What observational feature strongly suggests an SNR has entered or nears the Radiative Phase?
Answer
Bright, knotty optical filaments tracing the decelerating shock front where gas is cooling.
The Radiative Phase begins when the interior gas cools sufficiently that the energy lost through radiation surpasses the energy input from the shock wave's adiabatic heating. As the gas cools, it becomes denser, leading to observable changes in emission characteristics. Optical light is typically emitted by cooler, denser material. Therefore, the appearance of bright, knotty optical filaments is a strong indicator that the necessary cooling has occurred, allowing for visible light emission from ionized atoms along the original shock boundary as the gas slows down significantly.
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