  We report on theoretical model and experimental results of the experiment
made in a limit of absolute zero temperature ($\sim$ 600\,$\mu$K) studying the
spin wave analogue of black/white hole horizon using spin (magnonic)
superfluidity in superfluid $^3$He-B. As an experimental tool simulating the
properties of the black/white horizon we used the spin-precession waves
propagating on the background of the spin super-currents between two
Bose-Einstein condensates of magnons in form of homogeneously precessing
domains. We provide experimental evidence of the white hole formation for spin
precession waves in this system, together with observation of an amplification
effect. Moreover, the estimated temperature of the spontaneous Hawking
radiation in this 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.
