The death of massive stars produces a variety of supernovae, which are linked to the structure of the exploding stars. The detection of several precursor stars of type II supernovae has been reported (see, for example, ref. 3), but we do not yet have direct information on the progenitors of the hydrogen-deficient type Ib and Ic supernovae. Here we report that the peculiar type Ib supernova SN 2006jc is spatially coincident with a bright optical transient that occurred in 2004. Spectroscopic and photometric monitoring of the supernova leads us to suggest that the progenitor was a carbon-oxygen Wolf-Rayet star embedded within a helium-rich circumstellar medium. There are different possible explanations for this pre-explosion transient. It appears similar to the giant outbursts of luminous blue variable stars (LBVs) of 60-100 solar masses, but the progenitor of SN 2006jc was helium- and hydrogen-deficient (unlike LBVs). An LBV-like outburst of a Wolf-Rayet star could be invoked, but this would be the first observational evidence of such a phenomenon. Alternatively, a massive binary system composed of an LBV that erupted in 2004, and a Wolf-Rayet star exploding as SN 2006jc, could explain the observations.

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Ultraluminous x-ray sources (ULXs) in nearby galaxies shine brighter than any X-ray source in our Galaxy. ULXs are usually modeled as stellar-mass black holes (BHs) accreting at very high rates or intermediate-mass BHs. We present observations showing that NGC 5907 ULX is instead an x-ray accreting neutron star (NS) with a spin period evolving from 1.43 s in 2003 to 1.13 s in 2014. It has an isotropic peak luminosity of ∼1000 times the Eddington limit for a NS at 17.1 Mpc. Standard accretion models fail to explain its luminosity, even assuming beamed emission, but a strong multipolar magnetic field can describe its properties. These findings suggest that other extreme ULXs (x-ray luminosity ≥1041 erg s −1 ) might harbor NSs.Ultraluminous x-ray sources (ULXs) are observed in off-nucleus regions of nearby galaxiesand have x-ray luminosities in excess of a few 10 39 erg s −1 , which is the Eddington luminosity (L Edd ) for a black hole (BH) of 10 M (1). The L Edd sets an upper limit on the accretion luminosity (L acc ) of a compact object steadily accreting, since for L acc > L Edd accretion will be halted by radiation forces. For spherical accretion of fully ionized hydrogen, the limit can be written as, where σ T is the Thomson scattering cross section, m p is the proton mass, and M/M is the compact object mass in solar masses; for a 1.4 M neutron star (NS), the maximum accreting luminosity is ∼2×10 38 erg s −1 .The high luminosity of ULXs has thus been explained as accretion at or above the Eddington luminosity onto BHs of stellar origin (<80-100 M ), or onto intermediate-mass (10BHs (2, 3). However, if the emission of ULXs were beamed over a fraction b < 1 of the sky, their true luminosity, and thus also the compact object mass required not to exceed L Edd , would be reduced by the same factor. This possibility, together with the recent identification of two accreting NSs associated with the ∼10 40 erg s −1 M82 X-2 (4) and NGC 7793 P13 (5, 6) x-ray sources, have brought support to the view that most low-luminosity ULXs likely host a NS (7) 2 or a stellar-mass BH (8). For the most extreme ULXs with x-ray luminosity exceeding a few ×10 40 erg s −1 , BHs with masses in excess of 100 M are still commonly considered (9, 10).Despite several searches for coherent x-ray pulsations,no other ultraluminous x-ray source has been found to host a NS so far (11).Within the framework of "Exploring the X-ray Transient and variable Sky", EXTraS (12) Fig. 1 and Table 1). In all cases, a strong first period derivative term is present (see Table 1). The pulse shape is nearly sinusoidal, while the pulsed fraction (the semi-amplitude of the sinusoid divided by the average count rate)is energy dependent and increases from about 12% at low energies (<2.5 keV) to ∼20% in the hard band (>7 keV; Fig. 1).To derive constraints on the orbital period (P orb ), we applied a likelihood analysis to the two 2014 NuSTAR observations (see supplementary online text), which have the longest baseline. 3By assuming a circular orbit (as in the case of M...

