1.

Introduction

The Lynx x-ray surveyor is one of four large missions currently being studied by a NASA Science and Technology Definition Team (STDT). At the conclusion of this study, the team will have developed a concept, along with technology development plans and cost estimates, as input to the 2020 Decadal Survey process. Lynx is a concept for an x-ray observatory that will directly observe the dawn of supermassive black holes, reveal the invisible drivers of galaxy and structure formation, and trace the energetic side of stellar evolution and stellar ecosystems. It will provide huge improvements in x-ray sensitivity relative to existing and planned x-ray missions, including Athena, Chandra, and XMM. The current notional design for Lynx utilizes nested x-ray mirrors providing $\sim 2.3{\mathrm{m}}^2$ effective area at 1 keV. The angular resolution of the mirror system is expected to be better than 0.5 arcsec on-axis and better than 1 arcsec within a 10-arcmin radius at the focal plane. The combination of this large effective area and fine angular resolution leads to an $\sim 100\times$ improvement in sensitivity relative to existing and planned missions. The fine angular resolution across the field of view enables deep surveys with little to no source confusion, enabling, e.g., the detection of very distant and dim AGN out to redshifts of $z\sim 10$. These unprecedented characteristics, along with the excellent spectral resolution for point-like and extended sources, will enable: detailed studies of the first seed black holes, as well as their growth and evolution with time in the universe, the impact of black hole system emission/feedback on multiple scales, the mapping of the intergalactic cosmic structures and the study of its impact on galactic growth and evolution, detailed studies of galaxy clusters, studies of stellar evolution and birth, along with the impacts of feedback on stellar systems, and the impact of x-ray stellar activity on extrasolar planet habitability. In addition to the above science topics, Lynx will be a large observatory class mission, which would enable myriad major-impact community-driven science studies through a vibrant general observer program. For a more thorough overview of the Lynx mission and its science motivation, refer to Ref. 1.

The current baseline Lynx design includes three instruments: (1) the high-definition x-ray imager (HDXI), (2) the Lynx x-ray microcalorimeter (LXM), and (3) the x-ray grating spectrometer (XGS). The HDXI and the LXM are mounted on a translation table (see Fig. 1), which allows one of them to be in the field of view at any given time. The grating array readout is mounted to the side and is always in position for readout of the diffracted and offset x-rays, with its utility dependent on whether or not the grating itself is in position. The LXM is discussed by Bandler et al.;² the XGS is discussed by McEntaffer et al.³ and Moritz et al.;⁴ and in this paper, we will briefly describe the HDXI instrument. The majority of the description in this paper was presented at the SPIE Astronomical Telescopes meeting in 2018 and was reported in the associated proceedings.⁵

Fig. 1

(a) Initial/notional design of Lynx x-ray surveyor telescope assembly, image credit to NASA, M. Baysinger; and (b) focal plane layout. The translation table moves HDXI or LXM into position along with the associated support structure, heat pipes, and radiators.

2.

HDXI Overview

The HDXI instrument is an imaging x-ray spectrometer that will achieve a moderately wide field of view while simultaneously achieving fine angular resolution. In order to adequately oversample the $<0.5\mathrm{arcsec}$ point-spread-function (PSF) of the Lynx mirrors, the HDXI is designed with pixels that span less than 0.33 arcsec per pixel, which corresponds to pixels with a pitch that is $\le 16\mu \mathrm{m}$ for the baseline focal length of 10 m. This pixel size ensures that individual photon landing locations will be constrained to a region size that is smaller than the PSF, and this region can be constrained to an even smaller size through the use of subpixel positioning when the charge distribution is estimated from the x-ray event grade. With a field of field-of-view requirement driven by the need to efficiently survey the x-ray sky for distant accreting active galactic nuclei, the notional focal plane instrument is designed to be $22\times 22\mathrm{arcmin}$. The combination of these two requirements leads to a focal plane camera with $\sim 16$ megapixels that spans $\sim 6.5\mathrm{cm}\times 6.5\mathrm{cm}$. The large format, combined with a need for good 0.2 to 10 keV quantum efficiency, moderate spectral resolution, and rapid readout of the high x-ray throughput, leads to silicon detectors as a natural choice for the HDXI focal plane instrument. Since the survey science also requires $<1\mathrm{arcsec}$ resolution near the edges of the field-of-view (FOV), we are baselining multiple detectors to cover this FOV since this allows the detectors to be tilted in order to better match the curvature of the focal surface. The notional design includes 21 silicon detectors, with a layout as shown in Fig. 2. It is also possible that a future concept for this array could include larger detectors that are intrinsically curved but that has not been baselined at this time. One could also consider a future concept that utilizes larger pixels that are capable of subpixel spatial resolution for the x-ray event localization,^6,7 but for simplicity and consistency with past implementation (e.g., Chandra) during this concept study, we have baselined a concept that utilizes pixels that directly oversample the PSF of the optics.

