In the canonical model of a pulsar, rotational energy is transmitted through the surrounding plasma via two electrical circuits, each connecting to the star over a small region known as a "polar cap." For a dipole-magnetized star, the polar caps coincide with the magnetic poles (hence the name), but in general, they can occur at any place and take any shape. In light of their crucial importance to most models of pulsar emission (from radio to X-ray to wind), we develop a general technique for determining polar cap properties. We consider a perfectly conducting star surrounded by a force-free magnetosphere and include the effects of general relativity. Using a combined numerical-analytical technique that leverages the rotation rate as a small parameter, we derive a general analytic formula for the polar cap shape and charge-current distribution as a function of the stellar mass, radius, rotation rate, moment of inertia, and magnetic field. We present results for dipole and quadrudipole fields (superposed dipole and quadrupole) inclined relative to the axis of rotation. The inclined dipole polar cap results are the first to include general relativity, and they confirm its essential role in the pulsar problem. The quadrudipole pulsar illustrates the phenomenon of thin annular polar caps. More generally, our method lays a foundation for detailed modeling of pulsar emission with realistic magnetic fields.

Millisecond pulsars (MSP) are old neutron stars that have been spun up to high spin frequencies(as fast as 716 Hz) through the accretion of matter from a companion star. The extreme stellar densities in the core of globular clusters creates numerous accreting neutron star systems through exchange interactions; this leads to the formation of MSPs in larger numbers than in the galactic disk. Over the course of this project, we have collected over 17 TB of data on the 3 globular clusters M28, NGC6440 and NGC6441 plus 2 observations on NGC6522 and NGC6624 as part of the recently begun S-band survey using the Green Bank telescope. I have analyzed and conducted acceleration searches on 70% of the data and discovered 7 of the 23 new millisecond pulsars reported in this work. One year of timing observations of the pulsars in M28 and NGC6440 has led to the phase connected solution for 12 of the 15 new pulsars in those two clusters, 7 of which are in binaries. We have measured the rate of advance of periastron for two highly eccentric binaries and assuming this is purely due to general relativity, this leads to total system masses of (1.616 - 0.014)M and (2.2 - 0.8)M for M28C and NGC6440B respectively. The small mass function combined with this information imply that the most likely neutron star mass of NGC6440B is either very large or else there could be significant contribution to the advance of periastron from a nonzero quadrupole moment due to tidal interaction with the companion. Measurements of the period derivatives for many of the pulsars show that they are dominated by the dynamical effect of the gravitational field of the clusters. Finally, we have discovered the potential presence of a Mars-mass planet orbiting the pulsar NGC6440C with a period of 21 days. A dedicated timing campaign will be necessary to confirm the presence of such an object. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

No description available.

Le Large Area Telescope à bord du satellite Fermi, lancé le 11 juin 2008, est un télescope spatial observant l'univers des hautes énergies. L'instrument couvre l'intervalle en énergie de 20MeV à 300GeV avec une sensibilité nettement améliorée et la capacité de localiser des sources ponctuelles. Il détecte les photons ? par leur conversion en paire électron- positron, et mesure leur direction et leur énergie grâce à un trajectographe et un calorimètre. Cette thèse présente les courbes de lumières et les mesures spectrales résolues en phase des pulsars radio et gamma détectés par le LAT. La mesure des paramètres spectraux (flux, indice spectral, et énergie de coupure) dépend des fonctions de réponse de l'instrument (IRFs). Une méthode développée pour la validation en orbite de la surface efficace est présentée en utilisant le pulsar de Vela. Les efficacités des coupures entre les données du LAT et les données simulées sont comparées à chaque niveau de la rejection du fond. Les résultats de cette analyse sont propagés vers les IRFs pour évaluer les systématiques des mesures spectrales. La dernière partie de cette thèse présente les découvertes de nouveaux pulsars ? individuels tels que PSR J0205+6449, J2229+6114, et J1048-5832 à partir des données du LAT et des éphémérides radio et X. Des analyses temporelles et spectrales sont investies dans le but de contraindre les modèles d'émission gamma. Finalement, nous discutons les propriétés d'une large population de pulsars gamma détectés par le LAT, incluant les pulsars normaux et les pulsars milliseconde. / The Large Area Telescope (LAT) on Fermi, launched on 2008 June 11, is a space telescope to explore the high energy ?-ray universe. The instrument covers the energy range from 20MeV to 300GeV with greatly improved sensitivity and ability to localize ?-ray point sources. It detects ?-rays through conversion to electron-positron pairs and measurement of their direction in a tracker and their energy in a calorimeter. This thesis presents the ?-ray light curves and the phase-resolved spectral measurements of radio-loud gamma-ray pulsars detected by the LAT. The measurement of pulsar spectral parameters (i.e. integrated flux, spectral index, and energy cut-off) depends on the instrument response functions (IRFs). A method developed for the on-orbit validation of the effective area is presented using the Vela pulsar. The cut efficiencies between the real data and the simulated data are compared at each stage of the background rejection. The results are then propagated to the IRFs, allowing the systematic uncertainties of the spectral parameters to be estimated. The last part of this thesis presents the discoveries, using both the LAT observations and the radio and X ephemeredes, of new individual ?-ray pulsars such as PSR J0205+6449, and the Vela-like pulsars J2229+6114 and J1048-5832. Timing and spectral analysis are investigated in order to constrain the ?-ray emission model. In addition, we discuss the properties of a large population of ?-ray pulsars detected by the LAT, including normal pulsars, and millisecond pulsars.

