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.

We report extensive observational data for five of the lowest redshift Super-Luminous Type Ic Supernovae (SL-SNe Ic) discovered to date, namely, PTF10hgi, SN2011ke, PTF11rks, SN2011kf, and SN2012il. Photometric imaging of the transients at +50 to +230 days after peak combined with host galaxy subtraction reveals a luminous tail phase for four of these SL-SNe. A high-resolution, optical, and near-infrared spectrum from xshooter provides detection of a broad He i λ10830 emission line in the spectrum (+50 days) of SN2012il, revealing that at least some SL-SNe Ic are not completely helium-free. At first sight, the tail luminosity decline rates that we measure are consistent with the radioactive decay of 56 Co, and would require 1-4 M of 56 Ni to produce the luminosity. These 56 Ni masses cannot be made consistent with the short diffusion times at peak, and indeed are insufficient to power the peak luminosity. We instead favor energy deposition by newborn magnetars as the power source for these objects. A semi-analytical diffusion model with energy input from the spin-down of a magnetar reproduces the extensive light curve data well. The model predictions of ejecta velocities and temperatures which are required are in reasonable agreement with those determined from our observations. We derive magnetar energies of 0.4 E(10 51 erg) 6.9 and ejecta masses of 2.3 M ej (M ) 8.6. The sample of five SL-SNe Ic presented here, combined with SN 2010gx-the best sampled SL-SNe Ic so far-points toward an explosion driven by a magnetar as a viable explanation for all SL-SNe Ic.

Gravitational waves were discovered with the detection of binary black-hole mergers and they should also be detectable from lower-mass neutron-star mergers. These are predicted to eject material rich in heavy radioactive isotopes that can power an electromagnetic signal. This signal is luminous at optical and infrared wavelengths and is called a kilonova. The gravitational-wave source GW170817 arose from a binary neutron-star merger in the nearby Universe with a relatively well confined sky position and distance estimate. Here we report observations and physical modelling of a rapidly fading electromagnetic transient in the galaxy NGC 4993, which is spatially coincident with GW170817 and with a weak, short γ-ray burst. The transient has physical parameters that broadly match the theoretical predictions of blue kilonovae from neutron-star mergers. The emitted electromagnetic radiation can be explained with an ejected mass of 0.04 ± 0.01 solar masses, with an opacity of less than 0.5 square centimetres per gram, at a velocity of 0.2 ± 0.1 times light speed. The power source is constrained to have a power-law slope of -1.2 ± 0.3, consistent with radioactive powering from r-process nuclides. (The r-process is a series of neutron capture reactions that synthesise many of the elements heavier than iron.) We identify line features in the spectra that are consistent with light r-process elements (atomic masses of 90-140). As it fades, the transient rapidly becomes red, and a higher-opacity, lanthanide-rich ejecta component may contribute to the emission. This indicates that neutron-star mergers produce gravitational waves and radioactively powered kilonovae, and are a nucleosynthetic source of the r-process elements.

