We present optical light curves, redshifts, and classifications for 365 spectroscopically confirmed Type Ia supernovae (SNe Ia) discovered by the Pan-STARRS1 (PS1) Medium Deep Survey. We detail improvements to the PS1 SN photometry, astrometry and calibration that reduce the systematic uncertainties in the PS1 SN Ia distances. We combine the subset of 279 PS1 SN Ia (0.03 < z < 0.68) with useful distance estimates of SN Ia from SDSS, SNLS, various low-z and HST samples to form the largest combined sample of SN Ia consisting of a total of 1048 SN Ia ranging from 0.01 < z < 2.3, which we call the 'Pantheon Sample'. When combining Planck 2015 CMB measurements with the Pantheon SN sample, we find Ω m = 0.307±0.012 and w = −1.026±0.041 for the wCDM model. When the SN and CMB constraints are combined with constraints from BAO and local H 0 measurements, the analysis yields the most precise measurement of dark energy to date: w 0 = −1.007 ± 0.089 and w a = −0.222 ± 0.407 for the w 0 w a CDM model. Tension with a cosmological constant previously seen in an analysis of PS1 and low-z SNe has diminished after an increase of 2× in the statistics of the PS1 sample, improved calibration and photometry, and stricter light-curve quality cuts. We find the systematic uncertainties in our measurements of dark energy are almost as large as the statistical uncertainties, primarily due to limitations of modeling the low-redshift sample. This must be addressed for future progress in using SN Ia to measure dark energy.

The Zwicky Transient Facility (ZTF) is a new optical time-domain survey that uses the Palomar 48 inch Schmidt telescope. A custom-built wide-field camera provides a 47 deg 2 field of view and 8 s readout time, yielding more than an order of magnitude improvement in survey speed relative to its predecessor survey, the Palomar Transient Factory. We describe the design and implementation of the camera and observing system. The ZTF data system at the Infrared Processing and Analysis Center provides near-real-time reduction to identify moving and varying objects. We outline the analysis pipelines, data products, and associated archive. Finally, we present on-sky performance analysis and first scientific results from commissioning and the early survey. ZTF's public alert stream will serve as a useful precursor for that of the Large Synoptic Survey Telescope.

In the past decade, several rapidly evolving transients have been discovered whose timescales and luminosities are not easily explained by traditional supernovae (SNe) models. The sample size of these objects has remained small due, at least in part, to the challenges of detecting short timescale transients with traditional survey cadences. Here we present the results from a search within the Pan-STARRS1 Medium Deep Survey (PS1-MDS) for rapidly evolving and luminous transients. We identify 10 new transients with a time above half-maximum (t 1/2 ) of less than 12 days and −16.5 > M > −20 mag. This increases the number of known events in this region of SN phase space by roughly a factor of three. The median redshift of the PS1-MDS sample is z = 0.275 and they all exploded in star-forming galaxies. In general, the transients possess faster rise than decline timescale and blue colors at maximum light (g P1 − r P1 −0.2). Best-fit blackbodies reveal photospheric temperatures/radii that expand/cool with time and explosion spectra taken near maximum light are dominated by a blue continuum, consistent with a hot, optically thick, ejecta. We find it difficult to reconcile the short timescale, high peak luminosity (L > 10 43 erg s −1 ), and lack of UV line blanketing observed in many of these transients with an explosion powered mainly by the radioactive decay of 56 Ni. Rather, we find that many are consistent with either (1) cooling envelope emission from the explosion of a star with a low-mass extended envelope that ejected very little (<0.03 M ) radioactive material, or (2) a shock breakout within a dense, optically thick, wind surrounding the progenitor star. After calculating the detection efficiency for objects with rapid timescales in the PS1-MDS we find a volumetric rate of 4800-8000 events yr −1 Gpc −3 (4%-7% of the core-collapse SN rate at z = 0.2).

The high luminosity and slow decline of their light curves ( Fig PTF12dam is not detected in z P1 images on 1 January 2012, 132 days before the peak.Although their light curves match the declining phases of SN 2007bi and the PISN models quite well, PTF12dam and PS1-11ap rise to maximum light a factor of ~2 faster than these models.The spectra of PTF12dam and PS1-11ap show them to be similar supernovae. After 50 days from the respective light curve peaks, these spectra are almost identical to that of SN 2007bi at the same epoch ( Particularly around and after maximum light, PISN colours are expected to evolve to the red owing to increasing blanketing by iron group elements 7,8 abundant in their ejecta. We see no evidence of line blanketing in our spectra, even down to 2,000 Å (rest frame) in PS1-11ap, which suggests lower iron group abundances and a higher degree of ionization than in PISN models. Such conditions are fulfilled in models of ejecta reheated by magnetars-highly magnetic, rapidly rotating nascent pulsars 13,16,17 . The pressure of the magnetar wind on the inner ejecta can form a dense shell 13,14,17 at near-constant photospheric velocity. ForPTF12dam, the velocities of spectral lines are close to 10,000 km s −1 at all times. Intriguingly, Page 4 of 26 the early spectra of our objects are very similar to those of superluminous supernovae of type I (refs 2, 11, 12) and evolve in the same way, but on longer timescales and with lower line velocities (Fig. 2).Nebular modelling of SN 2007bi spectra has been used to argue 1 for large ejected oxygen and magnesium masses of 8-15M ! and 0.07-0.13M ! , respectively (where M ! is the solar mass). Such masses are actually closer to values in massive core-collapse models 18 than in PISN models, which eject ~40M ! oxygen and ~4M ! magnesium 1,8,9 . In the work reported in ref.1, an additional 37M ! in total of Ne, Si, S, and Ar were added to the model, providing a total ejecta mass consistent with a PISN. However, this was not directly measured 1 , because these elements lack any identified lines. These constraints are important, so we investigated line formation in this phase using our own non-local thermodynamic equilibrium code We suggest here one model that can consistently explain the data. A magnetarpowered supernova can produce a light curve with the observed rise and decline rates as the neutron star spins down and reheats the ejecta 13,14,16,17 . It has been suggested that ~10% of core-collapses may form magnetars 14 . Although their initial-spin distribution is unknown, periods ≳1 ms are physically plausible. This mechanism has already been proposed for SN (Fig. 4), and found a good fit for magnetic field B ≈ 10 14 G and spin period P ≈ 2.6 ms, with an ejecta mass of ~10-16M ! . At peak, the r-band luminosities of PTF12dam and PS1-11ap are ~1.5 times that of SN 2007bi. Scaling our light curve by this factor, our model implies a similar ejected mass for SN 2007bi, with a slower-spinning magnetar (P ≈ 3.3 ms), comparable to previous models 14 . If the mag...