Second Data Release of the Hyper Suprime-Cam Subaru Strategic Program
  This paper presents the second data release of the Hyper Suprime-Cam Subaru
Strategic Program, a wide-field optical imaging survey on the 8.2 meter Subaru
Telescope. The release includes data from 174 nights of observation through
January 2018. The Wide layer data cover about 300 deg^2 in all five broadband
filters (grizy) to the nominal survey exposure (10min in gr and 20min in izy).
Partially observed areas are also included in the release; about 1100 deg^2 is
observed in at least one filter and one exposure. The median seeing in the
i-band is 0.6 arcsec, demonstrating the superb image quality of the survey. The
Deep (26 deg^2) and UltraDeep (4 deg^2) data are jointly processed and the
UltraDeep-COSMOS field reaches an unprecedented depth of i~28 at 5 sigma for
point sources. In addition to the broad-bands, narrow-band data are also
available in the Deep and UltraDeep fields. This release includes a major
update to the processing pipeline, including improved sky subtraction, PSF
modeling, object detection, and artifact rejection. The overall data quality
has been improved, but this release is not without problems; there is a
persistent deblender problem as well as new issues with masks around bright
stars. The user is encouraged to review the issue list before utilizing the
data for scientific explorations. All the image products as well as catalog
products are available for download. The catalogs are also loaded to a
database, which provides an easy interface for users to retrieve data for
objects of interest. In addition to these main data products, detailed galaxy
shape measurements withheld from the Public Data Release 1 (PDR1) are now
available to the community. The shape catalog is drawn from the S16A internal
release, which has a larger area than PDR1 (160 deg^2). All products are
available at the data release site, https://hsc-release.mtk.nao.ac.jp/.
