A SNR is the result of the interaction between the debris from an exploded star and its surrounding medium. The explosion can either arise as the detonation or deflagration of a white dwarf that has exceeded the Chandrasekhar limit as the result of mass accretion from a binary companion (Type Ia) or the gravitational collapse of the core of a massive star that has exhausted its nuclear fuel (Type II, Type Ib, and Type Ic). In the early stages of the remnant evolution, the differences between these explosions can lead to morphological differences, but these begin to blur as the remnant becomes increasingly dominated by the surrounding medium. In both cases the outer layers of the star are ejected with velocity of tens of thousands of kilometers per second. Interaction between the outer envelope and the ambient medium causes the development of a shock front that heats, compresses, and ionizes ambient gas. Undecelerated ejecta catching up with the shell of shocked material produce a reverse shock, so called because it propagates backwards in the ejecta frame of reference (though not in the observer's frame). A contact discontinuity separates the material swept up by the two shocks. For the first few hundred years, during the "free expansion" phase, the forward shock propagates essentially at constant velocity. Once the shock has swept up a mass comparable to that of the ejecta it starts to decelerate, signaling the onset of the so-called adiabatic evolutionary phase. In this phase the cooling time of swept up material exceeds the dynamic time scale, and so the remnant loses very little energy. Its idealized behavior is described by the Sedov-Taylor self-similarity equations. A recent, comprehensive analytic description of SNR evolution under various conditions can be found in .
During these two phases, X-ray emission is produced from the collisionally-ionized, shock heated plasma. Both shocks have sufficient velocity to heat the shocked material to high kinetic temperatures; subsequently, collisions with electrons ionizes the gas to predominantly their H-like and He-like states. X-rays are emitted as Bremsstrahlung continuum from the collisionally-heated electrons, plus a rich spectrum of lines from the materials composing the ejecta and the swept-up medium. During the initial phase, lines from the dense ejecta dominate, allowing X-ray observations to probe the properties of the progenitor star via the ejecta abundances and distribution, but as the material in the remnant becomes predominantly swept-up interstellar material, the line spectrum reflects the composition of the interstellar medium. Nonthermal components from electron synchrotron emission arise in central pulsar powered wind nebulae and at the forward shock from electrons accelerated to relativistic energies.
Once the radiative and dynamic timescales become comparable, the remnant enters the radiative or shell forming phase. Newly shocked material radiates its thermal energy and gets swept up into a thin, dense, cool shell. During this phase the shock velocity is too low to produce new X-ray emitting gas, but as described later, X-ray emission still arises from the interior.
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