage applied to the GEMs, in single photon regime with energies between 3.7 and 17 keV. Subsequently it has been tested at the ABC laser facility (ENEA, Frascati). In this case, we measured the X-rays produced when the ABC neodymium laser, with pulse of 50 J and 3 ns time width, hits plane targets of aluminum. 2-D images have been acquired by means of a pinhole configuration with magnification 1.5 and 50 μ m of spatial resolution. The results are encouraging regarding the capability of this imaging detector to work in experiments where soft X-ray emissivity varies over many orders of magnitude.

Resonant scattering in the Perseus Cluster: spectral model for constraining gas motions with Astro-H

X-ray spectra from cores of galaxy clusters can be strongly distorted by resonant scattering of line photons, affecting metal abundance and gas velocity measurements. We introduce simulated spectral models that take into account the resonant scattering effect, radial variations of thermodynamic properties of the hot gas, projection effects and small-scale isotropic gas motions. The key feature of the models is that all these effects are treated self-consistently for the whole spectrum, rather than for individual lines. The model spectra are publicly available and can be used for direct comparison with observed projected spectra. Comparison with the existing XMM–Newton and Chandra data of the Perseus Cluster shows that even though there is no strong evidence for the resonant scattering in Perseus, the low energy resolution of the X-ray CCDs is not sufficient to robustly distinguish spectral distortions due to the resonant scattering, different metal abundance profiles and different levels of gas turbulence. Future Astro-H data will resolve most of the problems we are facing with CCDs. With the help of our models, the resonant scattering analysis can be done self-consistently using the whole spectral information, constraining the level of gas turbulence already with a 100-ks observation with Astro-H.

Design and optimization of the readout system for X-ray CCDs

A readout system for X-ray CCDs based on an improved architecture is presented; by optimizing several critical circuit blocks along the analog signal chain, the conflict between the readout speed and readout noise is greatly alleviated. Using CCD47-10 as its target CCD, the readout system has achieved 8.6e− readout noise and 142 eV FWHM at 5.9 keV Mn Kα under a pixel rate of 80 kHz. Also its performance of imaging has been investigated.

A low-power, radiation-resistant, Silicon-Drift-Detector array for extraterrestrial element mapping

We are developing a modular Silicon Drift Detector (SDD) X-Ray Spectrometer (XRS) for measuring the abundances of light surface elements (C to Fe) fluoresced by ambient radiation on remote airless bodies. The value of fluorescence spectrometry for surface element mapping is demonstrated by its inclusion on three recent lunar missions and by exciting new data that have recently been announced from the Messenger Mission to Mercury.The SDD-XRS instrument that we have been developing offers excellent energy resolution and an order of magnitude lower power requirement than conventional CCDs, making much higher sensitivities possible with modest spacecraft resources. In addition, it is significantly more radiation resistant than x-ray CCDs and therefore will not be subject to the degradation that befell recent lunar instruments. In fact, the intrinsic radiation resistance of the SDD makes it applicable even to the harsh environment of the Jovian system where it can be used to map the light surface elements of Europa.In this paper, we first discuss our element-mapping science-measurement goals. We then derive the necessary instrument requirements to meet these goals and discuss our current instrument development status with respect to these requirements.

In-Orbit Performance of the MAXI/SSC onboard the ISS

We report here the in-orbit performance of the CCD camera (MAXI/SSC) onboard the International Space Station (ISS). It was commissioned in 2009 August. This is the first all-sky survey mission employing X-ray CCDs. It consists of 32 CCDs, each of which is 1 inch square. It is a slit camera with a field of view of 1$^\circ\!\!\!.$5 $\times$ 90$^\circ$, and scans the sky as the rotation of the ISS. The CCD on the SSC is cooled down to the working temperature around $-$60$^\circ$C by the combination of a peltier cooler, a loop heat pipe and a radiator. The standard observation mode of the CCD is in a parallel sum mode (64-binning). The CCD functions properly, although it suffers an edge glow when the Sun is near the field of view (FOV), which reduces the observation efficiency of the SSC down to about 30%. The performance of the CCD is continuously monitored both by the Mn-K X-rays and by the Cu-K X-rays. There are many sources detected, not only point sources, but extended sources. Due to the lack of an effective observation time, we need more observation time to obtain an extended emission analysis extraction process.

