Hypernova

Abstract: It is generally regarded that the bulk of cosmic rays originate in the Galaxy and that those below the 'knee' (the rapid steepening in the energy spectrum) at a few PeV come from Galactic supernovae, the particles being accelerated by the shocks in the supernova remnants. At higher energies, there are problems in that conventional SNR - which surely constitute the bulk of the sources - have a natural limit at a few tens of PeV (for iron nuclei). The question of the origin of particles above this limit is thus an open one. Here we examine a number of possibilities: a variety of supernovae and hypernovae, pulsars, a Giant Galactic Halo and an Extragalactic origin. A relevant property of any model is the extent to which it can provide the lack of significant irregularity of the energy spectrum. Although it is appreciated that spectral measurements are subject to systematic as well as random errors we consider that contemporary data are good enough to allow at least some progress in this field. In the search for origin above PeV energies we conclude that shocks in the Galactic Halo, whatever their source (Galactic wind, relativistic plasmoids - 'cannonballs', multiple shocks from supernovae etc.) are most likely, pulsrs such as B0656+14 and hypernovae come a close second although such a suggestion is not without its difficulties. What is most important is that trapping of particles in the Halo is needed to reduce irregularities of the energy spectra both below and above the 'knee' caused by the stochastic nature of supernova explosions and other potential (discrete) Galactic sources. We argue that precise experimental studies of spectral 'irregularities' will provide considerable help in the search for cosmic ray origin.

Abstract: Type Ia supernovae, used as distance indicators by cosmologists, result from thermonuclear explosions of white dwarf stars in binary systems. Important questions remain about how the explosions proceed and the nature of the progenitors. A nearby example would be a help in finding the answers; now we have one. The recent discovery of light echoing from Tycho Brahe's supernova of 1572, and now the determination of its optical spectrum, confirm the suspicion that 'SN 1572' is in fact a type Ia supernova in our cosmological backyard, the Milky Way. This puts stringent constraints on explosion models that can now be compared in detail to observations of both the explosion 436 years ago and the remnant as we see it today. This study reports an optical spectrum of Tycho Brahe's supernova near maximum brightness, obtained from a scattered-light echo more than four centuries after the direct light of the explosion swept past Earth. It is found that SN 1572 belongs to the majority class of normal type Ia supernovae. Type Ia supernovae are thermonuclear explosions of white dwarf stars in close binary systems1. They play an important role as cosmological distance indicators and have led to the discovery of the accelerated expansion of the Universe2,3. Among the most important unsolved questions4 about supernovae are how the explosion actually proceeds and whether accretion occurs from a companion or by the merging of two white dwarfs. Tycho Brahe’s supernova of 1572 (SN 1572) is thought to be one of the best candidates for a type Ia supernova in the Milky Way5. The proximity of the SN 1572 remnant has allowed detailed studies, such as the possible identification of the binary companion6, and provides a unique opportunity to test theories of the explosion mechanism and the nature of the progenitor. The determination of the hitherto unknown7,8,9 spectroscopic type of this supernova is crucial in relating these results to the diverse population of type Ia supernovae10. Here we report an optical spectrum of Tycho’s supernova near maximum brightness, obtained from a scattered-light echo more than four centuries after the direct light from the explosion swept past the Earth. We find that SN 1572 belongs to the majority class of normal type Ia supernovae.

Abstract: We calculate radio-to-X-ray light curves for afterglows caused by nonthermal emission from a highly relativistic blast wave, which is inferred from the γ-ray flux detected in GRB 980425 and from the very bright radio emission detected in SN 1998bw. We find that the observed γ-ray and radio light curves are roughly reproduced by the synchrotron emission from a relativistic fireball. The optical flux predicted for the nonthermal emission is well below that of the thermal emission observed for SN 1998bw so that it will not be seen at least for a few years. The model predicts the X-ray flux just above the detection limit of BeppoSAX for the epoch when it was pointed to the field of GRB 980425. Therefore, the nondetection of X-ray and optical afterglows is consistent with the model. The models presented here are consistent with the physical association between SN 1998bw and GRB 980425 and lend further support to the idea that this object might correspond to an event similar to the "hypernova" or "collapsar"—events in which the collapse of a massive star forms a rotating black hole surrounded by a disk of the remnant stellar mantle.

Abstract: The observation of a strong and extended positron-electron line annihilation emission in the central regions of the Galaxy by the SPectrometer on the International Gamma-Ray Astrophysical Laboratory (INTEGRAL/SPI), consistent with the Galactic bulge geometry and without any counterpart in the gamma-ray range, neither at high energy nor in the 1809 keV 26Al decay line, is challenging. Leaving aside the geometrical question, we address the problem of the adequate positron sources, showing the potentiality of a new category of Type Ic supernovae (SNe Ic), exemplified by SN 2003dh, that is associated with a gamma-ray burst (GRB). This kind of supernova/hypernova/GRB event is interpreted as the result of a bipolar Wolf-Rayet explosion, which produces a large amount of 56Ni and ejects it at high velocity along the rotation axis. The bulk of positrons resulting from 56Co decay escapes in the surrounding medium as a result of the rapid thinning of the ejecta in the polar direction. We show that a rate of about 0.02 SN 2003dh-like events per century in the central region of the Galaxy is sufficient to explain the positron flux detected by INTEGRAL/SPI. In order to explain this flux by SN Ia events alone, a rate of 0.5 per century is necessary, much higher than indicated by Galactic evolutionary models applied to the bulge. Further observations of late light curves of SNe Ia and SNe Ic in the bulge of spiral galaxies, together with three-dimensional hydrodynamic calculations of anisotropic ejections of 56Ni in SN Ic/GRB events, will allow us to estimate the separate contributions of SNe Ia and SNe Ic to positron injection.