  KAGRA, the kilometer-scale underground gravitational-wave detector, is
located at Kamioka, Japan. In April 2020, an astrophysics observation was
performed at the KAGRA detector in combination with the GEO 600 detector; this
observation operation is called O3GK. The optical configuration in O3GK is
based on a power recycled Fabry-P\'{e}rot Michelson interferometer; all the
mirrors were set at room temperature. The duty factor of the operation was
approximately 53%, and the strain sensitivity was
$3\times10^{-22}~/\sqrt{\rm{Hz}}$ at 250 Hz. In addition, the
binary-neutron-star (BNS) inspiral range was approximately 0.6 Mpc. The
contributions of various noise sources to the sensitivity of O3GK were
investigated to understand how the observation range could be improved; this
study is called a "noise budget". According to our noise budget, the measured
sensitivity could be approximated by adding up the effect of each noise. The
sensitivity was dominated by noise from the sensors used for local controls of
the vibration isolation systems, acoustic noise, shot noise, and laser
frequency noise. Further, other noise sources that did not limit the
sensitivity were investigated. This paper provides a detailed account of the
KAGRA detector in O3GK including interferometer configuration, status, and
noise budget. In addition, strategies for future sensitivity improvements such
as hardware upgrades, are discussed.
