Dissertations / Theses on the topic 'Galactic binaries'

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 2002.
Includes bibliographical references (p. 127-140).
The research presented herein is a theoretical investigation of the formation, evolution, and ultimate fate of low-, intermediate-, and high-mass X-ray binaries (L/I/HMXBs). The primary theoretical tool used throughout is binary population synthesis. Results of these calculations are used to account for the numbers and properties of observed X-ray binaries and their descendants, as well as to direct future observational and theoretical work. Combining binary population synthesis and binary stellar evolution calculations, I present a systematic population study of L/IMXBs in the Galactic plane. Since full stellar evolution calculations are used to model the X-ray binary phase, it is possible to make detailed comparisons between the theoretical models and observations. It is demonstrated quantitatively that IMXBs probably play a crucial role in shaping the population of LMXBs observed at the current epoch, as well as their descendant binary millisecond radio pulsars. Recently, a new class of HMXBs has emerged, distinguished from other HMXBs by their wide, nearly circular orbits. I show that the discovery of a significant number of such systems is at odds with the conventional wisdom that most neutron stars receive very large "kick" speeds at birth. This problem may be rectified in a self-consistent way if the kick speed depends on the rotation rate of the pre-collapse core, which I propose is strongly influenced by the evolution of the neutron-star progenitor in a binary system. The reasonable suggestion that certain globular clusters contain nearly 1000 neutron stars conflicts with the large mean kick speeds estimated from observations of isolated radio pulsars, which are 5 to 10 times the present cluster escape speeds.
(cont.) Therefore, most neutron stars born from single progenitors should have been ejected from their host clusters. I show that many more neutron stars are retained if a significant fraction are formed with massive stellar companions, but that the retained fraction is still too small to account for the inferred large numbers of neutron stars at the current epoch. Several alternative hypotheses are discussed, including the intriguing possibility that globular clusters we see today were ten times more massive in the distant past. The Chandra X-ray Observatory has revealed hundreds of previously undetected point sources in a small field about the Galactic center. I show that the majority of these sources may be neutron stars accreting from the winds of unevolved companion stars. Infrared observations are proposed to search for the stellar counterparts of the X-ray sources.
by Eric D. Pfahl.
Ph.D.

In this thesis I study the inner-most regions of Active Galactic Nuclei (AGN) using the reverberation mapping technique, and neutron star low-mass X-ray binaries in quiescence using X-ray observations. Using the 13-year optical monitoring data for the AGN NGC 5548, the luminosity dependence of the Hβ emitting radius was modelled using a delay map, finding that the radius scales with luminosity as predicted by recent theoretical models. Time-delays between the continuum at different wavelengths in AGN can be used to probe the accretion disc. Here, continuum time-delays in a sample of 14 AGN were used to measure the radial temperature profile of the accretion discs, determine the nuclear extinction, and measure distances to the objects. However, the distances measured correspond to a value for Hubble's constant that is a factor of ~2 lower than the accepted value. The implications of this on the thermal disc reprocessing model are discussed. I present two Chandra observations of the neutron star transient in the globular cluster NGC 6440 in quiescence, where the power-law component to the spectrum is seen to be variable between the observations, suggesting that there is ongoing residual accretion. From a Chandra observation of the globular cluster Terzan 1, I have identifed the likely quiescent counterpart to a transient previously observed in outburst, and discuss the other sources within the cluster. Using Chandra and XMM-Newton monitoring observations of two neutron star transients (KS 1731-260 and MXB 1659-29) in quiescence I have found that the neutron star crusts in both sources have now returned to thermal equilibrium with the core. These observations also indicate that the crusts in both sources may have a high thermal conductivity and that enhanced neutrino emission may be occurring in the core. Finally, the discovery of an X-ray transient with XMM-Newton is presented, and the other sources in this observation discussed.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2002.
Includes bibliographical references (p. 183).
