Superluminous X-ray emission from the interaction of supernova ejecta with dense circumstellar shells

For supernova (SN) powered by the conversion of kinetic energy into radiation due to the interactions of the ejecta with a dense circumstellar shell, we show that there could be X-ray analogues of optically superluminous SNe with comparable luminosities and energetics. We consider X-ray emission from the forward shock of SN ejecta colliding into an optically thin circumstellar material (CSM) shell, derive simple expressions for the X-ray luminosity as a function of the circumstellar shell characteristics, and discuss the different regimes in which the shock will be radiative or adiabatic, and whether the emission will be dominated by free–free radiation or line cooling. We find that even with normal SN explosion energies of 1051 erg, there exist CSM shell configurations that can liberate a large fraction of the explosion energy in X-rays, producing unabsorbed X-ray luminosities approaching 1044 erg s−1 events lasting a few months, or even 1045 erg s−1 flashes lasting days. Although the large column density of the circumstellar shell can absorb most of the flux from the initial shock, the most luminous events produce hard X-rays that are less susceptible to photoelectric absorption, and can counteract such losses by completely ionizing the intervening material. Regardless, once the shock traverses the entire circumstellar shell, the full luminosity could be available to observers.