The high luminosity and slow decline of their light curves ( Fig PTF12dam is not detected in z P1 images on 1 January 2012, 132 days before the peak.Although their light curves match the declining phases of SN 2007bi and the PISN models quite well, PTF12dam and PS1-11ap rise to maximum light a factor of ~2 faster than these models.The spectra of PTF12dam and PS1-11ap show them to be similar supernovae. After 50 days from the respective light curve peaks, these spectra are almost identical to that of SN 2007bi at the same epoch ( Particularly around and after maximum light, PISN colours are expected to evolve to the red owing to increasing blanketing by iron group elements 7,8 abundant in their ejecta. We see no evidence of line blanketing in our spectra, even down to 2,000 Å (rest frame) in PS1-11ap, which suggests lower iron group abundances and a higher degree of ionization than in PISN models. Such conditions are fulfilled in models of ejecta reheated by magnetars-highly magnetic, rapidly rotating nascent pulsars 13,16,17 . The pressure of the magnetar wind on the inner ejecta can form a dense shell 13,14,17 at near-constant photospheric velocity. ForPTF12dam, the velocities of spectral lines are close to 10,000 km s −1 at all times. Intriguingly, Page 4 of 26 the early spectra of our objects are very similar to those of superluminous supernovae of type I (refs 2, 11, 12) and evolve in the same way, but on longer timescales and with lower line velocities (Fig. 2).Nebular modelling of SN 2007bi spectra has been used to argue 1 for large ejected oxygen and magnesium masses of 8-15M ! and 0.07-0.13M ! , respectively (where M ! is the solar mass). Such masses are actually closer to values in massive core-collapse models 18 than in PISN models, which eject ~40M ! oxygen and ~4M ! magnesium 1,8,9 . In the work reported in ref.1, an additional 37M ! in total of Ne, Si, S, and Ar were added to the model, providing a total ejecta mass consistent with a PISN. However, this was not directly measured 1 , because these elements lack any identified lines. These constraints are important, so we investigated line formation in this phase using our own non-local thermodynamic equilibrium code We suggest here one model that can consistently explain the data. A magnetarpowered supernova can produce a light curve with the observed rise and decline rates as the neutron star spins down and reheats the ejecta 13,14,16,17 . It has been suggested that ~10% of core-collapses may form magnetars 14 . Although their initial-spin distribution is unknown, periods ≳1 ms are physically plausible. This mechanism has already been proposed for SN (Fig. 4), and found a good fit for magnetic field B ≈ 10 14 G and spin period P ≈ 2.6 ms, with an ejecta mass of ~10-16M ! . At peak, the r-band luminosities of PTF12dam and PS1-11ap are ~1.5 times that of SN 2007bi. Scaling our light curve by this factor, our model implies a similar ejected mass for SN 2007bi, with a slower-spinning magnetar (P ≈ 3.3 ms), comparable to previous models 14 . If the mag...

We report the results of a 3 year-long dedicated monitoring campaign of a restless Luminous Blue Variable (LBV) in NGC 7259. The object, named SN 2009ip, was observed photometrically and spectroscopically in the optical and near-infrared domains. We monitored a number of erupting episodes in the past few years, and increased the density of our observations during eruptive episodes. In this paper we present the full historical data set from 2009-2012 with multi-wavelength dense coverage of the two high luminosity events between August -September 2012. We construct bolometric light curves and measure the total luminosities of these eruptive or explosive events. We label them the 2012a event (lasting ∼ 50 days) with a peak of 3 × 10 41 ergs −1 , and the 2012b event (14 day rise time, still ongoing) with a peak of 8 × 10 42 ergs −1 . The latter event reached an absolute Rband magnitude of about -18, comparable to that of a core-collapse supernova (SN). Our historical monitoring has detected high-velocity spectral features (∼13000 km s −1 ) in September 2011, one year before the current SN-like event. This implies that the detection of such high velocity outflows cannot, conclusively, point to a core-collapse SN origin. We suggest that the initial peak in the 2012a event was unlikely to be due to a faint core-collapse SN. We propose that the high intrinsic luminosity of the latest peak, the variability history of SN 2009ip, and the detection of broad spectral lines indicative of high-velocity ejecta are consistent with a pulsational pair-instability event, and that the star may have survived the last outburst. The question of the survival of the LBV progenitor star and its future fate remain open issues, only to be answered with future monitoring of this historically unique explosion.

