The Second Data Release of the Beijing-Arizona Sky Survey
  This paper presents the second data release (DR2) of the Beijing-Arizona Sky
Survey (BASS). BASS is an imaging survey of about 5400 deg$^2$ in $g$ and $r$
bands using the 2.3 m Bok telescope. DR2 includes the observations as of July
2017 obtained by BASS and Mayall $z$-band Legacy Survey (MzLS). This is our
first time to include the MzLS data covering the same area as BASS. BASS and
MzLS have respectively completed about 72% and 76% of their observations. The
two surveys will be served for the spectroscopic targeting of the upcoming Dark
Energy Spectroscopic Instrument. Both BASS and MzLS data are reduced by the
same pipeline. We have updated the basic data reduction and photometric methods
in DR2. In particular, source detections are performed on stacked images, and
photometric measurements are co-added from single-epoch images based on these
sources. The median 5$\sigma$ depths with corrections of the Galactic
extinction are 24.05, 23.61, and 23.10 mag for $g$, $r$, and $z$ bands,
respectively. The DR2 data products include stacked images, co-added catalogs,
and single-epoch images and catalogs. The BASS website
(http://batc.bao.ac.cn/BASS/) provides detailed information and links to
download the data.
