Abstract

Bright outbursts from Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) are believed to be caused by instabilities in ultramagnetized neutron stars, powered by a decaying magnetic field. It was originally thought that these outbursts were due to reconnection instabilities in the magnetosphere, reached via slow evolution of magnetic footpoints anchored in the crust. Later models considered sudden shifts in the crust's structure. Recent observations of magnetars give evidence that at least some outburst episodes involve rearrangements and/or energy releases within the star. We suggest that bursting episodes in magnetars are episodes of rapid plastic yielding in the crust, which trigger “swarms” of reconnection instabilities in the magnetosphere. Magnetic energy always dominates; elastic energy released within the crust does not generate strong enough Alfvén waves to power outbursts. We discuss the physics of SGR giant flares, and describe recent observations that give useful constraints and clues.

Introduction: a neutron star's crust

The crust of a neutron star has several components: (1) a Fermi sea of relativistic electrons, which provides most of the pressure in the outer layers; (2) another Fermi sea of neutrons in a pairing-superfluid state, present only at depths below the “neutron drip” level where the mass-density exceeds ρdrip ≈ 4.6 × 1011 gm cm-3; and (3) an array of positively-charged nuclei, arranged in a solid (but probably not regular crystalline) lattice-like structure throughout much of the crust.

Abstract

Emission morphologies of young, Galactic supernova remnants can be used for investigating SN expansion dynamics, elemental distributions, and progenitor mass loss history and properties at the time of outburst. The remnants of two suspected Galactic Type Ia SNe, Tycho and SN 1006, show spherical morphologies, with Si-rich ejecta near the forward shock front suggestive of significant mixing. Searches for possible surviving binary companions near the centers of these remnants may help clarify the progenitor binary system(s) involved in SNe Ia. On the other hand, high mass, core collapse remnants, such as SNR 1987A and Cas A, exhibit strongly asymmetrical morphologies, with Cas A showing some evidence for bipolar ejecta jets. However, it is currently unclear if such ejecta jets are consistent with any of the recently proposed jet induced SN explosion models.

Introduction

For a workshop on the 3-D signatures of stellar explosions, it seems worth-while to first explain why one might be interested in the properties of supernova remnants (SNRs). Even the youngest Galactic SN remnants are hundreds and even thousands of years removed from the actual SN events, so SNRs may seem at first to be relatively poor tools for any meaningful testing of SN models or explosion theories. However, young supernova remnants, and especially the nearby Galactic ones, offer chemical and kinematic data on SN ejecta on much finer spatial scales than possible from extragalactic SN/SNR investigations.

Abstract

We present the first unrestricted, three-dimensional relativistic hydrodynamical calculations of the blob of gas associated with the jet producing a gamma-ray burst as applied to the time when afterglow radiation is produced. Our main findings are that (ⅰ) gas ahead of the advancing blob does not accrete onto and merge with the blob material but rather flows around the blob, (ⅱ) the decay light curve steepens at a time corresponding roughly to γ˗1 ≈ θ (in accord with earlier studies), and (ⅲ) the rate of decrease of the forward component of momentum in the blob is well-fit by a simple model in which the gas in front of the blob exerts a drag force on the blob, and the cross sectional area of the blob increases quadratically with laboratory time.

Introduction

Gamma-ray bursts are the most powerful explosions in the Universe. If GRBs were isotropic, then the measured redshifts would imply total explosion energies of ∼1052–1054 ergs (Frail et al. 2001). Theoretical work on relativistic jet expansion, however, shows that one expects a steepening in the decay light curve if one is looking down the axis of a jet as the flow decelerates from a bulk Lorentz factor γ˗1 < θ to γ˗1 > θ, where θ is the jet beaming angle (Rhoads 1999). The concept of a “break” corresponding to γ˗1 ≃ θ has been used to infer the presence of strong beaming in GRBs (Frail et al.

