## UNLV Theses, Dissertations, Professional Papers, and Capstones

8-1-2017

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Physics and Astronomy

Bing Zhang

Daniel Proga

George Rhee

Amei Amei

197

#### Abstract

Gamma-ray bursts (GRBs) are among the most brilliant explosions in the universe, named for their intense, seconds-duration $\gamma$-ray emission. They are generally classified into two categories according to the durations of their $\gamma$-ray emission: the long-duration GRBs (LGRBs) with durations more than 2 seconds, and the short-duration GRBs (SGRBs) with durations less than 2 seconds. Most SGRBs are believed to originate from mergers of compact stars (e.g., neutron star-neutron star and neutron star-black hole; also known as Type I GRBs by their physical mechanism), while the LGRBs are thought to originate from core-collapse of massive stars (Type II). In addition to providing clues about the extreme conditions of their sources, both SGRBs and LGRBs are useful probes of the universe. Because massive stars are in the star forming regions, Type II GRBs are effective to study the star formation history and perhaps to probe Pop III stars. Type I GRBs are very likely to be associated with gravitational wave signals. However, the classification by duration is not a direct reflection of the physical nature of GRBs. In this dissertation, I use multi-wavelength data to study the difference between these two populations (Type I and II), with the purpose of finding a more efficient way to classify GRBs and searching for special cases.

My dissertation consists of four parts. (1) A comprehensive comparison between LGRBs and SGRBs. In order to compare LGRBs and SGRB comprehensively, we gather the properties of prompt emission and host galaxies of 407 GRBs, which is by far the most complete sample of GRBs with such data, from literature. It is found that all of the parameters of Type II GRBs overlap with those of Type I GRBs, even in two-dimensional (2D) distributions. It indicates that a physical classification of GRBs requires a multi-parameter and multi-wavelength approach. (2) Classifying GRBs with their multi-wavelength properties. With the catalog we compiled, we develop a likelihood method to distinguish Type I GRBs from Type II ones. Only $\sim1.3\%$ of Type II GRBs and $\sim3\%$ of Type I GRBs overlap with each other, which is much better than any conventional one parameter method (e.g., the duration $T_{90}$). (3) A search for Neutron Star-Neutron Star mergers as SGRB-less extended emissions. With the likelihood ratio method, we search, from the LGRB sample, for GRBs with prompt emission properties similar to extended emissions of SGRBs, as well as host galaxy properties similar to that of SGRBs. These sources might be merger-origin GRBs without short spikes, due to off-axis jets. There are ten candidates found and it indicates a free zone/jet zone ratio $k_{\Omega} \sim 1$. (4) The redshift dependence of lethal GRBs. GRBs are powerful sources. The amount of energy and its form (gamma-ray) emitted by GRBs are so extreme that GRBs must affect the environment of planets nearby. GRBs are proposed to be one of the reasons of mass extinctions on the Earth. It is worthwhile to study the role of GRBs on life evolution in the framework of the universe. Lethal GRBs tend to locate in dwarf galaxies with intense star formation and low metallicity. We investigate the redshift and galaxy type dependence of the event rate of lethal GRBs, and find that there is a relatively low lethal GRB rate in low redshift and early type massive galaxies.

#### Keywords

classification; Gamma Ray Burst; host galaxy; multi-wavelength; progenitor; prompt emission

#### Disciplines

Astrophysics and Astronomy

pdf

English