A Complete X-ray View of Supernova Remnant W28 with Einstein Probe: Spatial Distribution of Parameters and Origin of the Thermal-Composite Morphology

TL;DR

This study provides a comprehensive X-ray analysis of supernova remnant W28, revealing its complex morphology, plasma states, and revised age, offering insights into the origins of its thermal-composite structure.

Contribution

It offers the first detailed spatial distribution of plasma parameters in W28 using Einstein Probe data, clarifying the remnant's morphology and ionization states, and revising its age.

Findings

W28 has a shell-like structure extending to 72'x45'

Interior plasma shows recombination coincident with Hα emissions

Revised dynamical age of W28 is approximately 8 kyr

Abstract

It has been an unsolved question what leads a supernova remnant (SNR) to a thermal composite rather than a typical shell-like morphology, and what causes recombining plasma inside it. With the 13-ks observation of the Following-up X-ray Telescope onboard the Einstein Probe, we give an overall X-ray picture of W28, one of the prototypical thermal composite SNRs. The observation revealed a shell-like structure west of W28 in radio, optical, and X-ray images, which may revise the known extent of the SNR to $72'\times45'$. Spectral analysis explicitly maps that the special relationship where the plasma experiences recombination in the interior of the remnant, spatially coincident with H$\alpha$ emissions, while in the other regions, the plasma is ionization-dominated. We found that W28 is generally isobaric from its center to the newly discovered shell, and it is even isothermal with a temperature of $\sim0.6$-0.7 keV in the center before the cooling of the plasma. Saturated thermal conduction and cloud evaporation may cool down the plasma within $\sim3$ kyr, the estimated recombination timescale. We revised the SNR dynamical age to $\sim8$ kyr, much younger than previous estimates. The complex structure and complex ionization state distribution may suggest that centrally filled and shell-like morphologies coexist in W28. This state may depend on the environment in which the SNR evolves.