Photoexcitation spectroscopy of highly charged ions for application to astronomy using a compact electron beam ion trap (EBIT) at the synchrotron radiation facility SPring-8

TL;DR

The paper tackles the need for precise atomic data to interpret high-resolution X-ray spectra in astrophysics by performing active photoexcitation spectroscopy on highly charged ions using a compact EBIT coupled to the SPring-8 synchrotron beamline. The authors measure centroid energies and oscillator strengths for the O6+ Heα w line and the Fe16+ 3C line, achieving a relative energy precision of about $\Delta E / E \sim 4 \times 10^{-6}$, and set a 95% upper limit of $f_{3G}/f_{3C} \le 0.322$ for the weak 3G line. They observe a systematic energy shift of roughly $0.17$ eV relative to literature values, highlighting calibration challenges and the need for external references; 3G remains undetected due to signal-to-noise limitations. The work demonstrates the viability of EBIT–synchrotron active spectroscopy for obtaining high-fidelity atomic data, while outlining detector improvements and brighter light sources as key steps toward comprehensive measurements that will enhance plasma diagnostics in X-ray astronomy.

Abstract

In the past few decades, X-ray astronomy satellites equipped with grating spectrometers and microcalorimeters have enabled high-resolution spectroscopic observations of astrophysical objects. The need for accurate atomic data has arose as we attempt detailed analysis of the high-resolution spectra they provide. This is because current spectral models, which heavily rely on theoretical calculations, entail non-negligible uncertainties. We employ a plasma spectroscopy device called electron beam ion trap (EBIT) to experimentally obtain precise atomic data. An EBIT with a design that allows combined operation with synchrotron radiation facilities was developed based on the Heidelberg Compact EBIT and installed at ISAS/JAXA for this purpose. We conducted a spectroscopic experiment using the JAXA-EBIT at the synchrotron radiation facility SPring-8, and successfully obtained high-resolution spectra of the L$α$ resonance transition of Ne-like Fe$^{16+}$ ions, 3C, as well as the K$α$ resonance transition of He-like O$^{6+}$ ions. We also measured another Ne-like Fe$^{16+}$ L$α$ resonance transition, 3G, and constrained an upper limit of the oscillator strength ratio of 3G to 3C, using our experimental results. The experimental values obtained in this study will be applied to observational studies of astrophysical objects as a part of the plasma spectral modeling.

Figures (6)

• Figure 1: Schematic diagram showing the mechanism of HCI production and trapping inside the JAXA-EBIT, as well as the detection of the photons emitted from the resonant photoexcitation processes of HCIs irradiated with the monochromatic X-ray photons of the synchrotron radiation, which is introduced coaxially through the EBIT drift tube.
• Figure 2: Photograph of the JAXA-EBIT connected to BL17SU, SPring-8, with key components in our experimental setup.
• Figure 3: Schematic diagram of the resonant photoexcitation process (top) and the conceptual drawing of the spectrum obtained by scanning incident photon energy (bottom). The scattered photon has the same energy as the incident photon.
• Figure 4: Phase-folded light curve of the detected Fe L$\alpha$ photons within the electron beam energy alternation period of 1 second, summed over the results of 9 scans (total exposure time of 3030 s) performed for 3C measurement.
• Figure 5: (top) Heat map of photon counts in each pulse-height channel, obtained by scanning the input synchrotron photon energy around the expected centroid energy of O He$\alpha$ w line. The dotted horizontal lines indicate the lower and upper limits of the region of interest for the O He$\alpha$ photons. Events registered to this region are selected to plot the projected spectrum shown in the bottom panel. (bottom) Count rate of the detected photons for O He$\alpha$ w line in the above-selected region, shown as a function of the input synchrotron photon energy. 3C line is detected and fitted with a Gaussian + linear model. The fit result is shown in a solid red line.