The Lynx mission concept, under development ahead of the 2020 Astrophysics Decadal Review, includes the Lynx X-ray Microcalorimeter (LXM) as one of its primary instruments. The LXM uses a microcalorimeter array at the focus of a high-throughput soft x-ray telescope to enable high-resolution nondispersive spectroscopy in the soft x-ray waveband (0.2 to 15 keV) with exquisite angular resolution. Similar to other x-ray microcalorimeters, the LXM uses a set of blocking filters mounted within the dewar that pass the photons of interest (x-rays) while attenuating the out-of-band long-wavelength radiation. Such filters have been successfully used on previous orbital and suborbital instruments; however, the Lynx science objectives, which emphasize observations in the soft x-ray band (<1 keV), pose more challenging requirements on the set of LXM blocking filters. We present an introduction to the design of the LXM optical/IR blocking filters and discuss recent advances in filter capability targeted at LXM. In addition, we briefly describe the external filters and the modulated x-ray sources to be used for onboard detector calibration.
Lynx is an x-ray telescope, one of four large satellite mission concepts currently being studied by NASA to be a flagship mission. One of Lynx’s three instruments is an imaging spectrometer called the Lynx x-ray microcalorimeter (LXM), an x-ray microcalorimeter behind an x-ray optic with an angular resolution of 0.5 arc sec and ∼2 m2 of area at 1 keV. The LXM will provide unparalleled diagnostics of distant extended structures and, in particular, will allow the detailed study of the role of cosmic feedback in the evolution of the Universe. We discuss the baseline design of LXM and some parallel approaches for some of the key technologies. The baseline sensor technology uses transition-edge sensors, but we also consider an alternative approach using metallic magnetic calorimeters. We discuss the requirements for the instrument, the pixel layout, and the baseline readout design, which uses microwave superconducting quantum interference devices and high-electron mobility transistor amplifiers and the cryogenic cooling requirements and strategy for meeting these requirements. For each of these technologies, we discuss the current technology readiness level and our strategy for advancing them to be ready for flight. We also describe the current system design, including the block diagram, and our estimate for the mass, power, and data rate of the instrument.
The Lynx x-ray microcalorimeter instrument on the Lynx X-ray Observatory requires a state-of-the-art cryogenic system to enable high-precision and high-resolution x-ray spectroscopy. The cryogenic system and components described provide the required environment using cooling technologies that are already at relatively high technology readiness levels and are progressing toward flight-compatible subsystems. These subsystems comprise a cryostat, a 4.5-K mechanical cryocooler, and an adiabatic demagnetization refrigerator that provides substantial cooling power at 50 mK.
One option for the detector technology to implement the Lynx x-ray microcalorimeter (LXM) focal plane arrays is the metallic magnetic calorimeter (MMC). Two-dimensional imaging arrays of MMCs measure the energy of x-ray photons by using a paramagnetic sensor to detect the temperature rise in a microfabricated x-ray absorber. While small arrays of MMCs have previously been demonstrated that have energy resolution better than the 3 eV requirement for LXM, we describe LXM prototype MMC arrays that have 55,800 x-ray pixels, thermally linked to 5688 sensors in “hydra” configurations, and that have sensor inductance increased to avoid signal loss from the stray inductance in the large-scale arrays when the detectors are read out with microwave superconducting quantum interference device multiplexers, and that use multilevel planarized superconducting wiring to provide low-inductance, low-crosstalk connections to each pixel. We describe the features of recently tested MMC prototype devices and simulations of expected performance in designs optimized for the three subarray types in LXM.
Lynx is an x-ray telescope that is one of four large satellite mission concepts currently being studied by NASA to be the next flagship mission. One of Lynx’s three instruments is the Lynx X-ray Microcalorimeter (LXM), an imaging spectrometer placed at the focus of an x-ray optic with 0.5 arc-second angular resolution and approximately 2 m2 area at 1 keV. It will be used for a wide variety of observations, and the driving performance requirements are met through different sub-regions of the array. It will provide an energy resolution of better than 3 eV over the energy range of 0.2 to 7 keV, with pixels sizes that vary in scale from 0.5 to 1 arc-seconds in the inner 5 arc-minute field-of-view, and to 5 arc-seconds in the extended 20 arc-minute field-of-view.
The Main Array consists mostly of 1 arc-second pixels in the central 5 arc-minutes with less than 3 eV energy resolution (FWHM) in the energy range of 0.2 to 7 keV. It is enhanced in the inner 1 arc-minute region with 0.5 arc-second pixels that will better sample the point spread function of the X-ray optic. The inner 5 arc-minute region is designed specifically for the observations related to cosmic feedback studies, investigating the interactions of AGN with the local regions surrounding them. The 0.5" pixel size allows detailed studies of winds and jets on a finer angular scale. It is also optimized for spatially resolved measurements of cluster cores.
