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

Intermediate-Mass Black Holes (IMBHs) of $10^3-10^6\, {\rm M_\odot }$ 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 ~ 3. We find that this population of black holes has large orbital inclinations (60° ± 22°) 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 $\sim 12{{\ \rm per\ cent}}$. Their typical spectral energy distribution peaks in the infrared at $\sim 11 \, \mu \rm m$ rest-frame. Assuming a standard value of $10{{\ \rm per\ cent}}$ for the matter-to-energy radiative efficiency, IMBHs reach 2 - 10 keV X-ray luminosities $\gt 10^{37} \, \mathrm{erg\, s^{-1}}$ for $\sim 10{{\ \rm per\ cent}}$ of the time. This luminosity corresponds to fluxes $\gt 10^{-15} \, \mathrm{erg \, s^{-1} \, cm^{-2}}$ within 10 Mpc. They could be challenging to detect because of competing emissions from X-ray binaries and the interstellar medium. X-ray luminosities $\gt 10^{41} \, \mathrm{erg \, s^{-1}}$, in the hyper-luminous X-ray sources (HLXs) regime, are reached by $\sim 7{{\ \rm per\ cent}}$ of the IMBHs. These findings suggest that HLXs are a small subset of the wandering IMBH population, which is characterized by luminosities 10³ - 10⁴ times fainter. Dedicated surveys are needed to assess the demographics of this missing population of black holes.