Abstract

This article will cover two topics at the intersection of Gamma-Ray Bursts and supernovae that have been much studied by the Texas group with relevance for the 3-D structure of core collapse explosions. The first topic is the high-velocity and high-excitation absorption lines seen in GRB 021004 (and other more recent events). These lines must come from (likely clumpy) shells around the progenitor star, and hence can provide a unique means of knowing the nature of the exploding star. In particular, the lines imply that normal GRBs form from the core collapse of a massive star, and thus that GRBs are closely related to supernovae. The second topic is the four luminosity indicators for Gamma-Ray Bursts and their implications for cosmology. The validity of the luminosity (and hence distance) indicators is already well demonstrated, although the current accuracy of the distances is roughly a factor of three times worse than for Type Ia supernovae. With GRBs serving as standard candles visible out to redshifts of >12 or farther, they can be used for many of the same purposes in cosmology now reserved for supernovae at low redshifts. With the launch of Swift in 2004, hundreds of bursts can then be used to construct Hubble diagrams to z ≥ 5, to measure the star formation rate to z ∼ 12 or farther, and to serve as beacons for discovering the Gunn-Peterson effect.

GRB/SN connections

Gamma-Ray Bursts (GRBs) and supernovae (SNe) have long been connected. Before the discovery of GRBs, S.

Abstract

Observations of gamma-ray burst (GRB) afterglows have yielded tantalizing hints that supernovae (SNe) and GRBs are related. The case had been circumstantial, though, relying on irregularities in the light curve or the colors of the afterglow. I will present observations of the optical afterglow of GRB 030329. The early spectra show a power-law continuum, consistent with other GRB afterglows. After approximately one week, broad peaks in the spectrum developed that were remarkably similar to those seen in the spectra of the peculiar Type Ic SN 1998bw. This is the first direct, spectroscopic confirmation that at least some GRBs arise from SNe.

Introduction

The mechanism that produces gamma-ray bursts (GRBs) has been the subject of considerable speculation during the four decades since their discovery (see Mészáros 2002 for a recent review of the theories of GRBs). Optical afterglows (e.g., GRB 970228: Groot et al. 1997; van Paradijs et al. 1997) opened a new window on the field (see, e.g., van Paradijs, Kouveliotou, & Wijers 2000). Subsequent studies of other bursts yielded the redshifts of several GRBs (e.g., GRB 970508: Metzger et al. 1997), providing definitive evidence for their cosmological origin.

Models that invoked supernovae (SNe) to explain GRBs were proposed from the very beginning (e.g., Colgate 1968; Woosley 1993; Woosley & MacFadyen 1999). A strong hint was provided by GRB 980425. In this case, no traditional GRB optical afterglow was seen, but a supernova, SN 1998bw, was found in the error box of the GRB (Galama et al. 1998a).

Abstract

Multi-dimensional models of supernovae show radial and non-radial variation in the density and composition not seen in one-dimensional models. Many of the questions about the flow of radiation through the expanding, multi-dimensional atmosphere will require multi-dimensional radiation transport calculations, but some may be tested with existing one-dimensional transport codes. So far, tests with models of Type Ia supernovae have shown that the unburned fuel (C+O) mixed down into deeper layers in multi-dimensional models have only a simple (C II lines) and modest signature in the spectra. This places only light constraints on the mixing of C+O into the lower layers of Type Ia supernovae.

3-D effects on supernova spectra

The proliferation of multi-dimensional explosion models for supernovae has made it clear that using one dimensional models for spectrum synthesis is not fully adequate. How adequate are the old 1-D spectral models? What are the multidimensional signals present in the light received from distant supernovae? Can we calculate spectra with 1-D codes that can explore the multi-dimensional effects?

The clearest multi-dimensional signal in spectrum of supernova is polarization. The number of polarization observations of supernovae have increased dramatically in the last decade, and polarization has now been detected in all types and sub-types of supernovae. Since it is impossible to calculate polarization in the 1-D models we calculate, we will not discuss polarization further, but Kasen discusses polarization elsewhere in this volume.

Abstract

As we investigate the manifestly multi-dimensional nature of core collapse supernovae, the connection between microscopic physics and macroscopic fluid motion must not be forgotten. As an example, we discuss nuclear electron capture and its impact on the supernova shock. Though electron capture on nuclei with masses larger than 60 is the most important nuclear interaction to the dynamics of stellar core collapse, in prior simulations of core collapse it has been treated in a highly parameterized fashion, if not ignored. With a realistic treatment of electron capture on heavy nuclei come significant changes in the hydrodynamics of core collapse and bounce. We discuss these as well as their ramifications for the post-bounce evolution in core collapse supernovae.

