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Author(s): E. Pian (corresponding author) [1]; P. DAvanzo [2]; S. Benetti [3]; M. Branchesi [4, 5]; E. Brocato [6]; S. Campana [2]; E. Cappellaro [3]; S. Covino [2]; V. DElia [...]

The discovery of a luminous transient, PTF 12dam, was first reported (15) by the Palomar Transient Factory on 23 May 2012. We recovered the transient in Pan-STARRS1 (Panoramic Survey Telescope [...], Super-luminous supernovae (1-4) that radiate more than [10.sup.44] ergs per second at their peak luminosity have recently been discovered in faint galaxies at redshifts of 0.1-4. Some evolve slowly, resembling models of 'pair-instability' supernovae (5,6). Such models involve stars with original masses 140-260 times that of the Sun that now have carbon-oxygen cores of 65-130 solar masses. In these stars, the photons that prevent gravitational collapse are converted to electron-positron pairs, causing rapid contraction and thermonuclear explosions. Many solar masses of [sup.56]Ni are synthesized; this isotope decays to [sup.56]Fe via [sup.56]Co, powering bright light curves (7,8). Such massive progenitors are expected to have formed from metal-poor gas in the early Universe (9). Recently, supernova 2007 bi in a galaxy at redshift 0.127 (about 12 billion years after the Big Bang) with a metallicity one-third that of the Sun was observed to look like a fading pair-instability supernova (1,10). Here we report observations of two slow-to-fade super-luminous supernovae that show relatively fast rise times and blue colours, which are incompatible with pair-instability models. Their late-time light-curve and spectral similarities to supernova 2007bi call the nature of that event into question. Our early spectra closely resemble typical fast-declining super-luminous supernovae (2,11,12), which are not powered by radioactivity. Modelling our observations with 10-16 solar masses of magnetar-energized (13,14) ejecta demonstrates the possibility of a common explosion mechanism. The lack of unambiguous nearby pair-instability events suggests that their local rate of occurrence is less than 6 x [10.sup.-6] times that of the core-collapse rate.

When a carbon-oxygen white dwarf reaches a critical limit known as the Chandrasekhar mass (~ 1.38 solar masses), its central density and temperature increase to a point where a thermonuclear [...], Type Ia supernovae form an observationally uniform class of stellar explosions, in that more luminous objects have smaller declinerates (1). This one-parameter behaviour allows type Ia supernovae to be calibrated as cosmological 'standard candles', and led to the discovery of an accelerating Universe (2,3). Recent investigations, however, have revealed that the true nature of type Ia supernovae is more complicated. Theoretically, it has been suggested (4-8) that the initial thermonuclear sparks are ignited at an offset from the centre of the white-dwarf progenitor, possibly as a result of convection before the explosion (4). Observationally, the diversity seen in the spectral evolution of type Ia supernovae beyond the luminosity-decline-rate relation is an unresolved issue (9,10). Here we report that the spectral diversity is a consequence of random directions from which an asymmetric explosion is viewed. Our findings suggest that the spectral evolution diversity is no longer a concern when using type Ia supernovae as cosmological standard candles. Furthermore, this indicates that ignition at an offset from the centre is a generic feature of type Ia supernovae.

SN 2008ha was discovered on 7 Nov. 2008 4.1 h uT in the late-type galaxy UGC 12682, at an unfiltered magnitude of 18.8 (ref. 10). A spectrum obtained on 18.18 [...], 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 ejects of low kinetic energy, a faint optical luminosity and a small mass fraction of radioactive nickel (1-3). An alternative origin for low-energy supernovae is the collapse of the oxygen-neon core of a star of 7-9 solar masses (4,5). 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 evens (6)). This discovery could link these faint supernovae to some long-duration γ-ray bursts, because extremely faint, hydrogen-stripped core-collapse supernovae have been proposed to produce such long γ-ray bursts, the afterglows of which do not show evidence of associated supernovae (7-9).

Author(s): P. J. Talling (corresponding author) [1]; R. B. Wynn [2]; D. G. Masson [2]; M. Frenz [2]; B. T. Cronin [3]; R. Schiebel [2]; A. M. Akhmetzhanov [2]; S. [...]

Author(s): A. Pastorello (corresponding author) [1]; S. J. Smartt [1]; S. Mattila [1]; J. J. Eldridge [1]; D. Young [1]; K. Itagaki [2]; H. Yamaoka [3]; H. Navasardyan [4]; S. [...]

Author(s): M. Nicholl (corresponding author); S. J. Smartt; A. Jerkstrand; C. Inserra; M. McCrum; R. Kotak; M. Fraser; D. Wright; T.-W. Chen; K. Smith; D. R. Young; S. A. Sim; [...]