  We report the discovery of an accreting supermassive black hole at z=8.679,
in CEERS_1019, a galaxy previously discovered via a Ly$\alpha$-break by Hubble
and with a Ly$\alpha$ redshift from Keck. As part of the Cosmic Evolution Early
Release Science (CEERS) survey, we observed this source with JWST/NIRSpec
spectroscopy, MIRI and NIRCam imaging, and NIRCam/WFSS slitless spectroscopy.
The NIRSpec spectra uncover many emission lines, and the strong [O III]
emission line confirms the ground-based Ly$\alpha$ redshift. We detect a
significant broad (FWHM~1200 km/s) component in the H$\beta$ emission line,
which we conclude originates in the broad-line region of an active galactic
nucleus (AGN), as the lack of a broad component in the forbidden lines rejects
an outflow origin. This hypothesis is supported by the presence of
high-ionization lines, as well as a spatial point-source component embedded
within a smoother surface brightness profile. The mass of the black hole is
log($M_{BH}/M_{\odot})=6.95{\pm}0.37$, and we estimate that it is accreting at
1.2 ($\pm$0.5) x the Eddington limit. The 1-8 $\mu$m photometric spectral
energy distribution (SED) from NIRCam and MIRI shows a continuum dominated by
starlight and constrains the host galaxy to be massive (log M/M$_{\odot}$~9.5)
and highly star-forming (SFR~30 M$_{\odot}$ yr$^{-1}$). Ratios of the strong
emission lines show that the gas in this galaxy is metal-poor
(Z/Z$_{\odot}$~0.1), dense (n$_{e}$~10$^{3}$ cm$^{-3}$), and highly ionized
(log U~-2.1), consistent with the general galaxy population observed with JWST
at high redshifts. We use this presently highest-redshift AGN discovery to
place constraints on black hole seeding models and find that a combination of
either super-Eddington accretion from stellar seeds or Eddington accretion from
massive black hole seeds is required to form this object by the observed epoch.
