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Submitted by Paula Guerreiro (passarinho@reitoria.ulisboa.pt) on 2021-11-16T15:49:44Z No. of bitstreams: 1 ulsd737054_td_Amarantidis_Stergios.pdf: 106019521 bytes, checksum: e5db9374c4fa284821e2eda870add5d5 (MD5) Made available in DSpace on 2021-12-13T15:49:23Z (GMT). No. of bitstreams: 1 ulsd737054_td_Amarantidis_Stergios.pdf: 106019521 bytes, checksum: e5db9374c4fa284821e2eda870add5d5 (MD5) Previous issue date: 2021-05

We report optical photometric and polarimetric observations of the blazar OJ 287 gathered during 2016/17. The high level of activity, noticed after the General Relativity Centenary flare, is argued to be part of the follow-up flares that exhibited high levels of polarization and originated in the primary black hole jet. We propose that the follow-up flares were induced as a result of accretion disk perturbations, travelling from the site of impact towards the primary SMBH. The timings inferred from our observations allowed us to estimate the propagation speed of these perturbations. Additionally, we make predictions for the future brightness of OJ 287. © 2017 by the authors.

In this talk, I will present the first direct detections of gravitational waves from binary stellar-mass black hole mergers during the first observing run of the two detectors of the Advanced Laser Interferometer Gravitational-wave Observatory, which opened the field of gravitational-wave astronomy, and then discuss prospects for observing gravitational waves from supermassive black holes with future detectors.

The main objective of the original work presented in this Thesis is to develop a theoretical framework to understand the growth, cosmological evolution and observational features of the first black holes, formed when the Universe was younger than \(\sim 1 \, \mathrm{Gyr}\). In the first part a growth model is assembled, based on the developed radiation hydrodynamic code GEMS (Growth of Early Massive Seeds). We find that the accretion onto a Direct Collapse Black Hole (DCBH) of initial mass \(M_0=10^5 \, \mathrm{M_{\odot}}\) occurs at an average, super-Eddington, rate \(\dot{M}_{\bullet} \simeq 1.35 \, \dot{M}_{Edd} \simeq 0.1 \, \mathrm{M_{\odot} \, yr^{-1}}\), is intermittent (duty-cycle \(\lesssim 50\%\)) and lasts \(\sim 100 \, \mathrm{Myr}\), during which the black hole can accrete only up to \(\sim 20\%\) of the available mass. Our model identifies a "feeding-dominated" accretion regime for massive DCBHs (\(\gtrsim 10^{5-6} \, \mathrm{M_{\odot}}\)) and a "feedback-limited" one for light ones (\(\lesssim 10^{3-4} \, \mathrm{M_{\odot}}\)), the latter being characterized by intermittent (duty cycles \(\lesssim 0.5\)) and inefficient growth, with recurring outflow episodes. We have also explored slim disk models, appropriate for super-Eddington accretion, in which outflows play a negligible role and a black hole can accrete 80%-100% of the gas mass of the host halo in \(\sim 10 \, \mathrm{Myr}\). We find that the differential growth of light and massive DCBH seeds leads to a bimodal cosmological evolution in mass. In the second part we investigate the observational properties of these sources. The time-evolving spectrum emerging from the host halo of a DCBH is analyzed: the emission occurs predominantly in the observed infrared-submm (\(1-1000 \, \mathrm{\mu m}\)) and X-ray (\(0.1 - 100 \, \mathrm{keV}\)) bands. Such signal should be easily detectable by the JWST at \(\sim 1 \, \mathrm{\mu m}\), and by ATHENA (between \(0.1\) and \(10 \, \mathrm{keV}\)). Deep X-ray surveys like the CDF-S could have already detected these systems. Based on this, we provide upper limits for the \(z \gtrsim 6\) black hole mass density for both accretion models. A photometric method to identify DCBH candidates in deep multi-wavelength surveys is developed: these sources are characterized by a steep spectrum in the infrared (\(1.6-4.5 \, \mathrm{\mu m}\)), i.e. by very red colors. The method selects the only 2 objects with a robust X-ray detection found in the CANDELS/GOODS-S survey with \(z \gtrsim 6\). To date, the selected objects represent the most promising black hole seed candidates, possibly formed via the DCBH scenario, with predicted mass \(>10^5 \, \mathrm{M_{\odot}}\). Finally, we note that the abrupt collapse of a massive and rotating object such as a DCBH is a powerful source of gravitational waves emission. We show that the predicted signal lies above the foreseen sensitivity of the DECIGO observatory in the frequency range \((0.8-300) \, \mathrm{mHz}\), with a peak amplitude \(\Omega_{gw} = 1.1 \times 10^{-54}\) at \(\nu_{max} = 0.9 \, \mathrm{mHz}\) and a peak Signal-to-Noise Ratio \(\sim 22\) at \(\nu = 20 \, \mathrm{mHz}\).

The origin of high velocity stars observed in the halo of our Galaxy is still unclear. In this work we test the hypothesis, raised by results of recent high precision $N$-body simulations, of strong acceleration of stars belonging to a massive globular cluster orbitally decayed in the central region of the host galaxy where it suffers of a close interaction with a super massive black hole, which, for these test cases, we assumed $10^8$ M$_\odot$ in mass., Comment: 4 pages, 5 figures, Proceedings of 14th Marcel Grossmann Meeting

Galactic microquasars are binaries with a neutron star or stellar-mass black hole accreting gas from a companion star. They can accelerate particles to relativistic energies and emit radio flares, but the mechanism of jet energization is not known. A survey of the Cygnus region between mid-2007 and mid-2009 has detected four major γ-ray flaring episodes with energies over 100 MeV, each lasting 1 or 2 days, emanating from the microquasar Cygnus X-3. There is a clear pattern of temporal correlation between the γ-ray flares and transitional spectral states of radio and X-ray emission. Particle acceleration occurred a few days before radio jet ejections for two of the flares, suggesting that jet formation involves the production of highly energetic particles. Super-massive black holes in active galaxies can accelerate particles to relativistic energies, producing jets with associated γ-ray emission. Galactic 'microquasars' also produce relativistic jets; however, apart from an isolated event detected in Cygnus X-1, there has hitherto been no systematic evidence for the acceleration of particles to gigaelectronvolt or higher energies in a microquasar. Here, a report of four γ-ray flares with energies above 100 MeV from the microquasar Cygnus X-3 illuminates this important problem. Super-massive black holes in active galaxies can accelerate particles to relativistic energies1, producing jets with associated γ-ray emission. Galactic ‘microquasars’, which are binary systems consisting of a neutron star or stellar-mass black hole accreting gas from a companion star, also produce relativistic jets, generally together with radio flares2. Apart from an isolated event detected3 in Cygnus X-1, there has hitherto been no systematic evidence for the acceleration of particles to gigaelectronvolt or higher energies in a microquasar, with the consequence that we are as yet unsure about the mechanism of jet energization. Here we report four γ-ray flares with energies above 100 MeV from the microquasar Cygnus X-3 (an exceptional X-ray binary4,5,6 that sporadically produces radio jets7,8,9). There is a clear pattern of temporal correlations between the γ-ray flares and transitional spectral states of the radio-frequency and X-ray emission. Particle acceleration occurred a few days before radio-jet ejections for two of the four flares, meaning that the process of jet formation implies the production of very energetic particles. In Cygnus X-3, particle energies during the flares can be thousands of times higher than during quiescent states.

The main goal of the research presented in this thesis is to gain a better insight into the dynamical structure and evolution of galaxies, globular clusters and other stellar systems. In nearby stellar systems, we look for the fossil record of their formation by constructing realistic models that fit their photometric and (two-dimensional) spectroscopic observations in detail. A large fraction of this thesis is devoted to the development and validation of triaxial dynamical Schwarzschild models, which we apply to several elliptical galaxies.