arXiv subjects

Federico Garcia

Publications and source records attributed to Federico Garcia.

12 recordsLinked to original sources

arXiv2025

A Hidden Pulse: Uncovering a New Timing Signal in Cygnus X-1 with AstroSat

The study of fast variability properties in X-ray binaries advances our understanding of the physical processes and geometric properties of the accretion flow around the compact object. In this work, we study the evolution of the timing properties of Cygnus X-1 with AstroSat/LAXPC, during the transition of the source from the hard to soft state in 2017. We use a novel frequency-segmented technique to fit simultaneously the cross spectra and parts of the power spectra and coherence function with a multi-Lorentzian model and predict the phase-lags and the complementary parts of the power spectra and coherence function. We study the evolution of the frequency and power of the main variability components that are present throughout all the states. In particular, we identify previously undetected variability components, one of which manifests as a narrow dip in the coherence function and a broad drop in the phase-lag spectrum at the same frequency. This dip in coherence, which we detected for the first time in Cygnus X-1 at energies above 3 keV, appears in a state in which the source shows high-amplitude radio variability and significant hard X-ray polarization. While the contribution of the compact jet in X-rays is debated in the literature, this study provides a new avenue for investigating jet properties as well as the geometry of the Comptonizing medium.

astro-ph.HE↗

arXiv2023

Unveiling hidden variability components in accreting X-ray binaries using both the Fourier power and cross spectra

We present a novel method for measuring the lags of (weak) variability components in neutron-star and black-hole low-mass X-ray binaries (LMXBs). For this we assume that the power and cross spectra of these sources consists of a number of components that are coherent in different energy bands, but are incoherent with one another. The technique is based on fitting simultaneously the power spectrum (PS) and the Real and Imaginary parts of the cross spectrum (CS) with a combination of Lorentzian functions. We show that, because the PS of LMXBs is insensitive to signals with a large Imaginary part and a small Real part in the CS, this approach allows us to uncover new variability components that are only detected in the CS. We also demonstrate that, contrary to earlier claims, the frequency of the type-C quasi-periodic oscillation (QPO) in the black-hole binary GRS 1915+105 does not depend on energy. Rather, the apparent energy dependence of the QPO frequency can be explained by the presence of a separate QPO component with a slightly higher frequency than that of the QPO, whose rms amplitude increases faster with energy than the rms amplitude of the QPO. From all the above we conclude that, as in the case of the PS, the CS of black-hole and neutron-star binaries can be fitted by a combination of Lorentzian components. Our findings provide evidence that the frequency-dependent part of the transfer function of these systems can be described by a combination of responses, each of them acting over relatively well-defined time scales. This conclusion challenges models that assume that the main contribution to the lags comes from a global, broadband, transfer function of the accreting system.

astro-ph.HE↗

arXiv2023

The X-ray corona in the black-hole binary GRO J1655--40 from the properties of non-harmonically related quasi-periodic oscillations

The study of quasi periodic oscillations (QPOs) plays a vital role in understanding the nature and geometry of the Comptonizing medium around black-hole X-ray binaries. The spectral-state dependence of various types of QPOs (namely A, B, & C) suggests that they could have different origins. The simultaneous presence of different types of QPOs would therefore imply the simultaneous occurrence of different mechanisms. In this work we study the radiative properties of two non-harmonically related QPOs in the black-hole binary GRO J1655--40 detected at the peak of the ultraluminous state during the 2005 outburst of the source. The two QPOs have been previously identified as types B & C, respectively. We jointly fit the phase-lag and rms spectra of the QPOs and the time-averaged spectrum of the source with the time-dependent Comptonization model vkompth to infer the geometry of the media producing the QPOs. The time-averaged spectrum required a hot disk of 2.3 keV and a steep power law with index 2.7, revealing that the source was in an ultraluminous state. The corona that drives the variability of the type-B QPO is smaller in size and has a lower feedback fraction than the one that drives the variability of the type-C QPO. This suggests the simultaneous presence of a horizontally extended corona covering the accretion disk and a vertically elongated jet-like corona that are responsible for the type-C & B QPOs, respectively.

