A complete X-ray view of supernova remnant W28 with the <i>Einstein Probe</i> : Spatial distribution of parameters, and the origin of the thermal-composite morphology

The question about what causes a supernova remnant (SNR) to be a thermal composite rather than have a typical shell-like morphology remains unsolved, as does what causes the plasma inside it to recombine. With the 13-ks observation of the Following-up X-ray Telescope on board 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. This may revise the known extent of the SNR to 72′ × 45′. A spectral analysis explicitly maps the special relation in which the plasma experiences recombination in the interior of the remnant, spatially coincident with H α emissions, while in other regions, the plasma is dominated by ionization. We found that W28 is generally isobaric from its center to the newly discovered shell, and it is even isothermal with a temperature of ∼0.6-0.7 keV in the center before the plasma cools. Saturated thermal conduction and cloud evaporation may cool down the plasma within ∼3 kyr, which is the estimated recombination timescale. We revised the SNR dynamical age to ∼8 kyr, which is 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.