Le satellite Fermi a été lancé le 11 juin 2008, avec à son bord le Large Area Telescope (LAT). Le LAT est un télescope sensible au rayonnement gamma de 20 MeV à plus de 300 GeV. Au début de l'activité de Fermi, neuf pulsars jeunes et énergétiques étaient connus dans le domaine gamma. Le nombre de détections de pulsars par le LAT prédit avant lancement était de plusieurs dizaines au moins. Le LAT permettait également l'étude des pulsars milliseconde (MSPs), jamais détectés avec certitude à très haute énergie jusqu'alors. Cette thèse aborde dans un premier temps la campagne de chronométrie des pulsars émetteurs radio et/ou X, candidats à la détection par le LAT, en collaboration avec les grands radiotélescopes et télescopes X. Cette campagne a permis la recherche de signaux gamma pulsés avec une grande sensibilité. En outre, la plupart des MSPs galactiques ont été suivis dans le cadre de cette campagne, sans biais de sélection a priori sur cette population d'étoiles. Pour la première fois, des pulsations ont été détectées pour huit MSPs galactiques au-dessus de 100 MeV. Quelques bons candidats à une détection prochaine apparaissent. Une recherche similaire a été conduite pour des MSPs d'amas globulaires, sans succès à présent. L'analyse des courbes de lumière et des propriétés spectrales des huit MSPs détectés révèle que leur rayonnement gamma est relativement similaire à celui des pulsars ordinaires, et est vraisemblablement produit dans la magnétosphère externe. Cette découverte suggère que certaines sources non identifiées sont des MSPs, pour l'instant inconnus.

Depuis la découverte du premier pulsar en 1967, nous connaissons désormais plus de 2500 pulsars aujourd’hui. Les pulsars offrent un champ d’études considérable : depuis l’étude des propriétés du milieu interstellaire et l’étude de la magnétosphère des pulsars jusqu’aux tests de la gravité en champ fort et la caractérisation d’un fond d’ondes gravitationnelles d’origine cosmologique. Cela explique pourquoi nous continuons de chercher de nouveaux pulsars de nos jours. Après des découvertes de pulsars millisecondes dans les sources non identifiées du Fermi Large Area Telescope, un programme de recherche de nouveaux pulsars a été mené à partir de 2012 par G. Desvignes. Observant à 1.4 GHz avec une haute résolution temporelle et fréquentielle, le programme SPAN512 a été conçu pour la recherche de pulsars rapides et lointains situés dans le plan Galactique. Nous décrirons les méthodes d’analyse mises en place pour traiter les données afin de trouver de nouveaux pulsars, méthodes soit basées sur la stabilité de la période de rotation des pulsars soit sur leur émission d’impulsions individuelles. Nous présenterons aussi l’état actuel de l’analyse du programme SPAN512 et les découvertes effectuées, plus particulièrement du pulsar trouvé au cours de ce travail de thèse, PSR J2055+3829, un pulsar milliseconde de période de rotation de 2.08 ms appartenant à un système de type « Veuve Noire ». Ce sera l’occasion de présenter les études chronométriques réalisées pour trouver l’éphéméride de ce pulsar et, dans le même temps, j’en profiterai pour parler d’une analyse similaire faite sur le pulsar J1618-3921, un pulsar dans une orbite excentrique. Enfin, nous présenterons des études polarimétriques de pulsars réalisées à la lumière d’un nouveau modèle, le modèle du vecteur tournant décentré (DRVM). Nous montrerons qu’un champ magnétique hautement décentré peut expliquer les variations brusques de l’angle de polarisation. / Since the discovery of the first pulsar in 1967, we know over 2500 pulsars today. Pulsars offer a broad range of studies: from the study of the properties of interstellar medium and of pulsar magnetospheres up to test of gravity in the strong-field regime and the characterisation of the cosmological Gravitation Waves background. This explains why we keep searching pulsars nowadays. After successful detections of new millisecond pulsars in Fermi Large Area Telescope unassociated sources at Nançay, a blind pulsar survey was initiated in 2012 by G. Desvignes. Conducted at 1.4 GHz with short sampling time and narrow frequency channels, the SPAN512 was designed to find fast and distant pulsars within the Galactic plane. We describe the methods to analyse data in order to find new pulsars, thanks to their spin stability or tto their single pulses. We will also describe the current status of the survey and the discoveries, more especially the pulsar discovered during this thesis, PSR J2055+3829, a 2.08 ms pulsar in a black widow system. It will be the opportunity to present the radio timing analysis of this pulsar and, in the same time, we will describe similar studies conducted on the pulsar J1618-3921, a pulsar in an eccentric orbit. Finally, we present some polarisation studies of pulsars in light of a new model, the Decentred Rotating Vector Model (DRVM). We will show that a highly decentred dipole may explain abrupt variations of polarisation profiles.

