Nella nuova era dell'esplorazione del cielo transiente, i progenitori di esplosioni relativistiche -come i lampi gamma (GRB) e le supernovae relativistiche (SN)- sono tra i protagonisti più importanti della ricerca. Le osservazioni multi-frequenza del vicino GRB170817A -le cui onde gravitazionali sono state rilevate dagli interferometri LIGO/VIRGO- e la recente scoperta alle altissime energie degli afterglow associati a due GRB brillanti (190114C e 180720B) con i telescopi Cherenkov MAGIC e HESS hanno suggellato la nascita dell'astronomia multi-messaggera. Lo studio dell'afterglow dei GRB, originato dall'interazione tra la materia espulsa e il mezzo circostante, è fondamentale per comprendere i meccanismi di emissione, la microfisica dello shock relativistico, le proprietà del mezzo circostante e del getto. Gli afterglow radio dei GRB -per quanto difficili da osservare per la loro intrinseca debolezza (dell'ordine del sub-mJy)- sono cruciali per comprendere appieno questi aspetti, e in particolare lo shock inverso, che a sua volta dipende dalle proprietà della materia espulsa e quindi dal progenitore stesso del GRB. Anche se diverse campagne osservative hanno migliorato la copertura della parte radio dello spettro di emissione, ad oggi manca un quadro esaustivo degli afterglow. In questo contesto mi sono occupato di modellizzare l'emissione multi-frequenza degli afterglow, con particolare attenzione al radio, con la realizzazione del codice Python sAGa (Software for AfterGlow Analysis). Dopo aver testato con successo {\sc sAGa} su vari afterglow di GRB (120521C, 090423 e 050904), l'ho utilizzato nel caso di GRB160131A, i cui dati suggeriscono l'iniezione di energia da parte del progenitore, e la presenza di un getto. Inoltre, l'insolita presenza di picchi nelle curve di luce radio potrebbe essere dovuta a effetti di scintillazione interstellare. I miei risultati mostrano che i dati multi-frequenza sono difficilmente spiegabili nell'ambito del modello standard degli afterglow. Al fine di incrementare i dati radio, ho coordinato una serie di campagne osservative con il Sardinia Radio Telescope (SRT) per gli afterglow di GRB181201A e GRB190114C. Nonostante queste non abbiano portato alla rivelazione degli stessi, mi hanno offerto la possibilità di confrontare tre metodi per rivelare sorgenti deboli: quick-look (il meno accurato), source extraction (tipico dell'analisi ad alte energie) e il fit con una Gaussiana bi-dimensionale. La messa a punto di una nuova metodologia per l'analisi dei dati di SRT (1) ottimizza la rivelazione di una sorgente debole ad un flusso minimo rivelabile di ~1.8 mJy, e (2) evidenzia l'importanza di un'accurata conoscenza del fondo. I GRB lunghi sono associati alle Ic broad-line (Ic-BL) SNe. Ad oggi il legame GRB/SN è testabile solo per i GRB a redshift z < 1 per la debolezza intrinseca delle SNe. In questo contesto ho analizzato Ic-BL SN2014ad, rivelata solo in ottico. La vicinanza di questa sorgente (~26 Mpc) e le osservazioni in radio e raggi-X hanno permesso di vincolare profondamente (1) il tasso di perdita di massa del progenitore, (2) l'energia totale della materia espulsa a grande velocità e (3) la geometria dell'esplosione. Ho considerato due regimi di emissione di sincrotrone (isotropa con espansione sub-relativistica, tipica delle normali SN; relativistica con getto osservato fuori asse, come nei GRB), dimostrando che i getti poco energetici osservati fuori asse in un mezzo a bassa densità non possono essere esclusi nemmeno per una BL-Ic SN così vicina.

