  The spectacular outbursts of energy associated with supernovae (SNe) have
long motivated research into their potentially hazardous effects on Earth and
analogous environments. Much of this research has focused primarily on the
atmospheric damage associated with the prompt arrival of ionizing photons
within days or months of the initial outburst, and the high-energy cosmic rays
that arrive thousands of years after the explosion. In this study, we turn the
focus to persistent X-ray emission, arising in certain SNe that have
interactions with a dense circumstellar medium, and observed months and/or
years after the initial outburst. The sustained high X-ray luminosity leads to
large doses of ionizing radiation out to formidable distances. We provide an
assessment of the threat posed by these X-ray luminous SNe by analyzing the
collective X-ray observations from Chandra, Swift-XRT, XMM-Newton, NuSTAR, and
others. We find that this threat is particularly acute for SNe showing evidence
of strong circumstellar interaction, such as Type IIn explosions, which have
significantly larger ranges of influence than previously expected, and lethal
consequences up to $\sim$ 50 pc away. Furthermore, X-ray bright SNe could pose
a substantial and distinct threat to terrestrial biospheres, and tighten the
Galactic habitable zone. We urge follow-up X-ray observations of interacting
SNe for months and years after the explosion to shed light on the physical
nature of the emission and its full time evolution, and to clarify the danger
that these events pose for life in our Galaxy and other star-forming regions.
