Hyper Suprime-Cam (HSC) is a wide-field imaging camera on the prime focus of the 8.2m Subaru telescope on the summit of Maunakea in Hawaii. A team of scientists from Japan, Taiwan and Princeton University is using HSC to carry out a 300-night multi-band imaging survey of the high-latitude sky. The survey includes three layers: the Wide layer will cover 1400 deg 2 in five broad bands (grizy), with a 5 σ point-source depth of r ≈ 26. The Deep layer covers a total of 26 deg 2 in four fields, going roughly a magnitude fainter, while the UltraDeep layer goes almost a magnitude fainter still in two pointings of HSC (a total of 3.5 deg 2). Here we describe the instrument, the science goals of the survey, and the survey strategy and data processing. This paper serves as an introduction to a special issue of the Publications of the Astronomical Society of Japan, which includes a large number of technical and scientific papers describing results from the early phases of this survey.

We present a stellar mass-metallicity relation at z ∼ 1.4 with an unprecedentedly large sample of ∼ 340 star-forming galaxies obtained with FMOS on the Subaru Telescope. We observed K-band selected galaxies at 1.2 ≤ z ph ≤ 1.6 in the SXDS/UDS fields with M * ≥ 10 9.5 M ⊙ , and expected F(Hα) ≥ 5 × 10 −17 erg s −1 cm −2 . Among the observed ∼ 1200 targets, 343 objects show significant Hα emission lines. The gasphase metallicity is obtained from [N ii]λ6584/Hα line ratio, after excluding possible active galactic nuclei (AGNs). Due to the faintness of the [N ii]λ6584 lines, we apply the stacking analysis and derive the mass-metallicity relation at z ∼ 1.4. Our results are compared to past results at different redshifts in the literature. The mass-metallicity relation at z ∼ 1.4 is located between those at z ∼ 0.8 and z ∼ 2.2; it is found that the metallicity increases with decreasing redshift from z ∼ 3 to z ∼ 0 at fixed stellar mass. Thanks to the large size of the sample, we can study the dependence of the mass-metallicity relation on various galaxy physical properties. The average metallicity from the stacked spectra is close to the local FMR in the higher metallicity part but > ∼ 0.1 dex higher in metallicity than the FMR in the lower metallicity part. We find that galaxies with larger E(B − V ), B − R, and R − H colours tend to show higher metallicity by ∼ 0.05 dex at fixed stellar mass. We also find relatively clearer size dependence that objects with smaller half light radius tend to show higher metallicity by ∼ 0.1 dex at fixed stellar mass, especially in the low mass part.

We report the results of a multi-band observing campaign on the famous blazar 3C 279 conducted during a phase of increased activity from 2013 December to 2014 April, including first observations of it with NuSTAR. The γ-ray emission of the source measured by Fermi-LAT showed multiple distinct flares reaching the highest flux level measured in this object since the beginning of the Fermi mission, with F(E > 100 MeV) of 10 −5 photons cm −2 s −1 , and with a flux doubling time scale as short as 2 hours. The γ-ray spectrum during one of the flares was very hard, with an index of Γ γ = 1.7 ± 0.1, which is rarely seen in flat spectrum radio quasars. The lack of concurrent optical variability implies a very high Compton dominance parameter L γ /L syn > 300. Two 1-day NuSTAR observations with accompanying Swift pointings were separated by 2 weeks, probing different levels of source activity. While the 0.5−70 keV X-ray spectrum obtained during the first pointing, and fitted jointly with Swift-XRT is well-described by a simple power law, the second joint observation showed an unusual spectral structure: the spectrum softens by ∆Γ X ≃ 0.4 at ∼ 4 keV. Modeling the broad-band SED during this flare with the standard synchrotron plus inverse Compton model requires: (1) the location of the γ-ray emitting region is comparable with the broad line region radius, (2) a very hard electron energy distribution index p ≃ 1, (3) total jet power significantly exceeding the accretion disk luminosity L j /L d 10, and (4) extremely low jet magnetization with L B /L j 10 −4 . We also find that single-zone models that match the observed γ-ray and optical spectra cannot satisfactorily explain the production of X-ray emission.

Context. Over the past few years, on several occasions, large, continuous rotations of the electric vector position angle (EVPA) of linearly polarized optical emission from blazars have been reported. These events are often coincident with high energy γ-ray flares and they have attracted considerable attention, since they could allow us to probe the magnetic field structure in the γ-ray emitting region of the jet. The flat-spectrum radio quasar 3C 279 is one of the most prominent examples showing this behaviour. Aims. Our goal is to study the observed EVPA rotations and to distinguish between a stochastic and a deterministic origin of the polarization variability. Methods. We have combined multiple data sets of R-band photometry and optical polarimetry measurements of 3C 279, yielding exceptionally well-sampled flux density and polarization curves that cover a period of [2008][2009][2010][2011][2012]. Several large EVPA rotations are identified in the data. We introduce a quantitative measure for the EVPA curve smoothness, which is then used to test a set of simple random walk polarization variability models against the data. Results. 3C 279 shows different polarization variation characteristics during an optical low-flux state and a flaring state. The polarization variation during the flaring state, especially the smooth ∼360• rotation of the EVPA in mid-2011, is not consistent with the tested stochastic processes. Conclusions. We conclude that, during the two different optical flux states, two different processes govern polarization variation, which is possibly a stochastic process during the low-brightness state and a deterministic process during the flaring activity.