Discovery and Follow-up Observations of the Young Type Ia Supernova
  2016coj
  The Type~Ia supernova (SN~Ia) 2016coj in NGC 4125 (redshift $z=0.004523$) was
discovered by the Lick Observatory Supernova Search 4.9 days after the fitted
first-light time (FFLT; 11.1 days before $B$-band maximum). Our first detection
(pre-discovery) is merely $0.6\pm0.5$ day after the FFLT, making SN 2016coj one
of the earliest known detections of a SN Ia. A spectrum was taken only 3.7 hr
after discovery (5.0 days after the FFLT) and classified as a normal SN Ia. We
performed high-quality photometry, low- and high-resolution spectroscopy, and
spectropolarimetry, finding that SN 2016coj is a spectroscopically normal SN
Ia, but with a high velocity of \ion{Si}{2} $\lambda$6355 ($\sim 12,600$\,\kms\
around peak brightness). The \ion{Si}{2} $\lambda$6355 velocity evolution can
be well fit by a broken-power-law function for up to a month after the FFLT. SN
2016coj has a normal peak luminosity ($M_B \approx -18.9 \pm 0.2$ mag), and it
reaches a $B$-band maximum \about16.0~d after the FFLT. We estimate there to be
low host-galaxy extinction based on the absence of Na~I~D absorption lines in
our low- and high-resolution spectra. The spectropolarimetric data exhibit weak
polarization in the continuum, but the \ion{Si}{2} line polarization is quite
strong ($\sim 0.9\% \pm 0.1\%$) at peak brightness.
