Date of Graduation

5-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Space & Planetary Sciences (PhD)

Degree Level

Graduate

Department

Space & Planetary Sciences

Advisor

Julia Kennefick

Committee Member

Larry Roe

Second Committee Member

William Oliver III

Third Committee Member

John Dixon

Fourth Committee Member

Bret Lehmer

Keywords

Fermi Gamma-ray Space Telescope, Gamma-ray Bursts, GRB model, Isotropic Equivalent Energy, Systematical biases, Time-integrated spectrum

Abstract

Gamma-ray Bursts (GRBs) are the most energetic and luminous explosions in the Universe since the Big Bang, enabling them to be observed out to extremely large redshifts (z~9). Consequently, this makes them a promising cosmological standard candle candidate. Unfortunately, however, they have proven to be quite challenging to standardize. The GRB community has worked tirelessly at this task, and to date, has put forth several luminosity-distance relations, some more propitious than others. The most prevailing problem with these relations is in their sizable amount of scatter, likely due to measurement inconsistencies and errors in the variables they employ. This arises when the results from many independent analysts are combined, which cumulates their individual choices in methods and methodologies. In this dissertation I examine the systematical uncertainties associated with the calculation of a GRB's isotropical equivalent energy, Eiso. This energetic is attributed to the total energy that is released by a GRB’s central engine in the form of X-rays and gamma-rays, before becoming reprocessed down-jet into radiation spanning the entire electromagnetic spectrum. Not only does Eiso have physical implications for GRBs, but it is employed by two of the most widely recognizable luminosity-distance relations: the Amati and the Ghirlanda relations. Our greatest concern with Eiso is that, historically, it has not been measured with a high level of consistency, which may be leading to significant scatter and bias for these two relations. Whenever a relation’s variables suffer from significant measurement errors, the relations themselves also accrue these errors, thereby increasing their scatter and decreasing their reliability. In this dissertation I present the most comprehensive exploration into the systematical uncertainties of Eiso that has ever been done. In this work I focus on a few particular analytical choices that any analyst can make differently from the next, leading to inconsistent Eiso values measured for the same GRB event. I act as two independent analysts by altering a single analytical choice between the two analyses that are performed on the same subset of GRBs. This provides us with a measure of uncertainty that is associated with each particular analytical choice. In addition to quantifying uncertainties, I examine and discuss the minutiae of the spectral modeling parameters that are used to characterize a GRB’s fluence spectrum and thus its Eiso. To achieve our goals, I employed Fermi Gamma-ray Space Telescope GRB data for a sample of events with redshifts measured from ground-based follow-up optical/NIR spectroscopy. To ensure that data preparation uncertainties remained minimal, we performed all of our own data reductions, fluence spectral modeling, and Eiso calculations for this work, rather than adopting values from the literature.

Available for download on Wednesday, May 17, 2023

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