In rare cases, optical observations of Galactic binary star systems which are bright in the X-ray portion of the electromagnetic spectrum dynamically constrain the mass of one component to be well above theoretical limits for a neutron star. These systems - and systems with similar X-ray properties - are classified as black hole binaries. In this thesis, I report on observations of black hole binaries made with satellite observatories in the X-ray band. The region closest to the black hole is revealed in X-rays due to the viscous heating of matter that is accreted from the companion star. X-ray observations of these systems may therefore reveal General Relativistic effects. A fundamental and testable prediction of General Relativity is that matter may orbit more closely around black holes with significant angular momentum. I have investigated the possibility of black hole "spin" and the geometry of accretion flows in these systems using X-ray continuum spectroscopy, fast variability studies, and the shape of iron fluorescent emission lines in this band. I present evidence for black hole spin in XTE J1550-564, XTE J1650-500, and XTE J1748-248. Spin is not required by high-resolution spectral analysis of the archetypical Galactic black hole - Cygnus X-1 but a thermal accretion disk plus hot corona geometry is confirmed. Studies of XTE J1118+480 and GRS 1758-258 at low X-ray luminosity reveal that models for radiatively-inefficient accretion do not satisfactorily describe the geometry in these systems.
y Jon Matthew Miller.
Ph.D.

A planetary nebula (PN) consists of material, mainly gas, that has been ejected from a star on the asymptotic giant branch of its life cycle. This material emits electromagnetic radiation due to photoionization and recombination, collisional and radiative excitation or free-free radiation. The envelope of material moves outwards from the central star and may take one of a variety of shapes. These shapes are believed to be sculpted by the stellar wind, magnetic fields and interactions with a binary companion. However, within a time scale of as little as 10 000 years the nebula fades from view and merges with the interstellar medium.Similar variations in the shape of planetary nebulae (PNe) can be seen in both the Galactic Bulge and Disc and in the Magellanic Clouds. It is therefore reasonable to assume that the shaping process is universal. By classifying PNe by morphology and relating those shapes to other nebular properties we have attempted to derive information about that shaping process.We have used photometric narrowband observations of a sample of PNe listed in the Strasbourg-ESO Catalogue of Galactic Planetary Nebulae to investigate the relationship between PN morphology and the other PN characteristics. The high resolution images were made using ESO's New Technology Telescope and the Hubble Space Telescope. The information we could obtain directly from the observations was augmented by information in the literature in order to address that question. The observations were used to classify the morphologies of 154 PNe, to estimate the sizes of 138 of those nebulae that we considered to lie within the Galactic Bulge, to determine the orientations of 130 of those Bulge nebulae and to derive photometric fluxes for the 69 PNe which had observations of standard stars made during the same night. Information on central star binarity, nebular abundances and radial and expansion velocity was obtained from the literature.Our photometrically derived PNe line fluxes were used to verify 59 H-beta and 69 [OIII] catalogued values (which were obtained using spectroscopy). We found sufficient discrepancy between the values for 9 PNe to merit a further check taking place.We found no distinguishing relationship between PN morphology and any of PN size, radial velocity, or angular location within the Bulge. The abundances of He and O, and the N/O ratio, are generally lower in bipolar nebulae than in those nebulae with no apparent internal structure. We are unable to come to any conclusion as to a relationship between PN morphology and stellar metallicity.Given the short lifespan of PNe and the age of the Bulge it appears that almost all PNe in the Bulge must be associated with low mass stars. The high ratio of bipolar PNe we found in our Bulge sample suggests that, at least within the Bulge, bipolar nebulae are not necessarily associated with high mass stars. Our results show that unlike the orientations of other types of PNe the orientations of the bipolar nebulae in the Bulge are not randomly distributed. Measured to a line tip to tip along the lobes they peak and have their mean approximately along the Galactic Plane. This suggests that the bipolar PNe originate in a different environment from other morphological types, perhaps related to binary separation. However, we find that bipolarity does not imply common-envelope evolution. If the hypothesis that bipolar nebulae are formed in binary star systems is correct, binary systems in the Galactic Bulge have angular momentum vectors that are preferentially aligned along the Galactic Plane. As the orientation appears to be unrelated to lobe size and hence nebular age, the alignment implies that the non-random nature of the angular momentum vectors originated at the time the Bulge stellar population formed. We suggest that it is due to the direction and strength of the ambient magnetic fields.

