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High-resolution spectroscopy in soft X-rays ($<2$ keV) requires diffractive elements to resolve any astrophysically relevant diagnostics, such as closely spaced lines, weak absorption lines, or line profiles. The Rowland torus geometry describes how gratings and detectors need to be positioned to optimize the spectral resolving power. We describe how an on-axis Rowland geometry can be tilted to accommodate blazed gratings. In this geometry, two channels with separate optical axes can share the same detectors (double tilted Rowland spectrograph, DTRS). Small offsets between the channels can mitigate the effect of chip gaps and reduce the alignment requirements during the construction of the instrument. The DTRS concept is especially useful for sub-apertured mirrors, because it allows an effective use of space in the entrance aperture of a spacecraft. One mission that applies this concept is the Arcus Probe., Comment: Accepted by ApJ, 13 pages, 9 figures. Interactive figures are not available on arxiv, but will be hosted by ApJ

Arcus is a concept for a probe class mission to deliver high-resolution FUV and X-ray spectroscopy. For X-rays, it combines cost-effective silicon pore optics (SPO) with high-throughput critical-angle transmission (CAT) gratings to achieve $R> 3000$ in a bandpass from 12-50 Angstroem. We show in detail how the X-ray and the UV spectrographs (XRS and UVS) on Arcus will be aligned to each other. For XRS we present ray-tracing studies to derive performance characteristics such as the spectral resolving power and effective area, study the effect of misalignments on the performance, and conclude that most tolerances can be achieved with mechanical means alone. We also present an estimate of the expected on-orbit background., Comment: submitted to SPIE UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XXIII (2023)

Arcus is a proposed soft x-ray grating spectrometer Explorer. It aims to explore cosmic feedback by mapping hot gases within and between galaxies and galaxy clusters and characterizing jets and winds from supermassive black holes, and to investigate the dynamics of protoplanetary discs and stellar accretion. Arcus features 12 m-focal-length grazing-incidence silicon pore optics (SPO) developed for the Athena mission. Critical-angle transmission (CAT) gratings efficiently disperse high diffraction orders onto CCDs. We report new and improved x-ray performance results for Arcus-like CAT gratings, including record resolving power for two co-aligned CAT gratings. Multiple Arcus prototype grating facets were illuminated by an SPO at the PANTER facility. The facets consist of $32\times32.5$ mm$^2$ patterned silicon membranes, bonded to metal frames. The bonding angle is adjusted according to the measured average tilt angle of the grating bars in the membrane. Two simultaneously illuminated facets show minor broadening of the Al-K$_{\alpha}$ doublet in 18$^{\rm th}$ and 21$^{\rm st}$ orders with a best fit record effective resolving power of $R_G \approx 1.3^{+\infty}_{-0.5}\times10^4$ ($3\sigma$), about 3-4 times the Arcus requirement. We measured the diffraction efficiency of quasi-fully illuminated gratings at O-K wavelengths in orders 4-7 in an Arcus-like configuration and compare results with synchrotron spot measurements. After corrections for geometrical effects and bremsstrahlung continuum we find agreement between full and spot illumination at the two different facilities, as well as with the models used for Arcus effective area predictions. We find that these flight-like gratings meet diffraction efficiency and greatly exceed resolving power Arcus requirements., Comment: 19 pages, 16 figures

Astronomers are increasingly faced with a deluge of information, and finding worthwhile targets of study in the sea of data can be difficult. Outlier identification studies are a method that can be used to focus investigations by presenting a smaller set of sources that could prove interesting because they do not follow the trends of the underlying population. We apply a principal component analysis (PCA) and an unsupervised random forest algorithm (uRF) to sources from the Chandra Source Catalog v.2 (CSC2). We present 119 high-significance sources that appear in all repeated applications of our outlier identification algorithm (OIA). We analyse the characteristics of our outlier sources and cross-match them with the SIMBAD data base. Our outliers contain several sources that were previously identified as having unusual or interesting features by studies. This OIA leads to the identification of interesting targets that could motivate more detailed study.

