249 research outputs found
X-ray emission and thermonuclear bursts in neutron star X-ray binaries
Explosies op neutronensterren
Neutronensterren zijn de dichtste objecten die nog rechtstreeks waarneembaar zijn. Vanwege hun extreme dichtheid vormen neutronensterren een unieke proefopstelling voor de deeltjesfysica. Guo-Bang Zhang bestudeerde de röntgenstraling afkomstig van neutronensterren in een dubbelstersysteem met massaoverdracht van een begeleidende ster, waarbij hij zich richt op de accretiestaat, oppervlakte-emissie en thermonucleaire röntgenuitbarstingen.
In het eerste deel van zijn proefschrift analyseert hij waarnemingen van het systeem EXO 0748-676 tijdens de overgang naar de rustige toestand. Hij ontdekte dat het compacte object in dit dubbelstersysteem waarschijnlijk een neutronenster is en geen vreemde quarkster.
Voor het tweede deel van zijn proefschrift keek hij naar de thermonucleaire explosies (röntgenuitbarstingen) op het oppervlak van de aanwassende neutronenster in het dubbelstersysteem 4U 1636-53. Hij vindt dat de eigenschappen van deze explosies afhangen van de snelheid waarmee massa instroomt op de neutronenster tijdens de aanvang van de explosie. Zhang ontdekte een zeldzame driepiekige röntgenuitbarsting in deze bron en bediscussieert in zijn proefschrift de mogelijke oorzaak van zo’n explosie. Hij laat zien dat de gemiddelde relatie tussen bolometrische flux en temperatuur tijdens de afkoelfase van de röntgenuitbarstingen substantieel verschilt met de wet van Stefan-Boltzmann (flux is evenredig met temperatuur tot de vierde macht). Deze ongerijmdheid verklaart hij vanuit het oogpunt van de chemische samenstelling van de neutronensteratmosfeer. Ook vindt hij een correlatie tussen de spectrale en tijdsafhankelijke röntgenstralingparameters tijdens de afnemende fase van de straalexpansieexplosies in 4U 1636-53.
Neutron stars are the most compact objects that can be directly observed. Due to their extreme densities, neutron stars are unique laboratories for particle physics. In this thesis, I study the X-ray emission from neutron stars accreting from a companion star in a binary system, focusing on their accretion states, surface emission and thermonuclear X-ray bursts. In the first part of my thesis, I analyze an observation from the system EXO 0748-676, when the source underwent a transition to quiescence, and I find that the compact object in this X-ray binary is likely a neutron star rather than a strange quark star. In the second part of my thesis I examine the thermonuclear explosions (X-ray bursts) on the surface of the accreting neutron star in the system 4U 1636-53. I find that the properties of these bursts depend upon mass accretion rate onto the neutron star at the time of the onset of the burst. I discovered a very rare triple-peaked X-ray burst in this source; I further discuss the possible origin of such a burst. I show that the average relation between bolometric flux and temperature during the cooling phase of the X-ray bursts in 4U 1636-53 is significantly different from the canonical Stefan-Boltzmann law (flux proportional to temperature to the fourth power). I explain this in terms of the chemical composition of the neutronstar atmosphere. Finally, I found a correlation between the X-ray spectral and timing parameters in the decaying phase of radius-expansion bursts in 4U 1636-53.
Relation between the properties of the kilohertz quasi-periodic oscillations and spectral parameters in 4U 163653
We investigate the relation between the parameters of the energy spectrum and
the frequency and amplitude of the kilohertz quasi-periodic oscillations (kHz
QPOs) in the low-mass X-ray binary 4U 163653. We fit the -keV
spectrum of this source with a model that includes a thermal Comptonisation
component. We show that the frequencies of both kHz QPOs follow the same
relation as a function of the parameters of this spectral component, except for
a systematic frequency shift, whereas the rms fractional amplitude of each QPO
follows a different relation with respect to those same parameters. This
implies that, while the dynamical mechanism that sets the frequencies of the
QPO can be the same for both kHz QPOs, the radiative mechanisms that set the
amplitudes of the lower and the upper kHz QPO are likely different. We discuss
the implications of these results to the modelling of the kHz QPOs and the
possibility that the lower kHz QPO reflects a resonance between the
Comptonising medium and the photons from the accretion disc and/or the neutron
star surface.Comment: 7 pages, 4 figures, 2 tables. Accepted for publication in MNRA
Relation between spectral changes and the presence of the lower kHz QPO in the neutron-star low-mass X-ray binary 4U 1636-53
We fitted the -keV spectrum of all the observations of the
neutron-star low-mass X-ray binary 4U 163653 taken with the {\it Rossi X-ray
Timing Explorer} using a model that includes a thermal Comptonisation
component. We found that in the low-hard state the power-law index of this
component, , gradually increases as the source moves in the
colour-colour diagram. When the source undergoes a transition from the hard to
the soft state drops abruptly; once the source is in the soft state
increases again and then decreases gradually as the source spectrum
