More than fifty years ago, astronomers discovered gamma-ray bursts (GRBs), the brightest transient phenomena in the Universe. These powerful cosmic explosions occur in distant galaxies, reaching redshifts as high as $z \simeq 9$, and are triggered by either the collapse of massive stars (collapsars) or the merger of two neutron stars in a binary system. Their central engine consists of a compact object (likely a black hole or a rapidly spinning neutron star) that launches an ultra-relativistic, collimated jet. This jet is responsible for the bright and highly variable MeV radiation emitted soon after the explosion, known as the prompt emission, which lasts from fractions of a second to several hundred seconds. The duration of this phase depends on the GRB progenitor: collapsars typically produce long bursts ($> 2$ s), while neutron star mergers result in short bursts ($< 2$ s). When the relativistic jet interacts with the surrounding medium, it generates a long-lasting emission across the entire electromagnetic spectrum, from radio to TeV energies, known as the afterglow.\\ Decades of observations using various instruments have significantly advanced our understanding of GRB phenomenology. However, many open questions remain, particularly regarding the intrinsic diversity of these events. GRBs arise from fundamentally different progenitors, depending on which their spectral and temporal features might significantly vary. Spectrally, GRBs exhibit non-thermal emission potentially from synchrotron process, though this remains a subject of debate. While the main spectral continuum is well described by a Band function, namely two power laws smoothly connected around a peak, its specific realization differs from burst to burst. Additionally, secondary spectral features can enhance or suppress the primary continuum, adding further complexity to the prompt spectrum and contributing to the observed diversity in GRB properties.\\ In this thesis, I explore the underlying causes of this intrinsic variety in GRB phenomenology, in light of the three exceptional bursts exploded after late 2021 that overturned our understanding on these cosmic sources: GRB 211211A, GRB 221009A, and GRB 230307A. I investigate the differences in the GRB spectral domain through the analysis of GRB 220101A and GRB 221009A. These bursts radiated an incredibly large amount of isotropic-equivalent energy ($E_{iso} \sim 10^{54}$ erg and $E_{iso} \sim 10^{55}$ erg, respectively), with GRB 221009A being the brightest GRB ever observed (for this reason renamed the \textit{BOAT}). The occurrence of these two bursts at relatively large ($z = 4.62$) and low ($z=0.15$) distance allowed the study of different aspects of their prompt emission. In GRB 220101A, the optimal spectral coverage of the prompt and afterglow phases, together with the identification of a high energy spectral break at $E_{cutoff} = 85_{-26}^{+16}$ MeV due to pair attenuation, made this burst an ideal benchmark to test prompt emission models. In particular, the combination of this rich dataset provided constraints among the most stringent ones on the bulk Lorentz factor $\Gamma_0$ and position of the emission site $R_\gamma$, namely $\log R_\gamma[{\rm cm}] = 13.7_{-0.4}^{+0.6}$ and $700<\Gamma_0<1160$. Despite not being precise, these constraints seem to prefer an internal shock scenario for prompt emission, highlighting the renown problem of marginally fast-cooling electrons at those radii. In the BOAT, the exceptional signal from this burst provides prompt spectra with unprecedented signal-to-noise ratio. In the BOAT prompt emission spectrum, an emission feature at tens of MeV is discovered, making it the first emission feature of a gamma-ray burst. This feature is not found in any other GRB to date, and it is possibly generated from pair annihilation in the prompt emission site.\\ I investigate the differences in the GRB temporal domain through the analysis of GRB 211211A. Along with GRB 230307A, these are long-duration GRBs that, according to the duration dichotomy, would typically have a collapsar origin. However, in both cases, a kilonova followed the bursts, pointing instead to a merger origin. This makes them the first known merger-driven long GRBs, challenging for the first time the established framework that links GRB progenitors to prompt duration. In particular, I discuss the discovery of a late-time GeV emission in GRB 211211A, which exceeds predictions from the standard afterglow model. This excess can be explained by external inverse Compton scattering between the relativistic jet and optical seed photons from the kilonova. This finding opens the possibility of a new high-energy ($>100$ MeV) counterpart for for gravitational-wave signal coming from binary compact object mergers, potentially offering a novel way to probe the interaction among relativistic jets and and sub-relativistic ejecta giving rise to kilonova emission.