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).

We present optical spectroscopy and optical/near-IR photometry of 31 host galaxies of hydrogenpoor superluminous supernovae (SLSNe), including 15 events from the Pan-STARRS1 Medium Deep Survey. Our sample spans the redshift range 0.1 z 1.6 and is the first comprehensive host galaxy study of this specific subclass of cosmic explosions. Combining the multi-band photometry and emission-line measurements, we determine the luminosities, stellar masses, star formation rates and metallicities. We find that as a whole, the hosts of SLSNe are a low-luminosity ( M B ≈ −17.3 mag), low stellar mass ( M * ≈ 2 × 10 8 M ⊙ ) population, with a high median specific star formation rate ( sSFR ≈ 2 Gyr −1 ). The median metallicity of our spectroscopic sample is low, 12 + log(O/H) ≈ 8.35 ≈ 0.45Z ⊙ , although at least one host galaxy has solar metallicity. The host galaxies of H-poor SLSNe are statistically distinct from the hosts of GOODS core-collapse SNe (which cover a similar redshift range), but resemble the host galaxies of long-duration gamma-ray bursts (LGRBs) in terms of stellar mass, SFR, sSFR and metallicity. This result indicates that the environmental causes leading to massive stars forming either SLSNe or LGRBs are similar, and in particular that SLSNe are more effectively formed in low metallicity environments. We speculate that the key ingredient is large core angular momentum, leading to a rapidly-spinning magnetar in SLSNe and an accreting black hole inLGRBs.

Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. ABSTRACT We present griz P1 light curves of 146 spectroscopically confirmed Type Ia supernovae (SNe Ia; 0.03 < z < 0.65) discovered during the first 1.5 yr of the Pan-STARRS1 Medium Deep Survey. The Pan-STARRS1 natural photometric system is determined by a combination of on-site measurements of the instrument response function and observations of spectrophotometric standard stars. We find that the systematic uncertainties in the photometric system are currently 1.2% without accounting for the uncertainty in the Hubble Space Telescope Calspec definition of the AB system. A Hubble diagram is constructed with a subset of 113 out of 146 SNe Ia that pass our light curve quality cuts. The cosmological fit to 310 SNe Ia (113 PS1 SNe Ia + 222 light curves from 197 low-z SNe Ia), using only supernovae (SNe) and assuming a constant dark energy equation of state and flatness, yields w = −1.120 +0.360 −0.206 (Stat) +0.269 −0.291 (Sys). When combined with BAO+CMB(Planck)+H 0 , the analysis yields Ω M = 0.280 +0.013 −0.012 and w = −1.166 +0.072 −0.069 including all identified systematics. The value of w is inconsistent with the cosmological constant value of −1 at the 2.3σ level. Tension endures after removing either the baryon acoustic oscillation (BAO) or the H 0 constraint, though it is strongest when including the H 0 constraint. If we include WMAP9 cosmic microwave background (CMB) constraints instead of those from Planck, we find w = −1.124 +0.083 −0.065 , which diminishes the discord to <2σ . We cannot conclude whether the tension with flat ΛCDM is a feature of dark energy, new physics, or a combination of chance and systematic errors. The full Pan-STARRS1 SN sample with ∼three times as many SNe should provide more conclusive results.

We present multi-band optical and near-infrared observations of 19 short γ-ray burst (GRB) host galaxies, aimed at measuring their stellar masses and population ages. The goals of this study are to evaluate whether short GRBs track the stellar mass distribution of galaxies, to investigate the progenitor delay time distribution, and to explore any connection between long and short GRB progenitors. Using single stellar population models we infer masses of log(M * /M ⊙ ) ≈ 8.8 − 11.6, with a median of log(M * /M ⊙ ) ≈ 10.1, and population ages of τ * ≈ 0.03 − 4.4 Gyr with a median of τ * ≈ 0.3 Gyr. We further infer maximal masses of log(M * /M ⊙ ) ≈ 9.7−11.9 by assuming stellar population ages equal to the age of the universe at each host's redshift. Comparing the distribution of stellar masses to the general galaxy mass function we find that short GRBs track the cosmic stellar mass distribution only if the late-type hosts generally have maximal masses. However, there is an apparent dearth of early-type hosts compared to the equal contribution of early-and late-type galaxies to the cosmic stellar mass budget. These results suggest that stellar mass may not be the sole parameter controlling the short GRB rate, and raise the possibility of a two-component model with both mass and star formation playing a role (reminiscent of the case for Type Ia supernovae). If short GRBs in late-type galaxies indeed track the star formation activity, the resulting typical delay time is ∼ 0.2 Gyr, while those in early-type hosts have a typical delay of ∼ 3 Gyr. Using the same stellar population models we fit the broad-band photometry for 22 long GRB host galaxies in a similar redshift range and find that they have significantly lower masses and younger population ages, with log(M * /M ⊙ ) ≈ 9.1 and τ * ≈ 0.06 Gyr, respectively; their maximal masses are similarly lower, log(M * /M ⊙ ) ≈ 9.6, and as expected do not track the galaxy mass function. Most importantly, the two GRB host populations remain distinct even if we consider only the star-forming hosts of short GRBs, supporting our previous findings (based on star formation rates and metallicities) that the progenitors of long GRBs and short GRBs in late-type galaxies are distinct. Given the much younger stellar populations of long GRB hosts (and hence of long GRB progenitors), and the substantial differences in host properties, we caution against the use of Type I and II designations for GRBs since this may erroneously imply that all GRBs which track star formation activity share the same massive star progenitors.