Models for Gamma-Ray Burst Progenitors and Central Engines

arXiv:1105.4193v1 [astro-ph.HE] 20 May 2011 10 Models for Gamma-ray Burst Progenitors and Central Engines Stan E. Woosley Department of Astronomy and Astrophysics, University of California, Santa Cruz CA 95060 10.1 Introduction For forty years theorists have struggled to understand gamma-ray bursts (GRBs), not only where they are and the systematics of their observed properties, but what they are and how they operate. These broad questions of origin are often referred to as the problem of the “central engine”. So far, this prime mover remains hidden from direct view, and will remain so until neutrino or gravitational-wave signatures are detected. As discussed elsewhere in this volume, there is compelling evidence that all GRBs require the processing of some small amount of matter into a very exotic state, probably not paralleled elsewhere in the modern Universe. This matter is characterized by an enormous ratio of thermal or magnetic energy to mass, and the large energy loading drives anisotropic, relativistic outflows. The burst itself is made far away from this central source, outside the star which would otherwise obscure it, by processes that are still being debated (Chapters 7, 8). The flow of energy is modulated by passing through the star, which also explodes as a supernova, and this modulation further obscures details of the central engine. The study of GRBs experienced spectacular growth after 1997 when the first cosmological counterparts were localized (Chapter 4), and with that growth in data came increased diversity. Still it is customary to segregate GRBs into “long-soft” (LSBs) and “short-hard” (SHBs) categories (Kouve- liotou 1993), though the distinction is not always clear (Chapters 3 and 5; Section 10.5.9). Currently, it is thought that most SHBs result from the merger of compact objects - black holes and neutron stars - in galaxies and regions where the star formation rate is low. There are exceptions, such as GRB 050709, a short hard burst that happened in a star forming galaxy (Covino et al. 2006). But then Type Ia supernovae also happen in spiral 1 2 S.E. Woosley galaxies as well as ellipticals, and there is no reason to expect the SHB progenitor population to be exclusively confined to old galaxies (Prochaska et al. 2006), or even to galaxies for that matter. A few SHBs may be giant flares from soft gamma-ray repeaters in galaxies that are relatively nearby (Palmer et al. 2005; Tanvir et al. 2005). GRB 070201 seems to have hap- pened in the Andromeda Galaxy (Mazets et al. 2008), but the event rate for compact mergers there is ∼10−5 - 10−4 y−1 (Kalogera et al. 2004), and LIGO saw no signal (Abbott et al. 2006). Most LSBs, on the other hand, are clearly associated with massive star formation and, since the lifetime of such stars is short, with massive star death. This connection is strengthened by the observation that many LSBs are accompanied by bright supernovae, when one might be detectable (Chap- ter 9; Woosley & Bloom 2006). Most LSBs must therefore be a consequence of neutron star or black hole birth, and that means that LSBs are some variety of core-collapse supernova. One of the greatest current challenges in the study of stellar evolution is separating out the physical conditions that lead to ordinary supernovae, which are much more frequent, and to LSBs. Are they a continuum, or is there something uniquely different about LSBs? It would help if we understood the mechanism of “ordinary” supernovae better. In fact, one of the greatest opportunities provided by LSBs is the possibility of an improved understanding of massive star death in general. This chapter focuses on massive star models and is thus concerned chiefly, though not exclusively, with LSBs. This does not imply that massive stars are incapable of producing SHBs, only that the connection with LSBs is more clearly demonstrated. Indeed, the observational distinction between LSBs and SHBs is becoming blurred. For a recent review of SHBs see Nakar (2007), and Section 10.5.9. 10.2 Observational constraints 10.2.1 The long-soft burst host environment and progenitor masses LSBs are not just extragalactic, they show a preference for high redshift. Prior to Swift, the mean redshift of LSBs was 1.3; now it is in the range of 2.2 (for 82 bursts, Jakobsson et al. 2006) to 2.6 (for 41 bursts with good redshift determinations, Fiore et al. 2007). Most bursts observed with Swift originate from a time when the Universe was only a few billion years old. This fixes LSBs to an epoch when galaxies and metallicity were both evolving rapidly and the star formation rate, at least in some galaxies, was high (Savaglio et al. 2008). Some bursts do come from relatively nearby galaxies, Models for Gamma-ray Burst Progenitors and Central Engines 3 but most of these are subenergetic (Kaneko et al. 2007). One possibility is that this reflects the evolution of metallicity in the Universe (Section 10.2.2 and Section 10.3.3). LSBs are concentrated in small,

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