X-ray absorption of cold gas: Simulating interstellar molecular clouds in the laboratory

Galactic and extragalactic sources produce x-rays that are often absorbed by molecules and atoms in giant molecular clouds (GMCs), which provides valuable information about their composition and physical state. We mimic this phenomenon with a laboratory Z-pinch x-ray source, which is impinged on neutral molecular gas. This technique produces a soft x-ray pseudocontinuum using a pulsed-current generator. The absorbing gas is injected from a 1-cm-long planar gas-puff without any window or vessel along the line of sight. An x-ray spectrometer with a resolving power of λ/Δλ∼420, comparable to that of astrophysical space instruments, records the absorbed spectra. This resolution clearly resolves the molecular lines from the atomic lines, thus motivating the search for a molecular signature in astrophysical x-ray spectra. The experimental setup enables different gas compositions and column densities. K-shell spectra of CO_{2}, N_{2}, and O_{2} reveal a plethora of absorption lines and photoelectric edges measured at molecular column densities between ∼10^{16} and 10^{18} cm^{-2} typical of GMCs. We find that the population of excited states, contributing to the edge, increases with gas density.