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Micro-X is an X-ray sounding rocket payload that had its first flight on July 22, 2018. The goals of the first flight were to operate a transition edge sensor (TES) X-ray microcalorimeter array in space and take a high-resolution spectrum of the Cassiopeia A supernova remnant. The first flight was considered a partial success. The array and its time-division multiplexing readout system were successfully operated in space, but due to a failure in the attitude control system, no time on-target was acquired. A re-flight has been scheduled for summer 2022. Since the first flight, modifications have been made to the detector systems to improve noise and reduce the susceptibility to magnetic fields. The three-stage SQUID circuit, NIST MUX06a, has been replaced by a two-stage SQUID circuit, NIST MUX18b. The initial laboratory results for the new detector system will be presented in this paper., Comment: LTD proceedings, 6 pages, 3 figures, 1 table

The Micro-X sounding rocket is a NASA funded X-ray telescope payload that completed its first flight on July 22, 2018. This event marked the first operation of Transition Edge Sensors (TESs) and their SQUID-based multiplexing readout system in space. Unfortunately, due to an ACS pointing failure, the rocket was spinning during its five minute observation period and no scientific data was collected. However, data collected from the internal calibration source marked a partial success for the payload and offers a unique opportunity to study the response of TESs and SQUIDs in space. Of particular interest is the magnetic field response of the NIST MUX06a SQUID readout system to tumbling through Earth's magnetic field. We present a model to explain the baseline response of the SQUIDs, which lead to a subset of pixels failing to "lock" for the full observational period. Future flights of the Micro-X rocket will include the NIST MUX18b SQUID system with dramatically reduced magnetic susceptibility., Comment: 6 pages, 6 figures, LTD19 conference proceedings

With its first flight in 2018, Micro-X became the first program to fly Transition-Edge Sensors and their SQUID readouts in space. The science goal was a high-resolution, spatially resolved X-ray spectrum of the Cassiopeia A Supernova Remnant. While a rocket pointing error led to no time on target, the data was used to demonstrate the flight performance of the instrument. The detectors observed X-rays from the on-board calibration source, but a susceptibility to external magnetic fields limited their livetime. Accounting for this, no change was observed in detector response between ground operation and flight operation. This paper provides an overview of the first flight performance and focuses on the upgrades made in preparation for reflight. The largest changes have been upgrading the SQUIDs to mitigate magnetic susceptibility, synchronizing the clocks on the digital electronics to minimize beat frequencies, and replacing the mounts between the cryostat and the rocket skin to improve mechanical integrity. As the first flight performance was consistent with performance on the ground, reaching the instrument goals in the laboratory is considered a strong predictor of future flight performance.

Micro-X is a sounding rocket-borne instrument that uses a microcalorimeter array to perform high-resolution X-ray spectroscopy. This instrument flew for the first time on July 22nd, 2018 from the White Sands Missile Range, USA. This flight marks the first successful operation of a Transition-Edge Sensor array and its time division multiplexing read-out system in space. This launch was dedicated to the observation of the supernova remnant Cassiopeia A. A failure in the attitude control system prevented the rocket from pointing and led to no time on target. The on-board calibration source provided X-rays in flight, and it is used to compare detector performance during pre-flight integration, flight, and after the successful post-flight recovery. This calibration data demonstrates the capabilities of the detector in a space environment as well as its potential for future flights., Comment: 9 pages, 6 figures, Accepted to the JLTP for LTD-18

The Micro-X sounding rocket flew for the first time on July 22, 2018, becoming the first program to fly Transition-Edge Sensors and multiplexing SQUID readout electronics in space. While a rocket pointing failure led to no time on-target, the success of the flight systems was demonstrated. The successful flight operation of the instrument puts the program in a position to modify the payload for indirect galactic dark matter searches. The payload modifications are motivated by the science requirements of this observation. Micro-X can achieve world-leading sensitivity in the keV regime with a single flight. Dark matter sensitivity projections have been updated to include recent observations and the expected sensitivity of Micro-X to these observed fluxes. If a signal is seen (as seen in the X-ray satellites), Micro-X can differentiate an atomic line from a dark matter signature., Comment: Proceedings of the 18th International Workshop on Low Temperature Detectors (LTD18)

