Rapidly accreting black hole of the Ly{\alpha}-luminous quasar PSO
  J006.1240+39.2219
  We present near-infrared 1.1-1.3 and 1.3-1.6 $\mu$m spectra of the
Ly$\alpha$-luminous quasar PSO J006.1240+39.2219 at $z=6.617$ obtained with the
NIRSPEC spectrograph at the Keck-II telescope. The spectra cover the CIV
$\lambda$1549, CIII] $\lambda$1909 emission lines and part of the UV continuum
of the quasar. From the NIRSPEC observations of PSO J006.1240+39.2219, we
constrain the spectral slope of its UV continuum to be
$\alpha_{\lambda}=-1.35\pm0.26$ and measure an absolute magnitude of
$M_{1450}=-25.60$. Using the scaling relation between black hole mass, width of
the CIV line and ultraviolet continuum luminosity, we derive a black hole mass
of $(2.19\pm0.30)\times 10^8 M_{sun}$, which is consistent but somewhat smaller
than the typical black hole masses of $z\gtrsim6$ quasars of similar
luminosities. The inferred accretion rate of $L_{bol}/L_{edd}\gtrsim2$
indicates that PSO J006.1240+39.2219 is in the phase of the rapid growth of its
supermassive black hole characterized by the high NV/CIV line ratio,
NV/CIV$>1$, and lower level of ionization of its circumnuclear gas than in
other high-redshift luminous quasars. The NV/CIV line ratio of PSO
J006.1240+39.2219 implies relatively high abundance of nitrogen in its
circumnuclear gas. This abundance might be produced by the post-starburst
population of stars that provide the fuel for black hole accretion.
