BROADBAND OBSERVATIONS OF GAMMA-RAY BURSTS AND FAST RADIO BURSTS: ENERGETICS, AFTERGLOWS, AND PHYSICAL ORIGINS

Abstract

Long gamma-ray bursts (GRBs) are produced during the deaths of massive stars. They are the most powerful explosions known in the Universe and release most of their energy via a narrow cone of emission. The long-lived afterglows of the brightest GRBs detected by the \textit{Fermi} Large Area Telescope (LAT) are visible from radio to gamma-rays, and this relative abundance of broadband data makes them excellent tools for constraining theoretical models regarding their origins. Here, we use our sample of bright GRBs to test emission models beyond the canonical on-axis, top-hat jet model which has historically been applied throughout the literature. We demonstrate that many GRBs are likely to produce emission via a structured jet. We also find that derived physical parameters are highly dependent upon the fraction, $\xi$, of electrons which contribute to the synchrotron emission. Our findings for $\xi$ are contrary to what is generally assumed during GRB modeling ($\xi=1.0$), but consistent with theoretical simulations which predict lower values. Lower predictions for $\xi$ would impact our current understanding of GRBs, implying denser environments and higher energetics than commonly assumed.

Fast radio bursts (FRBs) are extremely bright, short-duration pulses at radio frequencies that were only confirmed as true astrophysical sources a decade ago. Although the field has experienced major leaps in recent years, many questions regarding their progenitors and emission processes remain. The identification of counterparts at higher energies is critical to understanding the physical origins of FRBs. Here, we report on an archival search of previously identified FRBs with the \textit{Fermi} Gamma-ray Burst Monitor (GBM), the \textit{Fermi}-LAT, and the \textit{Swift} Burst Alert Telescope (BAT). We find no significant X-ray or gamma-ray counterparts but report upper limits on the high-energy fluence, $f_{\gamma}$, for each FRB in our sample. We also report lower limits on the ratio of radio to high-energy fluence ($\nicefrac{f_{r}}{f_{\gamma}}$). We discuss the implications of our results on several FRB progenitor theories, including pulsar-like analogs and magnetar flares.