In this paper we present spectroscopic and photometric observations for four core‐collapsed supernovae (SNe), namely SNe 1994N, 1999br, 1999eu and 2001dc. Together with SN 1997D, we show that they form a group of exceptionally low‐luminosity events. These SNe have narrow spectral lines (indicating low expansion velocities) and low luminosities at every phase (significantly lower than those of typical core‐collapsed supernovae). The very‐low luminosity during the 56Co radioactive decay tail indicates that the mass of 56Ni ejected during the explosion is much smaller (MNi≈ 2–8 × 10−3 M⊙) than the average (MNi≈ 6–10 × 10−2 M⊙). Two supernovae of this group (SN 1999br and SN 2001dc) were discovered very close to the explosion epoch, allowing us to determine the lengths of their plateaux (≈100 d) as well as establishing the explosion epochs of the other, less completely observed SNe. It is likely that this group of SNe represent the extreme low‐luminosity tail of a single continuous distribution of Type II plateau supernovae events. Their kinetic energy is also exceptionally low. Although an origin from low‐mass progenitors has also been proposed for low‐luminosity core‐collapsed SNe, recent work provides evidence in favour of the high‐mass progenitor scenario. The incidence of these low‐luminosity SNe could be as high as 4–5 per cent of all Type II SNe.

We present the results of the one‐year long observational campaign of the type II plateau SN 2005cs, which exploded in the nearby spiral galaxy M51 (the Whirlpool galaxy). This extensive data set makes SN 2005cs the best observed low‐luminosity, 56Ni‐poor type II plateau event so far and one of the best core‐collapse supernovae ever. The optical and near‐infrared spectra show narrow P‐Cygni lines characteristic of this SN family, which are indicative of a very low expansion velocity (about 1000 km s−1) of the ejected material. The optical light curves cover both the plateau phase and the late‐time radioactive tail, until about 380 d after core‐collapse. Numerous unfiltered observations obtained by amateur astronomers give us the rare opportunity to monitor the fast rise to maximum light, lasting about 2 d. In addition to optical observations, we also present near‐infrared light curves that (together with already published ultraviolet observations) allow us to construct for the first time a reliable bolometric light curve for an object of this class. Finally, comparing the observed data with those derived from a semi‐analytic model, we infer for SN 2005cs a 56Ni mass of about 3 × 10−3 M⊙, a total ejected mass of 8–13 M⊙ and an explosion energy of about 3 × 1050 erg.

The results of a worldwide coordinated observational campaign on the broad‐lined Type Ic supernova (SN Ic) 2003jd are presented. In total, 74 photometric data points and 26 spectra were collected using 11 different telescopes. SN 2003jd is one of the most luminous SN Ic ever observed. A comparison with other Type Ic supernovae (SNe Ic) confirms that SN 2003jd represents an intermediate case between broad‐line events (2002ap, 2006aj) and highly energetic SNe (1997ef, 1998bw, 2003dh, 2003lw), with an ejected mass of Mej= 3.0 ± 1 M⊙ and a kinetic energy of Ek(tot) = 7+3−2× 1051 erg. SN 2003jd is similar to SN 1998bw in terms of overall luminosity, but it is closer to SNe 2006aj and 2002ap in terms of light‐curve shape and spectral evolution. The comparison with other SNe Ic suggests that the V‐band light curves of SNe Ic can be partially homogenized by introducing a time‐stretch factor. Finally, because of the similarity of SN 2003jd to the SN 2006aj/XRF 060218 event, we discuss the possible connection of SN 2003jd with a gamma‐ray burst (GRB).

The final fate of massive stars depends on many factors. Theory suggests that some with initial masses greater than 25 to 30 solar masses end up as Wolf-Rayet stars, which are deficient in hydrogen in their outer layers because of mass loss through strong stellar winds. The most massive of these stars have cores which may form a black hole and theory predicts that the resulting explosion of some of them produces ejecta of low kinetic energy, a faint optical luminosity and a small mass fraction of radioactive nickel. An alternative origin for low-energy supernovae is the collapse of the oxygen-neon core of a star of 7-9 solar masses. No weak, hydrogen-deficient, core-collapse supernovae have hitherto been seen. Here we report that SN 2008ha is a faint hydrogen-poor supernova. We propose that other similar events have been observed but have been misclassified as peculiar thermonuclear supernovae (sometimes labelled SN 2002cx-like events). This discovery could link these faint supernovae to some long-duration gamma-ray bursts, because extremely faint, hydrogen-stripped core-collapse supernovae have been proposed to produce such long gamma-ray bursts, the afterglows of which do not show evidence of associated supernovae.

A number of supernovae, classified as Type II, show remarkably peculiar properties such as an extremely low expansion velocity and an extraordinarily small amount of 56Ni in the ejecta. We present a joint analysis of the available observations for two of these peculiar Type II supernovae, SN 1997D and SN 1999br, using a comprehensive semi‐analytic method that can reproduce the light curve and the evolution of the line velocity and continuum temperature. We find that these events are underenergetic with respect to a typical Type II supernova and that the inferred mass of the ejecta is relatively large. We discuss the possibility that these supernovae originate from the explosion of a massive progenitor in which the rate of early infall of stellar material on the collapsed core is large. Events of this type could form a black hole remnant, giving rise to significant fallback and late‐time accretion.