Le satellite Fermi a été lancé le 11 juin 2008, avec à son bord le Large Area Telescope (LAT). Le LAT est un télescope sensible au rayonnement ? de 20 MeV à plus de 300 GeV. Avant la mise en service du LAT, six pulsars jeunes et énergétiques étaient connus dans le domaine ? . Le nombre de détections de pulsars par le LAT prédit avant lancement était de plusieurs dizaines au moins. Le LAT permettait également l’étude des pulsars milliseconde (MSPs), jamais détectés avec certitude à très haute énergie jusqu’alors. Cette thèse aborde dans un premier temps la campagne de chronométrie des pulsars émetteurs radio et/ou X, candidats à la détection par le LAT, en collaboration avec les grands radiotélescopes et télescopes X. Cette campagne a permis la recherche de signaux ? pulsés avec une grande sensibilité. En outre, la plupart des MSPs galactiques ont été suivis dans le cadre de cette campagne, sans biais de sélection a priori sur cette population d’étoiles. Pour la première fois, des pulsations ont été détectées pour huit MSPs galactiques au-dessus de 100 MeV. Quelques bons candidats à une détection prochaine apparaissent. Une recherche similaire a été conduite pour des MSPs d’amas globulaires, sans succès à présent. L’analyse des courbes de lumière et des propriétés spectrales des huit MSPs détectés révèle que leur rayonnement ? est relativement similaire à celui des pulsars ordinaires, et est vraisemblablement produit dans la magnétosphère externe. Cette découverte suggère que certaines sources non-identi?ées sont des MSPs, pour l’instant inconnus. / The Fermi observatory was launched on June 11, 2008. It hosts the Large Area Telescope (LAT), sensitive to ? -ray photons from 20 MeV to over 300 GeV. Before the LAT began its activity, six young and energetic pulsars were known in ? rays. At least several tens of pulsar detections by the LAT were predicted before launch. The LAT also allowed the study of millisecond pulsars (MSPs), never ?rmly detected in ? rays before Fermi. This thesis ?rst presents the pulsar timing campaign for the LAT, in collaboration with large radiotelescopes and X-ray telescopes, allowing for high sensitivity pulsed searches. Furthermore, it lead to quasi-homogeneous coverage of the galactic MSPs, so that the search for pulsations in LAT data for this population of stars was not affected by an a priori bias. We present a search for pulsations from these objects in LAT data. For the ?rst time, eight galactic MSPs have been detected as sources of pulsed ? -ray emission over 100 MeV. In addition, a couple of good candidates for future detection are seen. A similar search for globular cluster MSPs has not succeeded so far. Comparison of the phase-aligned ? -ray and radio light curves, as well as the spectral shapes, leads to the conclusion that their ? -ray emission is similar to that of normal pulsars, and is probably produced in the outer-magnetosphere. This discovery suggests that many unresolved ?-ray sources are unknown MSPs.

X-ray binaries are binary stellar systems containing a compact object and a normal companion star which are gravitationally bound and rotate about a common center of mass. The compact object accretes matter from the companion star. The accreted matter may have a high angular momentum and hence follow a Keplarian orbit about the compact object. It slowly spirals inward as its angular momentum is redistributed via viscous forces and forms an accreting disk before being finally accreted onto the compact object. The compact object that is accreting matter may either be a neutron star or a black hole. X-ray binaries can be broadly classified into two classes depending on the mass of the companion star. Low Mass X-ray Binaries (LMXBs) have companion star masses and accrete mass via Roche lobe overflow of the companion star. High Mass X-ray Binaries (HMXBs) have companion star masses and in these systems the compact object accretes matter from the high velocity stellar winds of the companion star. For the work and results that are presented in the thesis we have studied the orbital evolution, apsidal motion and long term flux variations in High mass X-ray binaries which have a neutron star compact object with very high magnetic field of the order of B ~ 1012 G. Due to the high magnetic field, the accretion disk is disrupted at the Alfven radius where the magnetic field pressure equals the ram pressure of the infalling matter. From that boundary, the flow of the infalling matter will be guided by the magnetic field lines. The infalling matter will follow these lines, finally falling onto the magnetic poles with velocity nearly equal to the free fall velocity and form an accretion column over the magnetic poles. A hot spot is formed at both the magnetic poles and high energy photons are emitted from these regions. Inverse Compton scattering of these photons by high energy electrons in the accretion column can produce hard X-rays. If the optical depth of the accretion column is low, the radiation comes along the magnetic axis forming a pencil beam whereas if the optical depth is high, radiation escapes tangential to the accretion column forming a fan beam. Since the neutron star is rotating about its rotation axis, the radiation beam directed along magnetic axis non-aligned with the rotation axis will sweep across the sky. Whenever this beam of rotating radiation is aligned with the line of sight, a pulse of X-ray radiation is detected. Hence these systems are also called X-ray Binary Pulsars (XBP). These pulses are emitted at equal intervals of time, where the time between the emission of two pulses is the spin period of the neutron star. But since the neutron star is in a binary orbit, the arrival time of pulses as recorded by an observer will be delayed or advanced due to the motion of the neutron star. When