Origin of the thermal radiation in a solid-state analogue of a black hole

An effective black-hole-like horizon occurs for electromagnetic waves in matter at a surface of singular electric and magnetic permeabilities. In a physical dispersive medium the horizon disappears for wave numbers with $k>\mathrm{}{k}_c,$ and the effective geometry becomes flat. Nevertheless, it is shown that Hawking radiation is still emitted if free field modes with $k>\mathrm{}{k}_c$ are in their ground state.