Introduction

The many observations of asymmetries in core collapse supernovae, coupled with the failure of spherically symmetric simulations of the neutrino reheating paradigm to produce explosions, has persuaded the community that multidimensional effects like convection and other fluid instabilities must be vital elements of the supernova mechanism (Wilson & Mayle 1993, Herant et al. 1994, Burrows et al. 1995, Fryer & Warren 2002) though, even with these convective enhancements, explosions are not guaranteed (Janka & Müller 1996, Mezzacappa et al. 1998, Buras et al. 2003). This view has been reinforced in recent years by the failure of more accurate spherically symmetric multigroup Boltzmann simulations to produce explosions (Rampp & Janka 2000, Mezzacappa et al. 2001, Liebendörfer et al. 2001, Thompson et al. 2003).

Abstract

We review recent progress in the theory of jet production, with particular emphasis on the possibility of 1) powerful jets being produced in the first few seconds after collapse of a supernova core and 2) those jets being responsible for the asymmetric explosion itself. The presently favored jet-production mechanism is an electrodynamic one, in which charged plasma is accelerated by electric fields that are generated by a rotating magnetic field anchored in the protopulsar. Recent observations of Galactic jet sources provide important clues to how all such sources may be related, both in the physical mechanism that drives the jet and in the astrophysical mechanisms that create conditions conducive to jet formation. We propose a grand evolutionary scheme that attempts to unify these sources on this basis, with MHD supernovae providing the missing link. We also discuss several important issues that must be resolved before this (or another scheme) can be adopted.

Introduction: a cosmic zoo of galactic jet sources

The last few decades have seen the discovery of a large number of different types of Galactic sources that produce jets. The purpose of this talk is to show that all of these jets sources are related, in both a physical sense and an astrophysical sense. Furthermore, Craig Wheeler's idea that most core collapse supernovae (SNe) are driven by MHD jets from a protopulsar provides the missing link in an attractive unified scheme of all stellar jet sources.

Abstract

Supernovae can be polarized by an asymmetry in the explosion process, an off-center source of illumination, scattering in an envelope distorted by rotation or a binary companion, or scattering by the circumstellar dust. Careful polarimetry should thus provide insights to the nature of supernovae. Spectropolarimetry is the most powerful tool to study the 3-D geometry of supernovae. A deep understanding of the 3-D geometry of SNe is critical in using them for calibrated distance indicators.

Introduction

Polarimetry of supernovae (SNe) reveals the intrinsic ejecta asymmetries (Shapiro & Sutherland 1982, McCall 1984, Höflich 1991, 1996). SN 1987A represented a breakthrough in this area, by providing the first detailed record of the spectropolarimetric evolution (e.g. Mendez et al. 1988; Cropper et al. 1988). SN 1993J also provided a wealth of data (Trammell, Hines, & Wheeler 1993; Tran et al. 1997). Most of the theoretical interpretations of the polarimetry data are based on oblate or prolate spheroid geometries. A very different picture of SN polarization is discussed in Wang & Wheeler (1996) where time-dependent dust scattering is shown to be a potential mechanism. New attempts are being made with more complicated geometrical structures (Kasen et al. 2003; 2004).

We started a systematic program of supernova spectropolarimetry in 1995 using the 2.1 meter telescope of the McDonald Observatory which nearly doubled the number of SNe with polarimetry measurements in the first year of the program.

Introduction

This conference was packed with interesting and relevant developments regarding the three-dimensional nature of both thermonuclear and core-collapse supernovae. Before summarizing those presentations, I would like to summarize some of the developments regarding rotation and magnetic fields that were on my mind during the conference.

Dynamo theory and saturation fields

There has been a major breakthrough in the conceptual understanding of astrophysical dynamos in the last few years. In traditional mean field dynamo theory, the turbulent velocity field that drives the “alpha” portion of the α – Ω dynamo was specified and held fixed. A weakness of the original theory was that the turbulent velocity field cannot be constant. The buildup of small scale magnetic field tends to inhibit turbulence, cutting off the dynamo process for both small and large scale fields. Since the small scale field tended to grow faster than the large scale field, it appeared that the growth of the large scale field would be suppressed (Kulsrud & Anderson 1992; Gruzinov & Diamond 1994). In these theories, the magnetic field energy cascades to smaller length scales where it is ultimately dissipated at the resistive scale. Large scale fields tend to build up slowly, if at all.