the neutron star is moving towards the observer, the pulses arrive faster and when the neutron star is moving away from the observer, the pulses are delayed. These delays or advances of the arrival time of pulses can be measured accurately which allows us to measure the orbital elements (ax sin i, Porb, e, ω, Tω ) of the neutron star orbit. The neutron star orbit may evolve with time due to mass loss from the system, mass transfer from the companion star onto the neutron star and due to tidal interaction between the neutron star and the companion star. Gravitational wave radiation may also cause orbital evolution. However, in HMXBs this effect is likely to be much weaker compared to the effect of mass loss, mass exchange and tidal interaction. Rossi X-ray Timing Explorer (RXTE) is an X-ray astronomy satellite launched in 1995 by NASA. It has two pointed instruments, the Proportional Counter Array (PCA) and the High Energy X-ray Timing Experiment (HEXTE). PCA has a large effective area of 6500 sq cm and works in the energy range of 2-60 keV. It has a very good time resolution of 1 microsec. HEXTE observes in the energy range of 15-250 keV and has a time resolution of 8 microsec. RXTE also has an All Sky Monitor (ASM) which scans 80% of the sky in 90 minutes. We have used RXTE-PCA data for timing and spectral studies and ASM data for the long term flux variation studies of Cen X-3. The thesis presents details of our work, the analysis of the data, results of the analysis and our conclusions from these results. The first chapter of the thesis gives an overview of X-ray binaries, their orbital evolution and the instrument details of RXTE. In the second chapter we have presented our work of timing analysis of three persistent sources, namely Cen X-3, SMC X-1 and 4U 1538–52. For the SMC X-1 system, we have for the first time measured the eccentricity and the angle of periastron (ω). We found that the accuracy of pulse timing analysis is limited by the dependence of pulse profile on orbital phase. The new measurement of the orbit ephemeris of Cen X-3 when combined with the previous measurements of orbit ephemeris obtained by observations from other X-ray missions, gave an improved measurement of the rate of orbital decay P˙orb/Porb ~ -1.8 x 10−6yr−1 . A long observation of SMC X-1 made by RXTE in 2000 during the high state of SMC X-1 allowed us to measure the very small orbit eccentricity e ~ 0.00021 in this system. SMC X-1 was again observed for a long time by RXTE during 2003 during its low state. The SMC X-1 pulse fraction depends on the flux state of the source such that the pulse fraction decreases with decrease in the source flux. Thus the 2003 observations of SMC X-1 have higher error in measurement of pulse arrival times compared to the 2000 observations and could not be used to measure the eccentricity of the orbit. But combining the orbital ephemeris of SMC X-1 measured using the 2000 and the 2003 observation with the epoch history allowed us to improve the measurement of rate of orbit decay by an order of magnitude compared to previous observations P˙orb/Porb ~ - 3.4 x 10−6yr−1 . We observed 4U 1538–52 with RXTE under the guest observer program to measure the orbital evolution of this system. From observations of this system with BeppoSAX , a circular orbit similar to the SMC X-1 system was inferred. 4U1538–52 was observed with RXTE again in 1997 and analysis of this observation showed it to have eccentric orbit with a marginal evidence for an orbital decay. Our analysis carried out using the 2003 RXTE observation data confirmed that the orbit is eccentric with e ~ 0.18. But the new orbital ephemeris measured clearly shows that the orbit is not evolving with time as reported earlier. We have derived an upper limit on the rate of change of orbital period of this system to be P˙orb/Porb = 2.5 x 10−6yr−1 . 4U 1538–52 is similar to SMC X-1 in many respects, both have similar orbital period of Porb(SMC X -1) = 3.89 days and Porb(4U1538 - 52) = 3.72 days and companion star mass. But tidal interactions between the neutron star and the companion star have almost circularised the orbit of SMC X-1 where as the orbit of 4U 1538–52 is quite eccentric. Therefore we conclude that 4U 1538–52 is a young system and hence the orbit has not circularised by tidal interaction. The neutron star orbit also precesses due to tidal interaction and rotation of the companion star, which causes the longitude of periastron ω to change with time. The rate of change of ω can be measured by comparing the orbital elements of the neutron star orbit measured at different epochs of time. This rate of change of ω is directly related to the mass distribution of the companion star and hence the apsidal motion constant that are predicted by the theoretical models for stellar structure. Therefore measuring ˙ω will be a direct test for the stellar structure models. But ω can be measured only when the orbit is eccentric and for this purpose the Be-star/X-ray binary pulsars are the most suitable objects. The Be-star/X-ray binary pulsars are transient systems and have wide eccentric orbits of Porb > 10 days. The Be-stars are fast rotating stars with rotational velocity near to the break-up velocity. They eject matter along their equator in a circumstellar disk. When the neutron star intercepts this circumstellar disk during its periastron passage, the rate of mass accretion increases and the system becomes bright in X-rays. These short outburst are called the type-I X-ray bursts. The Be-star also has episodes of high mass ejection when the neutron star may accrete a larger amount of matter and can be seen over several binary orbits. These long duration outbursts are called type-II X-ray bursts. In the third chapter of the the thesis we have reported the analysis and results of three Be-/X-ray