K. Futamoto

Detection of Highly Ionized O and Ne Absorption Lines in the X-Ray Spectrum of 4U 1820–303 in the Globular Cluster NGC 6624

We searched for absorption lines of highly ionized O and Ne in the energy spectra of two low-mass X-ray binaries, 4U 1820-303 in the globular cluster NGC 6624 and Cyg X-2, observed with the Chandra LETG, and detected O VII, O VIII, and Ne IX absorption lines for 4U 1820-303. The equivalent width of the O VII Kα line was 1.19 eV (90% errors), and the significance was 6.5 σ. Absorption lines were not detected for Cyg X-2 with a 90% upper limit on the equivalent width of 1.06 eV for O VII Kα. The intrinsic line width was not resolved, and an upper limit corresponding to a velocity dispersion of b = 420 km s-1 was obtained for the O VII Kα line of 4U 1820-303. The ion column densities were estimated from the curve-of-growth analysis, assuming several different values of b. The absorption lines observed in 4U 1820-303 are likely due to hot interstellar medium, because O will be fully photoionized if the absorbing column is located close to the binary system. The velocity dispersion is restricted to b = 200-420 km s-1 from consistency between O VII Kα and Kβ lines, the Ne/O abundance ratio, and H column density. The average temperature and the O VII density are estimated to be log T(K) = 6.2-6.3 and n = (0.7-2.3) × 10-6 cm-3, respectively. The difference of O VII column densities for the two sources may be connected to the enhancement of the soft X-ray background (SXB) toward the Galactic bulge region. Using the polytrope model of hot gas to account for the SXB, we corrected for the density gradient and estimated the midplane O VII density at the solar neighborhood. The scale height of hot gas is then estimated using the active galactic nuclei (AGN) absorption lines. It is suggested that a significant portion of both the AGN absorption lines and the high-latitude SXB emission lines can be explained by the hot gas in our Galaxy.

Present performance of a single pixel Ti/Au bilayer TES calorimeter

We are developing a superconducting transition-edge sensor (TES) calorimeter for future Japanese X-ray astronomy missions (e.g. NeXT mission). The performance of our single pixel TES calorimeter is presented. We fabricated a Ti/Au (40 nm/110 nm) bilayer TES on a thin silicon-nitride membrane, which is adjusted to have a transition temperature of about 100 mK. The size of the TES is 500μm × 500μm, and 300μm × 300μm gold with a thickness of 300 nm is deposited with sputtering as an X-ray absorber. The TES calorimeter was installed in a dilution refrigerator operated at about 40 mK, with a combination of 400-series SQUID array as an ammeter. Collimated 5.9 keV X-rays (200 um in diameter) from 55Fe isotope were irradiated and X-ray pulses were obtained. Simultaneously with a fast falling time constant of 74.2 us, the energy resolution of 6.6+-0.4 eV was attained, while the baseline noise was 6.4 eV. The contents of the energy resolution are 5.1 eV of the excess noise, 3.3 eV of the readout noise, 1.6 eV of the pulse by pulse variation, and 1.9 eV of the intrinsic noise. The baseline noise are dominated by an unknown excess noise, which increases roughly in proportion to the inverse of the TES resistance. The pulse height is sensitive to the operating conditions, and the superconducting shield appears to have improved it by a factor of about 2. The calorimeter works fine over six months surviving five thermal cycles, even though it is kept in air.

AC calorimeter bridge; a new multi-pixel readout method for TES calorimeter arrays

In order to realize a large format (e.g. ∼32×32) calorimeter array, it is essential to multiplex calorimeter signals at cryogenic temperatures without losing signal to noise ratio. For this purpose we propose a brand-new readout method, the CABBAGE (Cal¯orimeter B_ridge B_iased by an A_C Ge¯nerator) where an AC biased calorimeters are placed in resistance bridges. In this paper we first describe the principles of CABBAGE and investigate its response and noise. We propose the large format calorimeter array readout using CABBAGEs, and discuss the new TES microcalorimeter readout method without using SQUIDs.

High sensitive X-ray microcalorimeter using Bi-Au microabsorber for imaging applications

An X-ray microcalorimeter is a cryogenic energy-dispersive spectrometer, which has an energy resolution almost comparable to that of conventional wavelength-dispersive spectrometers. Using a transition edge sensor (TES) as a temperature sensor, the energy resolution can be further improved. We have developed a new method of achieving an array of TES microcalorimeters for the purpose of X-ray imaging. To achieve this, mushroom-shaped X-ray microabsorbers formed using electrodeposition were applied. The temperature of the TES, which is easily degraded by thermal diffusion, was kept sufficiently low throughout the process to achieve practical use. On the bases of this new method, a 2×2 (× 4) array of TES microcalorimeters was fabricated and tested. A high energy resolution of 13.0 eV at 6 keV was achieved and the filling factor was improved to 83%. Although several issues still need to be investigated, we verified that our method is useful for fabricating a Ti–Au TES microcalorimeter array.

Status of X-ray microcalorimeter development at ISAS

A superconducting phase-transition microcalorimeter is a promising detector for high resolution X-ray spectroscopy. We are developing such a detector for future Japanese X-ray astronomy missions. In our design, a tin absorber is electrodeposited as a mushroom structure to achieve a high covering fraction. We have succeeded in detecting X-rays with the first model of our calorimeter with an electrodeposited absorber, though the energy resolution was limited due to residual resist. We are also developing a phase-transition microcalorimeter with a low transition temperature. We describe the results of these experiments, and discuss the limiting factors of their performance.

