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Keith Jahoda
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We describe Cal X-1, a SmallSat mission concept to establish X-ray standard candles in the sky, which will enable absolute calibration of the current and future X-ray observatories. It consists of two CubeSats flying in formation, one containing an absolutely calibrated X-ray source and another, 1-2 km away, a simple X-ray telescope.
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We describe a process for cross-calibrating the effective areas of X-ray telescopes that observe common targets. The targets are not assumed to be standard candles in the classic sense, in that we assume that the source fluxes have well-defined, but {it a priori} unknown values. Using a technique developed by Chen et al. (2019, arXiv:1711.09429) that involves a statistical method called {em shrinkage estimation}, we determine effective area correction factors for each instrument that brings estimated fluxes into the best agreement, consistent with prior knowledge of their effective areas. We expand the technique to allow unique priors on systematic uncertainties in effective areas for each X-ray astronomy instrument and to allow correlations between effective areas in different energy bands. We demonstrate the method with several data sets from various X-ray telescopes.
Methods.There are no experimental data about the background experienced by microcalorimeters in the L2 orbit, and thus the particle background levels were calculated by means of Monte Carlo simulations: we considered the original design configuration and an improved configuration aimed to reduce the unrejected background, and tested them in the L2 orbit and in the low Earth orbit, comparing the results with experimental data reported by other X-ray instruments.To show the results obtainable with the improved configuration we simulated the observation of a faint, high-redshift, point source (F[0.5-10 keV]~6.4E-16 erg cm-2 s-1, z=3.7), and of a hot galaxy cluster at R200 (Sb[0.5-2 keV]=8.61E-16 erg cm-2 s-1 arcmin-2,T=6.6 keV). Results.First we confirm that implementing an active cryogenic anticoincidence reduces the particle background by an order of magnitude and brings it close to the required level.The implementation and test of several design solutions can reduce the particle background level by a further factor of 6 with respect to the original configuration.The best background level achievable in the L2 orbit with the implementation of ad-hoc passive shielding for secondary particles is similar to that measured in the more favorable LEO environment without the passive shielding, allowing us to exploit the advantages of the L2 orbit.We define a reference model for the diffuse background and collect all the available information on its variation with epoch and pointing direction.With this background level the ATHENA mission with the X-IFU instrument is able to detect ~4100 new obscured AGNs with F>6.4E-16 erg cm-2 s-1 during three years, to characterize cluster of galaxies with Sb(0.5-2 keV)>9.4E-16 erg cm-2 s-1 sr-1 on timescales of 50 ks (500 ks) with errors <40% (<12%) on metallicity,<16% (4.8%) on temperature,2.6% (0.72%) on the gas density, and several single-element abundances.
NICER has a comparatively low background rate, but it is highly variable, and its spectrum must be predicted using measurements unaffected by the science target. We describe an empirical, three-parameter model based on observations of seven pointing directions that are void of detectable sources. An examination of 3556 good time intervals (GTIs), averaging 570 s, yields a median rate (0.4-12 keV; 50 detectors) of 0.87 c/s, but in 5 percent (1 percent) of cases, the rate exceeds 10 (300) c/s. Model residuals persist at 20-30 percent of the initial rate for the brightest GTIs, implying one or more missing model parameters. Filtering criteria are given to flag GTIs likely to have unsatisfactory background predictions. With such filtering, we estimate a detection limit, 1.20 c/s (3 sigma, single GTI) at 0.4-12 keV, equivalent to 3.6e-12 erg/cm^2/s for a Crab-like spectrum. The corresponding limit for soft X-ray sources is 0.51 c/s at 0.3-2.0 keV, or 4.3e-13 erg/cm^2/s for a 100 eV blackbody. Faint-source filtering selects 85 percent of the background GTIs, and higher rates are expected for targets scheduled more favorably. An application of the model to 1 s timescale makes it possible to distinguish source flares from possible surges in the background.
The detection of photons above 10 keV through MeV and GeV energies is challenging due to the penetrating nature of the radiation, which can require large detector volumes, resulting in correspondingly high background. In this energy range, most detectors in space are either scintillators or solid-state detectors. The choice of detector technology depends on the energy range of interest, expected levels of signal and background, required energy and spatial resolution, particle environment on orbit, and other factors. This section covers the materials and configurations commonly used from 10 keV to > 1 GeV.
I describe a method for synthesizing photometric passbands for use with current and future X-ray instruments. The method permits the standardisation of X-ray passbands and thus X-ray photometry between different instruments and missions. The method is illustrated by synthesizing a passband in the XMM-Newton EPIC pn which is similar to the ROSAT PSPC 0.5-2 keV band.
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