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Register a new userPlasma code for astrophysical charge exchange emission at X-ray wavelengths
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Charge exchange X-ray emission provides unique insights into the interactions between cold and hot astrophysical plasmas. Besides its own profound science, this emission is also technically crucial to all observations in the X-ray band, since charge exchange with the solar wind often contributes a significant foreground component that contaminates the signal of interest. By approximating the cross sections resolved to $n$ and $l$ atomic subshells, and carrying out complete radiative cascade calculation, we create a new spectral code to evaluate the charge exchange emission in the X-ray band. Comparing to collisional thermal emission, charge exchange radiation exhibits enhanced lines from large-$n$ shells to the ground, as well as large forbidden-to-resonance ratios of triplet transitions. Our new model successfully reproduces an observed high-quality spectrum of comet C/2000 WM1 (LINEAR), which emits purely by charge exchange between solar wind ions and cometary neutrals. It demonstrates that a proper charge exchange model will allow us to probe remotely the ion properties, including charge state, dynamics, and composition, at the interface between the cold and hot plasmas.
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The reported observations of an unidentified X-ray line feature at $sim$3.5 keV have driven a lively discussion about its possible dark matter origin. Motivated by this, we have measured the emph{K}-shell X-ray spectra of highly ionized bare sulfur ions following charge exchange with gaseous molecules in an electron beam ion trap, as a source of or a contributor to this X-ray line. We produce $mathrm{S}^{16+}$ and $mathrm{S}^{15+}$ ions and let them capture electrons in collision with those molecules with the electron beam turned off while recording X-ray spectra. We observed a charge-exchanged-induced X-ray feature at the Lyman series limit (3.47 $pm$ 0.06 keV). The inferred X-ray energy is in full agreement with the reported astrophysical observations and supports the novel scenario proposed by Gu and Kaastra (A & A textbf{584}, {L11} (2015)).
A new and more comprehensive model of charge-exchange induced X-ray emission, due to ions precipitating into the Jovian atmosphere near the poles, has been used to analyze spectral observations made by the Chandra X-ray Observatory. The model includes for the first time carbon ions, in addition to the oxygen and sulfur ions previously considered, in order to account for possible ion origins from both the solar wind and the Jovian magnetosphere. By comparing the model spectra with newly reprocessed Chandra observations, we conclude that carbon ion emission provides a negligible contribution, suggesting that solar wind ions are not responsible for the observed polar X-rays. In addition, results of the model fits to observations support the previously estimated seeding kinetic energies of the precipitating ions (~0.7-2 MeV/u), but infer a different relative sulfur to oxygen abundance ratio for these Chandra observations.
We present the first high-resolution laboratory spectra of X-ray emission following L-shell charge exchange between nickel ions and neutral H2 and He. We employ the commonly used charge exchange models found in XSPEC and SPEX, ACX and SPEX-CX, to simulate our experimental results. We show that significant differences between data and models exist in both line energies and strengths. In particular, we find that configuration mixing may play an important role in generating lines from core-excited states, and may be improperly treated in models. Our results indicate that if applied to astrophysical data, these models may lead to incorrect assumptions of the physical and chemical parameters of the region of interest.
Though theoretically expected, the charge exchange emission from galaxy clusters has not yet been confidently detected. Accumulating hints were reported recently, including a rather marginal detection with the Hitomi data of the Perseus cluster. As suggested in Gu et al. (2015), a detection of charge exchange line emission from galaxy clusters would not only impact the interpretation of the newly-discovered 3.5 keV line, but also open up a new research topic on the interaction between hot and cold matter in clusters. We aim to perform the most systematic search for the O VIII charge exchange line in cluster spectra using the RGS on board XMM. We introduce a sample of 21 clusters observed with the RGS. The dominating thermal plasma emission is modeled and subtracted with a two-temperature CIE component, and the residuals are stacked for the line search. The systematic uncertainties in the fits are quantified by refitting the spectra with a varying continuum and line broadening. By the residual stacking, we do find a hint of a line-like feature at 14.82 A, the characteristic wavelength expected for oxygen charge exchange. This feature has a marginal significance of 2.8 sigma, and the average equivalent width is 2.5E-4 keV. We further demonstrate that the putative feature can be hardly affected by the systematic errors from continuum modelling and instrumental effects, or the atomic uncertainties of the neighbouring thermal lines. Assuming a realistic temperature and abundance pattern, the physical model implied by the possible oxygen line agrees well with the theoretical model proposed previously to explain the reported 3.5 keV line. If the charge exchange source indeed exists, we would expect that the oxygen abundance is potentially overestimated by 8-22% in previous X-ray measurements which assumed pure thermal lines.
It has been proposed that the charge exchange (CX) process at the interface between hot and cool interstellar gases could contribute significantly to the observed soft X-ray emission in star forming galaxies. We analyze the XMM-Newton/RGS spectrum of M82, using a newly developed CX model combined with a single-temperature thermal plasma to characterize the volume-filling hot gas. The CX process is largely responsible for not only the strongly enhanced forbidden lines of the K$alpha$ triplets of various He-like ions, but also good fractions of the Ly$alpha$ transitions of C VI (~87%), O VIII and N VII ($gtrsim$50%) as well. In total about a quarter of the X-ray flux in the RGS 6-30 AA band originates in the CX. We infer an ion incident rate of $3times10^{51},rm{s^{-1}}$ undergoing CX at the hot and cool gas interface, and an effective area of the interface as $sim2times10^{45},{rm cm^2}$ that is one order of magnitude larger than the cross section of the global biconic outflow. With the CX contribution accounted for, the best fit temperature of the hot gas is 0.6 keV, and the metal abundances are approximately solar. We further show that the same CX/thermal plasma model also gives an excellent description of the EPIC-pn spectrum of the outflow Cap, projected at 11.6 kpc away from the galactic disk of M82. This analysis demonstrates that the CX is potentially an important contributor to the X-ray emission from starburst galaxies and also an invaluable tool to probe the interface astrophysics.
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