The solution to this problem has been the recognition (Blackman & Field 2000; Vishniac & Cho 2001; Field & Blackman 2002; Blackman & Brandenburg, 2002; Blackman & Field 2002; Kleeorin et al. 2002) that the magnetic helicity, H = A · B is conserved in ideal MHD and that this conservation had not been treated explicitly in mean field dynamo theory.

Abstract

The basic explosion mechanisms of core-collapse supernovae (SNe) have not been clarified yet. The discovery of hypernovae with the isotropic kinetic energy (E) E51 ≡ E ∕ 1051 ergs ≳ 5-10 brought us a new light on this issue. Observational properties of hypernovae indicate that asymmetry may play an important role in the explosion. We discuss two classes of asymmetry effects related to hypernovae. (1) Effects of asymmetric ejecta on observed properties. Interpreting (late phase) optical light curves and spectra of hypernovae suggests that these objects are aspherical in nature. (2) Effects of asymmetric bipolar explosions on nucleosynthetic yields. An aspherical bipolar explosion provides high-velocity Fe-rich materials and low-velocity O-rich materials, which are in agreement with the observations. The unique yields of the bipolar explosions, e.g., enhanced (Zn, Co)/Fe and suppressed (Mn, Cr)/Fe, can account for the peculiar abundance patterns of extremely metal-poor stars, suggesting that they could have significantly contributed to the early Galactic chemical evolution.

Properties of hypernovae

Type Ic Hypernova SN 1998bw was probably linked to GRB 980425 (Galama et al. 1998), thus establishing for the first time a connection between gamma-ray bursts (GRBs) and core-collapse SNe. However, SN 1998bw was exceptional for a SN Ic: it was as luminous at peak as a SN Ia, indicating that it synthesized ∼0.5 M⊙ of 56Ni, and its isotropic E was estimated as E51 ≳ 30 (Iwamoto et al. 1998; Woosley, Eastman, & Schmidt 1999; see, however, Höflich, Wheeler, & Wang. 1999 for a different interpretation).

Abstract

Near-infrared (NIR) spectroscopy of several stripped-envelope core-collapse supernovae (SNe) are presented. NIR spectra of these objects are quite rich, exhibiting a large number of emission features. Particularly important are strong lines of He I and C I, which probe the outermost ejecta and constrain the pre-collapse mass-loss. Interestingly, the SN 1998bw-like broad-line Type Ic SN 2002ap does not exhibit the strong C I features seen in other Type Ic SNe. NIR spectra also exhibit strong, relatively isolated lines of Mg I, Si I, Ca II, and O I that provide clues into the kinematics and mixing in the ejecta. Finally, late-time NIR spectra of two Type Ic events: SN 2000ew and SN 2002ap show strong first-overtone carbon monoxide (CO) emission, providing the first observational evidence that molecule formation may not only be common in Type II SNe, but perhaps in all core-collapse events.

Introduction

Near-infrared (NIR) spectroscopy is a powerful tool for the study of supernovae (SNe), offering new insights into the kinematic, chemical, and evolutionary properties of these events. Here we present applications of NIR spectroscopy for the study of three stripped-envelope supernovae, the Type Ib SN 2001B, the Type Ic SN 2000ew and the broad-line Type Ic SN 2002ap. All of the data presented here were obtained using TIFKAM on the 2.4 m Hiltner telescope at MDM Observatory, except for the SN 2002ap data set which also includes spectra obtained at Lick Observatory, IRTF, and Subaru. The reduced spectra are presented in Figures 6.1–6.3.

Abstract

I review several topics in the structure of supernova remnants. Hydro-dynamic instabilities in young remnants may give rise to the cellular structure that is sometimes observed, although structure in the ejecta might also play a role. The presence of ejecta close to the forward shock front of a young remnant can be the result of ejecta clumps or the dynamical effects of cosmic rays. Slower moving ejecta clumps can affect the outer shock structure of older remnants such as Vela. Young remnants typically show a circular structure, but often have a one-sided asymmetry; the likely reasons are an asymmetric circumstellar medium, or pulsar velocities in the case of pulsar wind nebulae. In older remnants, asymmetric pulsar wind nebulae can result from asymmetric reverse shock flows and/or pulsar velocities.

Introduction

Observations of supernova remnants frequently show complex structure that can have its origin in several ways: structure in the freely expanding ejecta, structure in the surrounding medium, and the growth of instabilities that result from the interaction of the supernova with its surroundings. If we are to infer properties of the initial explosion from the supernova remnant, consideration of these various influences is necessary. Pulsar wind nebulae (PWNe) provide an additional probe inside a supernova remnant and can lead to an asymmetry because of a pulsar velocity. Here, I review studies of these phenomena.

