In this thesis the Gamma-Ray Bursts (GRBs) are studied, the brightest explosions in the universe. GRBs have been observed since year 1967, but there are several unsolved problems. In the first chapter there is an introduction to the issue of GRBs, and the history of observations are briefly described. The Fermi satellite, the latest satellite devoted to gamma-ray burst observations is described in chapter two. Characteristics of the Fermi instruments are also described. The observed data of GRBs are characterized in the third chapter. The distribution of GRB durations, distances, and spectral hardnesses are described. The characteristics of long and short GRBs (distance, isotropy of distribution, metalicity dependence, isotropic energy) are described. A chance of the appearance of a GRB in the Milky Way is discussed. New Fermi observations are described too. Fourth chapter is about models of GRBs. The fireball and canonball models are described. Fifth chapter is focused on the exposure function of CGRO-BATSE, Fermi-GBM, Swift. I have created the exposure function for GBM on Fermi satellite. It is quite difficult, and I have assumed some simplified hypotheses. Information of the satellite's position, position of detectors on the Fermi satellite, have been found on the Fermi web pages and in the article...

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.

The detailed nature of radio emission processes of pulsars and the exact location and distribution of the pulse emitting regions are still shrouded with mystery. Pulsars with drifting subpulses are considered as an important key for unlocking the mystery of how radio pulsars work. The phenomenon of subpulse drifting (first reported by Drake & Craft (1968)) is manifested as an organized subpulse behavior - subpulses appear at progressively changing longitude in the pulse window following some particular path. The path followed by the subpulses is specific to the individual pulsar concerned and is known as drift band. Drifting is generally characterized by Pm2(horizontal separation between the drift bands, i.e. in pulse longitude) and Pm3(vertical separation between the drift bands, i.e. in pulse numbers). The subpulse drifting phenomenon finds a natural explanation in the model of Ruderman & Sutherland (1975). According to this model, the subpulse drifting is produced from a system of sub-beams (subpulse associated plasma columns). Sparks (sparking discharges within the vacuum gap) rotating around the magnetic axis under the action of an E x B drift, give rise to a circulating pattern of sub-beams, and the time for one full circulation is referred to as the carousel rotation period, which we designate as P4. As pulsar radiation beams are widely believed to be arranged in concentric cones, it is natural to expect the circulating sparks to be distributed in annular rings on the polar cap. Each of these rings gives rise to one cone in the nested cones of emission. It has been recently shown that, subpulse drift may be fairly common among pulsars (Weltevrede et al. (2006) and (2007)). Hence, the pulsar radio emission mechanism is most likely closely connected with mechanism for drifting. In spite of significant progress both in high quality observations of drifting (e.g. Weltevrede et al. (2006) and (2007)) and attempts for confronting the results from the observations with existing models (e.g. Deshpande & Rankin (1999) and Gupta et al. (2004) etc), the pulsar emission mechanism is still an unsolved puzzle. Backer (1970b) first reported that emission from certain pulsars abruptly switches off for several periods, and suddenly comes back. This phenomenon is known as nulling. Nulling appears to be random, broadband and intrinsic to the concerned pulsar. Nulling is quite common in pulsars (Biggs, 1992). Although different aspects of the phenomenon of nulling are investigated in detail for many pulsars by several authors using high sensitivity observations, nulling is not yet explained by the existing theoretical models for pulsar radio radiation. In this thesis, I have mainly studied phenomenon of subpulse drifting and nulling, with the aim to probe the radio emission processes of pulsars. Most of the pulsars have a narrow duty cycle of emission (5-10 % of pulsar period). This is generally consistent with the expectations of the angular width of the polar cap, for typical viewing geometries. However, there are small but significant number of pulsars with unusually wide profiles where the emission is seen for a wide range of longitude (≥ 90 degrees). These are expected to be pulsars which are highly aligned, i.e. the magnetic dipole axis is almost parallel to the spin axis. In such a case, the line of sight (LOS) is very close to both the rotation and the magnetic axes, and consequently, we sample a large region of the polar cap. This