Neutron stars are some of the most magnetic objects that have ever been observed, and so they provide physicists with unique environments where fundamental laws of physics can be tested. Neutron stars are typically thought to have magnetic fields between 108 and 1014 G. The effects of the quantum electrodynamics are important above the quantum critical field (BQED) of 4.4×1013 G. In this thesis, I provide evidence that there may be many more neutron stars with B > BQED than previously thought, and that all neutron stars in binary systems that are close to spin equilibrium follow the same relationship between spin period (P) and magnetic field. In Chapter 2, I determine the long-term average X-ray luminosity, spin period, and rate of change of spin period for 42 Be X-ray binaries (BeXB) in the Small Magellanic Cloud (SMC). I use this information, combined with orbital data, to show that the neutron stars in all of these systems are disc-accreting, and that 85% are close to spin equilibrium. All systems with P & 100 s are predicted to have B > BQED. This applies to 2/3 systems. These predicted magnetic fields are higher than those of neutron stars in Galactic BeXB that have had their magnetic fields directly measured via cyclotron resonance scattering features (CRSF). I conclude that this is because the CRSF sources are not close to spin equilibrium. In Chapter 3, I look at pulse-profiles for the neutron stars discussed in Chapter 2 and find that they contain an array of features that vary both across and within individual systems. I suggest that BeXB containing neutron stars with relatively longer spin periods transition from a pencil to a fan beam at lower luminosities. In Chapter 4, I apply the methods used in Chapters 2 and 3 to LXP187, a BeXB in the Large Magellanic Cloud (LMC) that is not close to spin equilibrium. Results for LXP187 help confirm the conclusions of Chapter 2 - that � 2/3 BeXB contain neutron stars with B > BQED.

Le futur observatoire spatial d’ondes gravitationnelles LISA ouvrira une nouvelle fenêtre pour la mesure des ondes gravitationnelles, permettant d’observer des sources difficilement visibles avec les observatoires terrestres actuels. Parmi ces sources, la détection des binaires galactiques promet une richesse d’informations sans précédent, mais soulève également plusieurs défis d’analyse de données. En particulier, le grand nombre de sources attendues et la présence à la fois de bruit et d’artefacts entachant les données nécessitent le développement de méthodes d’analyse robustes.Grâce à une modélisation simple des signaux recherchés, nous montrons qu’il est possible de détecter les signaux en présence de bruit et de les estimer. Nous expliquons ensuite comment ce modèle peut être utilisé pour atténuer efficacement l’impact des données manquantes sur l’analyse. Enfin, nous étudierons ce qu’un nouveau modèle appris peut apporter en termes de caractérisation du signal
The forthcoming space-based gravitational wave observatory LISA will open a new window for the measurement of gravitational waves, making it possible to observe emitting systems hardly visible with the current Earth-based observatories. Among these sources, the detection of galactic binaries promises an unprecedented wealth of information about these systems, but also raises several challenges in signal processing. In particular the large number of expected sources and the presence of both complex instrumental noise and artifacts tainting the data call for the development of robust methods. Through simple modeling of the sought signals, we show that it is possible to detect them accurately in presence of instrumental noise and to recover the signals. We then explain how this model can be used to efficiently mitigate the impact of missing data on the analysis. Finally, we investigate what a new learning-based model can bring in terms of signal characterization

La population galactique des nébuleuses planétaires (NP) permet d'apporter de précieuses contraintes pour améliorer notre compréhension de multiples problèmes astrophysiques tant à l'échelle de la Galaxie qu'à celle de l'évolution stellaire. Les NP sont détectables à de grandes distances, car leur rayonnement est concentré en de brillantes raies d'émission, dont la position et l'intensité permettent de déterminer leurs vitesses radiales et abondances chimiques. Appartenant à une population stellaire plutôt âgée, les NP sont particulièrement abondantes dans le bulbe galactique, et leur cinématique est un robuste traceur de la dynamique de cette région. Les gradients chimiques permettent aussi de contraindre les modèles chemico-dynamiques de la Galaxie. A une échelle toute différente, les étoiles centrales des NP (ECNP) permettent une description améliorée des stades finaux très mal connus de l'évolution des systèmes d'étoiles doubles. Pour que ces diverses études puissent être conduites correctement, un échantillon significatif de la population des NP doit être constitué. Le nombre de NP