\\ Finally, I investigate a common pattern in GRB prompt emission by employing both temporal and spectral information through time-resolved empirical relations. It is well established that the energy of the spectral peak, $E_{p,z}$, correlates with the total luminosity of the burst, $L_{iso}$, according to the Yonetoku relation. While almost all GRBs follow this trend, the underlying physical reason remains unclear. This correlation was initially identified by fitting large GRB samples, specifically during their brightest pulse, using the phenomenological Band function. However, it holds even when other pulses are analyzed. Considering the increasing evidence that the prompt emission mechanism is synchrotron radiation from shock-accelerated electrons, we test the $E_{p,z}-L_{iso}$ relation using a physical model rather than a purely phenomenological one. Through this approach, we uncover a novel relation between $L_{iso}$ and the synchrotron cooling frequency, $\nu_{c,z}$, in fast-cooling spectra: $\nu_{c,z} \propto L_{iso}^{0.5}$. This finding suggests that luminosity is not directly tied to the spectral peak but rather to the low-energy spectral break. I discuss the physical implications of this new relation and how it remains consistent with over two decades of observations that have suggested a different correlation.\\ In conclusion, the exceptional bursts observed since the end of 2021 have renewed focus on the heterogeneous nature of gamma-ray bursts, revealing entirely new phenomenological insights. The mechanisms behind prompt emission remain elusive, as does the reason why its spectrum exhibits such diverse shapes and components. However, highly energetic bursts provide excellent datasets for testing theoretical models, enabling to impose stringent constraints and uncover spectral features previously unaccounted for. Determining GRB progenitors remains a challenge, and recent discoveries have further complicated the picture. However, a key signature of merger-driven bursts has emerged in late-time GeV emission, offering a potential new diagnostic tool. Last, while both long and short GRBs continue to follow the Yonetoku relation, its reinterpretation through synchrotron radiation marks a significant step forward in uncovering its physical origin.
On the common properties across the gamma-ray burst population: discovering a standard behavior and unique events / Mei, Alessio. - (2025 Jun 05).
On the common properties across the gamma-ray burst population: discovering a standard behavior and unique events
MEI, ALESSIO
2025-06-05
Abstract
More than fifty years ago, astronomers discovered gamma-ray bursts (GRBs), the brightest transient phenomena in the Universe. These powerful cosmic explosions occur in distant galaxies, reaching redshifts as high as $z \simeq 9$, and are triggered by either the collapse of massive stars (collapsars) or the merger of two neutron stars in a binary system. Their central engine consists of a compact object (likely a black hole or a rapidly spinning neutron star) that launches an ultra-relativistic, collimated jet. This jet is responsible for the bright and highly variable MeV radiation emitted soon after the explosion, known as the prompt emission, which lasts from fractions of a second to several hundred seconds. The duration of this phase depends on the GRB progenitor: collapsars typically produce long bursts ($> 2$ s), while neutron star mergers result in short bursts ($< 2$ s). When the relativistic jet interacts with the surrounding medium, it generates a long-lasting emission across the entire electromagnetic spectrum, from radio to TeV energies, known as the afterglow.\\ Decades of observations using various instruments have significantly advanced our understanding of GRB phenomenology. However, many open questions remain, particularly regarding the intrinsic diversity of these events. GRBs arise from fundamentally different progenitors, depending on which their spectral and temporal features might significantly vary. Spectrally, GRBs exhibit non-thermal emission potentially from synchrotron process, though this remains a subject of debate. While the main spectral continuum is well described by a Band function, namely two power laws smoothly connected around a peak, its specific realization differs from burst to burst. Additionally, secondary spectral features can enhance or suppress the primary continuum, adding further complexity to the prompt spectrum and contributing to the observed diversity in GRB properties.