Search results for "Accretion"
showing 10 items of 385 documents
Spectral Evolution of Scorpio X‐1 along its Color‐Color Diagram
2007
We analyze a large collection of RXTE archive data of the bright X‐ray source Scorpius X‐1 in order to study the broadband spectral evolution of the source for different values of the inferred mass accretion rate by selecting energy spectra from its Color‐Color Diagram. We model the spectra with the combination of two absorbed components: a soft thermal component, which can be interpreted as thermal emission from an accretion disk, and a hybrid Comptonization component, which self‐consistently includes the Fe Kα fluorescence line and the Compton reflected continuum. The presence of hard emission in Scorpius X‐1 has been previously reported, however, without a clear relation with the accreti…
PSR J1012+5307: younger than it looks?
1998
Lorimer et al. have recently reported that the spin-down age (∼7 × 109 yr) of the low-mass binary pulsar PSR J1012+5307 is much higher than the cooling age (3 × 108 yr) of its white dwarf companion. The proposed solutions for this discrepancy are outlined and discussed. In particular, the revised cooling age estimate proposed by Alberts et al. agrees with data from other low-mass binary pulsar systems if a transition to the ‘classical’ cooling regime occurs between ∼0.14 and ∼0.28 M⊙. If this transition is excluded, PSR J1012+5307 seems to have finished its accretion phase far from the spin-up line.
Spin down of an Accreting Millisecond Pulsar, the case of XTE J1814‐338
2007
We report about a timing analysis performed on the data gathered by RXTE of the accreting millisecond pulsar XTE J1814-338 during its 2003 outburst. The first full orbital solution of this binary system is given. Moreover the evolution of the phase of the pulsed emission reveals that the rotating compact object is spinning down at a rate ν˙ = (-6.7 +/- 0.7) × 10-14 Hz/s, while accreting. This behavior is considered as a result of the braking effect due to the interaction between the magnetosphere and the inner parts of the accretion disc, in the case of an accretion rate low enough to allow the expansion of the magnetospheric radius to the corotation limit. In this context we derive an esti…
3D numerical modeling of YSO accretion shocks
2013
International audience; The dynamics of YSO accretion shocks is determined by radiative processes as well as the strength and structure of the magnetic field. A quasi-periodic emission signature is theoretically expected to be observed, but observations do not confirm any such pattern. In this work, we assume a uniform background field, in the regime of optically thin energy losses, and we study the multi-dimensional shock evolution in the presence of perturbations, i.e. clumps in the stream and an acoustic energy flux flowing at the base of the chromosphere. We perform 3D MHD simulations using the PLUTO code, modeling locally the impact of the infalling gas onto the chromosphere. We find t…
Timing Properties and Spectral States in Aquila X‐1
2003
We have analyzed five X-ray outbursts of the neutron-star soft X-ray transient Aql X-1 and investigated the timing properties of the source in correlation with its spectral states as defined by different positions in the color-color and hardness-intensity diagrams. The hard color and the source count rate serve as the distinguishing parameters giving rise to three spectral states: a low-intensity hard state, an intermediate state and a high-intensity soft state. These states are respectively identified with the extreme island, island and banana states that characterize the atoll sources. The large amount of data analyzed allowed us to perform for the first time a detailed timing analysis of…
Three-dimensional simulation of polytropic accretion discs
1991
Three-dimensional simulations of the formation and evolution of accretion discs in close binary systems,realised with the Smoothed Particle Hydrodynamics method to solve the fluid dynamic equations, are presented. Although the runs presented here refer to an ideal gas with different polytropic indexes, and constitute the first stage of more physically complex forthcoming simulations, they nervertheless give some interesting results: the disc structure and dynamics are in agreement with standard models only for small γ-values; as a consequence of the z-resolution it is found that disc formation is inhibited for γ ≥ 1.2, which means that some 2 D simulations of polytropic discs are meaningles…
X-ray emission from dense plasma in classical T Tauri stars: hydrodynamic modeling of the accretion shock
2008
Context: High spectral resolution X-ray observations of classical T Tauri stars (CTTSs) demonstrate the presence of plasma at temperature T~2-3×10^6 K and density n_e~10^11-10^13 cm^-3, which are unobserved in non-accreting stars. Stationary models suggest that this emission is due to shock-heated accreting material, but do not allow us to analyze the stability of the material and its position in the stellar atmosphere. Aims: We investigate the dynamics and stability of shock-heated accreting material in classical T Tauri stars and the role of the stellar chromosphere in determining the position and thickness of the shocked region. Methods: We perform one-dimensional hydrodynamic simulation…
Effects of Flaring Activity on Dynamics of Accretion Disks in YSOs
2009
We investigate the effects of strong flares on the accretion phenomena in YSOs. Among all classical assumptions, the model accounts magnetic-field oriented thermal conduction. We study the global dynamics of the system for two positions of the heating release triggering the flare.
Timing of accreting millisecond pulsars
2008
We review recent results from the X-ray timing of accreting millisecond pulsars in LMXBs. This is the first time a timing analysis is performed on accreting millisecond pulsars, and for the first time we can obtain information on the behavior of a very fast pulsar subject to accretion torques. We find both spin-up and spin-down behaviors, from which, using available models for the accretion torques, we derive information on the mass accretion rate and magnetic field of the neutron star in these systems. We also report here the first measure of the orbital period derivative for an accreting millisecond pulsar, derived for SAX J1808.4-3658 over a timespan of more 7 years.
Accretion disc evolution in cataclysmic variables (the 1988 international time project in La Palma
1990
A summary of data collected during a sixty night international campaign devoted to cataclysmic variables is presented.