connues a été récemment doublé par le"Macquarie/AAO/Strasbourg Halpha (MASH) PNe catalogue" et atteint environ 2 700 NP en 2009. Le catalogue MASH-II est un supplément de MASH et contient plus de 360 nouvelles NP galactiques, découvertes suite à un sondage profond de tous les 233 champs du relevé AAO/UKST SuperCOSMOS Halpha en sa version digitale. De nouvelles techniques semi-automatiques de traitement de données et de visualisation multi-longueur d'ondes ont été developpées afin de maximiser la sensibilité de la recherche. Les NP de MASH-II se révèlent soit très petites et de brillance de surface élevée, soit très étendues avec une brillance de surface extrêmement faible. Plus de 90\% du catalogue a été confirmé spectroscopiquement au cours d'une vaste campagne d'observation. Le catalogue est disponible pour la communauté via le service vizir du Centre de Données astronomiques de Strasbourg (CDS). Cette thèse est basée sur l'exploitation des catalogues MASH et MASH-II représentant l'échantillon le plus vaste, le plus homogène, et le plus représentatif des NP dans la direction du centre galactique, offrant une opportunité unique de contribuer efficacement à deux domaines de recherche encore peu explorés : (i) La cinématique du bulbe galactique, et (ii) Le rôle de la binarité des étoiles centrales de NP. De nouvelles vitesses radiales ont été mesurées pour des centaines de NP dans la direction du bulbe, avec le spectrographe à fente du télescope ANU 2. 3-m et la spectroscopie à haute sensibilité des équipements multi-objets AAT 2dF/AAOmega et VLT FLAMES. De nombreuses mesures ont été effectuées pour beaucoup de NP résultant en un catalogue plus précis de 1200 NP dans la zone de longitude inférieure à 30° qui est couverte à 95%. L'étude cinématique a conduit à une courbe de la rotation du bulbe de pente 104 km/s/kpc en excellent agrément avec la valeur de 100 km/s/kpc determinée pour les étoiles géantes M. Des distributions cinématiques diverses ont été calculées et comparées en bon agrément avec celles d'autres populations traceurs, afin de mieux contraindre un modèle dynamique du bulbe. Dans le domaine des étoiles centrales (ECNP), une approche nouvelle et très performante a été conçue pour découvrir un grand nombre de ECNP binaires. Il s'agit d'analyser les séries de relevés photométriques d'environ 300 NP du bulbe sur le relevé OGLE-III (microlensing survey). Des variations périodiques ont été trouvées pour 21 ECNP binaires possibles, après l'élimination de 27 fausses identifications (NP mimics) détectées grâce à la spectroscopie à haute résolution. La distribution des périodes est dominée par des périodes plus courtes qu'un jour, ce qui implique que ces binaires très rapprochées auraient été produites via la phase d'enveloppe commune (common-envelope CE) de l'évolution de systèmes binaires. Ces découvertes ont doublé la population de ECNP binaires serrées, ce qui permettra pour la première fois une importante avancée dans la connaissance de l'évolution de la phase CE des binaires. La spectrométrie Gemini GMOS de 14 ECNP de l'échantillon OGLE confirme 10 « bona fide » ECNP binaires, plus 2 probablement binaires, et 2 à rejeter. De plus trois candidates au centre de petites NP devront être confirmées par une spectrométrie future, ainsi que quatre autres candidates situées au centre de NP plus étendues. Des étoiles géantes froides ont été révélées comme compagnons d'au moins deux ECNP binaires, et pour d'une d'elles la photométrie UV a révélé l'existence de la primaire invisible dans le spectre optique. Ceci suggère que des compagnons froids seraient plus fréquents que ce que l'on supposait, et permet d'éliminer des scénarios plus exotiques souvent évoqués (p. Ex. Born-again scénario). La distribution des périodes observée apparaît biaisée vers les périodes plus courtes que celles prédites par les modèles courants de synthèse de population CE. Un seul modèle de la littérature est compatible avec la distribution observée - à l'inverse de modèles plus récents. Les binaires serrées représenteraient une fraction d'au moins 10—20 % de l'ensemble des ECNP. Après une attentive prise en compte d'effets de sélection et autres limites des relevés, nos estimations apparaissent plus robustes que les estimations conduites au cours des 20--30 ans dans le passé avec des biais très incertains. Le rôle de la binarité est certainement primordial pour comprendre la morphologie des nébuleuses. Au vu de la proportion élevée de binaires serrées, au moins 10 à 20% des NP auraient du être modelée par la binarité, mais à ce jour aucune morphologie spécifique n'avait été identifiée pour les rares NP à noyau binaire serré connues. Nous avons découvert qu'environ 30% d'un échantillon de 30 NP post-CE soigneusement sélectionnées présentent une morphologie bipolaire typique. Une fraction d'au moins 60% de NP bipolaires est atteinte si on considère les variations d'inclinaison et d'autres effets. Il s'agit de l'étude la plus convaincante à ce jour montrant que la