Aspherical Explosion Models for SN 1998bw/GRB 980425

The recent discovery of the unusual supernova SN1998bw and its apparent correlation with the gamma-ray burst GRB 980425 has raised new issues concerning both the GRB and supernovae. Although the spectra resemble those of TypeIc supernovae, there are distinct differences at early times and SN1998bw appeared to be unusually bright and red at maximum light. The apparent expansion velocities inferred by the Doppler shift of (unidentified) absorption features appeared to be high, making SN1998bw a possible candidate for a "hypernova" with explosion energies between 20 and 50E51 erg and ejecta masses in excess of 6 - 15 M_o. Based on light curve calculations for aspherical explosions and guided by the polarization observations of "normal" SNIc and related events, we present an alternative picture that allows SN1998bw to have an explosion energy and ejecta mass consistent with core collapse supernovae (although at the 'bright' end). We show that the LC of SN1998bw can be understood as result of an aspherical explosion along the rotational axis of a basically spherical, non-degenerate C/O core of massive star with an explosion energy of 2foe and a total ejecta mass of 2 M_o if it is seen from high inclinations with respect to the plane of symmetry. In this model, the high expansion velocities are a direct consequence of an aspherical explosion which, in turn, produces oblate iso-density contours. It suggests that the fundamental core-collapse explosion process itself is strongly asymmetric.Comment: 12 pages, 8 figures, latex, aas2pp4.sty, submitted to Ap

Gamma-Ray Light Curves and Spectra of Models for Type-Ia Supernovae

Based on detailed Monte Carlo calculations, we present gamma-ray energy deposition functions, gamma-ray light curves, and gamma-ray spectra for a large set of theoretical models of Type Ia supernovae including ''classical'' detonation and deflagration, delayed detonation, explosions of low mass white dwarfs, and tamped detonation scenarios. Our computations show that models for Type Ia supernovae can be discriminated and the absolute amount of Ni-56 synthesized in the event can be determined on the basis of the gamma-ray light curves and spectra if gamma-ray measurements are combined with observations at other wavelengths, e.g., in the optical band. We discuss at which times gamma-ray observations are most suitable and needed from the theoretical point of view. The implication of the upper limit in the gamma-ray flux by CGRO experiment for our understanding of SN 1991 T is discussed. We find that this limit is consistent with both the optical light curve and the implied distance (12.5 Mpc), i.e., several models can be ruled out by the gamma-ray observations.Astronom

Properties of Deflagration Fronts and Models for Type Ia Supernovae

Detailed models of the explosion of a white dwarf, which include self-consistent calculations of the light curve and spectra, provide a link between observational quantities and the underlying explosion.These calculations assume spherical geometry and are based on parameterized descriptions of the burning front during the deflagration phase. Recently, first multi-dimensional calculations for nuclear burning fronts have been performed. Although a fully consistent treatment of the burning fronts is beyond the current state of the art, these calculations provided a new and better understanding of the physics, and new descriptions for the flame propagation have been proposed. Here, we have studied the influence on the results of previous analyses of Type Ia Supernovae, namely, the nucleosynthesis and structure of the expanding envelope. Our calculations are based on a set of delayed detonation models with parameters that give a good account of the optical and infrared light curves, and of the spectral evolution. In this scenario, the burning front propagates first in a deflagration mode and, subsequently, turns into a detonation. The explosions and light curves are calculated using a one-dimensional Lagrangian radiation-hydro code, including a detailed nuclear network.Comment: 9 pages, 4 figures, macros 'crckapb.sty'. The Astrophysical Journal (accepted

On the gamma-ray emission of Type Ia Supernovae

A multi-dimension, time-dependent Monte Carlo code is used to compute sample gamma-ray spectra to explore whether unambiguous constraints could be obtained from gamma-ray observations of Type Ia supernovae. Both spherical and aspherical geometries are considered and it is shown that moderate departures from sphericity can produce viewing-angle effects that are at least as significant as those caused by the variation of key parameters in one-dimensional models. Thus gamma-ray data could in principle carry some geometrical information, and caution should be applied when discussing the value of gamma-ray data based only on one-dimensional explosion models. In light of the limited sensitivity of current gamma-ray observatories, the computed theoretical spectra are studied to revisit the issue of whether useful constraints could be obtained for moderately nearby objects. The most useful gamma-ray measurements are likely to be of the light curve and time-dependent hardness ratios, but sensitivity higher than currently available, particularly at relatively hard energies (~2-3 MeV), is desirable.Comment: 10 pages, 8 figures. Accepted by MNRAS. Minor changes to clarify discussion in Section

