'Discovery'). No activity had been detected at the same location in the images taken on the previous night and earlier, indicating that the SN likely exploded between May 2.29 and 3.29. Our follow-up spectroscopic campaign (See Extended Data Table 1 for the observation log) established that iPTF14atg was a Type Ia supernova (SN Ia). 3Upon discovery we triggered observations with the Ultraviolet/Optical Telescope (UVOT) and the X-ray Telescope (XRT) onboard the Swift space observatory 11 (observation and data reduction is detailed in Methods subsection 'Data acquisition'; raw measurements are shown in Extended Data Table 2). As can be seen in Figure 1, the UV brightness of iPTF14atg declined substantially in the first two observations. A rough energy flux measure in the UV band is provided by ν f ν ≈ 3×10 −13 ergs cm −2 s −1 in the uvm2 band. Starting from the third epoch, the UV and optical emission began to rise again in a manner similar to that seen in other SNe Ia. The XRT did not detect any X-ray signal at any epoch (Methods subsection 'Data acquisition'). We thus conclude that iPTF 14atg emitted a pulse of radiation primarily in the UV band. This pulse with an observed luminosity of L UV ≈ 3×10 41 ergs s −1 was probably already declining by the first epoch of the Swift observations (within four days of its explosion).Figure 1 also illustrates that such an early UV pulse from a SN Ia within four days of its explosion is unprecedented 12,13 . We now seek an explanation for this early UV emission.As detailed in Methods subsection 'Spherical models for the early UV pulse', we explored models in which the UV emission is spherically symmetric with the SN explosion (such as shock cooling and circumstellar interaction). These models are unable to explain the observed UV pulse. Therefore we turn to asymmetric models in which the UV emission comes from particular directions.A reasonable physical model is UV emission arising in the ejecta as the ejecta encounters a companion 9,14 . When the rapidly moving ejecta slams into the companion, a strong 4 reverse shock is generated in the ejecta that heats up the surrounding material. Thermal radiation from the hot material, which peaks in the ultraviolet, can then be seen for a few days until the fast-moving ejecta engulfs the companion and hides the reverse shock region. We compare a semi-analytical model 9 to the Swift/UVOT lightcurves. For simplicity, we fix the explosion date at May 3. We assume that the exploding white dwarf is close to the Chandrasekhar mass limit (1.4 solar mass) and that the SN explosion energy is 10 51 ergs. These values lead to a mean expansion velocity of 10 4 km s −1 for the ejecta. Since the temperature at the collision location is so high that most atoms are ionized, the opacity is probably dominated by electron scattering. To further simplify the case, we assume that the emission from the reverse shock region is blackbody and isotropic. In order to explain the UV lightcurves, the companion star should be located 60 solar radii away from the w...

We report the discovery of a multiply-imaged gravitationally lensed Type Ia supernova, iPTF16geu (SN 2016geu), at redshift $z=0.409$. This phenomenon could be identified because the light from the stellar explosion was magnified more than fifty times by the curvature of space around matter in an intervening galaxy. We used high spatial resolution observations to resolve four images of the lensed supernova, approximately 0.3" from the center of the foreground galaxy. The observations probe a physical scale of $\sim$1 kiloparsec, smaller than what is typical in other studies of extragalactic gravitational lensing. The large magnification and symmetric image configuration implies close alignment between the line-of-sight to the supernova and the lens. The relative magnifications of the four images provide evidence for sub-structures in the lensing galaxy.Comment: Matches published versio

Supernovae (SNe) embedded in dense circumstellar material (CSM) may show prominent emission lines in their early-time spectra ( 10 days after the explosion), owing to recombination of the CSM ionized by the shockbreakout flash. From such spectra ("flash spectroscopy"), we can measure various physical properties of the CSM, as well as the mass-loss rate of the progenitor during the year prior to its explosion. Searching through the Palomar Transient Factory (PTF and iPTF) SN spectroscopy databases from 2009 through 2014, we found 12 SNe II showing flash-ionized (FI) signatures in their first spectra. All are younger than 10 days. These events constitute 14% of all 84 SNe in our sample having a spectrum within 10 days from explosion, and 18% of SNeII observed at ages <5 days, thereby setting lower limits on the fraction of FI events. We classified as "blue/featureless" (BF) those events having a first spectrum that is similar to that of a blackbody, without any emission or absorption signatures. It is possible that some BF events had FI signatures at an earlier phase than observed, or that they lack dense CSM around the progenitor. Within 2 days after explosion, 8 out of 11 SNe in our sample are either BF events or show FI signatures. Interestingly, we found that 19 out of 21 SNe brighter than an absolute magnitude M R =−18.2 belong to the FI or BF groups, and that all FI events peaked above M R =−17.6 mag, significantly brighter than average SNeII.

Type Ibn supernovae (SNe) are a small yet intriguing class of explosions whose spectra are characterized by lowvelocity helium emission lines with little to no evidence for hydrogen. The prevailing theory has been that these are the core-collapse explosions of very massive stars embedded in helium-rich circumstellar material (CSM). We report optical observations of six new SNe Ibn: PTF11rfh, PTF12ldy, iPTF14aki, iPTF15ul, SN2015G, and iPTF15akq. This brings the sample size of such objects in the literature to 22. We also report new data, including a near-infrared spectrum, on the Type Ibn SN 2015U. In order to characterize the class as a whole, we analyze the photometric and spectroscopic properties of the full TypeIbn sample. We find that, despite the expectation that CSM interaction would generate a heterogeneous set of light curves, as seen in SNe IIn, most TypeIbn light curves are quite similar in shape, declining at rates around 0.1 mag day −1 during the first month after maximum light, with a few significant exceptions. Early spectra of SNe Ibn come in at least two varieties, one that shows narrow PCygni lines and another dominated by broader emission lines, both around maximum light, which may be an indication of differences in the state of the progenitor system at the time of explosion. Alternatively, the spectral diversity could arise from viewing-angle effects or merely from a lack of early spectroscopic coverage. Together, the relative light curve homogeneity and narrow spectral features suggest that the CSM consists of a spatially confined shell of helium surrounded by a less dense extended wind.