Resumen:

The evolution of the magnetic field in neutron stars is strongly related to their internal structure. In the NS core, there is a fluid mixture of neutrons, protons, and electrons (joined by other species at increasing densities) that scatter off each other through strong and electromagnetic interactions, causing effective friction forces, and can also convert into each other by weak interactions (‘‘Urca reactions’’). Likely, the dominant process evolving the magnetic field there is ambipolar diffusion, i.e., the joint motion of the charged particles and the magnetic field relative to the neutrons, driven by the Lorentz force and controlled by frictional forces and pressure gradients. Here, we present simulations of the long-term evolution of the magnetic field in the interior of an isolated, axially symmetric neutron star core, under ambipolar diffusion. Special attention is given to the characterization of the newly developed numerical approach, different physical processes involved, as well as their corresponding timescales, which happen to be in agreement with our numerical estimates.

Resumen:

The currently-growing LIGO-Virgo data provides an excellent opportunity to investigate the demography of gravitational-wave sources and their production channels. Gravitational-waves detected by LIGO-Virgo are originated in the coalescence of two compact objects, as black holes (BHs) or neutron stars (NSs). Compact-object binaries that merge in less than a Hubble time may arise as the last evolutionary stage of binary stars evolved in isolation. In addition, dynamically active and dense environments, such as star clusters, provide several mechanisms to produce or enhance these merger episodes via dynamical channels. Research in this topic is addressed performing population-synthesis simulations of binary systems and direct N-body star-cluster simulations. In this talk we will show our recently developed tool to compute both dynamics and binary evolution integrated together, which uses the group's population-synthesis code SEVN (Stellar EVolution N-body) within the state-of-the-art N-body code PeTar. SEVN uses a set of precomputed stellar tracks to interpolate the evolution of star properties on the fly. This is a change of paradigm in the field, since most population synthesis codes rely on fitting formulas to stellar evolution tracks run more than 20 years ago. We will show our results on the demography of binary compact objects, as binary BHs (BBHs) and BH-NSs, formed in young star clusters ($10^{4-5}~M_{\odot}$), and confront the results with those which use past stellar-evolution prescriptions.

Resumen:

With approximately a hundred detections to date, the discoveries of compact binary mergers by the LIGO and Virgo gravitational-wave detectors have opened a new observational window to the Universe. These data have been made publicly available, enabling their independent analysis by the community. In this talk I will describe various such analyses carried by our group. A first layer involves searching for signals. We have developed a detection pipeline implementing original solutions to mitigate systematics in the detectors, with which we were able to confirm previous detections and make new ones. A second stage requires estimating signal parameters (masses, spins, location...). We have developed algorithms to improve the efficiency and robustness of this process, and implemented them in an open-source code `cogwheel`. Lastly, considering the parameters of all events as a whole we can describe the statistical properties of the observed population of binary black holes and compare against predictions of candidate formation mechanisms. For example, I will show that the observed proportion of events with spins aligned versus anti-aligned with the orbit disfavors the hypothesis that the spin distribution is isotropic.

Resumen:

The binary-driven hypernova (BdHN) model is a proposed explanation for long gamma-ray bursts (GRBs) associated with type Ic supernovae (SNe) in which a binary system consisting of a carbon-oxygen (CO) star and a neutron star (NS) companion in close orbit undergoes a series of physical episodes. The event is triggered by the core collapse of the CO star, leading to a supernova (SN) explosion that forms a newborn NS (hereafter $\nu$NS) at its center. The SN accretes at super-Eddington rates on the $\nu$NS and the NS companion, leading to a binary system classified as type I when the NS forms a black hole (BH), type II when the NS becomes only a more massive NS, and type III when the orbital separation is too large for the companion NS to play any role in the GRB. With smooth hydrodynamics simulations, in this work, we investigate the binary parameters that determine whether the $\nu$NS or the NS companion reaches the point of gravitational collapse into a BH during the accretion process, as well as the time it takes for collapse to occur since the SN explosion. We also examine whether the system remains gravitationally bound after the SN explosion, leading to an NS-NS binary in BdHN II, an NS-BH binary in BdHN I, or if it disrupts altogether. We determine the maximum orbital period of the initial CO-NS binary that can remain bound after a BdHN event and calculate the characteristic parameters of the resulting NS-NS binaries.Overall, our findings shed light on the physical mechanisms driving BdHN and provide insight into the potential outcomes of such cataclysmic events in binary systems.

Resumen:

PKS2004-447 is a rare hybrid between a gamma-ray emitting narrow-line Seyfert 1 and a compact steep-spectrum radio source. It harbors a relatively small supermassive black hole accreting at typical NLS1s' Eddington ratio and, as our recent observations show, it connects these two classes of objects in the AGN evolution framework. PKS 2004-447 underwent, for the first time, a gamma-ray flare in 2019 mainly caused by the relativistic jet, which altered the broad line region with a flux excess redshifted by 250 km/s observed in the Balmer, Paschen, and He I permitted lines. This new emission feature was no longer visible 1.5 years later, suggesting a causal connection with the flare. The emission lines coming from the same atomic transition series show a similar velocity offset for this "red excess", but the offset changes for different line series. This discovery suggests that the relativistic jet can affect the physics of the BLR in this peculiar AGN, and that flaring activity can lead to the formation of additional and localized broad emission components. Our results highlight the importance of optical spectroscopy for flaring jetted AGN, and that our understanding of the jet-BLR -connection is still very limited.

Resumen:

Over the past few years, the steadily growing catalog of compact object mergers observed by the LVK Collaboration has allowed for a picture of the properties of these events as a population to start to emerge. One invaluable tool in the exploitation of this growing catalog has been rapid binary population synthesis (BPS), which allows for predictions for the rate of observed mergers and their properties, as well as for constraining the initial conditions of stellar/binary formation and evolutionary models by comparing the results with observations. While much has been done in BPS to constrain the evolutionary models, the initial conditions have been comparatively neglected. Chiefly, this ignores the long-expected trend of the initial mass function (IMF) to become top-heavy at high redshift, i.e., to produce more compact object progenitors. One such model for a varying IMF is the integrated-galaxy wide IMF (IGIMF). In this work, we employ the IGIMF and correlated orbital parameter distributions to perform the population synthesis of compact object mergers with the code COMPAS. We are able to generate populations consistent with star formation rate, metallicity and IMF evolution with redshift, such that we are able to study how observed merger rates and other properties vary with redshift. We find an overall tendency of merging binaries to be more massive the older they are and the higher the merger redshift is. We also find support for a peak in the merger rate of binary black holes (BBHs) between redshift ~2 and ~1. We compare this trend to the current LVK constraints on the merger evolution of the BBH merger rate. Our work provides a framework for redshift-dependent population synthesis broadly speaking, and for testing IMF variations in particular.