\\ In this thesis, I explore the underlying causes of this intrinsic variety in GRB phenomenology, in light of the three exceptional bursts exploded after late 2021 that overturned our understanding on these cosmic sources: GRB 211211A, GRB 221009A, and GRB 230307A. I investigate the differences in the GRB spectral domain through the analysis of GRB 220101A and GRB 221009A. These bursts radiated an incredibly large amount of isotropic-equivalent energy ($E_{iso} \sim 10^{54}$ erg and $E_{iso} \sim 10^{55}$ erg, respectively), with GRB 221009A being the brightest GRB ever observed (for this reason renamed the \textit{BOAT}). The occurrence of these two bursts at relatively large ($z = 4.62$) and low ($z=0.15$) distance allowed the study of different aspects of their prompt emission. In GRB 220101A, the optimal spectral coverage of the prompt and afterglow phases, together with the identification of a high energy spectral break at $E_{cutoff} = 85_{-26}^{+16}$ MeV due to pair attenuation, made this burst an ideal benchmark to test prompt emission models. In particular, the combination of this rich dataset provided constraints among the most stringent ones on the bulk Lorentz factor $\Gamma_0$ and position of the emission site $R_\gamma$, namely $\log R_\gamma[{\rm cm}] = 13.7_{-0.4}^{+0.6}$ and $700<\Gamma_0<1160$. Despite not being precise, these constraints seem to prefer an internal shock scenario for prompt emission, highlighting the renown problem of marginally fast-cooling electrons at those radii. In the BOAT, the exceptional signal from this burst provides prompt spectra with unprecedented signal-to-noise ratio. In the BOAT prompt emission spectrum, an emission feature at tens of MeV is discovered, making it the first emission feature of a gamma-ray burst. This feature is not found in any other GRB to date, and it is possibly generated from pair annihilation in the prompt emission site.\\ I investigate the differences in the GRB temporal domain through the analysis of GRB 211211A. Along with GRB 230307A, these are long-duration GRBs that, according to the duration dichotomy, would typically have a collapsar origin. However, in both cases, a kilonova followed the bursts, pointing instead to a merger origin. This makes them the first known merger-driven long GRBs, challenging for the first time the established framework that links GRB progenitors to prompt duration. In particular, I discuss the discovery of a late-time GeV emission in GRB 211211A, which exceeds predictions from the standard afterglow model. This excess can be explained by external inverse Compton scattering between the relativistic jet and optical seed photons from the kilonova. This finding opens the possibility of a new high-energy ($>100$ MeV) counterpart for for gravitational-wave signal coming from binary compact object mergers, potentially offering a novel way to probe the interaction among relativistic jets and and sub-relativistic ejecta giving rise to kilonova emission.\\ Finally, I investigate a common pattern in GRB prompt emission by employing both temporal and spectral information through time-resolved empirical relations. It is well established that the energy of the spectral peak, $E_{p,z}$, correlates with the total luminosity of the burst, $L_{iso}$, according to the Yonetoku relation. While almost all GRBs follow this trend, the underlying physical reason remains unclear. This correlation was initially identified by fitting large GRB samples, specifically during their brightest pulse, using the phenomenological Band function. However, it holds even when other pulses are analyzed. Considering the increasing evidence that the prompt emission mechanism is synchrotron radiation from shock-accelerated electrons, we test the $E_{p,z}-L_{iso}$ relation using a physical model rather than a purely phenomenological one. Through this approach, we uncover a novel relation between $L_{iso}$ and the synchrotron cooling frequency, $\nu_{c,z}$, in fast-cooling spectra: $\nu_{c,z} \propto L_{iso}^{0.5}$. This finding suggests that luminosity is not directly tied to the spectral peak but rather to the low-energy spectral break. I discuss the physical implications of this new relation and how it remains consistent with over two decades of observations that have suggested a different correlation.\\ In conclusion, the exceptional bursts observed since the end of 2021 have renewed focus on the heterogeneous nature of gamma-ray bursts, revealing entirely new phenomenological insights. The mechanisms behind prompt emission remain elusive, as does the reason why its spectrum exhibits such diverse shapes and components. However, highly energetic bursts provide excellent datasets for testing theoretical models, enabling to impose stringent constraints and uncover spectral features previously unaccounted for. Determining GRB progenitors remains a challenge, and recent discoveries have further complicated the picture. However, a key signature of merger-driven bursts has emerged in late-time GeV emission, offering a potential new diagnostic tool. Last, while both long and short GRBs continue to follow the Yonetoku relation, its reinterpretation through synchrotron radiation marks a significant step forward in uncovering its physical origin.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