binary pulsars we have studied, namely 4U 0115+62, V0332+52 and 2S 1417-624 which were observed by RXTE during their respective type-II bursts. The X-ray pulse profiles of the Be-/X-ray systems evolve as a function of the source flux. Generally a simple single peaked pulse profile is seen during the onset of the outburst, which evolves into a more complex multiple peaked pulse profile as the source flux increases. The pulse profile again returns to the simple single peaked profile as the outburst fades off and the source flux decreases to persistent X-ray flux levels. Also due to varying mass accretion rate, the spin period evolves during the outburst. Both these factors together reduce the accuracy of measuring the arrival time of pulses. Hence we have used the instantaneous spin period measurements to deduce the orbital parameters of these system. The apparent spin period (Pspin) of the neutron star is modified by the radial velocity of the neutron star due to Doppler effect. The radial velocity of neutron star is dependant on the neutron star orbit and hence measurement of the spin period of the neutron star at different orbital phases allows us to determine the orbital elements. 4U 0115+63 was observed with the RXTE during two of its recent type II outbursts in 1999 and 2004. We measured the orbital parameters during both these outbursts independently. We combined the previous measurements of ω with our two measurements and measure the rate of apsidal motion of the system to be ˙ω ~ 0o .06 yr−1. V0332+52 was seen in outburst during 2004. During its previous outburst of 1983 only nine spin period measurements had been obtained and the orbital parameters measured from them were erroneous. We have measured the orbital parameters of this system accurately and determined the correct projected semi-major axis ax sin i and orbital period. The new orbit parameters can now be used to compare with future orbital element measurements to estimate any apsidal motion and/or orbital evolution in this system. We also used the 1999 outburst of 2S 1417–624 to accurately measure the orbital parameters of this system. We have also investigated the long term flux variations in the X-ray light curves of X-ray Binaries. Our studies on the flux variations observed in Cen X-3 are described in the fourth chapter of the thesis. Long term light curves of X-ray binaries show variations due to many reasons. Periodic variations of few milliseconds to a few hours in the light curve are seen due to spin of the neutron star. Light curves show variations due to motion of the neutron star in its orbit at timescales of few minutes to several days. Many sources also show quasi periodic variations in their X-ray light curves at timescales smaller than the neutron star orbital period which are believed to arise due to some material inhomogeneity orbiting the neutron star. These variations are called quasi periodic oscillations (QPOs). QPOs in X-ray binaries are observed between a frequency range of few millihertz to a few kilohertz. Long term X-ray light curves of many sources also reveal flux variations at time scales greater than the respective orbital period of the source. These variations are called superorbital variations. Systems like Her X-1, LMC X-4, 2S0114+650, SS 433, XTE J1716–389, 4U 1820–303 and Cyg X-1 show periodic superorbital variations whereas other systems like SMC X-1, GRS 1747-312, Cyg X2, LMC X-3 and the Rapid Burster show quasi periodic superorbital flux variations. These superorbital flux variations are understood as arising either due to a changing mass accretion rate which could be aperiodic in nature or as due to obscuration of the central X-ray source by a tilted, warped and precessing accretion disk. Many theoretical models have been proposed to explain the disk precession. The long term flux variations in the X-ray light curves of bright persistent X-ray binaries like Her X-1, SMC X-1 and LMC X-4 have been understood to be due to a periodic (in case of Her X-1 and LMC X-4) or a quasi periodic (for SMC X-1) precession of a warped accretion disk. We analysed the light curves of Cen X-3 obtained with the RXTE-ASM. The Cen X-3 light curves show aperiodic X-ray flux variations in all the energy bands of 1.5-3, 3-5 and 5-12 keV. The high and low states last for a few to upto a hundred days. The source also shows two spectral modes during the observations carried out with the ASM. The source was in a hard state during December 2000 to April 2004. At first look the aperiodic variations seen in Cen X-3 light curves seem to be arising due to a changing mass accretion rate. To investigate the cause of these aperiodic flux variations of Cen X-3 we studied the orbital modulation and the pulsed fraction as a function of source flux state. In the high state, the eclipse ingress and egress are found to be sharp whereas in the intermediate state, the transitions are more gradual. In the low state, instead of eclipse ingress and egress, the light curve shows a smooth intensity variation with orbital phase. The orbital modulation of the X-ray light curve in the low state shows that the X-ray emission observed in this state is from an extended object. The intensity dependent orbital modulations indicate that the different intensity states of Cen X-3 are primarily due to varying degree of obscuration. Measurements of the pulsed fraction in different intensity states are consistent with the X-ray emission of Cen X-3 having two components, one highly varying component with a constant pulsed fraction and a relatively stable component that is unpulsed and in the low state, the unpulsed component becomes dominant. The observed X-ray emission in the low state is likely to be due to scattering of X-rays from the stellar wind of the companion star. Though we can not ascertain