softens further. The changes in , together with changes of the electron
temperature, reflect changes of the optical depth in the corona. The lower
kilohertz quasi-periodic oscillation (kHz QPO) in this source appears only in
observations during the transition from the hard to the soft state, when the
optical depth of the corona is high and changes depends strongly upon the
position of the source in the colour-colour diagram. Our results are consistent
with a scenario in which the lower kHz QPO reflects a global mode in the system
that results from the resonance between, the disc and/or the neutron-star
surface, and the Comptonising corona.Comment: 9 pages, 6 figures. Accepted for publication in MNRA
Phase lags of quasi-periodic oscillations across source states in the low-mass X-ray binary 4U 1636-53
While there are many dynamical mechanisms and models that try to explain the
origin and phenomenology of the quasi-periodic oscillations (QPOs) seen in the
X-ray light curves of low-mass X-ray binaries, few of them address how the
radiative processes occurring in these extreme environments give rise to the
rich set of variability features actually observed in these light curves. A
step towards this end comes from the study of the energy and frequency
dependence of the phase lags of these QPOs. Here we used a methodology that
allowed us to study, for the first time, the dependence of the phase lags of
all QPOs in the range of 1 Hz to 1300 Hz detected in the low-mass X-ray binary
4U 1636-53 upon energy and frequency as the source changes its states as it
moves through the colour-colour diagram. Our results suggest that within the
context of models of up-scattering Comptonization, the phase lags dependencies
upon frequency and energy can be used to extract size scales and physical
conditions of the medium that produces the lags
Millihertz quasi-periodic oscillations in 4U 1636-53 associated with bursts with positive convexity only
We investigated the convexity of all type I X-ray bursts with millihertz quasi-periodic oscillations (mHz QPOs) in 4U 1636?53 using archival observations with the Rossi X-ray Timing Explorer. We found that, at a 3.5? confidence level, in all 39 cases in which the mHz QPOs disappeared at the time of an X-ray burst, the convexity of the burst is positive. The convexity measures the shape of the rising part of the burst light curve and, according to recent models, it is related to the ignition site of bursts on the neutron-star surface. This finding suggests that in 4U 1636-53 these 39 bursts and the marginally stable nuclear burning process responsible for the mHz QPOs take place at the neutron-star equator. This scenario could explain the inconsistency between the high accretion rate required for triggering mHz QPOs in theoretical models and the relatively low accretion rate derived from observations
Discovery of an Accretion-rate Independent Absolute RMS Amplitude of Millihertz Quasi-periodic Oscillations in 4U 1636-53
We investigate the frequency and amplitude of the millihertz quasi-periodic oscillations (mHz QPOs) in the neutron-star low-mass X-ray binary 4U 1636?53 using Rossi X-ray Timing Explorer observations. We find that no mHz QPOs appear when the source is in the hard spectral state. We also find that there is no significant correlation between the frequency and the fractional RMS amplitude of the mHz QPOs. Notwithstanding, for the first time, we find that the absolute rms amplitude of the mHz QPOs is insensitive to the parameter S a , which measures the position of the source in the color?color diagram and is usually assumed to be an increasing function of mass accretion rate. This finding indicates that the transition from marginally stable burning to stable burning or unstable burning could happen very rapidly since, before the transition, the mHz QPOs do not gradually decay as the ratefurther changes.Fil: Lyu, Ming. Xiangtan University; ChinaFil: Méndez, Mariano. University Of Groningen. Kapteyn Astronomical Institute; AlemaniaFil: Altamirano, D.. University of Southampton; Reino UnidoFil: Zhang, Guobao. Chinese Academy Of Sciences; ChinaFil: Mancuso, Giulio Cesare. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; Argentin
Study of the X-ray properties of the neutron-star binary 4U 172834 from the soft to hard state
We studied five XMM-Newton observations of the neutron-star binary 4U
172834 covering the hard, intermediate and soft spectral states. By jointly
fitting the spectra with several reflection models, we obtained an inclination
angle of 2553 and an iron abundance up to 10 times the solar. From the
fits with reflection models, we found that the fluxes of the reflection and the
Comptonised components vary inconsistently; since the latter is assumed to be
the illuminating source, this result possibly indicates the contribution of the
neutron star surface/boundary layer to the disc reflection. As the source
evolved from the relatively soft to the intermediate state, the disc inner
radius decreased, opposite to the prediction of the standard accretion disc
model. We also explore the possible reasons why the supersolar iron abundance
is required by the data and found that this high value is probably caused by
the absence of the hard photons in the XMM-Newton data.Comment: 14 pages, 6 figure
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