morphologie observée des NP post-CE est tout à fait compatible avec les conjectures théoriques de la formation de hauts contrastes de densité pendant la phase CE. Des structures de basse ionisation (Low Ionisation Structures LIS) sont souvent observées dans les NP post-CE. LIS apparaissent confinées soit comme des filaments ou noeuds distribués radialement dans le plan orbital, ou comme des jets à faible brillance de surface dans les régions polaires. La binarité de ECNP expliquerait aussi les LIS apparaissant autour de noyaux à émission, où une ou plusieurs phases CE auraient créé la morphologie observée et la chimie complexe des poussières (dual-dust-chemistry). Une oigine binaire expliquerait aussi la distribution (durant la phase CE) dans le plan orbital de condensations neutres de poussières et molécules d'hydrogène, qui seront ensuite photoionisés durant la phase NP. Si ce scénario binaire est avéré, les abondance d'azote des NP avec LIS doivent être revues, car les émissions [NII] observées ne seraient pas le résultat de matériau stellaire enrichi, mais seraient plutôt dues à des chocs proposés dans des modèles avec des abondances standard
The Galactic population of planetary nebulae (PNe) offers great potential in improving our understanding of many astrophysical problems on both large and small scales. They are revealed out to large distances by their bright emission line spectra from which their radial velocities and chemical abundances can be measured. As members of the old stellar population, PNe are particularly abundant towards the Galactic bulge where their kinematics are a valuable, relatively unbiased tracer of the dynamics of the region. Chemical abundance variations may also be traced by PNe to place constraints on chemodynamical models of the Galaxy. On much smaller scales their central stars (CSPN) are a powerful window into the poorly understood later stages of binary stellar evolution. The capacity of PNe to perform these studies is critically dependent on the size of the population. The current Galactic population of PNe was recently doubled by the Macquarie/AAO/Strasbourg Halpha (MASH) PNe catalogue. A supplement to MASH, the MASH-II catalogue, is presented with more than 360 new Galactic PNe found after a thorough search of all 233 AAO/UKST SuperCOSMOS Halpha Survey fields in digital format. Novel semi-automated data processing and multi-wavelength visualisation techniques are developed to maximise the sensitivity of the search. MASH-II PNe are notable for being either small, star-like PNe of relatively high surface brightness, or very large, extremely low surface brightness PNe. Over 90% of the catalogue is confirmed spectroscopically during extensive observing campaigns and the catalogue is available via the vizier catalogue service at the Centre de Donn\'ees Astronomiques de Strasbourg (CDS). This thesis is based on the exploitation of the MASH and MASH-II PNe catalogues that have provided the largest and most representative sample of PNe towards the Galactic bulge. This offers a unique opportunity to contribute towards two different, largely unexplored research domains: (i) The kinematics of the bulge, and (ii) The role of binary central stars of PNe. Radial velocities of hundred of Pne towards the Bulge were measured from ANU 2. 3-m longslit spectroscopy and from deep spectroscopy conducted with the AAT 2dF/AAOmega and VLT FLAMES multi-object spectroscopy facilities. Multiple measurements were recorded for many PNe resulting in a more accurate catalogue of about 1200 PNe within the longitude smaller than 30° region reaching a very high completeness of 95%. The kinematic study enabled a slope of 104 km/s/kpc for the rotation curve of the bulge that is in excellent agreement with 100 km/s/kpc determined from M-giants. General kinematic profiles were calculated and compared well with other tracer populations to bring new constraints on a dynamical model of the bulge. A completely new and powerful approach is conceived to discover large numbers of binary CSPN. The concept was employed to analyse the time-series photometry of nearly 300 Galactic bulge PNe from the OGLE-III microlensing survey. A total of 21 periodic binary CSPN candidates were found after careful elimination of 27 PN mimics identified using deep spectroscopy. The orbital period distribution is dominated by periods less than one day which indicates these binaries must have been produced via the common-envelope (CE) phase of binary stellar evolution. These discoveries have effectively doubled the population of close binary CSPN whose potential in advancing our knowledge of CE evolution has yet to be realised. Gemini GMOS spectroscopy of the 14 members of the OGLE sample produces 10 bona fide binary CSPN, 2 likely binary CSPN and 2 unlikely associations. There remains three candidates in the centre of small nebulae which leave little doubt of their bona fide status pending future spectroscopy, while four other candidates lie in larger nebulae awaiting