the origin and nature of the obscuring material that causes the aperiodic long term intensity variation, we point out that a precessing accretion disk driven by radiative forces is a distinct possibility. We also studied the QPOs in Cen X-3 that are seen at 40 mHz. The QPOs are explained by the Beat Frequency Model (BFM) as arising due to the beat between the Keplarian frequency of the inner accretion disk and the spin of the neutron star. Thus when the mass accretion rate is high the inner disk radius decreases, increasing the Keplarian frequency and hence the observed QPO frequency and vise versa when the mass accretion rate decreases. Thus if the flux variations of Cen X-3 were due to a changing mass accretion rate then the observed QPO frequency should have a positive correlation with the observed X-ray flux of the source. But we find in our study that the QPO frequency does not have any correlation with the observed X-ray flux and the QPO frequencies does not follow the Frequency-Flux relation as expected in the Beat frequency model. Thus the QPO behaviour is in agreement that the observed X-ray flux does not indicate the true X-ray intensity state and hence the mass accretion rate in Cen X-3. Therefore, we conclude that X-ray variations of Cen X-3 are not due to changing mass accretion rate but due to varying obscuration of the central X-ray source, possibly by an accretion disk which precesses aperiodically. The conclusions from our studies presented in chapter 2, 3 and 4 of the thesis are summarised in the final chapter. The improved measurements of the rate of change of orbital periods from our work can now help us to detect any small departures from a constant period derivative in the persistent HMXB systems. The improved measurements of the orbital elements of Be-/X-ray binaries can now be used to study orbital evolution and apsidal motion in these system. New outbursts of the transient systems observed by future satellites providing good timing accuracy and large effective area, like LAXPC (Large Area X-ray Proportional Counter) of the ASTROSAT mission will facilitate such studies. The long term X-ray light curves study as done for Cen X-3 can be extended to other X-ray binary systems observed by All Sky Monitor. The method of source flux state dependent studies developed to study the Cen X-3 system can be easily extended to other systems that show long term superorbital flux variations. These kind of studies can be done by future proposed X-ray missions like ASTROSAT which will have a Sky Monitor similar to ASM dedicated to monitor X-ray sources. More sensitive measurements of long term X-ray light curves with the MAXI mission will allow similar studies of a large number of X-ray binaries and we will be able to see if aperiodically precessing accretion disk is present in many X-ray binaries.

Radio emission from pulsars is believed to originate from charged particles streaming along the open magnetic field lines, radiating within a narrow cone at each of the two magnetic poles. In each rotation of the star, the emission beam sweeping across the observer’s line of sight, is seen as a pulse of radio emission. Average pulse profiles integrated over several hundreds of individual pulses, along with polarization information, reveal the viewing geometry and various emission properties(e.g., emission in multiple cones, frequency dependence of the emission altitude, notches in the average profiles, etc.), and provide some clues about the possible emission mechanisms. The sequence of individual pulses generally exhibit richer details, e.g., pulse-nulling, variety of subpulse drifting, polarization mode-changing, micro-structure and giant pulse emission, etc., and seem to be more crucial and promising in probing the underlying physical processes. The physical understanding of many of the above properties and phenomena is still far from complete. In first two parts of this thesis, we address a few of these aspects, and probe related details by mapping the pulsar polar emission patterns, while in the last part, we present our searches for dispersed signals(periodic as well as transient) at very low frequencies. More specifically, Part-I makes use of the present understanding of drifting subpulses phenomenon to reconstruct the emission patterns in nearly complete polar cap region of the pulsar B1237+25, and addresses the origin of emission in multiple cones using these reconstructed emission maps. In Part-II, we discuss a need for new instrumentation primarily motivated by the need for tomographic studies of pulsar polar emission regions. We report the consequent design and development of a novel, self-contained multi-band receiver (MBR)system, intended for use with a single large aperture to facilitate sensitive and high time-resolution observations simultaneously in 10 discrete frequency bands sampling a wide spectral span(100–1500MHz) in a nearly log-periodic fashion. Part-III presents our deep searches designed to detect radio transient as well as periodic signals from the (so far) “radio-quiet” gamma-ray pulsars — a population of radio silent pulsars recently discovered using the Large Area Telescope on the Fermi-satellite. Brief descriptions of the issues addressed in the three parts of the thesis, along with a summary of respective results, is as follows. 