1,721,159 research outputs found
High-Frequency Variabilityin Neutron-Star Low-Mass X-rayBinaries
Full text linkBinary systems with a neutron-star primary accreting from a companion star display variability in the X-ray band on time scales ranging from years to milliseconds. With frequencies of up to ∼1300 Hz, the kilohertz quasi-periodic oscillations (kHz QPOs) represent the fastest variability observed from any astronomical object. The sub-millisecond time scale of this variability implies that the kHz QPOs are produced in the accretion flow very close to the surface of the neutron star, providing a unique view of the dynamics of matter under the influence of some of the strongest gravitational fields in the Universe. This offers the possibility to probe some of the most extreme predictions of General Relativity, such as dragging of inertial frames and periastron precession at rates that are 16 orders of magnitude faster than those observed in the solar system and, ultimately, the existence of a minimum distance at which a stable orbit around a compact object is possible. Here we review the last 20 years of research on kHz QPOs, and we discuss the prospects for future developments in this field.<br/
Unveiling hidden variability components in accreting X-ray binaries using both the Fourier power and cross spectra
No full textWe 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
GRS 1915+105 and the disc-jet coupling in accreting black hole systems
No full textGRS 1915+105—the first stellar-scale, highly relativistic jet source identified—is a key system for our understanding of the disc-jet coupling in accreting black hole systems. Comprehending the coupling between inflow and outflow in this source not only is important for X-ray binary systems but has a broader relevance for studies of active galactic nuclei and gamma-ray bursts. In this paper, we present a detailed review of the observational properties of the system, as established in the decade since its discovery. We attempt to place it in context by a detailed comparison with other sources, and construct a simple model for the disc-jet coupling, which may be more widely applicable to accreting black hole systems
The evolution of the phase lags associated with the Type-C Quasi-periodic oscillation in GX 339–4 during the 2006/2007 Outburst
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The evolving properties of the corona of GRS 1915+105: a spectral-timing perspective through variable-Comptonization modelling
Full text linkThe inverse Compton process by which soft photons are up-scattered by hot electrons in a corona plays a fundamental role in shaping the X-ray spectra of black hole (BH) low-mass X-ray binaries (LMXBs), particularly in the hard and hard-intermediate states. In these states, the power-density spectra of these sources typically show Type-C low-frequency quasi-periodic oscillations (QPOs). Although several models have been proposed to explain the dynamical origin of their frequency, only a few of those models predict the spectral-timing radiative properties of the QPOs. Here, we study the physical and geometrical properties of the corona of the BH-LMXB GRS 1915+105 based on a large sample of observations available in the RXTE archive. We use a recently developed spectral-timing Comptonization model to fit simultaneously the energy-dependent fractional rms amplitude and phase-lag spectra of the Type-C QPO in 398 observations. For this, we include spectral information gathered from fitting a Comptonization model to the corresponding time-averaged spectra. We analyse the dependence of the physical and geometrical properties of the corona upon the QPO frequency and spectral state of the source, the latter characterized by the hardness ratio. We find consistent trends in the evolution of the corona size, temperature, and feedback (the fraction of the corona photons that impinge back on to the disc) that persist for roughly 15 yr. By correlating our observations with simultaneous radio-monitoring of the source at 15 GHz, we propose a scenario in which the disc–corona interactions connect with the launching mechanism of the radio jet in this source
A two-component Comptonization model for the type-B QPO in MAXI J1348−630
Full text linkSpectral-timing analysis of the fast variability observed in X-rays is a powerful tool to study the physical and geometrical properties of the accretion/ejection flows in black-hole binaries. The origin of type-B quasi-periodic oscillations (QPO), predominantly observed in black-hole candidates in the soft-intermediate state, has been linked to emission arising from the relativistic jet. In this state, the X-ray spectrum is characterised by a soft-thermal blackbody-like emission due to the accretion disc, an iron emission line (in the 6-7 keV range), and a power-law like hard component due to Inverse-Compton scattering of the soft-photon source by hot electrons in a corona or the relativistic jet itself. The spectral-timing properties of MAXI J1348-630 have been recently studied using observations obtained with the NICER observatory. The data show a strong type-B QPO at ~4.5 Hz with increasing fractional rms amplitude with energy and positive lags with respect to a reference band at 2-2.5 keV. We use a variable-Comptonisation model that assumes a sinusoidal coherent oscillation of the Comptonised X-ray flux and the physical parameters of the corona at the QPO frequency, to fit simultaneously the energy-dependent fractional rms amplitude and phase lags of this QPO. We show that two physically-connected Comptonisation regions can successfully explain the radiative properties of the QPO in the full 0.8-10 keV energy range
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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