Academic literature on the topic 'X-­ray pulsars'

In this thesis we analyse X-ray data of accretion powered low mass and high mass X-ray binaries to understand the nature of their accretion mechanisms by searching for some clues of viscous time-scales of their accretion discs, if they have, in their low frequency power density spectra created from their long-term X-ray observations, or by doing pulse timing analysis with much shorter X-ray data to detect the effects of torque fluctuations caused by the accreting material on the pulsar. The low mass and high mass X-ray binaries we analysed have breaks in their power density spectra, which are attributed to the role of viscosity in the formation of accretion discs. Although, the time-scales corresponding to these break frequencies are smaller than the predictions of the Standard theory of accretion discs, the sources give consistent results among themselves by displaying the expected correlation between their break and orbital frequencies. The correlation curve of LMXBs implies thicker appearing accretion discs than those assumed by the theory. The dichotomy of the HMXBs on this curve points out the different origins of accretion that these sources may have, and offers a way to distinguish the stellar-wind fed systems from the Roche-lobe overflow systems. The timing and spectral analysis of Swift J1626.6-5156 reveal a correlation between the spin-up rate and the luminosity of the source implying that the pulsar is accretion-powered. This correlation together with the characteristics of the X-ray spectra enables us to estimate the magnetic field and the distance of the source. The AXP 1E 2259+586 does not display any signs of viscous time-scale in its low frequency power density spectra, and its pulse timing analysis gives a much smaller torque noise value than that expected from accretion powered pulsars. In addition, the analysis results presented in this thesis reveal magnetar-like glitches which differ than those of radio pulsars, due to the presence of the strong magnetic field of the pulsar. These results eliminate the possibility that the AXP is an accretion-powered pulsar.

Anomalous X-ray Pulsars (AXPs) are a group of neutron stars that share several common properties. They are dubbed "anomalous'' because their observed luminosity exceeds the rate of loss of their rotational kinetic energy. According to the magnetar model, they are young, isolated neutron stars, powered by a large magnetic energy reservoir. Five of these AXPs (1E 2259+586, 4U 0142+61, RXS J170849.0-400910, 1E 1841-045, 1E 1048.1-5937) have been monitored with the Rossi X-ray Timing Explorer (RXTE) since 1997 (1998 for two of the sources). The goal of the monitoring program is to study the timing behaviour of these sources including glitches, and the radiative variability in the soft X-ray band including pulsed flux changes, pulse profile changes, and bursts. The program provides a detailed description of the evolving properties of these objects, which can then be used to test AXP models, such as the magnetar model. In this thesis, I present some of my co-authored contributions to the AXP monitoring program. <br><br> I first present a long-term (1998-2006) study of AXP 4U 0142+61 in which we show that from 2000 to 2006 the pulse profile of the pulsar slowly evolved, and the pulsed flux increased by 29+/-8%. <br><br> AXP 4U 0142+61 then entered an active phase in 2006 March that lasted several months and included several X-ray bursts. I present a study of the behaviour of AXP 4U 0142+61 during this active phase. <br><br> I then present a long-term (1998-2007) study of AXPs RXS J170849.0-400910 and 1E 1841-045 in which we report on four new glitches. We show that AXPs are very active glitchers and that some AXP glitch properties are challenging to interpret in standard glitch models. Furthermore, we note that AXP glitches appear to fall in two classes: radiatively loud and radiatively quiet. <br><br> Finally, I present a long-term (1997-2008) study of AXP 1E 1048.1-5937 in which we show that three timing events occurred cont<br>Les pulsars X anormaux (AXPs) sont des étoiles à neutrons qui partagent certaines propriétés spécifiques. Ils sont dits "anormaux'" car leur luminosité est supérieure au taux de perte de leur énergie de rotation. Selon le modèle des "magnétars'', ces pulsars sont des étoiles à neutrons jeunes, isolées, et dont la luminosité provient d'un grand réservoir d'énergie magnétique. Cinq de ces AXPs (1E 2259+586, 4U 0142+61, RXS J170849.0-400910, 1E 1841-045, 1E 1048.1-5937) sont sous observation depuis 1997 dans le cadre d'un programme de surveillance utilisant le Rossi X-ray Timing Explorer (RXTE). Le but de ce programme est d'étudier les propriétés temporelles de ces pulsars tels les changements soudains de fréquence, et d'étudier leurs propriétés radiatives tels les changements de flux pulsé, les changements de profil de pulse, et les sursauts d'énergie. Ce programme de surveillance aide à obtenir une description détaillée des propriétés de ces objets, qui servira à tester les modèles théoriques des AXPs, dont le modèle des "magnétars''. Dans cette thèse, je présente quelques-unes de mes contributions au programme de surveillance.<br><br>D'abord, je présente une étude à long terme (1998-2006) du AXP 4U 0142+61 où nous montrons qu'entre 2000 et 2006, le profil de pulse du pulsar a lentement évolué, et que le flux pulsé a augmenté de 29+/-8%.<br><br>Le AXP 4U 0142+61 est ensuite entré dans une phase active en mars 2006, dans laquelle il a émis plusieurs sursauts d'énergie. Je présente une étude du comportement du AXP 4U 0142+61 pendant cette phase active.<br><br>Ensuite, je présente une étude à long terme (1998-2007) des AXPs RXS J170849.0-400910 et 1E 1841-045. Dans cette étude, nous avons détecté quatre nouveaux changements soudains de fréquence. Nous montrons que dans le contexte de changements soudains de fréquence, les AXPs sont très actifs. Nous montrons auss