Extended optical and near-IR observations reveal that SN 2009dc shares a number of similarities with normal Type Ia supernovae (SNe Ia), but is clearly overluminous, with a (pseudobolometric) peak luminosity of log (L) = 43.47 (erg s −1 ). Its light curves decline slowly over half a year after maximum light [ m 15 (B) true = 0.71], and the early-time near-IR light curves show secondary maxima, although the minima between the first and the second peaks are not very pronounced. The bluer bands exhibit an enhanced fading after ∼200 d, which might be caused by dust formation or an unexpectedly early IR catastrophe. The spectra of SN 2009dc are dominated by intermediate-mass elements and unburned material at early times, and by iron-group elements at late phases. Strong C II lines are present until ∼2 weeks past maximum, which is unprecedented in thermonuclear SNe. The ejecta velocities are significantly lower than in normal and even subluminous SNe Ia. No signatures of interaction with a circumstellar medium (CSM) are found in the spectra. Assuming that the light curves are powered by radioactive decay, analytic modelling suggests that SN 2009dc produced ∼1.8 M of 56 Ni assuming the smallest possible rise time of 22 d. Together with a derived total ejecta mass of ∼2.8 M , this confirms that SN 2009dc is a member of the class of possible super-Chandrasekhar-mass SNe Ia similar to SNe 2003fg, 2006gz and 2007if. A Based on observations at ESO La Silla, Prog. 083.D-0970 and 184.D-1140 and ESO Paranal, Prog. 083.D-0728.

We present observations of SN 2015bn (= PS15ae = CSS141223-113342+004332 = MLS150211-113342+004333), a Type I superluminous supernova (SLSN) at redshift z = 0.1136. As well as being one of the closest SLSNe I yet discovered, it is intrinsically brighter (M U ≈ −23.1) and in a fainter galaxy (M B ≈ −16.0) than other SLSNe at z ∼ 0.1. We used this opportunity to collect the most extensive dataset for any SLSN I to date, including densely-sampled spectroscopy and photometry, from the UV to the NIR, spanning −50 to +250 days from optical maximum. SN 2015bn fades slowly, but exhibits surprising undulations in the light curve on a timescale of 30-50 days, especially in the UV. The spectrum shows extraordinarily slow evolution except for a rapid transformation between +7 and +20-30 days. No narrow emission lines from slow-moving material are observed at any phase. We derive physical properties including the bolometric luminosity, and find slow velocity evolution and non-monotonic temperature and radial evolution. A deep radio limit rules out a healthy off-axis gamma-ray burst, and places constraints on the pre-explosion mass loss. The data can be consistently explained by a 10 M stripped progenitor exploding with ∼ 10 51 erg kinetic energy, forming a magnetar with arXiv:1603.04748v3 [astro-ph.SR] 22 Jul 2016 a spin-down timescale of ∼ 20 d (thus avoiding a gamma-ray burst) that reheats the ejecta and drives ionization fronts. The most likely alternative scenario -interaction with ∼ 20 M of dense, inhomogeneous circumstellar material -can be tested with continuing radio follow-up.

We assemble a sample of 24 hydrogen-poor super-luminous supernovae (SLSNe). Parameterizing the light curve shape through rise and decline timescales shows that the two are highly correlated. Magnetar-powered models can reproduce the correlation, with the diversity in rise and decline rates driven by the diffusion timescale. Circumstellar interaction models can exhibit a similar rise-decline relation, but only for a narrow range of densities, which may be problematic for these models. We find that SLSNe are approximately 3.5 magnitudes brighter and have light curves 3 times broader than SNe Ibc, but that the intrinsic shapes are similar. There are a number of SLSNe with particularly broad light curves, possibly indicating two progenitor channels, but statistical tests do not cleanly separate two populations. The general spectral evolution is also presented. Velocities measured from Fe II are similar for SLSNe and SNe Ibc, suggesting that diffusion time differences are dominated by mass or opacity. Flat velocity evolution in most SLSNe suggests a dense shell of ejecta. If opacities in SLSNe are similar to other SNe Ibc, the average ejected mass is higher by a factor 2-3. Assuming κ = 0.1 cm 2 g −1 , we estimate a mean (median) SLSN ejecta mass of 10 M (6 M ), with a range of 3-30 M . Doubling the assumed opacity brings the masses closer to normal SNe Ibc, but with a high-mass tail. The most probable mechanism for generating SLSNe seems to be the core-collapse of a very massive hydrogen-poor star, forming a millisecond magnetar.