Detection of an Iron Emission Feature from the Lensed Broad Absorption Line QSO H1413+117 at [CLC][ITAL]z[/ITAL][/CLC] = 2.56

We present the X-ray energy spectrum of the lensed broad absorption line QSO H1413+117 (the Cloverleaf) at z = 2.56 observed with the Chandra X-ray observatory. We detected 293 photons in a 40 ks Advanced CCD Imaging Spectrometer (ACIS-S) observation. The X-ray image consists of four lensed image components, thus the photons are from the lensed QSO itself. The overall spectrum can be described with a power-law function heavily absorbed by neutral matter at a redshift consistent with the QSO redshift. This supports the idea that intrinsic absorption is significant for BAL QSOs. The spectral fit significantly (99% confidence) improves when we include an emission line. The centroid energy and intrinsic width (Gaussian σ) of the line are 6.21 ± 0.16 keV and 220 eV (90% errors), respectively, in the QSO rest frame, assuming the absorbed power law as the continuum. The equivalent width of the line in the QSO rest frame is 960 eV. We suggest that the large equivalent width, the centroid energy, and the line broadness can be explained by iron K emission arising from X-ray reprocessing in the BAL flow, assuming it has a conical thin-sheet structure.

Fabrication of an X-ray microcalorimeter with an electrodeposited X-ray microabsorber

For the next generation of astronomical X-ray imaging detectors, arrays with large numbers of microstructures (∼1000 pixels) will be required. To meet this requirement, a tin absorber for an X-ray microcalorimeter, which has a so-called “mushroom” shape, is fabricated by electrodeposition and polishing. This method enables the fabrication of a large number of arrayed microcalorimeters. Details of the fabrication process, the characteristics of the absorber and the fabricated microcalorimeters are reported.

Detection of an Iron Emission Feature from the Lensed Broad Absorption Line QSO H1413+117 at z = 2.56

We present the X-ray energy spectrum of the lensed broad absorption line QSO H1413+117 (the Cloverleaf) at z = 2.56 observed with the Chandra X-ray observatory. We detected 293 photons in a 40 ks Advanced CCD Imaging Spectrometer (ACIS-S) observation. The X-ray image consists of four lensed image components, thus the photons are from the lensed QSO itself. The overall spectrum can be described with a power-law function heavily absorbed by neutral matter at a redshift consistent with the QSO redshift. This supports the idea that intrinsic absorption is significant for BAL QSOs. The spectral fit significantly (99% confidence) improves when we include an emission line. The centroid energy and intrinsic width (Gaussian σ) of the line are 6.21 ± 0.16 keV and 220 eV (90% errors), respectively, in the QSO rest frame, assuming the absorbed power law as the continuum. The equivalent width of the line in the QSO rest frame is 960 eV. We suggest that the large equivalent width, the centroid energy, and the line broadness can be explained by iron K emission arising from X-ray reprocessing in the BAL flow, assuming it has a conical thin-sheet structure.

Multipixel readout of TES calorimeters

We are developing a superconducting Transition-Edge Sensor (TES) calorimeter array. We adopt calorimeter multiplex in frequency domain to read signals from the calorimeter array with a small number of front-end electronics and wirings. We further utilize Calorimeter Bridge Biased by an AC Generator (CABBAGE) approach to eliminate the AC carrier in the output. We tested the method using a TES calorimeter, which has a transition temperature of 390 mK. Because of the high operating temperature, energy resolution of the calorimeter is limited to 200 eV at 5.9 keV even when it is biased with a DC current. We operated the calorimeter in CABBAGE circuit with 30 kHz sinusoidal bias and obtained an energy resolution of 250 eV. We found that there remains a small-amplitude residual in the output even at the bridge balance point. The residual contains not only 30 kHz component but also odd-order harmonics. We consider that this is due to the variation of the TES resistance with bias current. The 50 eV degradation of the energy resolution from DC to AC biases can be explained by the fact that some of signal power is carried in the odd-order harmonics, which we did not utilize in the data reduction process. We also succeeded to operate the CABBAGE by 100 kHz, although the energy resolution was degraded to 380 eV probably due to low response of the signal readout circuit at the frequency.

Improvements of an X-ray microcalorimeter for detecting cosmic rays

An X-ray microcalorimeter that consists of an x-ray absorber to transfer the incident photon energy to the temperature rise, a temperature sensor to detect the temperature change and suspending beams for thermal isolation from the substrate have been fabricated. Titanium/Gold thin film transition edge sensor (TES) is used as the temperature sensor. We fabricated and tested the first prototype in the previous study and obtained the transition temperature of 0.52 K, energy resolution of 550 eV (FWHM) for 6 keV radiation. These values were smaller than that of expected. We applied a Sn absorber and redesigned the microstructure of the x-ray microcalorimeter. Consequently, we have obtained 158 eV at 5.9 keV radiation of the energy resolution, which is about 4 times higher than that of the first prototype. This value is nearly equal to the conventional X-ray CCD. The highest energy resolution of the x-ray microcalorimeter of our design is estimated to approximately 5 eV at the operating point of 0.2 K. To realize such a good energy resolution calorimeter array, we are going to improve the sensitivity of the TES by optimizing the process condition. A Sn absorber formed by electroplating is also under evaluating simultaneously. It is necessary to fabricate uniform array structures.