The outer regions of the array are designed to operate during a completely different set of observations. The Extended Array will be utilized for surveys over large regions of the sky, the 20 arc-minute field-of-view making it practical to make observations of the soft diffuse emission from larger scale-structure such as extended galaxies, the outer regions of galaxy groups and clusters and also cosmic filaments. This array is optimized for high energy resolution up to 2 keV through the use of thin (0.5 um) gold absorbers. The Ultra-High-Res Array is designed specifically to enable the study turbulent line broadening around individual through the study of the highly ionized oxygen lines. It is optimized for energy resolution for the oxygen VII and VIII lines, with better than 0.4 eV energy resolution.
In this paper we present the design of the baseline configuration and the scientific motivation. We discuss the technologies that are being developed for this instrument, in particular the transition-edge sensor (TES) and metallic magnetic calorimeter (MMC) sensor technologies. We place these technologies in the context of the required energy resolution, energy range, pixel size, and count-rate, as well as strategies for the pixel layout and wiring. We will discuss the use of microwave SQUIDs, HEMT amplifiers, and parametric amplifiers for the read-out and the implications for the cryogenic design. We also describe the design of the full instrument, including the strawman cryogenic design, as well as an estimate for the mass, power and data rate.
The X-ray integral field unit (X-IFU) proposed for ESA’s Athena X-ray observatory will consist of 3840 transition edge sensor (TES) microcalorimeters optimized for the energy range of 0.2 to 12 keV. The instrument will provide unprecedented spectral resolution of ~ 2.5 eV at energies of up to 7 keV and will accommodate photon fluxes of 10’s mcrab (1000’s cps).
Over the past two years the baseline configuration has evolved from the original proposal. The current baseline consists of a uniform large pixel array (LPA) of 5” pixels, AC-biased within their superconducting-to-normal transition and read out using frequency domain multiplexing (FDM). The baseline pixel design is approximately a factor of two times slower than in the original concept. High count-rate accommodation, needed for bright point source observations, is now achieved by defocusing the telescope optic to spread the photons over a larger number of pixels. In this paper we report on Mo/Au TES designs that are being optimized to meet the baseline pixel parameters and performance goals. This includes detailed studies on the optimization of the thermal heat sink and the impact of different TES geometries (including TES size and normal metal feature geometries) on the DC-biased transition shape. We discuss how these geometric effects ultimately impact important performance metrics such as energy resolution, decay time, slew-rate and array scale uniformity.
Our Mo/Au TESs have historically been designed and optimized for DC-biased operation, however, the primary readout technology uses an AC drive to bias the TES. Depending upon the drive frequency, the AC bias affects the TES transition shape in two ways. Firstly, due to losses from the bias current coupling to metallic components in close proximity to the TES and secondly introducing fine structure in the transition due to Josephson effects. We present latest pixel design optimizations targeted at mitigating these frequency dependent effects in order to achieve commensurate performance with that obtained in the DC case.
Lynx is a concept under study for prioritization in the 2020 Astrophysics Decadal Survey. Providing orders of magnitude increase in sensitivity over Chandra, Lynx will examine the first black holes and their galaxies, map the large-scale structure and galactic halos, and shed new light on the environments of young stars and their planetary systems. In order to meet the Lynx science goals, the telescope consists of a high-angular resolution optical assembly complemented by an instrument suite that may include a High Definition X-ray Imager, X-ray Microcalorimeter and an X-ray Grating Spectrometer. The telescope is integrated onto the spacecraft to form a comprehensive observatory concept. Progress on the formulation of the Lynx telescope and observatory configuration is reported in this paper.
The focal plane of the X-ray integral field unit (X-IFU) for ESA’s Athena X-ray observatory will consist of ~ 4000 transition edge sensor (TES) x-ray microcalorimeters optimized for the energy range of 0.2 to 12 keV. The instrument will provide unprecedented spectral resolution of ~ 2.5 eV at energies of up to 7 keV and will accommodate photon fluxes of 1 mCrab (90 cps) for point source observations. The baseline configuration is a uniform large pixel array (LPA) of 4.28” pixels that is read out using frequency domain multiplexing (FDM). However, an alternative configuration under study incorporates an 18 × 18 small pixel array (SPA) of 2” pixels in the central ~ 36” region. This hybrid array configuration could be designed to accommodate higher fluxes of up to 10 mCrab (900 cps) or alternately for improved spectral performance (< 1.5 eV) 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.
Four astrophysics missions are currently being studied by NASA as candidate large missions to be chosen in the 2020 astrophysics decadal survey.1 One of these missions is the “X-Ray Surveyor” (XRS), and possible configurations of this mission are currently under study by a science and technology definition team (STDT). One of the key instruments under study is an X-ray microcalorimeter, and the requirements for such an instrument are currently under discussion. In this paper we review some different detector options that exist for this instrument, and discuss what array formats might be possible. We have developed one design option that utilizes either transition-edge sensor (TES) or magnetically coupled calorimeters (MCC) in pixel array-sizes approaching 100 kilo-pixels. To reduce the number of sensors read out to a plausible scale, we have assumed detector geometries in which a thermal sensor such a TES or MCC can read out a sub-array of 20-25 individual 1” pixels. In this paper we describe the development status of these detectors, and also discuss the different options that exist for reading out the very large number of pixels.