Magnonic Analogue Black/White Hole Horizon in Superfluid $^3$He-B: experiment

We provide experimental details of the first experiment made in zero temperature limit ($\sim$ 600\,$\mu$K) studying the magnonic black/white hole horizon analogue using absolutely pure physical system based on the spin superfluidity in superfluid $^3$He-B. We show that spin precession waves propagating on the background of the spin super-currents in a channel between two Bose-Einstein condensates of magnons in form of homogeneously precessing domains mimic the properties of the black/white horizon. Once the white hole horizon is formed and blocks the propagation of the spin-precession waves between two domains, we observed an amplification effect, i.e. when the energy of the spin precession waves reflected from the horizon is higher than the energy of the excited spin precession waves before horizon was formed. Moreover, the estimated temperature of the spontaneous Hawking radiation in this model system is about four orders of magnitude lower than the system's background temperature what makes it a promising tool to study the effect of spontaneous Hawking radiation.