1. Origin of Radio Emission in Multiple Cones Many pulsars exhibit systematic variations in position and intensity of their subpulses, a phenomenon now well known as “subpulse drifting”. Ruderman & Sutherland(1975) suggested this regular modulation to be a manifestation of a carousel of “spark” discharges in the acceleration zone of the star, circulating around the magnetic axis because of the E×B drift. In the qualitative framework of the above carousel model, the coherent modulation in a subpulse sequence can be mapped back to the underlying pattern of sub-beams/emission-columns (see, for example, Deshpande & Rankin, 1999). However, the completeness with which the underlying configuration of sub-beams can be sampled depends on how close our line of sight approaches the magnetic axis. The bright pulsar B1237+25 has a special viewing geometry where the sightline traverses almost through the magnetic axis, thus providing an excellent opportunity to map and study the underlying patterns across the full transverse slice of its polar emission region. However, the rich variety in pulse-to-pulse fluctuations in this pulsar makes this task challenging. In Chapter 2, we present our analysis of a number of pulse-sequences from this star observed with the Arecibo telescope, wherein we search for, and use, coherent modulation in sub-sequences, to map the underlying emission patterns. The reconstructed maps provide a convenient way to study the details in multiple emission cones, and any inter-relationship between them. More specifically, we have utilized these maps to explore whether the multiple cones of this pulsar originate from a common seed pattern or not. A summary of results The results obtained from our study of B1237+25 are summarized below: 1 The underlying carousel of sparks for this pulsar appears to lack stability over long durations. The circulation period, deduced using smaller length sub-sequences, appears to vary over a large range(about18 to34 times the rotation period). 2. The emission patterns corresponding to the outer and the inner cones are found to be significantly correlated with each other, implying that the emission in the two cones share a common seed pattern of sparks. This main result is consistent with the same radio frequency emission in the two cones, originating from a common seed pattern of sparks at two different altitudes. 3 The emission patterns corresponding to the outer and the inner cones are found to be offset from each other, consistently across various sub-sequences, by about 10◦ in magnetic azimuth. This large offset indicates certainly a twist in the emission columns, and most likely in the magnetic field geometry, between the two different emission altitudes. 4. The core component also seems to share its origin with the conal counterparts. Presence of a compact, diffuse and further-in carousel of sub-beams is consistent with the observed modulation in the core component of this pulsar. The featureless spectrum observed for many core-single pulsars can be explained readily when the diffuse pattern approaches uniformity. 2.Tomography of the Pulsar Magnetosphere: Development of a Multi-band Receiver Although drifting subpulses are now routinely interpreted in the qualitative framework of the carousel model, estimation of circulation time associated with the system of emission columns has been possible so far in only a handful of pulsars, and the important details determining such configurations, their evolution across the magnetosphere, and the pattern circulation are yet to be understood. Radius-to-frequency mapping in pulsars suggests that the lower frequency emission originates farther away from the surface of the star than the higher frequency emission. Hence, the sub-beam configuration mapped at a particular frequency provides a view of a single slice of the polar emission region at the corresponding emission altitude. Mapping of the underlying emission patterns simultaneously at a number of frequencies would amount to viewing a “tomograph” of the pulsar magnetosphere. Such tomographic studies would reveal not only the evolution of sub-beams across the magnetosphere but can also provide much needed clues about the generation of the sub-beam patterns, and their possible connection with the profile/polarization mode changes observed in various pulsars. Simultaneous multi-frequency observations, which are required for many other interesting astronomical studies as well, are usually carried out by using several telescope, each observing at different frequency. Such an approach has inherent complexity in coordinating various telescopes, in addition to numerous other difficulties which limit the desired advantages of such observations. Some of these difficulties, which we faced in our attempt of carrying out simultaneous multi-frequency observations using five different telescopes, are discussed in Chapter 3. We suggest an optimum approach to carry out simultaneous multi-frequency observations, using a single large aperture. In Chapter 4, we present the design of a novel, “self-contained” multi-band receiver(MBR) system developed for this purpose. The MBR system includes a suitable feed, broadband front-end, parallel analog and digital receiver pipelines, along with appropriate monitoring, synchronization and data recording systems. When used with a large aperture, the MBR facilitates high time-resolution observations simultaneouslyin10discretefrequencybandssampling a wide spectral span(100–1500MHz) in a nearly log-periodic fashion. The raw voltage time sequences corresponding to each of the two linear polarization channels for each of the 10 spectral bands are simultaneously recorded, each sampling a bandwidth of 16 MHz at the Nyquist rate. The dual-polarization multi-band feed, a key component of the MBR, is designed to have good responses only overthe10discretebandspre-selected as relatively RFI-free, and hence provides preliminary immunity against RFI. The MBR also offers significant tunability of the center frequencies of each of the 16-MHz sub-bands separately, within the spectral spans of respective bands. Similarity of the 10 sub-band receiver chains provides desired compatibility, in addition to an easy inter-changeability of these units, if required, and an overall modularity to the system. The MBR was used with the 110 meter Green Bank Telescope to conduct test observations on a few bright continuum sources, and about 20 hours of observations on a number of bright pulsars. Using these observations, we have constructed a preliminary tomograph of the polar emission region of B0809+74, and studied the spectral evolution of emission altitudes and flux density ofB0329+54(Chapter5). Although the MBR system design is optimized for tomographic studies of pulsar polar emission regions, the simultaneous multi-frequency observations with such a system offer particular advantages in fast transient searches. The MBR is also suitable for several other astronomical investigations, e.g., studying the spectral evolution of average properties of pulsars and propagation effects, single-dish continuum studies and surveys/studies of recombination lines. 