Gamma-ray bursts (GRBs) are brief flashes of photons that trigger current space-based hard X-ray and gamma-ray detectors every two or three days. During a time ranging from less than one to several thousand seconds, a highly variable photon flux with an unpredictable time structure is recorded by the detector. Fifty years have flown since the first observation of this kind, during which a long series of technological and theoretical breakthroughs paved the way for the current, widely-accepted paradigm that relates these flashes to accretion of matter on a newborn stellar-mass black hole or neutron star. Two are the natural birthplaces of such relativistic beasts: the collapse of a massive star and the coalescence of two compact objects. The latter, perhaps the most intriguing of the two, was the first to be proposed as a candidate progenitor of GRBs, but in 1998 the association of GRB 980425 with supernova 1998bw provided compelling evidence for the former. Nevertheless, no supernova has been associated so far – in some cases down to very stringent limits – to members of a particular subclass of these events, known as short gamma-ray bursts (SGRBs). Several pieces of evidence support the idea that the progenitor of SGRBs is indeed the coalescence of two neutron stars, or of a black hole and a neutron star. If this is true, then SGRBs are also intimately related to gravitational waves (GW). The advanced network of ground-based GW detectors – which at present consists of the two Advanced LIGO interferometers in the USA and of Advanced Virgo in Italy – is especially sensitive in the frequency range of GW produced by the inspiral and merger of a stellar mass compact object binary, so that we are right in the position to start testing the SGRB–GW connection. In August of this year, the first observation of GW from a neutron star binary coalescence, followed by the first observation of a kilonova – the UV/Optical/Infrared emission from the expanding material ejected during the merger and post-merger phases of the coalescence, powered by nuclear decay of unstable nuclei synthesized by the r-process – and an associated SGRB-like transient marked the start of a revolution, whose effect on our understanding of these subjects still needs to be completely unfolded. For this reason, in this thesis I do not to draw firm conclusions about these observations, but rather I discuss some possible interpretations and implications, leaving many questions open to future investigation.

I gamma-ray burst (GRB) sono brevi lampi di fotoni che vengono intercettati dagli odierni rivelatori spaziali di raggi X duri e di raggi gamma ogni due o tre giorni. Per un tempo che può durare da meno di un secondo a diverse migliaia di secondi, un flusso di fotoni altamente variabile, con una struttura temporale imprevedibile, investe il rivelatore. Sono trascorsi cinquant'anni dalla prima osservazione di questo tipo, durante i quali una lunga serie di avanzamenti tecnologici teorici ha lastricato la strada che ha condotto all'odierno paradigma secondo il quale questi lampi sono legati all'accrescimento di materia su un buco nero o una stella di neutroni appena nati. Due sono i luoghi di nascita naturali di queste bestie relativistiche: il collasso di una stella massiva o la coalescenza di due oggetti compatti. Il secondo processo, forse il più affascinante dei due, è stato il primo ad essere proposto come possibile progenitore dei GRB, ma nel 1998 l'associazione del GRB 980425 con la supernova 1998bw ha costituito la prima prova schiacciante del primo scenario. Nonostante ciò, nessuna supernova è stata ancora associata - in alcuni casi con dei limiti molto stringenti - ad un membro di una sottoclasse di questi eventi, quella degli short gamma-ray bursts (SGRB). Diversi indizi infatti supportano l'idea che il progenitore dei SGRB sia proprio la coalescenza di due stelle di neutroni, o di un buco nero e una stella di neutroni. Se questo dovesse essere vero, allora c'è un legame fondamentale tra SGRB e onde gravitazionali (GW). La rete di rivelatori terrestri avanzati di GW - che al momento consiste dei due Advanced LIGO negli Stati Uniti, e di Advanced Virgo in Italia - è sensibile in particolare alla banda di frequenze in cui sono emesse le GW prodotte dallo spiraleggiare e fondersi di oggetti compatti di massa stellare, per cui tutto è pronto per poter testare la connessione SGRB-GW. Nell'agosto di quest'anno, la prima osservazione di GW dalla coalescenza di due stelle di neutroni, seguita dall'osservazione in associazione di una kilonova - l'emissione ultravioletta, ottica e infrarossa proveniente da materiale in espansione, lanciato durante la fase di fusione e quella successiva alla fusione della coalescenza, la cui sorgente di energia è il decadimento radioattivo di nuclei instabili sintetizzati attraverso la cattura rapida di neutroni - e di un lampo simile ad un SGRB hanno segnato l'inizio di una rivoluzione, il cui effetto sulla nostra comprensione di questi fenomeni deve ancora dispiegarsi completamente. Per questa ragione, in questa tesi non traggo ferme conclusioni da queste osservazioni, ma piuttosto discuto alcune possibili interpretazioni e conseguenze, lasciando molte domande aperte a future investigazioni.

(2018). short gamma-ray bursts as electromagnetic counterparts of compact binary mergers. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2018).

short gamma-ray bursts as electromagnetic counterparts of compact binary mergers

SALAFIA, OM SHARAN
2018

Abstract

Gamma-ray bursts (GRBs) are brief flashes of photons that trigger current space-based hard X-ray and gamma-ray detectors every two or three days. During a time ranging from less than one to several thousand seconds, a highly variable photon flux with an unpredictable time structure is recorded by the detector. Fifty years have flown since the first observation of this kind, during which a long series of technological and theoretical breakthroughs paved the way for the current, widely-accepted paradigm that relates these flashes to accretion of matter on a newborn stellar-mass black hole or neutron star. Two are the natural birthplaces of such relativistic beasts: the collapse of a massive star and the coalescence of two compact objects. The latter, perhaps the most intriguing of the two, was the first to be proposed as a candidate progenitor of GRBs, but in 1998 the association of GRB 980425 with supernova 1998bw provided compelling evidence for the former. Nevertheless, no supernova has been associated so far – in some cases down to very stringent limits – to members of a particular subclass of these events, known as short gamma-ray bursts (SGRBs). Several pieces of evidence support the idea that the progenitor of SGRBs is indeed the coalescence of two neutron stars, or of a black hole and a neutron star. If this is true, then SGRBs are also intimately related to gravitational waves (GW). The advanced network of ground-based GW detectors – which at present consists of the two Advanced LIGO interferometers in the USA and of Advanced Virgo in Italy – is especially sensitive in the frequency range of GW produced by the inspiral and merger of a stellar mass compact object binary, so that we are right in the position to start testing the SGRB–GW connection. In August of this year, the first observation of GW from a neutron star binary coalescence, followed by the first observation of a kilonova – the UV/Optical/Infrared emission from the expanding material ejected during the merger and post-merger phases of the coalescence, powered by nuclear decay of unstable nuclei synthesized by the r-process – and an associated SGRB-like transient marked the start of a revolution, whose effect on our understanding of these subjects still needs to be completely unfolded. For this reason, in this thesis I do not to draw firm conclusions about these observations, but rather I discuss some possible interpretations and implications, leaving many questions open to future investigation.
COLPI, MONICA
GHISELLINI, GABRIELE
GHIRLANDA, GIANCARLO
gamma-ray; bursts; compact; binary; mergers
gamma-ray; bursts; compact; binary; mergers
FIS/05 - ASTRONOMIA E ASTROFISICA
English
14-mar-2018
FISICA E ASTRONOMIA - 86R
30
2016/2017
open
(2018). short gamma-ray bursts as electromagnetic counterparts of compact binary mergers. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2018).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/198962
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