Low Carbon Abundance in Type Ia Supernovae

We investigate the quantity and composition of unburned material in the outer layers of three normal Type Ia supernovae (SNe Ia): 2000dn, 2002cr and 20 04bw. Pristine matter from a white dwarf progenitor is expected to be a mixture of oxygen and carbon in approximately equal abundance. Using near-infrared (NIR, 0.7-2.5 microns) spectra, we find that oxygen is abundant while carbon is severely depleted with low upper limits in the outer third of the ejected mass. Strong features from the OI line at rest wavelength = 0.7773 microns are observed through a wide range of expansion velocities approx. 9,000 - 18,000 km/s. This large velocity domain corresponds to a physical region of the supernova with a large radial depth. We show that the ionization of C and O will be substantially the same in this region. CI lines in the NIR are expected to be 7-50 times stronger than those from OI but there is only marginal evidence of CI in the spectra and none of CII. We deduce that for these three normal SNe Ia, oxygen is more abundant than carbon by factors of 100 - 1,000. MgII is also detected in a velocity range similar to that of OI. The presence of O and Mg combined with the absence of C indicates that for these SNe Ia, nuclear burning has reached all but the extreme outer layers; any unburned material must have expansion velocities greater than 18,000 km/s. This result favors deflagration to detonation transition (DD) models over pure deflagration models for SNe Ia.Comment: accepted for publication in Ap

Three Dimensional Simulation of Gamma Ray Emission from Asymmetric Supernovae and Hypernovae

Hard X- and $\gamma$-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 $\gamma$-ray opacity for computing the energy deposition rate in the aspherical SN ejecta. The 3D detailed computations show that the effective $\gamma$-ray opacity $\kappa_{\gamma} \sim 0.025 - 0.027$ cm$^{2}$ g$^{-1}$ reproduces the detailed energy-dependent transport for both spherical and aspherical (jet-like) geometry.Comment: 24 pages, 13 figures. Figure 7 added in the accepted version. ApJ, 644 (01 June 2006 issue), in press. Resolution of figures lower than the published versio

Asymmetric Supernovae, Pulsars, Magnetars, and Gamma-Ray Bursts

We outline the possible physical processes, associated timescales, and energetics that could lead to the production of pulsars, jets, asymmetric supernovae, and weak gamma-ray bursts in routine circumstances and to a magnetar and perhaps stronger gamma-ray burst in more extreme circumstances in the collapse of the bare core of a massive star. The production of a LeBlanc-Wilson MHD jet could provide an asymmetric supernova and result in a weak gamma-ray burst when the jet accelerates down the stellar density gradient of a hydrogen-poor photosphere. The matter-dominated jet would be formed promptly, but requires 5 to 10 s to reach the surface of the progenitor of a Type Ib/c supernova. During this time, the newly-born neutron star could contract, spin up, and wind up field lines or turn on an alpha-Omega dynamo. In addition, the light cylinder will contract from a radius large compared to the Alfven radius to a size comparable to that of the neutron star. This will disrupt the structure of any organized dipole field and promote the generation of ultrarelativistic MHD waves (UMHDW) at high density and Large Amplitude Electromagnetic Waves (LAEMW) at low density. The generation of the these waves would be delayed by the cooling time of the neutron star about 5 to 10 seconds, but the propagation time is short so the UMHDW could arrive at the surface at about the same time as the matter jet. In the density gradient of the star and the matter jet, the intense flux of UMHDW and LAEMW could drive shocks, generate pions by proton-proton collision, or create electron/positron pairs depending on the circumstances. The UMHDW and LAEMW could influence the dynamics of the explosion and might also tend to flow out the rotation axis to produce a collimated gamma-ray burst.Comment: 31 pages, LaTeX, revised for referee comments, accepted for ApJ, July 10 issu

SN 2005hj: Evidence for Two Classes of Normal-Bright SNe Ia and Implications for Cosmology

HET Optical spectra covering the evolution from about 6 days before to about 5 weeks after maximum light and the ROTSE-IIIb unfiltered light curve of the "Branch-normal" Type Ia Supernova SN 2005hj are presented. The host galaxy shows HII region lines at redshift of z=0.0574, which puts the peak unfiltered absolute magnitude at a somewhat over-luminous -19.6. The spectra show weak and narrow SiII lines, and for a period of at least 10 days beginning around maximum light these profiles do not change in width or depth and they indicate a constant expansion velocity of ~10,600 km/s. We analyzed the observations based on detailed radiation dynamical models in the literature. Whereas delayed detonation and deflagration models have been used to explain the majority of SNe Ia, they do not predict a long velocity plateau in the SiII minimum with an unvarying line profile. Pulsating delayed detonations and merger scenarios form shell-like density structures with properties mostly related to the mass of the shell, M_shell, and we discuss how these models may explain the observed SiII line evolution; however, these models are based on spherical calculations and other possibilities may exist. SN 2005hj is consistent with respect to the onset, duration, and velocity of the plateau, the peak luminosity and, within the uncertainties, with the intrinsic colors for models with M_shell=0.2 M_sun. Our analysis suggests a distinct class of events hidden within the Branch-normal SNe Ia. If the predicted relations between observables are confirmed, they may provide a way to separate these two groups. We discuss the implications of two distinct progenitor classes on cosmological studies employing SNe Ia, including possible differences in the peak luminosity to light curve width relation.Comment: ApJ accepted, 31 page