confirmation. Cool giant companions are revealed in at least two binary CSPN and in one instance UV photometry proves the existence of the primary invisible in the optical spectrum. This suggests cool central stars may be more common than previously thought and more exotic scenarios explaining their presence can be ruled out. The observed orbital period distribution is found to be biased towards shorter periods than predicted by CE population synthesis models. Only one model in the literature matches the distribution reasonably well, but more recent models could not reproduce its predictions. A close binary fraction of at least 10--20% is estimated for PNe. After consideration of selection effects and other limitations of the survey, our estimate is found to be more robust than the previous estimate obtained from previous a survey conducted over 20--30 years with uncertain biases. Of particular interest is elucidating the role of binarity in the shaping of nebular morphologies. The close binary fraction imposes that at least 10--20% of PNe have been heavily shaped by a close companion, however no clear morphological properties have been identified amongst PNe with close binary CSPN. Nearly 30% of a carefully selected sample of 30 post-CE nebulae are found to have canonical bipolar morphologies. A very plausible bipolar fraction of at least 60% is reached once the inclination and other effects are considered. This is the strongest indication yet that the morphologies of post-CE nebulae largely satisfy theoretical expectations of a high density contrast established during the CE phase. Low ionisation structures (LIS) are common amongst post-CE nebulae suggesting they have a binary origin. LIS seem confined to either the orbital plane as radially distributed knots or filaments, or to the polar regions as (mostly) low surface brightness jets triggered by a dynamo effect. A binary origin may also be responsible for LIS around emission-line nuclei whereby one or more CE phases created the identifiable morphology and dual-dust chemistry. A likely binary formation scenario for LIS includes the distribution of neutral clumps of dust and H2 during the CE phase into the orbital plane that are then photo-ionised by winds during the PN phase. If the binary scenario holds then Nitrogen abundances for PNe with LIS would be rendered meaningless since the [NII] emission observed is not a result of enriched stellar material, but rather reflects shocked emission generated with models that assume standard abundances

Etude d'etoiles de type a-f au voisinage solaire, de noyaux brillants de nebuleuses planetaires et des sous naines les plus brillantes. On s'interesse plus specialement a leur repartition galactique: jeune disque, vieux disque, halo. Les relations theoriques entre binarite, rotation, pulsation et structure d'enveloppe chez les etoiles a-f ont ete confirmees par analyse statistique. Un noyau a spectre composite a ete decouvert dans la nebuleuse planetaire lts et des variations d'eclat periodiques decelees dans le noyau abell 35. Six nouvelles binaires spectroscopiques ont par ailleurs ete decouvertes parmi les sous naines brillantes du halo

L'elaborato prende in esame il fenomeno della bremsstrahlung, ovvero la radiazione prodotta da una carica decelerata dall'interazione con il campo coulombiano di un'altra carica. Nel primo capitolo è presentata una descrizione fisica del fenomeno. Il caso base di una singola particella viene poi ampliato al caso di più ampio interesse astrofisico di una nube di particelle interagenti, con particolare attenzione al caso in cui, all'equilibrio termico, è presente autoassorbimento da parte degli stessi elettroni che emettono la radiazione. Nel secondo capitolo si studiano alcune applicazioni astrofisiche che mostrano come la bremsstrahlung sia uno dei meccanismi di radiazione più diffusi nel cosmo quando sono presenti particelle libere, e dunque gas ionizzati. Analizziamo le fasi HIM e WIM del mezzo interstellare, portando la nebulosa di Orione come esempio di regione WIM particolarmente ionizzata, detta regione HII. Studiamo le galassie con tasso di formazione stellare particolarmente intenso, chiamate galassie starburst, e le esplosioni di supernovae che le alimentano e che sono a loro volta fonti di bremsstrahlung termica e relativistica. Come altro esempio di bremsstrahlung relativistica, si citano gli oggetti di Herbig-Haro, causati dall'interazione fra onde d'urto e nubi di materia. Si parla poi di ambienti astrofisici che emettono nei raggi X: sistemi di stelle binarie, per le quali si discutono anche i meccanismi che possono portare al trasferimento di materia da uno dei due corpi all'altro; corone galattiche alimentate da nebulose planetarie ed esplosioni di supernovae; e per concludere si parla di ammassi di galassie e di come in questo contesto lo studio dell'emissione per bremsstrahlung abbia portato alla soluzione del "problema della massa mancante", dando prova dell'esistenza della materia oscura.