astro-ph.HE↗

arXiv2023

Fast-varying time lags in the Quasi-periodic Oscillation in GRS 1915+105

The properties of sub-second time variability of the X-ray emission of the black-hole binary GRS 1915+105 are very complex and strictly connected to its patterns of variability observed on long time scales. A key aspect for determining the geometry of the accretion flow is the study of time lags between emission at different energies, as they are associated to key time scales of the system. In particular, it is important to examine the lags associated to the strong low-frequency Quasi-periodic Oscillations (QPOs), as the QPOs provide unambiguous special frequencies to sample the variability. We have analyzed data from an observation with the AstroSat satellite, in which the frequency of the low-frequency QPO varies smoothly between 2.5 and 6.6 Hz on a time scale of ~10 hours. The derived phase lags show the same properties and evolution of those observed on time scales of a few hundred days, indicating that changes in the system geometry can take place on times below one day. We fit selected energy spectra of the source and rms and phase-lag spectra of the QPO with a time-variable Comptonization model, as done previously to RossiXTE data of the same source, and find that indeed the derived parameters match those obtained for variations on much longer time scales.

astro-ph.HE↗

arXiv2023

A variable corona during the transition from type-C to type-B quasi-periodic oscillations in the black hole X-ray binary MAXI J1820+070

We analyze a Neutron Star Interior Composition Explorer (NICER) observation of the black hole X-ray binary MAXI J1820+070 during a transition from type-C to type-B quasi-periodic oscillations (QPOs). We find that below ~2 keV, for the type-B QPOs the rms amplitude is lower and the magnitude of the phase lags is larger than for the type-C QPOs. Above that energy, the rms and phase-lag spectra of the type-B and type-C QPOs are consistent with being the same. We perform a joint fit of the time-averaged spectra of the source, and the rms and phase-lag spectra of the QPOs with the time-dependent Comptonization model vkompth to study the geometry of the corona during the transition. We find that the data can be well-fitted with a model consisting of a small and a large corona that are physically connected. The sizes of the small and large coronae increase gradually during the type-C QPO phase whereas they decrease abruptly at the transition to type-B QPO. At the same time, the inner radius of the disc moves inward at the QPO transition. Combined with simultaneous radio observations showing that discrete jet ejections happen around the time of the QPO transition, we propose that a corona that expands horizontally during the type-C QPO phase, from ~10^{4} km (~800 Rg) to ~10^{5} km (~8000 Rg) overlying the accretion disc, transforms into a vertical jet-like corona extending over ~10^{4} km (~800 Rg) during the type-B QPO phase.

astro-ph.HE↗

arXiv2023

A catalogue of high-mass X-ray binaries in the Galaxy: from the INTEGRAL to the Gaia era

High-mass X-ray binaries (HMXBs) are a particular class of high-energy sources that require multi-wavelength observational efforts to be properly characterised. New identifications and the refinement of previous measurements are regularly published in the literature by independent teams of researchers and might, when they are collected in a catalogue, offer a tool for facilitating further studies of HMXBs. We update previous instances of HMXB catalogues in the Galaxy and provide the community easy access to the most complete set of observables on Galactic HMXBs. In addition to the fixed version that is available in Vizier, we also aim to host and maintain a dynamic version that can be updated upon request from users. Any modification will be logged in this version. Using previous HMXB catalogues supplemented by listings of hard X-ray sources detected in the past 20 years, we produced a base set of HMXBs and candidates by means of identifier and sky coordinate cross matches. We queried in Simbad for unreferenced HMXBs. We searched for as many hard X-ray, soft X-ray, optical, and infrared counterparts to the HMXBs as we could in well-known catalogues and compiled their coordinates. Each HMXB was subjected to a meticulous search in the literature to find relevant measurements and the original reference. We provide a catalogue of 152 HMXBs in the Galaxy with their best known coordinates, the spectral type of the companion star, systemic radial velocities, component masses, orbital period, eccentricity, and spin period when available. We also provide the coordinates and identifiers for each counterpart we found from hard X-rays to the near-infrared, including 111 counterparts from the recent Gaia DR3 catalogue.

astro-ph.HE↗

arXiv2023

Generating Synthetic Clinical Data that Capture Class Imbalanced Distributions with Generative Adversarial Networks: Example using Antiretroviral Therapy for HIV