Readout ASIC With SOI Technology for X-Ray CCDs

We developed an analog front-end application specific integrated circuit (ASIC) with a fully depleted (FD) silicon-on-insulator (SOI) technology for readout of X-ray CCDs. The ASIC contains seven readout channels, each of which is equipped with the correlated double sampling circuit followed by an amplitude-to-pulse width conversion circuit. We combined the ASIC with an X-ray CCD for performance evaluation tests. We succeeded in processing analog signals from the CCD and confirmed an X-ray imaging and photon-counting capabilities by irradiating a radioisotope <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">55</sup> Fe. The energy resolution is 305 eV at 5.9 keV (full-width at half maximum) and the readout noise is 53.7 μV for power consumption of 33 mW per chip. The ASIC proves that the FD-SOI process can be a practically usable option for front-end applications.

The MAXI Mission on the ISS: Science and Instruments for Monitoring All-Sky X-Ray Images

Abstract The Monitor of All-sky X-ray Image (MAXI) mission is the first astronomical payload to be installed on the Japanese Experiment Module — Exposed Facility (JEM-EF or Kibo-EF) on the International Space Station. It has two types of X-ray slit cameras with wide FOVs and two kinds of X-ray detectors consisting of gas proportional counters covering the energy range of 2 to 30 keV and X-ray CCDs covering the energy range of 0.5 to 12 keV. MAXI will be more powerful than any previous X-ray All Sky Monitor payloads, being able to monitor hundreds of Active Galactic Nuclei. A realistic simulation under optimal observation conditions suggests that MAXI will provide all-sky images of X-ray sources of $\sim $20 mCrab ($\sim $7 $\times$ 10$^{-10} $erg cm$^{-2} $s$^{-1}$ in the energy band of 2–30 keV) from observations during one ISS orbit (90 min), $\sim $4.5 mCrab for one day, and $\sim $2 mCrab for one week. The final detectability of MAXI could be $\sim $0.2 mCrab for two years, which is comparable to the source confusion limit of the MAXI field of view (FOV). The MAXI objectives are: (1) to alert the community to X-ray novae and transient X-ray sources, (2) to monitor long-term variabilities of X-ray sources, (3) to stimulate multi-wavelength observations of variable objects, (4) to create unbiased X-ray source cataloges, and (5) to observe diffuse cosmic X-ray emissions, especially with better energy resolution for soft X-rays down to 0.5 keV.

New CTI Correction Method for Spaced-Row Charge Injection of the Suzaku X-Ray Imaging Spectrometer

The charge transfer inefficiency (CTI) of the X-ray CCDs aboard the Suzaku satellite (X-ray Imaging Spectrometers: XIS) has increased since the launch due to radiation damage, and the energy resolution has been degraded. To improve the CTI, we have applied a spaced-row charge injection (SCI) technique to the XIS in orbit; by injecting charges into CCD rows periodically, the CTI is actively decreased. The CTI in the SCI mode depends on the distance between a signal charge and a preceding injected row, and the pulse height shows periodic positional variations. Using in-flight data of onboard calibration sources and of the strong iron line from the Perseus cluster of galaxies, we studied the variation in detail. We developed a new method to correct the variation. By applying the new method, the energy resolution (FWHM) at 5.9 keV in 2008 March is $\sim$155 eV for the front-illuminated CCDs and $\sim$175 eV for the back-illuminated CCD.

