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.

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 − 8 + 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M ⊙ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 Mpc ) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

We combine the CfA3 supernova Type Ia (SN Ia) sample with samples from the literature to calculate improved constraints on the dark energy equation of state parameter, w. The CfA3 sample is added to the Union set of Kowalski et al. (2008) to form the Constitution set and, combined with a BAO prior, produces 1 + w = 0.013 +0.066 −0.068 (0.11 syst), consistent with the cosmological constant. The CfA3 addition makes the cosmologically-useful sample of nearby SN Ia between 2.6 and 2.9 times larger than before, reducing the statistical uncertainty to the point where systematics play the largest role. We use four light curve fitters to test for systematic differences: SALT, SALT2, MLCS2k2 (R V = 3.1), and MLCS2k2 (R V = 1.7). SALT produces high-redshift Hubble residuals with systematic trends versus color and larger scatter than MLCS2k2. MLCS2k2 overestimates the intrinsic luminosity of SN Ia with 0.7 < ∆ < 1.2. MLCS2k2 with R V = 3.1 overestimates host-galaxy extinction while R V ≈ 1.7 does not. Our investigation is consistent with no Hubble bubble. We also find that, after lightcurve correction, SN Ia in Scd/Sd/Irr hosts are intrinsically fainter than those in E/S0 hosts by 2σ, suggesting that they may come from different populations. We also find that SN Ia in Scd/Sd/Irr hosts have low scatter (0.1 mag) and equation of state, p = wρ, where the equation of state parameter, w, relates the dark energy density, ρ, to the dark energy pressure, p. In a Friedman universe, ρ depends on 1 + w and the scale factor of the universe, a, as ρ ∼ a −3(1+w) . The first question that arises is whether the dark energy density is constant (1 + w = 0, a cosmological constant) or not. We choose to use the notation, 1 + w, since it is then easier to think about values of w larger than −1 (1 + w > 0) or more negative than −1 (1 + w < 0). In the case of 1 + w < 0 the dark energy grows in density as the universe expands! The second question is whether the dark energy properties, as described by w, are constant in time or not.The first study on the equation of state produced a 95%-confidence limit of 1 + w < 0.3, assuming Ω M ∼ 0.2 and zero possibility of 1 + w < 0 (Garnavich et al. 1998). Knop et al. (2003) found 1 + w = −0.05 +0.15 −0.20 . Riess et al. (2005) reported 1 + w = −0.02 +0.13 −0.19 . The SNLS and ESSENCE surveys were designed to narrow the constraints on 1+w and their first reports showed significant improvement in statistical uncertainty over the previous values, bringing them down to the range where systematic uncertainties,which they try to reduce as well, are of roughly equal importance. Astier et al. (2006, A06, hereafter) found 1 + w = −0.02 ± 0.09 while Wood-Vasey et al. (2007, WV07, hereafter) found 1+w = −0.07±0.09. Most recently, Kowalski et al. (2008) (K08, hereafter) made a compilation of the literature SN Ia, plus several new nearby ones that they present, and found 1 + w = −0.01 ± 0.08 when using the same priors as A06 and WV07. All of these studies are consistent with a cosmological constant. On the time-evolution...