HSC-XD 52: An X-Ray Detected AGN in a Low-mass Galaxy at z ∼ 0.56

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 ∼ 0.56. 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⋆ ≈ 3 × 109M⊙, placing it in the dwarf regime. We estimate a central black hole mass of MBH ∼ 106M⊙. With an average X-ray luminosity of ${L}_{{\rm{X}}}\approx 3.5\times {10}^{43}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$, 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.