* Astronomy

Title: Discovery and Follow-up Observations of the Young Type Ia Supernova 2016coj
Author: WeiKang Zheng, Alexei V. Filippenko, Jon Mauerhan, Melissa L. Graham, Heechan Yuk, Griffin Hosseinzadeh, Jeffrey M. Silverman, Liming Rui, Ron Arbour, Ryan J. Foley, Bela Abolfathi, Louis E. Abramson, Iair Arcavi, Aaron J. Barth, Vardha N. Bennert, Andrew P. Brandel, Michael C. Cooper, Maren Cosens, Sean P. Fillingham, Benjamin J. Fulton, Goni Halevi, D. Andrew Howell, Tiffany Hsyu, Patrick L. Kelly, Sahana Kumar, Linyi Li, Wenxiong Li, Matthew A. Malkan, Christina Manzano-King, Curtis McCully, Peter E. Nugent, Yen-Chen Pan, Liuyi Pei, Bryan Scott, Remington Oliver Sexton, Isaac Shivvers, Benjamin Stahl, Tommaso Treu, Stefano Valenti, H. Alexander Vogler, Jonelle L. Walsh, Xiaofeng Wang

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±0.5 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 Si2 \lambda 6355 (~12,600kms around peak brightness). The ion Si2 \lambdab6355 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±0.2 mag), and it reaches a B-band maximum \about 16.0~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 Si2 line polarization is quite strong (~0.9%±0.1%) at peak brightness.

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