HSC-XD 52: An X-ray detected AGN in a low-mass galaxy at $z\sim0.56$
  The properties of low-mass galaxies hosting central black holes provide clues
about the formation and evolution of the progenitors of supermassive black
holes. In this letter, we present HSC-XD 52, a spectroscopically confirmed
low-mass active galactic nucleus (AGN) at an intermediate redshift of
$z\sim0.56$. We detect this object as a very luminous X-ray source coincident
with a galaxy observed by the Hyper Suprime-Cam (HSC) as part of a broader
search for low-mass AGN. We constrain its stellar mass through spectral energy
distribution modeling to be LMC-like at $M_\star \approx 3 \times 10^9
M_\odot$, placing it in the dwarf regime. We estimate a central black hole mass
of $M_\mathrm{BH} \sim 10^{6} M_\odot$. With an average X-ray luminosity of
$L_X \approx 3.5 \times 10^{43}~\mathrm{erg}~\mathrm{s}^{-1}$, HSC-XD 52 is
among the most luminous X-ray selected AGN in dwarf galaxies. The spectroscopic
and photometric properties of HSC-XD 52 indicate that it is an intermediate
redshift counterpart to local low-mass AGN.