has the exciting potential to allow a detailed study of the distribution and behavior of emission regions located in an annular ring around the magnetic axis. The study of pulsars showing systematic subpulse drift patterns provides important clues for the understanding of the unsolved problems of pulsar emission mechanism. Constraints provided by such observations can have far reaching implications for the theoretical models, as exemplified by some of the recent results in this area (e.g. Deshpande & Rankin (1999) and Gupta et al. (2004)). In this context, wide profile drifting pulsars can provide extra insights because of the presence of simultaneous multiple drift bands. During the thesis period, I have mainly concentrated on the study of single pulse properties of two wide profile drifting pulsars, PSR B0818-41 and PSR B0826-34. In depth study of PSR B0818-41 We have studied single pulse properties of a relatively less studied wide profile pulsar, B0818-41 using highly sensitive multi-frequency observations with the GMRT in full polar mode. Detailed investigation of PSR B0818-41 are reported in Chapters 2, 3 and 4 of this thesis. New results from our study are described in the following. We estimate the mean flux of PSR B0818-41 at 5 different frequencies and show that the spectrum flattens at frequencies lower than 325 MHz (at 244 or 157 MHz), providing indication of a low frequency turn-over. Significant linear polarization is observed at 325, 610 and 1060 MHz. Average linear polarization falls off much faster than the total intensity and decreases to zero near the outer edge of the profile. This can be explained by the orthogonal polarization mode jump at the edges of the profile observed at 325 MHz. Polarization angle sweep across the pulse profile evolves remarkably with frequency (between 325, 610 and 1060 MHz), which is not generally observed in other pulsars. Very less circular polarization without any signature of changing handedness is observed at 325 and 610 MHz. But circular polarization changes sign at the middle of the pulse profile at 1060 MHz. We report the discovery of a remarkable subpulse drift pattern in PSR B0818-41, using the high sensitivity GMRT observations. We find simultaneous occurrence of three drift regions with two different drift rates: an inner region with steeper apparent drift rate flanked on each side by a region of slower apparent drift rate. The closely spaced drift bands always maintain a constant phase relationship: the subpulse emission from the inner drift region is in phase with that from the outer drift region on the right hand side, and at the same time the emission in the inner drift region is out of phase with the outer drift region situated on the left hand side. This phase locked relationship (hereafter PLR) is maintained for the entire stretch of the data (for all the epochs of observations at 325 and 610 MHz) and does not appear to get perturbed after intermittent nulling or during changes in the drift rate. We observe frequent changes of drift rates. We see extreme examples of changing drift rates such as transitions from negative to stationary or stationary to negative drift rates, many of which appear to have some connection with nulls. We investigate changes in drift rates for about 10,000 pulses from two different epochs of observations at 325 MHz and observe frequent occurrences of small changes in the drift rate, seven transitions from negative to stationary drift rates, five transitions from stationary to negative drift rates, and two possible signatures of curved drift bands. In addition to the remarkable subpulse drift observed at 325 MHz, we report subpulse drifting at 244 and 610 MHz. At 244 MHz subpulse drifting is observed only in the leading and trailing outer regions, but not in the inner region. Though the drift bands are weaker, subpulse drifting is observed in both inner and outer region at 610 MHz. Pm2, Pm3 and ΔΦs(subpulse width) are determined for the inner and the outer drift regions for different frequencies. Though Pm3 is observed to be the same for the inner and outer drift regions, pm2 and ΔΦsare different for different drift regions. The unique drift pattern of this pulsar can be naturally explained as being created by the intersection of our LOS with two conal rings on the polar cap of a fairly aligned rotator. Based on the frequency evolution of the average profile, observed polarization angle (PA) swing and results from subpulse drifting, we converged on two possible choices of emission geometry: G-1 (inclination angle α= 11 deg and impact angle β= -5.4 deg; which incidentally reproduces the middle part of the PA sweep at 610 MHz) and G-2 (α=175.4 deg and β=- 6.9 deg; geometry derived from RVM fit to 325 MHz PA sweep). Pulsar radiation pattern simulated with both the geometries reproduces the