Clinical data usually cannot be freely distributed due to their highly confidential nature and this hampers the development of machine learning in the healthcare domain. One way to mitigate this problem is by generating realistic synthetic datasets using generative adversarial networks (GANs). However, GANs are known to suffer from mode collapse thus creating outputs of low diversity. This lowers the quality of the synthetic healthcare data, and may cause it to omit patients of minority demographics or neglect less common clinical practices. In this paper, we extend the classic GAN setup with an additional variational autoencoder (VAE) and include an external memory to replay latent features observed from the real samples to the GAN generator. Using antiretroviral therapy for human immunodeficiency virus (ART for HIV) as a case study, we show that our extended setup overcomes mode collapse and generates a synthetic dataset that accurately describes severely imbalanced class distributions commonly found in real-world clinical variables. In addition, we demonstrate that our synthetic dataset is associated with a very low patient disclosure risk, and that it retains a high level of utility from the ground truth dataset to support the development of downstream machine learning algorithms.

cs.LG↗

arXiv2022

Finding the birthplace of HMXBs in the Galaxy using Gaia EDR3: kinematical age determination through orbit integration

High-Mass X-ray Binaries (HMXBs) are produced after the first supernova event in a massive binary. These objects are intrinsically young, and can suffer from a significant natal kick. As such, the progenitors of HMXBs are likely to have formed away from the current location of the X-ray emitting systems. We aim to find the birthplace of the known HMXBs of our Milky Way. Specifically, we want to answer the question whether the formation of HMXBs can be associated to open stellar clusters and/or Galactic spiral structures, and infer from that the time elapsed since the first supernova event. We use astrometric data from the Gaia EDR3 to initialize the position and velocity of each known HMXBs from the Galaxy, and integrate their motion back in time. In parallel, we perform the same calculations on a sample of 1381 open clusters detected by Gaia as well as for four Galactic spiral arms which shape and motion have also been recently modelled using Gaia data. We report on all the encounter candidates between HMXBs and clusters or spiral arms in the past 100 Myr. In our sample of 26 HMXBs, we infer that 7 were born in clusters, 8 were born near a Galactic spiral arm, and conclude that 7 others could have formed isolated. The birthplaces of the remaining 4 HMXBs are still inconclusive due to a combination of great distance, poor astrometric data and lack of known open cluster in the vicinity. We provide the kinematical age since supernova of 15 HMXBs. The astrometry from Gaia and the orbit integration we employ are effective at finding the birthplaces of HMXBs in the Milky Way. By considering the biases in our data and method, we find it is likely that the progenitors of HMXBs preferentially formed alongside other massive stars in open clusters.

astro-ph.GA↗

arXiv2022

Constraints to neutron-star kicks in High-Mass X-ray Binaries with Gaia EDR3

All neutron star progenitors in neutron-star High-Mass X-ray Binaries (NS HMXBs) undergo a supernova event that may lead to a significant natal kick impacting the motion of the whole binary system. The space observatory Gaia performs a deep optical survey with exquisite astrometric accuracy, for both position and proper motions, that can be used to study natal kicks in NS HMXBs. We aim to survey the observed Galactic NS HMXB population and to quantify the magnitude of the kick imparted onto their NSs, and to highlight any possible differences arising in between the various HMXB types. We perform a census of Galactic NS HMXBs and cross-match existing detections in X-rays, optical and infrared with the Gaia Early Data Release 3 database. We retrieve their parallaxes, proper motions, and radial velocities (when available), and perform a selection based on the quality of the parallax measurement. We then compute their peculiar velocities with respect to the rotating reference frame of the Milky Way, and including their respective masses and periods, we estimate their kick velocities through Markov Chain Monte Carlo simulations of the orbit undergoing a supernova event. We infer the posterior kick distributions of 35 NS HMXBs. After an inconclusive attempt at characterising the kick distributions with Maxwellian statistics, we find that the observed NS kicks are best reproduced by a Gamma distribution of mean $116^{+18}_{-15}$km.s$^{-1}$. We note that supergiant systems tend to have higher kick velocities than Be High-Mass X-ray Binaries. The peculiar velocity versus non-degenerate companion mass plane hints at a similar trend, supergiant systems having a higher peculiar velocity independently of their companion mass.