3. Searches for Decameter-wavelength Counterparts of Radio-quiet Gamma-ray Pulsars Before the launch of the Fermi gamma-ray space telescope, the “radio-quiet” gamma-ray pulsar population consisted of only one pulsar ,i.e., Geminga (for example, see Bignami& Caraveo,1996; Abdo etal.,2009). High sensitivity of the Large Area Telescope(LAT) on the Fermi-satellite made it possible, for the first time, to perform blind searches for pulsars in γ-rays. Since the Fermi-operation started, the number of pulsars known to emit in γ-rays has seen an extraordinary increase — from less than 10 to 117 pulsars. About one-third of these pulsars have been discovered in blind searches of the LAT data. Despite deep radio searches, only 4 of these LAT-discovered pulsars could be detected, suggesting the rest of these to be “radio-quiet” gamma-ray pulsars. One of the possible explanations for the apparent absence of radio emission from these pulsars is that their narrow radio beams miss the line of sight towards earth (Brazier & Johnston, 1999), and hence appear as “radio-quiet”. The radius-to-frequency mapping in radio pulsars suggests that the emission beam becomes wider at low frequencies, increasing the probability of our line of sight passing through the beam. However, all of the deep searches mentioned above were carried out at higher radio frequencies(∼1GHz and above, and some at300MHz,Ray etal.,2011;Pletsch etal.,2012),and the lower frequency domain(<≈100 MHz) has remained relatively unexplored. Given the expected widening of emission beam, follow-up searches of the radio-quiet pulsars at low radiofrequencies could also be revealing. With this view, we searched the archival data of the pulsar/transient survey at 34.5 MHz, carried out using the Gauribidanur telescope during 2002-2006,for any periodic or transient dispersed signal along the direction of many of the LAT-discovered pulsars. Motivated by an intriguing possible detection of the pulsar J1732−3131 from the above search, we carried out further extensive follow-up observations and deep searches for pulsed(periodic as well as transient) radio emission from a selected sample of radio-quiet pulsars. Chapters 6 and 7 present details of our observations, detection strategies and methodologies, and interesting results obtained in a few of the target directions. The results obtained from these searches include: 1 A possible detection of periodic radio pulses from J1732−3131 was made, using the archival data, at a dispersion measure(DM) of15.44 ±0.32 pc/cc. We also detected 10 individual bright pulses in the same observing session, although marginally above the detection threshold, at a DM consistent with that associated with the periodic signal. The apparent brightness of these single pulses, and similarity of their apparent distribution in pulse-longitude with that of giant pulses in J0218+4232, suggest that these might be giant pulses. Our DM-based distance estimate, using Cordes & Lazio electron density model(2002),matches well with earlier estimates based on gamma-ray emission efficiency. 2 In our follow-up deep searches, we could not detect any readily apparent pulsed radio signal(neither periodic nor single pulses) from J1732−3131, i.e., above a detection threshold of 8σ. However, when we time-aligned and co-added data from observing sessions at 21different epochs, and dedispersed using the DM estimated from the candidate detection, the average profile shape is found to be completely consistent with that from the candidate detection. Finding the same profile shape after 10 years of the original detection suggests that the signal is unlikely to be due to RFI or a mere manifestation of random noise. 3.In a couple of the observing sessions towards the telescope pointing direction of RA=06:34:30, DEC=10◦ , we detected a few ultra-bright pulses at two different DMs of about2pc/cc and3.3 pc/cc, respectively. However, when dedispersed at the DMs suggested by the bright single pulses, no significant signal was found at the expected periodicities of our targetpulsarsJ0633+0632 andJ0633+1746,which would be in the telescope beam centered at above coordinates. Energies of these strong pulses in the two observing sessions are comparable to typical energies of giant pulses from the Crab pulsar at decameter wavelengths. 4. No significant pulsed signal(periodic or transient), above a detection threshold of 8σ,was found towards the directions of other selected radio-quiet gamma-ray pulsars. Time-aligning and combining of observations at different epochs allowed us to carry out deep searches for signals at the expected periodicities of these pulsars. Despite the large background sky-temperature at decameter wavelengths, the minimum detectable flux density in our deep searches are comparable with those from previous searches at higher frequencies, when scaled using a spectral index of −2.0 and assuming no turn-over in the spectrum.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy