Physics, Astrophysics and Cosmology with Gravitational Waves

Gravitational wave detectors are already operating at interesting sensitivity levels, and they have an upgrade path that should result in secure detections by 2014. We review the physics of gravitational waves, how they interact with detectors (bars and interferometers), and how these detectors operate. We study the most likely sources of gravitational waves and review the data analysis methods that are used to extract their signals from detector noise. Then we consider the consequences of gravitational wave detections and observations for physics, astrophysics, and cosmology.Comment: 137 pages, 16 figures, Published version <http://www.livingreviews.org/lrr-2009-2

Monsters in the Dark: High Energy Signatures of Black Hole Formation with Multimessenger Astronomy

When two compact objects inspiral and violently merge it is a rare cosmic event, producing fantastically “luminous” gravitational wave emission. It is also fleeting, staying in the Laser Interferometer Gravitational-wave Observatory’s (LIGO) sensitive band only for somewhere between tenths of a second and several tens of minutes. However, when there is at least one neutron star, disk formation during the merger may power a slew of potentially detectable electromagnetic counterparts, such as short gamma-ray bursts (GRBs), afterglows, and kilonovae. These explosions span the full electromagnetic spectrum and are expected within seconds, hours or days of the merger event. To learn as much astrophysics as possible requires targeted observations at every stage of this process, demanding a coordinated worldwide effort across many facilities and multiple astronomical disciplines, all in nearly real-time. In this dissertation I outline some of the major obstacles facing the multimessenger astronomy effort, including computation, data analysis and sky localization for LIGO source candidates, as well as disseminating this information quickly to the astronomical community. I also report on the performance of some of these services during Advanced LIGO’s first Observing Run, and on my experience at LIGO Livingston Observatory during the first Observing Run of LIGO’s Advanced stage, during which the instruments directly detected gravitational waves for the very first time. (The transient source GW150914 was observed 14 September 2015, and is consistent with a binary black hole merger at redshift 0.09.) I also participate in time-domain optical astronomy with the intermediate Palomar Transient Factory (iPTF) collaboration, searching for orphaned afterglow candidates to better understand the nature of relativistic outbursts such as GRBs

The impact of stellar envelopes on the formation of merging compact-object binaries

The common envelope (CE) phase play a key role in the formation of many astrophysical systems, including merging compact-object binaries. In a tight binary system, the CE phase happens when one star overfills its Roche lobe and initiates a process of dynamically unstable mass transfer. In this scenario, the mass transfer rate increases with time, the secondary star cannot accrete all the incoming material, and the latter surrounds the entire binary. The gas surrounding the binary star is known as CE. During the CE phase, the binary system rotates at a different rate than the CE. The orbital energy decreases due to the friction between the binary system and the CE. Due to orbital energy loss, the core of the donor and the companion star spiral toward one another within the CE (spiral-in phase). The orbital semi-major axis of binary systems can shrink by orders of magnitude during the spiral-in phase. The lost fraction of orbital energy is transferred to the envelope, which heats up and expands. The CE phase can end with two different outcomes. In the first scenario, the envelope is ejected, leaving the binary system with quite small semi-major axes. In the other scenario, during the spiral-in phase, the two stars merge and become an (evolved) massive star. Self-consistent hydrodynamical simulations of CE are very complex and computationally expensive. In fast population-synthesis studies, the CE phase is simulated via the (α,λ)- formalism, where α parameterizes the fraction of orbital energy transferred to the envelope, and λ is the envelope’s binding-energy parameter. The time required for a binary system to merge is highly dependent on the α and λ parameters, so their values have a crucial impact on the interpretation of many astrophysical systems, including merging compact-object binaries. While constraining the α parameter is challenging, we can calculate the λ values and consider λ as a physical quantity instead of a parameter. In this thesis, we present new results on self-consistent calculations of the binding energy parameter for a large set of hydrogen and helium stars, using the up-todate tracks from the PARSEC stellar evolution code. We demonstrate how the definition of the core-envelope boundary, the nature of the energy sources, metallicity, stellar mass, and evolutionary stage influence the value of λ parameters. We show that the new λ values are up to one order of magnitude lower than those obtained in previous studies and we discuss the associated implication for the formation of merging compact-object binaries. We present fitting formulas for the new binding energy parameters for hydrogen and helium stars obtained in this work, and we evaluate their accuracy with respect to self-consistent data. The SEVN population-synthesis code is the ground for implementing the new bindingenergy prescriptions obtained in this thesis and for an up-to-date astrophysical interpretation of present and forthcoming gravitational-wave sources. Since the SEVN code is based on the star tracks of the PARSEC stellar evolution code, it will be self-consistent to test our new CE prescriptions and study their impact on the evolutionary pathways of binary systems. In this thesis, we focus mainly on introducing several technical improvements in the SEVN code (e.g., adaptive data loading, single and multi-node parallelization), which is the preparatory work that will be crucial to perform efficient simulations of large populations of binary stars and testing the new binding-energy prescriptions

Portuguese SKA white book

Sem resumo disponível.publishe

The Fermi-LAT high-latitude Survey: Source Count Distributions and the Origin of the Extragalactic Diffuse Background

This is the first of a series of papers aimed at characterizing the populations detected in the high-latitude sky of the {\it Fermi}-LAT survey. In this work we focus on the intrinsic spectral and flux properties of the source sample. We show that when selection effects are properly taken into account, {\it Fermi} sources are on average steeper than previously found (e.g. in the bright source list) with an average photon index of 2.40$\pm0.02$ over the entire 0.1--100\,GeV energy band. We confirm that FSRQs have steeper spectra than BL Lac objects with an average index of 2.48$\pm0.02$ versus 2.18$\pm0.02$. Using several methods we build the deepest source count distribution at GeV energies deriving that the intrinsic source (i.e. blazar) surface density at F$_{100}\geq10^{-9}$\,ph cm$^{-2}$ s$^{-1}$ is 0.12$^{+0.03}_{-0.02}$\,deg$^{-2}$. The integration of the source count distribution yields that point sources contribute 16$(\pm1.8)$\,\% ($\pm$7\,\% systematic uncertainty) of the GeV isotropic diffuse background. At the fluxes currently reached by LAT we can rule out the hypothesis that point-like sources (i.e. blazars) produce a larger fraction of the diffuse emission.Comment: Version replaced to match the published one. Contact authors: M. Ajello and A. Tramacer

Sergio de Benedetti Manuscript (English translation)

The autobiography of a young physicist who left Italy after the passage of the Racial Laws in 1938. Written in 1941, this account covers the tumultuous years between the two world wars. English translation.https://digitalcommons.chapman.edu/debenedetti/1001/thumbnail.jp

Seeing Double: ASASSN-18bt Exhibits a Two-component Rise in the Early-time K2 Light

On 2018 February 4.41, the All-Sky Automated Survey for SuperNovae (ASAS-SN) discovered ASASSN-18bt in the K2 Campaign 16 field. With a redshift of z = 0.01098 and a peak apparent magnitude of B max = 14.31, ASASSN-18bt is the nearest and brightest SNe Ia yet observed by the Kepler spacecraft. Here we present the discovery of ASASSN-18bt, the K2 light curve, and prediscovery data from ASAS-SN and the Asteroid Terrestrial-impact Last Alert System. The K2 early-time light curve has an unprecedented 30-minute cadence and photometric precision for an SN Ia light curve, and it unambiguously shows a ~4 day nearly linear phase followed by a steeper rise. Thus, ASASSN-18bt joins a growing list of SNe Ia whose early light curves are not well described by a single power law. We show that a double-power-law model fits the data reasonably well, hinting that two physical processes must be responsible for the observed rise. However, we find that current models of the interaction with a nondegenerate companion predict an abrupt rise and cannot adequately explain the initial, slower linear phase. Instead, we find that existing published models with shallow 56Ni are able to span the observed behavior and, with tuning, may be able to reproduce the ASASSN-18bt light curve. Regardless, more theoretical work is needed to satisfactorily model this and other early-time SNe Ia light curves. Finally, we use Swift X-ray nondetections to constrain the presence of circumstellar material (CSM) at much larger distances and lower densities than possible with the optical light curve. For a constant-density CSM, these nondetections constrain ρ < 4.5 × 105 cm−3 at a radius of 4 × 1015 cm from the progenitor star. Assuming a wind-like environment, we place mass loss limits of for v w = 100 km s−1, ruling out some symbiotic progenitor systems. This work highlights the power of well-sampled early-time data and the need for immediate multiband, high-cadence follow-up for progress in understanding SNe Ia

Machine learning in astronomy

The search to find answers to the deepest questions we have about the Universe has fueled the collection of data for ever larger volumes of our cosmos. The field of supernova cosmology, for example, is seeing continuous development with upcoming surveys set to produce a vast amount of data that will require new statistical inference and machine learning techniques for processing and analysis. Distinguishing between real objects and artefacts is one of the first steps in any transient science pipeline and, currently, is still carried out by humans - often leading to hand scanners having to sort hundreds or thousands of images per night. This is a time-consuming activity introducing human biases that are extremely hard to characterise. To succeed in the objectives of future transient surveys, the successful substitution of human hand scanners with machine learning techniques for the purpose of this artefact-transient classification therefore represents a vital frontier. In this thesis we test various machine learning algorithms and show that many of them can match the human hand scanner performance in classifying transient difference g, r and i-band imaging data from the SDSS-II SN Survey into real objects and artefacts. Using principal component analysis and linear discriminant analysis, we construct a grand total of 56 feature sets with which to train, optimise and test a Minimum Error Classifier (MEC), a naive Bayes classifier, a k-Nearest Neighbours (kNN) algorithm, a Support Vector Machine (SVM) and the SkyNet artificial neural network