astro-ph.HE↗

arXiv2022

The Health Gym: Synthetic Health-Related Datasets for the Development of Reinforcement Learning Algorithms

In recent years, the machine learning research community has benefited tremendously from the availability of openly accessible benchmark datasets. Clinical data are usually not openly available due to their highly confidential nature. This has hampered the development of reproducible and generalisable machine learning applications in health care. Here we introduce the Health Gym - a growing collection of highly realistic synthetic medical datasets that can be freely accessed to prototype, evaluate, and compare machine learning algorithms, with a specific focus on reinforcement learning. The three synthetic datasets described in this paper present patient cohorts with acute hypotension and sepsis in the intensive care unit, and people with human immunodeficiency virus (HIV) receiving antiretroviral therapy in ambulatory care. The datasets were created using a novel generative adversarial network (GAN). The distributions of variables, and correlations between variables and trends over time in the synthetic datasets mirror those in the real datasets. Furthermore, the risk of sensitive information disclosure associated with the public distribution of the synthetic datasets is estimated to be very low.

cs.LG↗

arXiv2022

Coupling between the accreting corona and the relativistic jet in the micro quasar GRS 1915+105

GRS 1915+105 was the first stellar-mass black-hole in our Galaxy to display a superluminal radio jet, similar to those observed in active galactic nuclei with a supermassive black hole at the centre. It has been proposed that the radio emission in GRS 1915+105 is fed by instabilities in the accretion disc by which the inner parts of the accretion flow is ejected in the jet. Here we show that there is a significant correlation between: (i) the radio flux, coming from the jet, and the flux of the iron emission line, coming from the disc and, (ii) the temperature of the corona that produces the high-energy part of the X-ray spectrum via inverse Compton scattering and the amplitude of a high-frequency variability component coming from the innermost part of the accretion flow. At the same time, the radio flux and the flux of the iron line are strongly anti-correlated with the temperature of the X-ray corona and the amplitude of the high-frequency variability component. These correlations persist over ~10 years, despite the highly variable X-ray and radio properties of the source in that period. Our findings provide, for the first time, incontrovertible evidence that the energy that powers this black-hole system can be directed either to the X-ray corona or the jet. When this energy is used to power the corona, raising its temperature, there is less energy left to fuel the jet and the radio flux drops, and vice versa. These facts, plus the modelling of the variability in this source show conclusively that in GRS 1915+105 the X-ray corona morphs into the jet.

astro-ph.HE↗

arXiv2020

Direct evidence of two-component ejecta in supernova 2016gkg from nebular spectroscopy

Spectral observations of the type-IIb supernova (SN) 2016gkg at 300-800 days are reported. The spectra show nebular characteristics, revealing emission from the progenitor star's metal-rich core and providing clues to the kinematics and physical conditions of the explosion. The nebular spectra are dominated by emission lines of [O I] $λ\lambda6300, 6364$ and [Ca II] $λ\lambda7292, 7324$. Other notable, albeit weaker, emission lines include Mg I] $\lambda4571$, [Fe II] $\lambda7155$, O I $\lambda7774$, Ca II triplet, and a broad, boxy feature at the location of H$α$. Unlike in other stripped-envelope SNe, the [O I] doublet is clearly resolved due to the presence of strong narrow components. The doublet shows an unprecedented emission line profile consisting of at least three components for each [O I]$\lambda6300, 6364$ line: a broad component (width $\sim2000$ km s$^{-1}$), and a pair of narrow blue and red components (width $\sim300$ km s$^{-1}$) mirrored against the rest velocity. The narrow component appears also in other lines, and is conspicuous in [O I]. This indicates the presence of multiple distinct kinematic components of material at low and high velocities. The low-velocity components are likely to be produced by a dense, slow-moving emitting region near the center, while the broad components are emitted over a larger volume. These observations suggest an asymmetric explosion, supporting the idea of two-component ejecta that influence the resulting late-time spectra and light curves. SN 2016gkg thus presents striking evidence for significant asymmetry in a standard-energy SN explosion. The presence of material at low velocity, which is not predicted in 1D simulations, emphasizes the importance of multi-dimensional explosion modeling of SNe.

astro-ph.SR↗