Fig. 2

Schematic of detector layout. The circle shows the 22-arcmin diameter requirement on the field of view, and the layout of the detectors is shown to cover the field with considerable margin.

The above-stated HDXI parameters and more detailed instrument requirements are driven by the science requirements derived by the Lynx STDT. These initial requirements and parameters are summarized in Table 1. In addition to the FOV and pixel size requirements that were described above, the high throughput of the x-ray mirrors necessitates fast readout of the detectors in order to avoid pile-up of the individual x-rays that would otherwise lead to loss of energy resolution and flux measurement accuracy. The energy range is driven by the typical soft x-ray flux of the sources required to achieve the prime science goals of the mission.

Table 1

Requirements and baseline parameters table for HDXI.

The Marshall Space Flight Center Advanced Concept Office (MSFC–ACO) and the Goddard Space Flight Center Instrument Design Lab (GSFC–IDL) have carried out engineering and design studies. These studies have led to significant refinement of the notional design for HDXI, as well as initial estimates of mass, power, volume, and cost. A notional instrument configuration that resulted from these studies is shown in Figs. 3 and 4. This configuration includes the 21-sensor array that is mounted to a mosaic plate, which is cooled to $-90°\mathrm{C}$ through a cold strap connection to the instrument chamber housing. This housing, along with its thermal load from the rest of the instrument, is passively cooled via external radiators that are connected to the instrument through heat pipes that move along with the translation table. Thermal modeling indicated that this design was feasible for cooling the detectors, as well as an array of ASICs that were mounted on the front-end mother board.

Fig. 3

Schematic block diagram of HDXI from MSFC-ACO and GSFC-IDL combined studies.

Fig. 4

Detector assembly diagram for notional HDXI instrument, showing the filter assembly housing (designed at the GSFC - IDL) above the detector plane, which resides inside a vacuum enclosure (shown with transparent walls).

This sensor assembly vacuum chamber also contains an array of retractable filters: two for blocking UV/optical light, one protective blocking filter, and one with calibration x-ray sources. The electronics box contains boards for main processing, power, heater and filter control, housekeeping, and FPGA boards for grading and packaging x-ray event data. These FPGA boards are for event recognition processing, and they operate on parallel data streams to extract the x-ray event pixel data from the full-frames; thus enabling the telemetering of x-ray event data without the need to telemeter full frames in normal observing mode. Occasional full frame data can also be telemetered in order to enable calibration data. Based on these conceptual design studies, we expect the HDXI instrument to have a total power of $\sim 178\mathrm{W}$, a mass of $\sim 80\mathrm{kg}$, and an average telemetry rate of $\sim 600\mathrm{kBytes}/\mathrm{s}$. This initial design is intended as a reference point to estimate required resources and feasibility.

3.

HDXI Detectors

Three primary detector types are being considered for the notional HDXI instrument design. These detectors include two forms of active pixel silicon sensors, namely x-ray hybrid CMOS detectors (HCDs) and x-ray monolithic CMOS detectors, as well as one form of charge-coupled device (CCD) known as a digital CCD (see Fig. 5). Each of the three sensor technologies has a clear path toward reaching TRL 5 by the time of mission adoption and phase B, which is projected to be in $\sim 2026$, but each of the technologies needs further research and development in order to reach this point.

Fig. 5

Three silicon detector technologies are currently being used to help specify HDXI parameters. They are: (1) hybrid CMOS active pixel sensors being developed by Teledyne and the Pennsylvania State University, (2) monolithic CMOS active pixel sensors being developed by Sarnoff and Harvard-Smithsonian Astrophysical Observatory, and (3) digital CCDs being developed by Lincoln Labs and the Massachusetts Institute of Technology.

4.

Summary

The HDXI instrument on Lynx would enable breakthrough science by providing massive improvements in x-ray source sensitivity through the use of fine angular resolution detectors over a wide enough field of view to enable survey science at the high redshifts and for low luminosity targets. Each of the detector technologies described above is capable of achieving some of the requirements of HDXI, but none of these detector technologies can currently achieve all of the requirements simultaneously. Technology development over the next $\sim 6$ years will bring at least one of these technologies to TRL 5 for the start of the Lynx mission. While other detectors may be capable of achieving the requirements in the future and while further trade studies will be carried out to choose the optimum detector, these three options have enabled the notional instrument to be specified in a way that has proven feasibility. The notional HDXI design has shown that it can accommodate any of these three detector types and that reasonable mass, power, and cost are achievable. Through multiple MSFC-ACO and GSFC-IDL runs, we have developed a notional design for the HDXI instrument, and we have shown that it can be accommodated within reasonable constraints while fulfilling requirements that satisfy the groundbreaking science drivers identified by the Lynx Science and Technology Definition Team.