Orbital and radiative properties of wandering intermediate-mass black holes in the ASTRID simulation

Intermediate-Mass Black Holes (IMBHs) of 10(3) - 10(6) M-circle dot are commonly found at the centre of dwarf galaxies. Simulations and observations convincingly show that a sizeable population of IMBHs could wander off-centre in galaxies. We use the cosmological simulation ASTRID to study the orbital and radiative properties of wandering IMBHs in massive galaxies at z similar to 3. We find that this population of black holes has large orbital inclinations (60 degrees +/- 22 degrees) with respect to the principal plane of the host. The eccentricity of their orbits is also significant (0.6 +/- 0.2) and decreases with time. Wandering IMBHs undergo spikes of accretion activity around the pericentre of their orbits, with rates 10(-3) - 10(-5) times the Eddington rate and a median accretion duty cycle of similar to 12 per cent. Their typical spectral energy distribution peaks in the infrared at similar to 11 mu m rest-frame. Assuming a standard value of 10 per cent for the matter-to-energy radiative efficiency, IMBHs reach 2 - 10 keV X-ray luminosities > 10(37) erg s(-1) for similar to 10 per cent of the time. This luminosity corresponds to fluxes > 10(-15) erg s(-1) cm(-2) within 10 Mpc. They could be challenging to detect because of competing emissions from X-ray binaries and the interstellar medium. X-ray luminosities > 10(41) erg s(-1), in the hyper-luminous X-ray sources (HLXs) regime, are reached by similar to 7 per cent of the IMBHs. These findings suggest that HLXs are a small subset of the wandering IMBH population, which is characterized by luminosities 10(3) - 10(4) times fainter. Dedicated surveys are needed to assess the demographics of this missing population of black holes.