Instabilities in young remnants

The basic instability that results from the deceleration of the supernova ejecta by the surrounding medium is related to the Rayleigh-Taylor instability.

Abstract

We briefly review the existing database of supernova spectropolarimetry, concentrating on recent data and on results from our group's research. Spectropolarimetry provides the only direct known probe of early-time supernova geometry. To obtain reliable conclusions, however, it is very important to correctly account for interstellar polarization. We find that Type IIn supernovae (SNe IIn) tend to be highly polarized, perhaps in part because of the interaction of the ejecta with an asymmetric circumstellar medium. In contrast, SNe II-P are not polarized much, at least shortly after the explosion. At later times, however, there is evidence for increasing polarization, as one views deeper into the expanding ejecta. Moreover, core-collapse SNe that have lost part (SN IIb) or all (SN Ib) of their hydrogen (or even helium; SN Ic) layers prior to the explosion tend to show substantial polarization; thus, the deeper we probe into core-collapse events, the greater the asphericity. There is now conclusive evidence that at least some SNe Ia are intrinsically polarized, although only by a small amount. Finally, SN spectropolarimetry provides the opportunity to study the fundamental properties of the interstellar dust in external galaxies. For example, we have found evidence for extremely high polarization efficiency for the dust along the line-of-sight to SN 1999gi in NGC 3184.

Introduction

Since extragalactic supernovae (SNe) are spatially unresolvable during the very early phases of their evolution, explosion geometry has been a difficult question to approach observationally.

Abstract

Recent observations have extended the range of diversity among spectra of Type Ia supernovae. I briefly discuss SN Ia explosion models in the spectroscopic context, the observed diversity, and some recent results obtained with the Synow code for one normal and two peculiar SNe Ia. Relating the observational manifestations of diversity to their physical causes is looking like an ever more challenging problem.

Introduction

“Surprises” refers not only to some recent developments in Type Ia supernova (SN Ia) spectroscopy that will be discussed below, but also to additional recent discoveries that I will be able only to mention, such as the polarization signal in SN 2001el (Wang et al. 2003; see also the chapter by Wang); the unusual properties of SN 2001ay (see the chapter by Howell); and the circumstellar Hα emission of SN 2002ic (Hamuy et al. 2003; see also the chapter by Hamuy). The scope of this chapter is restricted to photospheric—phase optical spectra. For recent results on infrared spectra see, e.g., Marion et al. (2003).

Some background, including mention of the various kinds of SN Ia explosion models in the spectroscopic context, is in §15.2. An overview and update of the SN Ia spectroscopic diversity is in §15.3. Some recent results from direct analysis of the spectra of three events (the normal SN 1998aq and the peculiar SNe 2000cx and 2002cx), obtained with the parameterized, resonance scattering code Synow, are discussed in §15.4. The final section (§15.5) contains more questions than conclusions.

Abstract

We have argued that MHD turbulence in an accretion disk naturally produces hoop-stresses, and that in a geometrically-thick flow these stresses could both drive and collimate an outflow. We based this argument on an analogy of turbulent MHD fluids to viscoelastic fluids, in which azimuthal shear flow creates hoop-stresses that cause a variety of flow phenomena, including the Weissenberg effect in which a fluid climbs a spinning rod.

One of the more important differences between the Weissenberg effect and astrophysical jets is the source of power. In our previous analysis, we only considered the power due to the spin-down torque on the central object, and thus found that we could only drive an outflow if the central object were maximally rotating. Here we take into account the energy that is liberated by the accreting matter, and describe a scenario in which this energy couples to the outflow to create a thermodynamic engine.

Introduction

We wish to discuss here in simple language some of our ideas regarding jet collimation and acceleration. In this paper, we will concentrate on the basic intuitive notions rather than the mathematics, which we have discussed in print elsewhere (see references below).

Review: turbulence models and jets

We have argued (Williams 2001; see also Ogilvie 2001) that the stress due to magnetohydrodynamic (MHD) turbulence in ionized accretion disks — such as, but not limited to, the turbulence driven by the magnetorotational instability (MRI) — behaves more like the stress in a viscoelastic fluid than the stress in a viscous fluid.