Hypernova

Abstract: Hard X- and $γ$-ray spectra and light curves resulting from radioactive decays are computed for aspherical (jet-like) and energetic supernova models (representing a prototypical hypernova SN 1998bw), using a 3D energy- and time-dependent Monte Carlo scheme. The emission is characterized by (1) early emergence of high energy emission, (2) large line-to-continuum ratio, and (3) large cut-off energy by photoelectric absorptions in hard X-ray energies. These three properties are not sensitively dependent on the observer's direction. On the other hand, fluxes and line profiles depend sensitively on the observer's direction, showing larger luminosity and larger degree of blueshift for an observer closer to the polar ($z$) direction. Strategies to derive the degree of asphericity and the observer's direction from (future) observations are suggested on the basis of these features, and an estimate on detectability of the high energy emission by the {\it INTEGRAL} and future observatories is presented. Also presented is examination on applicability of a gray effective $γ$-ray opacity for computing the energy deposition rate in the aspherical SN ejecta. The 3D detailed computations show that the effective $γ$-ray opacity $κ_γ \sim 0.025 - 0.027$ cm$^{2}$ g$^{-1}$ reproduces the detailed energy-dependent transport for both spherical and aspherical (jet-like) geometry.

Abstract: We perform numerical hydrodynamic modeling of various physical processes that can form an HI ring as is observed in Holmberg I. Three energetic mechanisms are considered: multiple supernova explosions (SNe), a hypernova explosion associated with a gamma ray burst (GRB), and the vertical impact of a high velocity cloud (HVC). The total released energy has an upper limit of 10^54 ergs. We find that multiple SNe are in general more effective in producing shells that break out of the disk than a hypernova explosion of the same total energy. As a consequence, multiple SNe form rings with a high ring-to-center contrast K<100 in the HI column density, whereas single hypernova explosions form rings with K<10. Only multiple SNe can reproduce both the size (diameter \~1.7 kpc) and the ring-to-center contrast (K ~ 15-20) of the HI ring in Hoolmberg I. High velocity clouds create HI rings that are much smaller in size (< 0.8 kpc) and contrast (K < 4.5) than seen in Holmberg I. We construct model position-velocity (pV) diagrams and find that they can be used to distinguish among different HI ring formation mechanisms. The observed pV-diagrams of Holmberg I are best reproduced by multiple SNe. We conclude that the giant HI ring in Holmberg I is most probably formed by multiple SNe. We also find that the appearance of the SNe-driven shell in the integrated HI image depends on the inclination angle of the galaxy. In nearly face-on galaxies, the integrated HI image shows a ring of roughly constant HI column density surrounding a deep central depression, whereas in considerably inclined galaxies (i > 45 deg) the HI image is characterized by two kidney-shaped density enhancements and a mild central depression.

Abstract: Using a deep Chandra AO-1 observation of the face-on spiral galaxy M101, we examine three of five previously optically identified X-ray sources that are spatially correlated with optical supernova remnants (MF 54, MF 57, and MF 83). The X-ray fluxes from these objects, if due to diffuse emission from the remnants, are bright enough to require a new class of objects, with the possible attribution by Wang to diffuse emission from hypernova remnants. Of the three, MF 83 was considered the most likely candidate for such an object because of its size, nature, and close positional coincidence. However, we find that MF 83 is clearly ruled out as a hypernova remnant by both its temporal variability and spectrum. The bright X-ray sources previously associated with MF 54 and MF 57 are seen by Chandra to be clearly offset from the optical positions of the supernova remnants by several arcseconds, confirming a result suggested by the previous work. MF 54 does have a faint X-ray counterpart, however, with a luminosity and temperature consistent with a normal supernova remnant of its size. The most likely classifications of the sources are as X-ray binaries. Although counting statistics are limited, over the 0.3–5.0 keV spectral band the data are well fitted by simple absorbed power laws with luminosities in the 1038–1039 ergs s-1 range. The power-law indices are softer than those of Milky Way low-mass X-ray binaries of similar luminosities and are more consistent with those of the Large Magellanic Cloud. Both the high luminosity and the soft spectral shape favor these being accreting black hole binaries in high soft states.

Abstract: Based on an ultradeep (230 ks) ROSAT HRI imaging of M101, we have detected five X-ray sources that coincide spatially with optical emission line features previously classified as supernova remnants (SNRs) in this nearby galaxy. Two of these coincidences (SNR MF 83 and NGC 5471B) most likely represent the true physical association of X-ray emission with shock-heated interstellar gas. MF 83, with a radius of ~134 pc, is one of the largest remnants known. NGC 5471B, with a radius of 30 pc and a velocity of at least 350 km s-1 (FWZI), is extremely bright in both radio and optical. The X-ray luminosities of these two shell-like remnants are ~1 and 3×1038 ergs s-1 (0.5-2 keV), about an order of magnitude brighter than the brightest supernova remnants known in our Galaxy and in the Magellanic Clouds. The inferred blast-wave energy is ~3×1052 ergs for NGC 5471B and ~3×1053 ergs for MF 83. Therefore, the remnants likely originate in hypernovae, which are a factor of 10 more energetic than canonical supernovae and are postulated as being responsible for gamma-ray bursts observed at cosmological distances. The study of such hypernova remnants in nearby galaxies has the potential to provide important constraints on the progenitor type, rate, energetics, and beaming effect of gamma-ray bursts.