In the new transient sky era, the progenitors of relativistic explosions, such as gamma-ray bursts (GRBs) and relativistic supernovae (SNe), are the focus of forefront research. Broadband observations of nearby short GRB170817A -whose gravitational waves were detected by LIGO/VIRGO- and the recent detection at TeV energies of two bright GRB afterglows (190114C and 180720B) with the MAGIC and HESS Cherenkov telescopes heralded the birth of so-called multi-messenger astronomy. GRB afterglows, originating from the interaction between the ejecta and the circumburst medium, help constrain the radiation mechanism, the relativistic shock microphysics, the circumburst environment, the structure and geometry of the relativistic jet. Observations of radio afterglows are key to understand the reverse shock, which links directly to the nature of the outflow and, consequently, to the progenitor itself. On the other hand, they can hardly be observed with current radiotelescopes because of their faintness (mJy or sub-mJy). Recently, several radio followup campaigns improved the observational coverage of the lower part of the emission spectrum, but an exhaustive picture of GRB afterglows is still missing. I developed an approach focused on broadband modelling (with particular attention to the radio frequencies) through the Python code called sAGa (Software for AfterGlow Analysis). After successfully testing it on various GRB afterglows (120521C, 090423, and 050904), I applied it to long GRB160131A, whose data show evidence for energy injection and jetted emission. Radio light curves are characterised by several peaks, that could be due to either interstellar scintillation (ISS) effects or multi-component structure. My results show that the data can hardly be explained self-consistently with the standard model of GRB afterglows. To help collect more radio data on GRB afterglows, I coordinated radio campaigns with Sardinia Radio Telescope (SRT) to observe two GRB radio afterglows (181201A and 190114C). Although they have come up with no detection, they fostered the definition and the comparative analysis of three detection methods for faint radio sources through single-dish imaging, in terms of sensitivity and robustness: quick-look (a smart but rough approach), source extraction (typical of high-energy astronomy), and fitting procedure with a 2-Dimensional Gaussian (a more sophisticated approach). The new methodology for the SRT data analysis (1) pushes down the sensitivity limits of this radiotelescope -with respect to more traditional techniques- at ~1.8 mJy, and (2) highlights the need for accurate knowledge of the background. L-GRBs are associated with type Ic broad-line supernovae (Ic-BL SNe). This connection can be observed only at redshift z < 1 because of the intrinsic faintness of SNe. In this context, I analysed Ic-BL SN2014ad, detected only in optical. The proximity of the source (~26 Mpc) and the radio/X-ray observations turned into very deep constraints on (1) the progenitor mass-loss rate, (2) the total energy of the ejecta, and (3) the explosion geometry. I considered two synchrotron emission regimes (uncollimated non-relativistic ejecta, typical of ordinary SNe); off-axis relativistic jet, such as those seen in GRBs), showing that off-axis low-energy jets expanding in a low-density medium cannot be ruled out even in the most nearby BL-Ic SNe.

Broadband modelling of relativistic explosions

MARONGIU, MARCO
2020

Abstract

In the new transient sky era, the progenitors of relativistic explosions, such as gamma-ray bursts (GRBs) and relativistic supernovae (SNe), are the focus of forefront research. Broadband observations of nearby short GRB170817A -whose gravitational waves were detected by LIGO/VIRGO- and the recent detection at TeV energies of two bright GRB afterglows (190114C and 180720B) with the MAGIC and HESS Cherenkov telescopes heralded the birth of so-called multi-messenger astronomy. GRB afterglows, originating from the interaction between the ejecta and the circumburst medium, help constrain the radiation mechanism, the relativistic shock microphysics, the circumburst environment, the structure and geometry of the relativistic jet. Observations of radio afterglows are key to understand the reverse shock, which links directly to the nature of the outflow and, consequently, to the progenitor itself. On the other hand, they can hardly be observed with current radiotelescopes because of their faintness (mJy or sub-mJy). Recently, several radio followup campaigns improved the observational coverage of the lower part of the emission spectrum, but an exhaustive picture of GRB afterglows is still missing. I developed an approach focused on broadband modelling (with particular attention to the radio frequencies) through the Python code called sAGa (Software for AfterGlow Analysis). After successfully testing it on various GRB afterglows (120521C, 090423, and 050904), I applied it to long GRB160131A, whose data show evidence for energy injection and jetted emission. Radio light curves are characterised by several peaks, that could be due to either interstellar scintillation (ISS) effects or multi-component structure. My results show that the data can hardly be explained self-consistently with the standard model of GRB afterglows. To help collect more radio data on GRB afterglows, I coordinated radio campaigns with Sardinia Radio Telescope (SRT) to observe two GRB radio afterglows (181201A and 190114C). Although they have come up with no detection, they fostered the definition and the comparative analysis of three detection methods for faint radio sources through single-dish imaging, in terms of sensitivity and robustness: quick-look (a smart but rough approach), source extraction (typical of high-energy astronomy), and fitting procedure with a 2-Dimensional Gaussian (a more sophisticated approach). The new methodology for the SRT data analysis (1) pushes down the sensitivity limits of this radiotelescope -with respect to more traditional techniques- at ~1.8 mJy, and (2) highlights the need for accurate knowledge of the background. L-GRBs are associated with type Ic broad-line supernovae (Ic-BL SNe). This connection can be observed only at redshift z < 1 because of the intrinsic faintness of SNe. In this context, I analysed Ic-BL SN2014ad, detected only in optical. The proximity of the source (~26 Mpc) and the radio/X-ray observations turned into very deep constraints on (1) the progenitor mass-loss rate, (2) the total energy of the ejecta, and (3) the explosion geometry. I considered two synchrotron emission regimes (uncollimated non-relativistic ejecta, typical of ordinary SNe); off-axis relativistic jet, such as those seen in GRBs), showing that off-axis low-energy jets expanding in a low-density medium cannot be ruled out even in the most nearby BL-Ic SNe.
GUIDORZI, Cristiano
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Descrizione: Broadband modelling of relativistic explosions
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/2488013
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