Toward an Understanding of the Progenitors of Gamma‐Ray Bursts

Abstract

The various possibilities for the origin ("progenitors") of gamma-ray bursts (GRBs) manifest in differing observable properties. Through deep spectroscopic and high-resolution imaging observations of some GRB hosts, I demonstrate that well-localized long-duration GRBs are connected with otherwise normal star-forming galaxies at moderate redshifts of order unity. Using high-mass binary stellar population synthesis models, I quantify the expected spatial extent around galaxies of coalescing neutron stars, one of the leading contenders for GRB progenitors. I then test this scenario by examining the offset distribution of GRBs about their apparent hosts making extensive use of ground-based optical data from Keck and Palomar and space-based imaging from the Hubble Space Telescope. The offset distribution appears to be inconsistent with the coalescing neutron star binary hypothesis (and, similarly, blackhole-neutron star coalescences); instead, the distribution is statistically consistent with a population of progenitors that closely traces the ultraviolet light of galaxies. This is naturally explained by bursts which originate from the collapse of massive stars ("collapsars"). This claim is further supported by the unambiguous detections of intermediate-time (approximately three weeks after the bursts) emission "bumps" which appear substantially more red than the afterglows themselves. I claim that these bumps could originate from supernovae that occur at approximately the same time as the associated GRB; if true, GRB 980326 and GRB 011121 provide strong observational evidence connecting cosmological GRBs to high-redshift supernovae and implicate massive stars as the progenitors of at least some long-duration GRBs. Regardless of the true physical origin of these bumps, it appears that all viable alternative models of these bumps (such as dust scattering of the afterglow light) require a substantial amount of circumburst matter that is distributed as a wind-stratified medium; this too, implicates massive stars. Also suggested herein are some future observations which could further solidify or refute the supernova claim. In addition to the observational and modeling work, I also constructed the Jacobs Camera (JCAM), a dual-beam optical camera for the Palomar 200-inch Telescope designed to follow-up rapid GRB localizations. viii

Quantifying the Luminosity Evolution in Gamma‐Ray Bursts

“…Unfortunately, a simple increase in the overall energy budget cannot by itself explain the deduced luminosity evolution. Frail et al (2001) and Bloom et al (2003) have recently shown observational evidence suggesting that the collimation corrected GRB energies E γ are actually narrowly cluster around the 10 51 ergs typical of SNe explosions. They come to this conclusion by correcting the observed prompt isotropic equivalent energy release E iso of several GRBs with known redshift by a factor of 1 − cos θ j , where θ j is the canonical jet opening angle.…”

“…Furthermore, an observer viewing the GRB at a small θ v would see an extremely powerful burst, with the observed luminosity declining as some function of increasing θ v . A jet structure with a functional form of ǫ(θ) −2 is required to reproduce the observed t j ∝ E iso , i.e the Frail et al (2001) and Bloom et al (2003) results. If the requirement of a narrow E γ and L γ distribution is broken, then any power law structure θ k could still produce the observed steepening in the afterglow light curve.…”

“…It is interesting then to compare our demographic results to that of various types of supernovae. There is overwhelming observational evidence and theoretical discussion suggesting a GRBs-SNe connection, including observations of supernovae bumps in afterglow lightcurves (Stanek et al 2003;Hjorth et al 2003) and a deduced collimation corrected energy that is narrowly clustered around the typical SNe energy of 10 51 ergs (Frail et al 2001;Bloom 2003). Although the intrinsic luminosity of type Ia SNe are a priori assumed to be constant with redshift (hence no luminosity evolution), we can still compare the formation rates of SNe Ia/b/c to GRBs, although the b/c events are obviously of more relevance to GRB models.…”

Understanding the engines and progenitors of gamma-ray bursts

“…A growing set of observations seem to support this paradigm: long bursts appear to be concentrated near the peak emission in star-forming galaxies, are even more centrally located than supernovae [Vreeswijk et al, 2001, Bloom, 2003, Fruchter et al, 2006. Full confirmation of the accretion disk paradigm required observations demonstrating short GRBs arise from mergers, e.g.…”

Host galaxies and electromagnetic counterparts to binary neutron star mergers across the cosmic time: Detectability of GW170817-like events

“…Observational and theoretical studies of the host galaxies of binary compact objects pre-date the detection of gravitational waves (e.g. Perna & Belczynski 2002;Voss & Tauris 2003;Bloom 2003;Fan et al 2005;Belczynski et al 2006;Covino et al 2006;Nakar et al 2006;O'Shaughnessy et al 2008;Piranomonte et al 2008;Guetta & Stella 2009;D'Avanzo et al 2009;Antonelli et al 2009;Berger 2010;Kopač et al 2012;Margutti et al 2012;Fong & Berger 2013;Fong et al 2015). Since the short gamma-ray bursts (SGRBs) have long been suspected to be associated with BNS and neutron star-black hole (NSBH) mergers, studies of the galaxy hosts have provided remarkable clues onto the evolutionary channels leading to the observed SGRBs.…”