We report on the development of multi-absorber transition edge sensors (TESs), referred to as hydras. A hydra consists of multiple x-ray absorbers each with a different thermal conductance to a TES. Position information is encoded in the pulse shape. With some trade-off in performance, hydras enable very large format arrays without the prohibitive increase in bias and read-out components associated with arrays of individual TESs. Hydras are under development for the next generation of space telescope such as Lynx. Lynx is a NASA concept under study that will combine a < 1 arcsecond angular resolution optic with 100,000-pixel microcalorimeter array with energy resolution of deltaE_FWHM ~ 3 eV in the soft x-ray energy range. We present first results from hydras with 25-pixels for Lynx. Designs with absorbers on a 25 micron and 50 micron pitch are studied. Arrays incorporate, for the first time, microstrip buried wiring layers of suitable pitch and density required to readout a full-scale Lynx array. The resolution from the coadded energy histogram including all 25-pixels was deltaE_FWHM = 1.66+/-0.02 eV and 3.34+/-0.06 eV at an energy of 1.5 keV for the 25 micron and 50 micron absorber designs respectively. Position discrimination is demonstrated from parameterization of the rise-time., Comment: 7 pages, 5 figures. Low Temperature Detectors 18 Conference, Milan Italy

ATHENA is a European led x-ray observatory due for launch in the early 2030s. The X-ray Integral Field Unit (X-IFU) instrument on ATHENA will have an array of 3168 TES microcalorimeters, 2.5 eV resolution (at 7 keV), 5 arc second angular resolution and a 5 arc minute field of view. As one example, will allow unprecedented views of composition and dynamics of galaxy clusters.

In many superconducting transition-edge sensor (TES) microcalorimeters, the measured electrical noise exceeds theoretical estimates based on a thermal model of a single body thermally connected to a heat bath. Here, we report on noise and complex impedance measurements of a range of designs of TESs made with a Mo/Au bilayer. We have fitted the measured data using a two-body model, where the x-ray absorber and the TES are connected by an internal thermal conductance Gae. We find that the so-called excess noise measured in these devices is consistent with the noise generated from the internal thermal fluctuations between the x-ray absorber and the TES. Our fitted parameters are consistent with the origin of Gae being from the finite thermal conductance of the TES itself. These results suggest that even in these relatively low resistance Mo/Au TESs, the internal thermal conductance of the TES may add significant additional noise and could account for all the measured excess noise. Furthermore, we find that around regions of the superconducting transition with rapidly changing derivative of resistance with respect to temperature, an additional noise mechanism may dominate. These observations may lead to a greater understanding of TES devices and allow the design of TES microcalorimeters with improved performance.

The X-ray integral field unit (X-IFU) for ESA's Athena X-ray observatory will consist of 3840 AC-biased transition edge sensors (TESs), read out using frequency domain multiplexing (FDM). In this paper we describe details of the latest pixels geometries that are being designed for the current baseline array configuration. This includes details on how important TES properties (transition parameters, thermal design of the pixels, absorber composition) are being optimized to meet the target energy resolution, count-rate and quantum efficiency. We also present the latest design optimizations specifically targeted at mitigating AC-bias phenomena in Mo/Au TESs that can degrade performance.

Exploring the effects of size and geometry of normal metal features on the transition shapes and performance of transition-edge sensor microcalorimeters. The spectral resolution of transition-edge sensor (TES) microcalorimeters is very sensitive to the specific dependencies of the resistance R in the superconducting transition on the current I, magnetic field B, and temperature T. In particular, it has been shown that transitions that are very steep in (R,T) space lead to a significant noise term, in excess of conventional expectations. This so-called unexplained noise is known to be reduced by the addition of normal metal stripes across the TES perpendicular to the direction of current flow. These normal metal stripes have been shown to drastically alter the oscillatory patterns seen in measurements of the critical current as a function of magnetic field. However, there are many remaining questions about the exact impact of the stripes on current distributions within the TES, the Fraunhofer pattern and, therefore, the shape of the R(I, B, T) surface. Through measurements of the resistance under DC bias of TES devices of various sizes, with different stripe patterns and dimensions, we will discuss how these stripes can affect the R(I, B, T) surface. In addition, using measurements and analysis of the noise spectra of various devices we will present how these changes to the stripe pattern may affect the performance of the TES. In particular, we will discuss strategies to reduce the presence of localized discontinuities in the derivative of R, associated with increased noise, while maintaining the globally low levels of unexplained noise currently achieved with conventional metal stripe patterns. Implementing these strategies is a path towards producing large arrays with highly uniform transitions and high spectral resolution. These large uniform arrays will be required for future x-ray astronomy applications, such as the X-IFU on ATHENA.

Micro-X is a sounding rocket borne X-ray telescope that utilizes transition edge sensors to perform imaging spectroscopy with a high level of energy resolution. Its 2.1m focal length X-ray optic has an effective area of 300 sq cm, a field of view of 11.8 arcmin, and a bandpass of 0.12.5 keV. The detector array has 128 pixels and an intrinsic energy resolution of 4.5 eV FWHM. The integration of the system has progressed with functional tests of the detectors and electronics complete, and performance characterization of the detectors is underway. We present an update of ongoing progress in preparation for the upcoming launch of the instrument.