We present a new, inexpensive, bench-top method for measuring groove period over large areas with high mapping resolution and high measurement accuracy, dubbed the grating mapper for accurate period (GMAP). The GMAP has the ability to measure large groove period changes and non-parallel grooves, both of which cannot be measured via optical interferometry. In this paper, we detail the calibration and setup of the GMAP, and employ the instrument to measure three distinct gratings. Two of these measured gratings have customized groove patterns that prevent them from being measured via other traditional methods, such as optical interferometry. Our implementation of this tool achieves a spatial resolution of 0.1 mm$\times$0.1 mm and a period error of 1.7 nm for a 3 $\mu$m size groove period., Comment: 14 pages, 16 figures

Recently CMOS (complementary metal-oxide semiconductor) sensors have progressed to a point where they may offer improved performance in imaging x-ray detection compared to the CCDs often used in x-ray satellites. We demonstrate x-ray detection in the soft x-ray band (250-1700 eV) by a commercially available back-illuminated Sony IMX290LLR CMOS sensor using the Advanced Photon Source at the Argonne National Laboratory. While operating the device at room temperature, we measure energy resolutions (FWHM) of 48 eV at 250 eV and 83 eV at 1700 eV which are comparable to the performance of the Chandra ACIS and the Suzaku XIS. Furthermore, we demonstrate that the IMX290LLR can withstand radiation up to 17.1 krad, making it suitable for use on spacecraft in low earth orbit.

The soft X-ray grating spectrometer on board the Off-plane Grating Rocket Experiment (OGRE) hopes to achieve the highest resolution soft X-ray spectrum of an astrophysical object when it is launched via suborbital rocket. Paramount to the success of the spectrometer are the performance of the $>250$ reflection gratings populating its reflection grating assembly. To test current grating fabrication capabilities, a grating prototype for the payload was fabricated via electron-beam lithography at The Pennsylvania State University's Materials Research Institute and was subsequently tested for performance at Max Planck Institute for Extraterrestrial Physics' PANTER X-ray Test Facility. Bayesian modeling of the resulting data via Markov chain Monte Carlo (MCMC) sampling indicated that the grating achieved the OGRE single-grating resolution requirement of $R_{g}(\lambda/\Delta\lambda)>4500$ at the 94% confidence level. The resulting $R_g$ posterior probability distribution suggests that this confidence level is likely a conservative estimate though, since only a finite $R_g$ parameter space was sampled and the model could not constrain the upper bound of $R_g$ to less than infinity. Raytrace simulations of the system found that the observed data can be reproduced with a grating performing at $R_g=\infty$. It is therefore postulated that the behavior of the obtained $R_g$ posterior probability distribution can be explained by a finite measurement limit of the system and not a finite limit on $R_g$. Implications of these results and improvements to the test setup are discussed., Comment: 25 pages, 16 figures, preprint of an article accepted for publication in the Journal of Astronomical Instrumentation \copyright 2020 [copyright World Scientific Publishing Company] [https://www.worldscientific.com/worldscinet/jai]

Gratings enable dispersive spectroscopy from the X-ray to the optical, and feature prominently in proposed flagships and SmallSats alike. The exacting performance requirements of these future missions necessitate assessing whether the present state-of-the-art in grating manufacture will limit spectrometer performance. In this work, we manufacture a 1.5 mm thick, 1000 nm period at grating using electron-beam lithography (EBL), a promising lithographic technique for patterning gratings for future astronomical observatories. We assess the limiting spectral resolution of this grating by interferometrically measuring the diffracted wavefronts produced in +/-1st order. Our measurements show this grating has a performance of at least R ~ 14,600, and that our assessment is bounded by the error of our interferometric measurement. The impact of EBL stitching error on grating performance is quantifed, and a path to measuring the period error of customized, curved gratings is presented.

