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.

Sensitive high angular and linear resolution radio images of the 240-pc radio jet in NGC 4151, imaged at linear resolutions of 0.3 to 2.6 pc using the VLBA and phased VLA at λ21 cm, are presented and reveal for the first time a faint, highly collimated jet (diameter ∼ <1.4 pc) underlying discrete components, seen in lower 1 Royal Society University Research Fellow resolution MERLIN and VLA images, that appear to be shock-like features associated with changes in direction as the jet interacts with small gas clouds within the central ∼100 pc of the galaxy. In addition, λ21-cm spectral line imaging of the neutral hydrogen in the nuclear region reveals the spatial location, distribution and kinematics of the neutral gas detected previously in a lower resolution MER-LIN study. Neutral hydrogen absorption is detected against component C4W (E+F) as predicted by Mundell et al, but the absorption, extending over 3 pc, is spatially and kinematically complex on sub-parsec scales, suggesting the presence of small, dense gas clouds with a wide range of velocities and column densities. The main absorption component matches that detected in the MERLIN study, close to the systemic velocity (998 km s −1 ) of the galaxy, and is consistent with absorption through a clumpy neutral gas layer in the putative obscuring torus, with higher velocity blue-and red-shifted systems with narrow linewidths also detected across E+F. In this region, average column densities are high, lying in the range 2.7 × 10 19 T S < N H < 1.7 × 10 20 T S cm −2 K −1 (T S is the spin temperature), with average radial velocities in the range 920 < V r < 1050 km s −1 . The spatial location and distribution of the absorbing gas across component E+F rules out component E as the location of the AGN (as suggested by Ulvestad et al.) and, in combination with the well-collimated continuum structures seen in component D, suggests that component D (possibly subcomponent D3) is the most likely location for the AGN. We suggest that components C and E are shocks produced in the jet as the plasma encounters, and is deviated by, dense clouds with diameters smaller than ∼1.4 pc.Comparison of the radio jet structure and the distribution and kinematics of ionized gas in the narrow line region (NLR) suggests that shock-excitation by passage of the radio jet is not the dominant excitation mechanism for the NLR. We therefore favour nuclear photoionization to explain the structure of the NLR, although it is interesting to note that a small number of clouds with low velocity and high velocity dispersion are seen to bound the jet, particularly at positions of jet direction changes, suggesting that some NLR clouds are responsible for bending the jet. Alternatively, compression by a cocoon around the radio jet due to pressure stratification in the jet bow shock could explain the bright, compressed optical line-emitting clouds surrounding the cloud-free channel of the radio jet, as modelled by Steffen et al.

We present multi-wavelength observations and modeling of the exceptionally bright long γ-ray burst GRB 160625B. The optical and X-ray data are well fit by synchrotron emission from a collimated blastwave with an opening angle of 3 . 6 j q »  and kinetic energy of E 2 10»´erg, propagating into a low-density (n 5 10 5 »´-cm −3 ) medium with a uniform profile. The forward shock is sub-dominant in the radio band; instead, the radio emission is dominated by two additional components. The first component is consistent with emission from a reverse shock, indicating an initial Lorentz factor of 100 0  G and an ejecta magnetization of R 1 100 B » -. The second component exhibits peculiar spectral and temporal evolution and is most likely the result of scattering of the radio emission by the turbulent Milky Way interstellar medium (ISM). Such scattering is expected in any sufficiently compact extragalactic source and has been seen in GRBs before, but the large amplitude and long duration of the variability seen here are qualitatively more similar to extreme scattering events previously observed in quasars, rather than normal interstellar scintillation effects. High-cadence, broadband radio observations of future GRBs are needed to fully characterize such effects, which can sensitively probe the properties of the ISM and must be taken into account before variability intrinsic to the GRB can be interpreted correctly.

We present ground-based and HST optical and infrared observations of Swift XRF 100316D / SN 2010bh. It is seen that the optical light curves of SN 2010bh evolve at a faster rate than the archetype GRB-SN 1998bw, but at a similar rate to SN 2006aj, a supernova that was spectroscopically linked with XRF 060218, and at a similar rate to non-GRB associated type Ic SN 1994I. We estimate the rest-frame extinction of this event from our optical data to be E(B − V ) = 0.18 ± 0.08 mag. We find the V -band absolute magnitude of SN 2010bh to be M V = −18.62 ± 0.08, which is the faintest peak V -band magnitude observed to-date for a spectroscopicallyconfirmed GRB-SNe. When we investigate the origin of the flux at t − t o = 0.598 days, it is shown that the light is not synchrotron in origin, but is likely coming from the supernova shock break-out. We then use our optical and infrared data to create a quasi-bolometric light curve of SN 2010bh which we model with a simple analytical formula. The results of our modeling imply that SN 2010bh synthesized a nickel mass of M Ni ≈ 0.1M ⊙ , ejected M ej ≈ 2.2M ⊙ and has an explosion energy of E k ≈ 1.4 × 10 52 erg. Thus, while SN 2010bh is an energetic explosion, the amount of nickel created during the explosion is much less than that of SN 1998bw, and only marginally more than SN 1994I. Finally, for a sample 22 GRB-SNe we check for a correlation between the stretch factors and luminosity factors in the R band and conclude that no statisticallysignificant correlation exists.