BICEP2 / Keck Array x: Constraints on Primordial Gravitational Waves
  using Planck, WMAP, and New BICEP2/Keck Observations through the 2015 Season
  We present results from an analysis of all data taken by the BICEP2/Keck CMB
polarization experiments up to and including the 2015 observing season. This
includes the first Keck Array observations at 220 GHz and additional
observations at 95 & 150 GHz. The $Q/U$ maps reach depths of 5.2, 2.9 and 26
$\mu$K$_{cmb}$ arcmin at 95, 150 and 220 GHz respectively over an effective
area of $\approx 400$ square degrees. The 220 GHz maps achieve a
signal-to-noise on polarized dust emission approximately equal to that of
Planck at 353 GHz. We take auto- and cross-spectra between these maps and
publicly available WMAP and Planck maps at frequencies from 23 to 353 GHz. We
evaluate the joint likelihood of the spectra versus a multicomponent model of
lensed-$\Lambda$CDM+$r$+dust+synchrotron+noise. The foreground model has seven
parameters, and we impose priors on some of these using external information
from Planck and WMAP derived from larger regions of sky. The model is shown to
be an adequate description of the data at the current noise levels. The
likelihood analysis yields the constraint $r_{0.05}<0.07$ at 95% confidence,
which tightens to $r_{0.05}<0.06$ in conjunction with Planck temperature
measurements and other data. The lensing signal is detected at $8.8 \sigma$
significance. Running maximum likelihood search on simulations we obtain
unbiased results and find that $\sigma(r)=0.020$. These are the strongest
constraints to date on primordial gravitational waves.