In response to the need for the Astro2020 Decadal Survey to explicitly engage early career astronomers, the National Academies of Sciences, Engineering, and Medicine hosted the Early Career Astronomer and Astrophysicist Focus Session (ECFS) on October 8-9, 2018 under the auspices of Committee of Astronomy and Astrophysics. The meeting was attended by fifty six pre-tenure faculty, research scientists, postdoctoral scholars, and senior graduate students, as well as eight former decadal survey committee members, who acted as facilitators. The event was designed to educate early career astronomers about the decadal survey process, to solicit their feedback on the role that early career astronomers should play in Astro2020, and to provide a forum for the discussion of a wide range of topics regarding the astrophysics career path. This white paper presents highlights and themes that emerged during two days of discussion. In Section 1, we discuss concerns that emerged regarding the coming decade and the astrophysics career path, as well as specific recommendations from participants regarding how to address them. We have organized these concerns and suggestions into five broad themes. These include (sequentially): (1) adequately training astronomers in the statistical and computational techniques necessary in an era of "big data", (2) responses to the growth of collaborations and telescopes, (3) concerns about the adequacy of graduate and postdoctoral training, (4) the need for improvements in equity and inclusion in astronomy, and (5) smoothing and facilitating transitions between early career stages. Section 2 is focused on ideas regarding the decadal survey itself, including: incorporating early career voices, ensuring diverse input from a variety of stakeholders, and successfully and broadly disseminating the results of the survey., Comment: 9 pages; Astro2020 APC White Paper: State of the Profession Consideration

Future soft X-ray (10 - 50 Angstrom) spectroscopy missions require higher effective areas and resolutions to perform critical science that cannot be done by instruments on current missions. An X-ray grating spectrometer employing off-plane reflection gratings would be capable of meeting these performance criteria. Off-plane gratings with blazed groove facets operated in the Littrow mounting can be used to achieve excellent throughput into orders achieving high resolutions. We have fabricated two off-plane gratings with blazed groove profiles via a technique which uses commonly available microfabrication processes, is easily scaled for mass production, and yields gratings customized for a given mission architecture. Both fabricated gratings were tested in the Littrow mounting at the Max-Planck-Institute for extraterrestrial Physics PANTER X-ray test facility to assess their performance. The line spread functions of diffracted orders were measured, and a maximum resolution of 800 $\pm$ 20 is reported. In addition, we also observe evidence of a `blaze' effect from measurements of relative efficiencies of the diffracted orders., Comment: 25 pages including references

An X-ray spectrograph consisting of radially ruled off-plane reflection gratings and silicon pore optics was tested at the Max Planck Institute for extraterrestrial Physics PANTER X-ray test facility. The silicon pore optic (SPO) stack used is a test module for the Arcus small explorer mission, which will also feature aligned off-plane reflection gratings. This test is the first time two off-plane gratings were actively aligned to each other and with a SPO to produce an overlapped spectrum. The gratings were aligned using an active alignment module which allows for the independent manipulation of subsequent gratings to a reference grating in three degrees of freedom using picomotor actuators which are controllable external to the test chamber. We report the line spread functions of the spectrograph and the actively aligned gratings, and plans for future development., Comment: Draft Version March 19, 2015

Future NASA X-ray spectroscopy missions will require high throughput, high resolution grating spectrometers. Off-plane reflection gratings are capable of meeting the performance requirements needed to realize the scientific goals of these missions. We have identified a novel grating fabrication method that utilizes common lithographic and microfabrication techniques to produce the high fidelity groove profile necessary to achieve this performance. Application of this process has produced an initial pre-master that exhibits a radial (variable line spacing along the groove dimension), high density (>6000 grooves/mm), laminar profile. This pre-master has been tested for diffraction efficiency at the BESSY II synchrotron light facility and diffracts up to 55% of incident light into usable spectral orders. Furthermore, tests of spectral resolving power show that these gratings are capable of obtaining resolutions well above 1300 ($\lambda/\Delta\lambda$) with limitations due to the test apparatus, not the gratings. Obtaining these results has provided confidence that this fabrication process is capable of producing off-plane reflection gratings for the next generation of X-ray observatories., Comment: 17 pages, 10 figures, Submitted to Experimetal Astronomy

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.

Developments in grating spectroscopy are paramount for meeting the soft X‐ray science goals of future NASA X‐ray Observatories. While developments in the laboratory setting have verified the technical feasibility of using off‐plane reflection gratings to reach this goal, flight heritage is a key step in the development process toward large missions. To this end we have developed a design for a suborbital rocket payload employing an Off‐Plane X‐ray Grating Spectrometer. This spectrometer utilizes slumped glass Wolter‐1 optics, an array of gratings, and a CCD camera. We discuss the unique capabilities of this design, the expected performance, the science return, and the perceived impact to future missions.