We present UV, optical, and near-infrared (NIR) photometry of the first electromagnetic counterpart to a gravitational wave source from Advanced Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo, the binary neutron star merger GW170817. Our data set extends from the discovery of the optical counterpart at 0.47-18.5 days post-merger, and includes observations with the Dark Energy Camera (DECam), Gemini-South/ FLAMINGOS-2 (GS/F2), and the Hubble Space Telescope (HST). The spectral energy distribution (SED) inferred from this photometry at 0.6 days is well described by a blackbody model with » T 8300 K, a radius of »Ŕ 4.5 10 14 cm (corresponding to an expansion velocity of » v c 0.3 ), and a bolometric luminosity of »Ĺ 5 10 bol 41 erg s −1 . At 1.5 days we find a multi-component SED across the optical and NIR, and subsequently we observe rapid fading in the UV and blue optical bands and significant reddening of the optical/ NIR colors. Modeling the entire data set, we find that models with heating from radioactive decay of 56 Ni, or those with only a single component of opacity from r-process elements, fail to capture the rapid optical decline and red optical/NIR colors. Instead, models with two components consistent with lanthanide-poor and lanthanide-rich ejecta provide a good fit to the data; the resulting "blue" component has » . These ejecta masses are broadly consistent with the estimated r-process production rate required to explain the Milky Way r-process abundances, providing the first evidence that binary neutron star (BNS) mergers can be a dominant site of r-process enrichment.

We present new observations of the binary neutron star merger GW170817 at ∆t ≈ 220 − 290 days postmerger, at radio (Karl G. Jansky Very Large Array; VLA), X-ray (Chandra X-ray Observatory) and optical (Hubble Space Telescope; HST) wavelengths. These observations provide the first evidence for a turnover in the X-ray light curve, mirroring a decline in the radio emission at 5σ significance. The radio-to-X-ray spectral energy distribution exhibits no evolution into the declining phase. Our full multi-wavelength dataset is consistent with the predicted behavior of our previously published models of a successful structured jet expanding into a low-density circumbinary medium, but pure cocoon models with a choked jet cannot be ruled out. If future observations continue to track our predictions, we expect that the radio and X-ray emission will remain detectable until ∼ 1000 days post-merger.

We present simulated observations to assess the ability of LSST and the WFD survey to detect and characterize kilonovae -the optical emission associated with binary neutron star (and possibly black hole -neutron star) mergers. We expand on previous studies in several critical ways by exploring a range of kilonova models and several choices of cadence, as well as by evaluating the information content of the resulting light curves. We find that, depending on the precise choice of cadence, the WFD survey will achieve an average kilonova detection efficiency of ≈ 1.6 − 2.5% and detect only ≈ 3 − 6 kilonovae per year. The detected kilonovae will be within the detection volume of Advanced LIGO/Virgo (ALV). By refitting the best resulting LSST light curves with the same model used to generate them we find the model parameters are generally weakly constrained, and are accurate to at best a factor of 2 − 3. Motivated by the finding that the WFD will yield a small number of kilonova detections, with poor light curves and marginal information content, and that the detections are in any case inside the ALV volume, we argue that target-of-opportunity follow-up of gravitational wave triggers is a much more effective approach for kilonova studies. We outline the qualitative foundation for such a program with the goal of minimizing the impact on LSST operations. We argue that observations in the gz-bands with a total time investment per event of ≈ 1.5 hour per 10 deg 2 of search area is sufficient to rapidly detect and identify kilonovae with 90% efficiency. For an estimated event rate of ∼ 20 per year visible to LSST, this accounts for ∼ 1.5% of the total survey time. In this regime, LSST has the potential to be a powerful tool for kilonovae discovery, with detected events handed off to other narrow-field facilities for further monitoring.

We present an exquisite 30 minute cadence Kepler (K2) light curve of the Type Ia supernova (SN Ia) 2018oh (ASASSN-18bt), starting weeks before explosion, covering the moment of explosion and the subsequent rise, and continuing past peak brightness. These data are supplemented by multi-color Panoramic Survey Telescope (Pan-STARRS1) and Rapid Response System 1 and Cerro Tololo Inter-American Observatory 4 m Dark Energy Camera (CTIO 4-m DECam) observations obtained within hours of explosion. The K2 light curve has an unusual twocomponent shape, where the flux rises with a steep linear gradient for the first few days, followed by a quadratic rise as seen for typical supernovae (SNe)Ia. This "flux excess" relative to canonical SNIa behavior is confirmed in our i-band light curve, and furthermore, SN 2018oh is especially blue during the early epochs. The flux excess peaks 2.14±0.04 days after explosion, has a FWHM of 3.12±0.04 days, a blackbody temperature of T 17, 500 9,000 11,500 =-+ K, a peak luminosity of 4.3 0.2 10 erg s 37 1 ´-, and a total integrated energy of 1.27 0.01 10 erg 43 ´. We compare SN 2018oh to several models that may provide additional heating at early times, including collision with a companion and a shallow concentration of radioactive nickel. While all of these models generally reproduce the early K2 light curve shape, we slightly favor a companion interaction, at a distance of ∼2 10 cm 12 based on our early color measurements, although the exact distance depends on the uncertain viewing angle. Additional confirmation of a companion interaction in future modeling and observations of SN 2018oh would provide strong support for a single-degenerate progenitor system.

We discovered two transient events in the Kepler field with light curves that strongly suggest they are type II-P supernovae. Using the fast cadence of the Kepler observations we precisely estimate the rise time to maximum for KSN2011a and KSN2011d as 10.5±0.4 and 13.3±0.4 rest-frame days respectively. Based on fits to idealized analytic models, we find the progenitor radius of KSN2011a (280±20 R ) to be significantly smaller than that for KSN2011d (490±20 R ) but both have similar explosion energies of 2.0±0.3 × 10 51 erg.The rising light curve of KSN2011d is an excellent match to that predicted by simple models of exploding red supergiants (RSG). However, the early rise of KSN2011a is faster than the models predict possibly due to the supernova shockwave moving into pre-existing wind or mass-loss from the RSG. A mass loss rate of 10 −4 M yr −1 from the RSG can explain the fast rise without impacting the optical flux at maximum light or the shape of the post-maximum light curve.No shock breakout emission is seen in KSN2011a, but this is likely due to the circumstellar interaction suspected in the fast rising light curve. The early light curve of KSN2011d does show excess emission consistent with model predictions of a shock breakout. This is the first optical detection of a shock breakout from a type II-P supernova.