A Dyson Sphere around a black hole
  The search for extraterrestrial intelligence (SETI) has been conducted for
nearly 60 years. A Dyson Sphere, a spherical structure that surrounds a star
and transports its radiative energy outward as an energy source for an advanced
civilisation, is one of the main targets of SETI. In this study, we discuss
whether building a Dyson Sphere around a black hole is effective. We consider
six energy sources: (i) the cosmic microwave background, (ii) the Hawking
radiation, (iii) an accretion disk, (iv) Bondi accretion, (v) a corona, and
(vi) relativistic jets. To develop future civilisations (for example, a Type II
civilisation), $4\times10^{26}\,{\rm W}$($1\,{\rm L_{\odot}}$) is expected to
be needed. Among (iii) to (vi), the largest luminosity can be collected from an
accretion disk, reaching $10^{5}\,{\rm L_{\odot}}$, enough to maintain a Type
II civilisation. Moreover, if a Dyson Sphere collects not only the
electromagnetic radiation but also other types of energy (e.g., kinetic energy)
from the jets, the total collected energy would be approximately 5 times
larger. Considering the emission from a Dyson Sphere, our results show that the
Dyson Sphere around a stellar-mass black hole in the Milky Way ($10\,\rm kpc$
away from us) is detectable in the ultraviolet$(\rm 10-400\,{\rm nm)}$,
optical$(\rm 400-760\,{\rm nm)}$, near-infrared($\rm 760\,{\rm nm}-5\,{\rm \mu
m}$), and mid-infrared($\rm 5-40\,{\rm \mu m}$) wavelengths via the waste heat
radiation using current telescopes such as Galaxy Evolution Explorer
Ultraviolet Sky Surveys. Performing model fitting to observed spectral energy
distributions and measuring the variability of radial velocity may help us to
identify these possible artificial structures.
