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Compact binary merger searches in the era of multimessenger high energy astrophysics

Abstract : Neutron stars and stellar mass black holes are the remnants of massive stars, caused by the gravitational collapse taking place at the end of their nuclear fusion based life cycles. They can be found gravitationally isolated or in binary systems, forming binary neutron stars (BNS), neutron star-black holes (NSBH) and binary black holes (BBH). In the latter case, when the compact objects orbit each other, the system is loosing angular momentum by emission of gravitational waves (GW). Thus the orbital separation shrinks over time and, if the initial separation is not too high, it might lead to a merger during the Universe lifetime. The amplitude of the GWs increases with the approach of the coalescence time, making them detectable by the GW interferometers Advanced LIGO and Advanced Virgo. If in addition, at least one of the binary components is a neutron star, a relativistic beamed high energy electromagnetic (EM) emission, called short gamma ray burst (GRB), activated by the infalling matter into the accretion disc surrounding the newly born black hole, is expected in the seconds following or preceding the merger. A more isotropic radiation in both the optical and near infrared spectrum, called kilonova and powered by the radioactive decay of the synthesized r-process heavy elements, is emitted on days timescale. Finally the short GRB afterglows spanning a wide energy spectrum going from X-rays to radio wavelengths might be detectable on timescales varying from hours to months. GW170817 was a remarkable astrophysical event, showing clear evidence for all these GW/EM signatures. Moreover, theoretical work predict the existence of some other signals emitted by these violent phenomena such as the emission of neutrinos or gamma-ray burst precursors. In this thesis, several procedures whose aim is to increase the chances of detecting some GW/EM signals linked to a compact binary merger are presented. We propose a statistical method for the detection of the association between the prompt GRB emission, identifiable in the Fermi-GBM data, and the GW trigger, reported by Advanced LIGO and/or Advanced Virgo. The idea behind it is to take advantage of the low coincident rate of backgrounds in two distinct detectors. We also introduce a tool aimed to search for a modulated gamma-ray signal detectable by the Fermi-GBM scintillators, prior to a GW event identified by the Advanced interferometers. Whereas the emissions of gamma-ray radiation and GWs are expected to be almost simultaneous, the kilonova counterpart follows the merger, can last for days and is tracked most of the time by the terrestrial telescopes. We propose here two methods aiming to help the EM observers community in trying to detect kilonovae. Firstly, we suggest the use of a machine learning based algorithm in order to distinguish between photometric lightcurves attributed to the desired kilonovae, the dominating background supernovae and other background optical transients. Secondly, GW low-latency based kilonovae photometric lightcurves are proposed with delays of the order of minutes. The near future will be characterized by a plethora of signals of different natures, making the data interpretation very challenging. In this exciting world, there will be place only for the most efficient data analysis. methods.
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Submitted on : Thursday, October 28, 2021 - 9:29:36 AM
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  • HAL Id : tel-03406790, version 1

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Cosmin Stachie. Compact binary merger searches in the era of multimessenger high energy astrophysics. Astrophysics [astro-ph]. Université Côte d'Azur, 2021. English. ⟨NNT : 2021COAZ4056⟩. ⟨tel-03406790⟩

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