El objetivo principal de esta tesis ha sido el estudio del comportamiento transitorio durante estallidos de distinto tipo, de una selección de pulsares acretores en rayos X, localizados en el plano galáctico.Con ello se pretende haber avanzado hacia una explicación más clara de la naturaleza de estas objetos binarios de alta masa (con estrella de neutrones como objeto compacto), así como de los mecanismos físicos que operan en este escenario. Para todo esto se han analizado datos de dos misiones espaciales:INTEGRA Y RXTE.El segundo objetivo ha sido el estudio de la zona del brazo galactico de Scutum. Al ser esta una región de fuerte absorción se cree de la existencia de sistemas del tipo anteriormente mencionado este todavía escondidos, y los cuales solo pueden ser descubiertos a altas energías. En esta tesis se han podido presentar resultados esperanzadores, con la detección de varios objetos posibles nuevos candidatos a pulsares acretores de rayos X.

We report on the long-term monitoring of five anomalous X-ray Pulsars (AXPs) using the Rossi X-ray Timing Explorer (RXTE ). We present a phase-coherent timing ephemeris for 4U 0142+61, and show that it has rotated with high stability over 4.4 yr, with RMS phase deviation of 7% of the pulse period from a simple fit including only n and n&d2; . We show that 1E 1048.1-5937 shows significant deviations from simple spin-down such that phase-coherent timing has not been possible over time spans longer than a few months. We find that the deviations from simple spin down are not consistent with single "glitch" type events, nor are they consistent with radiative precession. We suggest that 1E 1048.1-5937 may be a transition object between the soft gamma-ray repeater and AXP populations, and the AXP most likely to one day undergo an outburst. We also report on the continued timing stability of 1E 2259.1+586, for which phase coherence has now been maintained over 4.5 yr, as well as on the detection of a significant n&d3; in RXS J170849.0-400910, consistent with recovery following a glitch. We note a possible correlation in which timing stability decreases with increasing n&d2; . We find no large variability in pulse morphology as a function of time. We present high signal-to-noise ratio average pulse profiles for each AXP, and consider them as a function of energy. We find a variety of different behaviors, and consider possible trends in the data. We also find no large variations in pulsed flux, and set la upper limits of &sim;20--30% of the mean.