Development of an n-channel CCD, CCD-NeXT1, for Soft X-ray Imager onboard the NeXT satellite

NeXT (New X-ray Telescope) is the next Japanese X-ray astronomical satellite mission after the Suzaku satellite. NeXT aims to perform wide band imaging spectroscopy. Due to the successful development of a multilayer coated mirror, called a supermirror, NeXT can focus X-rays in the energy range from 0.1 up to 80 keV. To cover this wide energy range, we are in the process of developing a hybrid X-ray camera, Wideband X-ray Imager (WXI) as a focal plane detector of the supermirror. The WXI consists of X-ray CCDs (SXI) and CdTe pixelized detectors (HXI), which cover the lower and higher X-ray energy bands of 0.1–80 keV, respectively. The X-ray CCDs of the SXI are stacked above the CdTe pixelized detectors of the HXI. The X-ray CCDs of the SXI detect soft X-rays below ∼ 10 keV and allow hard X-rays pass into the CdTe detectors of the HXI without loss. Thus, we have been developing a “back-supportless CCD” with a thick depletion layer, a thinned silicon wafer, and a back-supportless structure. In this paper, we report the development and performances of an evaluation model of CCD for the SXI, “CCD-NeXT1”. We successfully fabricated two types of CCD-NeXT1, unthinned CCDs with 625- μ m thick wafer and 150- μ m thick thinned CCDs. By omitting the polishing process when making the thinned CCDs, we confirmed that the polishing process does not impact the X-ray performance. In addition, we did not find significant differences in the X-ray performance between the two types of CCDs. The energy resolution and readout noise are ∼ 140 eV (FWHM) at 5.9 keV and ∼ 5 electrons (RMS), respectively. The estimated thickness of the depletion layer is ∼ 80 μ m . The performances almost satisfy the requirements of the baseline plan of the SXI.

Development of an analog LSI for readout of X-ray CCDs

We integrated analog readout electronics for X-ray CCDs to an LSI. One LSI chip has two channels, each of which consists of an integrated correlated-double-sampling circuit (iCDS) and an analog-to-digital converter (ADC). We designed the LSI using “OPEN IP”, which is a library of reliable circuit blocks open to public developed by ISAS/JAXA. We fabricated the LSI through 0.25 μ m complimentary metal-oxide semiconductor (CMOS) process provided by Taiwan Semiconductor Manufacturing Company (TSMC). First, We tested the LSI with false signals of X-ray CCDs to check if the circuits function as designed and confirmed the expected function of the LSI. We finally succeeded in processing analog signals from an X-ray CCD using the LSI and confirmed an imaging and photon-counting capabilities. The energy resolution was 710 ± 10 eV at 5.9 keV (full-width at half-maximum) and the readout noise was 55 e - r . m . s . , which is 11 times larger than that processed with the conventional system made of discrete electronic parts. We found by SPICE simulations which part was the main noise source, and concluded that we could redesign an LSI with lower noise developed from this first prototype for the next try.

Performance of back supportless CCDs for the NeXT mission

New X-ray Telescope (NeXT) will be the next Japanese X-ray astronomical satellite, which will be launched around 2010. A multilayer super mirror will give NeXT a large effective area across the 0.1–80 keV band. This wide bandpass requires a focal plane detector that is also sensitive along the entire energy band. As the focal plane detector for NeXT, we have been developing a Wideband hybrid X-ray Imager (WXI) consisting of X-ray CCDs and pixelized CdTe detector. The X-ray CCD of the WXI is required to maintain high quantum efficiency up to the high-energy band while allowing hard X-rays to pass undetected through the depletion (sensitive) layer. In order to meet these requirements, we have been developing a back supportless CCD (BS-CCD) which has a thick depletion layer, a thinned Si wafer and a back supportless structure. As the first step, we manufactured a test model of BS-CCD in order to (1) learn the handling and thinning process and (2) confirm that there is no change of the performance. For the mechanical strength and the safe handling of the wafer, we decided to thin the wafer to ∼ 200 μ m . We verified the thickness of the depletion layer and wafer of the BS-CCD to be about 70 and 190 μ m , respectively. The energy resolution at 5.9 keV of 144 eV and the read-out noise of 7 electrons (rms) are equal to those of an un-thinned CCD, hence, we confirmed that our thinning process has no effect on the performance.

Pile-up on X-ray CCD instruments

Pile-up occurs on X-ray CCDs when several photons hit the detector at the same place between two readouts. In that case they are counted as one (or not at all) and their energies are summed. Pile-up thus affects both flux measurements and spectral characterisation of bright sources. Here I extend a previous work on flux loss, and present a simple formula for spectral distortion, in a form directly applicable when the pixel size is much smaller than the telescope's point spread function, but which can be extended to a more general case.