We present observations and analysis of 18 stripped-envelope supernovae observed during 2013 -2018. This sample consists of 5 H/He-rich SNe, 6 H-poor/He-rich SNe, 3 narrow lined SNe Ic and 4 broad lined SNe Ic. The peak luminosity and characteristic time-scales of the bolometric light curves are calculated, and the light curves modelled to derive 56 Ni and ejecta masses (M Ni and M ej ). Additionally, the temperature evolution and spectral line velocity-curves of each SN are examined. Analysis of the [O i] line in the nebular phase of eight SNe suggests their progenitors had initial masses < 20 M . The bolometric light curve properties are examined in combination with those of other SE events from the literature. The resulting dataset gives the M ej distribution for 80 SE-SNe, the largest such sample in the literature to date, and shows that SNe Ib have the lowest median M ej , followed by narrow lined SNe Ic, H/He-rich SNe, broad lined SNe Ic, and finally gamma-ray burst SNe. SNe Ic-6/7 show the largest spread of M ej ranging from ∼ 1.2 − 11 M , considerably greater than any other subtype. For all SE-SNe = 2.8 ± 1.5 M which further strengthens the evidence that SE-SNe arise from low mass progenitors which are typically < 5 M at the time of explosion, again suggesting M ZAMS < 25 M . The low and lack of clear bimodality in the distribution implies < 30 M progenitors and that envelope stripping via binary interaction is the dominant evolutionary pathway of these SNe.

This Supplement provides supporting material for Abbott et al. (2016a). We briefly summarize past electromagnetic (EM) follow-up efforts as well as the organization and policy of the current EM follow-up program. We compare the four probability sky maps produced for the gravitational-wave transient GW150914, and provide additional details of the EM follow-up observations that were performed in the different bands.

We describe the development of a low cost, low resolution (R ∼ 350), high throughput, long slit spectrograph covering visible (4000-8000Å) wavelengths. The spectrograph has been developed for fully robotic operation with the Liverpool Telescope (La Palma). The primary aim is to provide rapid spectral classification of faint (V ∼ 20) transient objects detected by projects such as Gaia, iPTF (intermediate Palomar Transient Factory), LOFAR, and a variety of high energy satellites. The design employs a volume phase holographic (VPH) transmission grating as the dispersive element combined with a prism pair (grism) in a linear optical path. One of two peak spectral sensitivities are selectable by rotating the grism. The VPH and prism combination and entrance slit are deployable, and when removed from the beam allow the collimator/camera pair to re-image the target field onto the detector. This mode of operation provides automatic acquisition of the target onto the slit prior to spectrographic observation through World Coordinate System fitting. The selection and characterisation of optical components to maximise photon throughput is described together with performance predictions.

The optical observations of Ic-4 supernova (SN) 2016coi/ASASSN-16fp, from ∼ 2 to ∼ 450 days after explosion, are presented along with analysis of its physical properties. The SN shows the broad lines associated with SNe Ic-3/4 but with a key difference. The early spectra display a strong absorption feature at ∼ 5400Å which is not seen in other SNe Ic-3/4 at this epoch. This feature has been attributed to He i in the literature. Spectral modelling of the SN in the early photospheric phase suggests the presence of residual He in a C/O dominated shell. However, the behaviour of the He i lines is unusual when compared with He-rich SNe, showing relatively low velocities and weakening rather than strengthening over time. The SN is found to rise to peak ∼ 16 d after core-collapse reaching a bolometric luminosity of L p ∼ 3 × 10 42 erg s −1 . Spectral models, including the nebular epoch, show that the SN ejected 2.5 − 4 M of material, with ∼ 1.5 M below 5000 km s −1 , and with a kinetic energy of (4.5 − 7) × 10 51 erg. The explosion synthesised ∼ 0.14 M of 56 Ni. There are significant uncertainties in E (B − V) host and the distance however, which will affect L p and M Ni . SN 2016coi exploded in a host similar to the Large Magellanic Cloud (LMC) and away from star-forming regions. The properties of the SN and the host-galaxy suggest that the progenitor had M ZAMS of 23 − 28 M and was stripped almost entirely down to its C/O core at explosion.

We present the discovery, classification, and radio-to-X-ray follow-up observations of iPTF17cw, a broad-lined (BL) type Ic supernova (SN) discovered by the intermediate Palomar Transient Factory (iPTF). Although it is unrelated to the gravitational wave trigger, this SN was discovered as a happy by-product of the extensive observational campaign dedicated to the follow-up of Advanced LIGO event GW 170104. The spectroscopic properties and inferred peak bolometric luminosity of iPTF17cw are most similar to the gamma-ray-burst (GRB)-associated SN, SN 1998bw, while the shape of the r-band light curve is most similar to that of the relativistic SN, SN 2009bb. Karl G. Jansky Very Large Array (VLA) observations of the iPTF17cw field reveal a radio counterpart ≈10 times less luminous than SN 1998bw, and with a peak radio luminosity comparable to that of SN 2006aj/ GRB 060218 and SN 2010bh/GRB 100316D. Our radio observations of iPTF17cw imply a relativistically expanding outflow. However, further late-time observations with the VLA in its most extended configuration are needed to confirm fading of the iPTF17cw radio counterpart at all frequencies. X-ray observations carried out with Chandra reveal the presence of an X-ray counterpart with a luminosity similar to that of SN 2010bh/ GRB 100316D. Searching the Fermi catalog for possible γ-rays reveals that GRB 161228B is spatially and temporally compatible with iPTF17cw. The similarity to SN 1998bw and SN 2009bb, the radio and X-ray detections, and the potential association with GRB 161228B all point to iPTF17cw being a new candidate member of the rare sample of optically discovered engine-driven BL-Ic SNe associated with relativistic ejecta.