More than half a decade of X-ray astronomy with various balloon borne and space orbiting X-ray instruments, have led to discoveries and detailed studies of X-ray binaries. An important property of X-ray binaries is intensity variations of different magnitudes in a wide range of timescales from milliseconds (quasi-periodic oscillations, millisecond pulsations), to a few weeks (orbital and super-orbital modulations) or longer (outbursts etc). In this thesis, different types of variabilities of X-ray binaries are considered in X-ray binary population studies and to investigate certain aspects of some individual systems. In Chapter 1, we provide an introduction to various types of variabilities seen in different classes of X-ray binaries. We mention in detail the various periodic and aperiodic variabilities seen in X-ray binaries. In Chapter 2, we describe, in some detail, the various X-ray all sky monitors and X-ray observatories, data from which has been utilized in the work carried out in this thesis. We also describe the various data analysis techniques that we have used. The rest of the thesis is divided into two major sections: Variability studies of indi-vidual systems and X-ray binary population studies Variability studies of individual systems In Chapter 3, we report results from an investigation of energy resolved orbital in-tensity pro les and from exhaustive orbital phase resolved spectroscopic measurements of GX 301{2 with MAXI{GSC . The orbital variation of the spectral parameters, es-pecially the relation between the equivalent width of Fe line and the column density of absorbing matter are then utilized to examine the models for the mode of accretion onto the neutron star in GX 301{2: circumstellar disk model by Pravdo & Ghosh (2001), and the accretion stream model by Leahy & Kostka (2008). A very large equivalent width of the iron line along with a small value of the column density in the orbital phase range 0.10-0.30 after the periastron passage indicates an asymmetry in the distribution of the matter around the neutron star, strongly favoring the accretion stream model by Leahy & Kostka (2008). Presence of an eclipse in an X-ray binary can be useful in determining orbital param-eters like inclination and in estimating the orbital evolution by eclipse timing method, which is reported in Chapter 4. For the HMXB system IGR J16393{4643, we found a short eclipse in the Swift{BAT light-curve and utilized it to constrain the orbital in-clination of the system. We have also studied, for the rst time, broad-band pulsation and spectral characteristic of the system with a Suzaku observation, showing sub-orbital intensity variations. For another eclipsing and non-pulsing HMXB 4U 1700{37, the orbital evolution is studied using mid-eclipse times from observations with narrow eld instruments as well as from long term light-curves of X-ray all sky monitors. The orbital period decay rate is estimated to be 5 10 7 /yr, an order slower than a previous measurement by Rubin et al.(1996). Since no pulsations are detected in this system, it is difficult to estimate its orbital parameters, especially its eccentricity. Using mid-eclipse times from 10 years of Swift{BAT data, we have independently constrained the eccentricity of the binary system. X-ray binary population studies In Chapter 5, we report results from an analysis of the 16 years light-curves of X-ray binaries in 2-10 keV energy band of RXTE{ASM , used to construct the differential and integral probability distributions of count-rates. These distributions are then employed to construct multiple snapshots of X-ray binary luminosity functions of the Milky Way instead of averaging the luminosities, an improvement over previous analysis by Grimm et al. (2002). We found that the averaged luminosities of highly variable X-ray binaries do not represent their true positions in XLFs and the variability of X-ray binaries do indeed signi cantly affect the luminosity functions. In Chapter 6, the measurements of the averaged spectra of X-ray binaries using MAXI{GSC data are reported and are used for constructing the composite X-ray spec-trum. These composite X-ray binary spectra are useful in estimating the contribution of X-ray binaries in extra-galactic SEDs constructed from the simultaneous Chandra / XMM{Newton and NuSTAR observations of these galaxies. These SEDs will also serve as a useful input in estimating the contribution of X-ray binary heating at high redshift IGM during the Epoch of Re-ionization. In Chapter 7, we summarize the main conclusions of the work carried out in this thesis and discuss some future prospects related to this thesis.