The Astro-H (Hitomi) Soft X-ray Spectrometer (SXS) was a pioneering imaging x-ray spectrometer with 5 eV energy resolution at 6 keV. The instrument used a microcalorimeter array at the focus of a high-throughput soft x-ray telescope to enable high-resolution non-dispersive spectroscopy in the soft x-ray waveband (0.3-12 keV). We present the suite of ground calibration measurements acquired from 2012-2015, including characterization of the detector system, anti-coincidence detector, optical blocking filters, and filter-wheel filters. The calibration of the 36-pixel silicon thermistor microcalorimeter array includes parameterizations of the energy gain scale and line spread function for each event grade over a range of instrument operating conditions, as well as quantum efficiency measurements. The x-ray transmission of the set of five Al/polyimide thin-film optical blocking filters mounted inside the SXS dewar has been modeled based on measurements at synchrotron beamlines, including with high spectral resolution at the C, N, O, and Al K-edges. In addition, we present the x-ray transmission of the dewar gate valve and of the filters mounted on the SXS filter wheel (external to the dewar), including beryllium, polyimide, and neutral density filters.

The focal plane of the X-ray integral field unit (X-IFU) for ESA's Athena X-ray observatory will consist of approximately 4000 transition edge sensor (TES) x-ray microcalorimeters optimized for the energy range of 0.2 to 12 kiloelectronvolts. The instrument will provide unprecedented spectral resolution of approximately 2.5 electronvolts at energies of up to 7 kiloelectronvolts and will accommodate photon fluxes of 1 milliCrab (90 counts per second) for point source observations. The baseline configuration is a uniform large pixel array (LPA) of 4.28 arcseconds pixels that is read out using frequency domain multiplexing (FDM). However, an alternative configuration under study incorporates an 18 by × 18 small pixel array (SPA) of 2 arcseconds pixels in the central approximately 36 arcseconds region. This hybrid array configuration could be designed to accommodate higher fluxes of up to 10 milliCrabs (900 counts per second) or alternately for improved spectral performance (less than 1.5 electronvolts) at low count-rates. In this paper we report on the TES pixel designs that are being optimized to meet these proposed LPA and SPA configurations. In particular we describe details of how important TES parameters are chosen to meet the specific mission criteria such as energy resolution, count-rate and quantum efficiency, and highlight performance trade-offs between designs. The basis of the pixel parameter selection is discussed in the context of existing TES arrays that are being developed for solar and x-ray astronomy applications. We describe the latest results on DC biased diagnostic arrays as well as large format kilo-pixel arrays and discuss the technical challenges associated with integrating different array types on to a single detector die.

The calorimeter array of the JAXA Astro-H (renamed Hitomi) Soft X-ray Spectrometer (SXS) was designed to provide unprecedented spectral resolution of spatially extended cosmic x-ray sources and of all cosmic x-ray sources in the Fe-K band around 6 keV. The properties that make the SXS a powerful x-ray spectrometer also make it sensitive to the entire electromagnetic band. If characterized as a bolometer, it would have a noise equivalent power (NEP) of < 4x10(exp -18) W/(Hz)0.5. Thus it was imperative to shield the detector from thermal radiation from the instrument and optical and UV photons from the sky. Additionally, it was necessary to shield the coldest stages of the instrument from the thermal radiation emanating from the warmer stages. These needs are addressed by a series of five thin-film radiation blocking filters that block long-wavelength radiation while minimizing x-ray attenuation. The SXS aperture assembly is a system of barriers, baffles, filter carriers, and filter mounts that supports the filters and inhibits their potential contamination. The three warmer filters also were equipped with thermometers and heaters for decontamination.

The traditional method of applying a digital optimal filter to measure X-ray pulses from transition-edge sensor (TES) devices does not achieve the best energy resolution when the signals have a highly non-linear response to energy, or the noise is non-stationary during the pulse. We present an implementation of a method to analyze X-ray data from TESs, which is based upon principal component analysis (PCA). Our method separates the X-ray signal pulse into orthogonal components that have the largest variance. We typically recover pulse height, arrival time, differences in pulse shape, and the variation of pulse height with detector temperature. These components can then be combined to form a representation of pulse energy. An added value of this method is that by reporting information on more descriptive parameters (as opposed to a single number representing energy), we generate a much more complete picture of the pulse received. Here we report on progress in developing this technique for future implementation on X-ray telescopes. We used an 55Fe source to characterize Mo/Au TESs. On the same dataset, the PCA method recovers a spectral resolution that is better by a factor of two than achievable with digital optimal filters.