What is a Supernova Explosion (SNe)?

super - 1: very large or powerful (a superatomic bomb) 
        2: exhibiting the characteristics of its type to an
                  extreme or excessive degree (supersecrecy)

nova - Etymology: New Latin, feminine of Latin novus,

new. 
Plural: novae
        1: a star that suddenly increases its light output
                  tremendously and then fades away to its former
                  obscurity in a few or years

                                        - Merrian-Webster Dictionary

A SNe is the instantaneous release of ~10⁵¹ ergs (10³¹ Megatons) of energy, the result of either the catastrophic collapse of a massive star or runnaway nuclear burning on the surface of a white dwarf. Although only a small fraction of the energy is released in the form of visible light, this is enough to make it appear as if a new star has appeared in the sky. The effects of the SNe are profound and far-reaching, and it is possible to see the remains of SNe that occured many hundreds of years ago.

How does the SNR Evolve?

Tycho supernova remnant (SN 1572)

While the material swept up by the shock is much less than the mass of the stellar ejecta, the expansion of the stellar ejecta proceeds at essentially a constant velocity equal to the initial shock wave speed, typically of the order of 10,000 km/s. This is known as the "free expansion" phase and may last for approximately 200 years, at which point the shock wave has swept up as much interstellar material as the initial stellar ejecta. The supernova remnant at this time will be about 10 light years in radius. Although the remnant is radiating thermal X-ray and synchrotron radiation across a broad range of the electromagnetic spectrum (from radio to X-rays), the initial energy of the shock wave will have diminished very little. Line emission from the radioactive isotopes generated in the supernovae contribute significantly to the total apparent brightness of the remnant in the early years, but do not significantly effect the shock wave.

As the remnant sweeps up ambient mass equal to the mass of the stellar ejecta, the wave will begin to slow and the remnant enters a phase known as adiabatic expansion, or the Sedov-Taylor or blast wave phase. The internal energy of the shock continues to be very large compared to radiation losses from thermal and synchrotron radiation, so the total energy remains nearly constant. The rate of expansion is determined solely the initial energy of the shock wave and the density of the interstellar medium.

As the shock wave cools, it will become more efficient at radiating energy. Once the temperature drops below 20000 K or so, some electrons will be able to recombine with carbon and oxygen ions, enabling ultraviolet line emission which is a much more efficient radiation mechanism than the thermal X-rays and synchrotron radiation. Hence this new phase is known as the radiative phase during which X-ray radiation becomes much less apparent and the remnant cools and disperses into the surrounding medium over the course of the next 10000 years.

Analytical models are few, since the equations governing the motion of the SNR gas are very complex and cannot be solved for except in very limited special cases (spherical geometry and constant energy, for example). In a numerical model, the evolution of the ejecta and ISM gas, fit onto a computational grid, is followed over time, by using the conservation equations of mass, momentum and energy . Numerical models are constrained by limits on the time and space it takes to run the simulation and then store the data. It is not difficult, for example, to perform simulations that have spherical or cylindrical symmetry (1-dimensional and 2-dimensional, respectively), but many of the more complicated structures found in real SNR are destroyed by the imposition of this kind of symmetry. Fully 3-dimensional simulations, on the other hand, are computationally very expensive, particularly if they are of sufficient resolution (small enough numerical grid size) so that small features such as small scale turbulence, do not get averaged out.

Despite the difficulties, many good SNR models have been developed, which increase our understanding of how they evolve and how they interact with the surrounding ISM, including the following:

How Does X-ray Astronomy Fit in?

The Puppis A remnant with the ROSAT neutron star discovery Optical image of the Crab Nebula

There are several mechanisms by which SNR generate X-rays. The ejecta, heated and comressed by the powerful forward shock, generates thermal and line emission in the X-ray region of the spectrum. Electrons, accelerated by the SNR shock to relativistic speeds emit synchrotron radiation when they change direction in the presence of a magnetic field. And a neutron star formed in a Type II SNe may also be a significant source of X-rays. The X-rays emitted by SNR give us many clues that help us understand SNR, their progenitor stars, and their interaction with the ISM. We usually know little about the progenitor star, however, X-ray spectra of the shock heated ejecta in the remnant offer direct information about the composition of the progenitor star. X-ray spectra can also give information on the composition of the ISM into which the remnant is expanding, if and to what extent the elemental layers of the progenitor star are mixed with the gas of the ISM. By studying the X-ray morphologies of SNR, we can learn about how thermal conduction acts in SNR and in the ISM. Looking for X-ray synchrotron radiation is a way of getting direct evidence for cosmic ray acceleration in SNR shocks.