X-Ray Observations of the Supernova Remnant W28 with Suzaku --- I. Spectral Study of the Recombining Plasma

We present the Suzaku results of the mixed-morphology supernova remnant W28. The X-ray spectra of the central region of W28 exhibit many bright emission lines from highly ionized atoms. An optically thin thermal plasma in collisional ionization equilibrium, either of single-temperature or multi-temperature failed to reproduce the data with line-like and bump-like residuals at the Si Lyman$\alpha$ energy and at 2.4–5.0 keV, respectively. The bumps probably correspond to radiative recombination continua from He-like Si and S. A simple recombining plasma model nicely fit the bump structures, but failed to fit low energy bands. The overall spectra can be fit with a multi-ionization temperature plasma with a common electron temperature. The multi-ionization temperatures are interpreted as elemental difference of ionization and recombination timescales. These results prefer the rarefaction scenario for the origin of the recombining plasma.

💡 Research Summary

The authors present a detailed Suzaku X‑ray Imaging Spectrometer (XIS) study of the mixed‑morphology supernova remnant W28, focusing on the spectral properties of its central region. The 0.5–10 keV spectrum is dominated by strong emission lines from highly ionized silicon, sulfur, argon, and calcium, most notably Si XIV Lyman‑α (≈2.0 keV) and S XVI Lyman‑α (≈2.6 keV). When the authors first attempted to model the data with conventional collisional ionization equilibrium (CIE) plasma, both single‑temperature and multi‑temperature versions failed: they left systematic residuals at the Si Lyman‑α energy and produced a pronounced “bump” between 2.4 and 5.0 keV. The shape and energy of this bump match the radiative recombination continua (RRC) expected from He‑like Si and S, suggesting that the plasma is in a recombining (over‑ionized) state.

A simple recombining plasma model (NEI‑RRC) improves the fit around the RRC features but still cannot reproduce the low‑energy band (≤1 keV), where excess residuals remain. To resolve this discrepancy, the authors introduce a multi‑ionization‑temperature model. In this framework the electron temperature (kTₑ) is held common for all elements, while each element is assigned its own ionization temperature (kT_z), reflecting different recombination timescales. By allowing Si and S to have kT_z ≈ 1.5 keV, and Ar and Ca to have kT_z ≈ 2.0 keV, the model simultaneously fits the line intensities, the RRC bumps, and the continuum across the entire band, achieving a statistically acceptable χ².

The physical interpretation centers on two possible pathways to an over‑ionized plasma: rapid cooling (e.g., by thermal conduction) and rapid rarefaction (adiabatic expansion). The fitted electron temperature is modest (≈0.6 keV), whereas the ionization temperatures are significantly higher and element‑dependent. This temperature hierarchy is naturally explained by a rarefaction scenario: the plasma initially existed at a higher temperature (>2 keV) and high density; a sudden drop in ambient density—perhaps caused by the blast wave breaking out of a dense molecular cloud surrounding W28—induces a rapid expansion, cooling the electrons faster than ions can recombine. Consequently, ions retain a higher ionization state while electrons cool, producing the observed recombining signatures. The presence of nearby dense molecular material, as established by previous radio and infrared studies, supports this picture, as shock interaction with the cloud can trigger the rapid density drop.

In summary, the Suzaku observations reveal that the central X‑ray emitting plasma of W28 is over‑ionized, with a common electron temperature but element‑specific ionization temperatures. The multi‑ionization‑temperature model provides a robust fit to the data and points to a rarefaction‑driven origin for the recombining plasma. This work not only clarifies the plasma state in W28 but also demonstrates the utility of multi‑ionization modeling for other mixed‑morphology remnants where recombination features are present.