average profile as well as the observed features in the drift pattern quite well. However, G-2 fits the PA sweep much better. We report that the peaks of the emission from the trailing and leading outer regions, as a function of the pulse number, are offset by a constant interval, P5~9P1. We also report a phase locked relation (PLR) between the inner and outer drift regions for PSR B0818-41. A new technique is introduced by us for resolving aliasing, using this constant offset (P5 ~ 9P1) between the peak emission from the leading and trailing outer regions. From the result of this technique, we propose that the subpulse drifting for PSR B0818 -41 is most likely first order aliased, and the corresponding carousel rotation period 4 =10 s. This implies that PSR B0818 -41 has the fastest known carousel. The drift pattern in the inner and outer rings are always phase locked for PSR B0818 -41. This could be a significant constraint for the theoretical models of pulsar radio emission, and favors a pan magnetospheric emission mechanism. We observe frequent nulling for PSR B0818-41. We calculate a nulling fraction ~30% at 325 MHz for this pulsar. Lengths of neighboring nulls and bursts are found to be independent. For the inner drift region, our investigations bring out the fact that the nature of the transitions from burst to null are different from the transitions from null to burst. Switching off of pulsar radiation during nulling for PSR B0818 -41 is not abrupt, but is gradual, whereas the transitions from null to burst are found to be rather abrupt for the inner drift region. This effect is not prominent in the outer drift regions. Although, the inner region of the last active pulses before nulls are dimmer, the first active pulses after nulls outshines the normal ones. The intensity of the inner region is maximum for the average profile from the first active pulse immediately after the nulls and then gradually goes down. This is consistent with the behavior of the individual nulls described above. However, this is not the case for the leading and trailing outer regions. The average profiles from the first active pulse immediately after the nulls follows similar shape as the normal profile but shows an increased intensity (in the form of a bump) in the inner region which is not present in the normal average profile. In addition, the leading and the trailing peaks appear to be of similar intensity, while trailing peak is significantly more intense for the normal profile. The average profiles from the pulses immediately after the nulls are wider than the normal profile. The average profiles of the first active pulses after the nulls are drastically similar between two epochs of observations. This is a very unique result which is not reported for any other pulsar so far and may imply that the phenomenon of nulling is associated with some systematic energy re-distribution in the pulsar magnetosphere. In depth study of PSR B0826-34 PSR B0826-34 is a pulsar with one of the widest known profile. The earlier studies of this pulsar (Durdin et al. (1979), Biggs et al. (1985) and Gupta et al. (2004)) have brought out some unique properties : strong evolution of the average profile with frequency, apparent nulling for 70% of time and a remarkable subpulse drift property- multiple curved drift bands with frequent changes and sign reversals of drift rate. We studied PSR B0826 -34 using the GMRT, simultaneously at 303 and 610 MHz, and individually at 157, 325, 610 and 1060 MHz. Detailed investigation of PSR B0826- 34 are reported in Chapter 5 of this thesis. Some of the interesting new results from our work are, As a natural out-come of the simultaneous dual frequency observations, we obtain an accurate DM value, equal to 52.2(6) pc/cm3, for this pulsar. Unlike most normal pulsars the DM determination for this pulsar is a difficult and trick exercise, mainly because the profile is quite complex, very wide and strongly evolving with frequency. The advantage of our method of DM determination is that the observations at a single epoch are self sufficient for obtaining the DM value at that epoch. Contrary to the earlier study by Esamdin et al. (2005), we find no evidence of weak emission during the typical long null states of this pulsar, simultaneously at 303 and 610 MHz, as well as from non simultaneous observations at 157, 325, 610 and 1060 MHz at separate epochs. We have also obtained absolute flux limits for the non-detection at various frequencies, which should be a useful comparison standard for any more sensitive studies in the future. We present the average profiles at five different frequencies. Main pulse (MP) and inter pulse (IP) emission observed for this pulsar span over wide pulse longitude. There is a remarkable frequency-evolution of pulse profile: IP becomes stronger with increasing frequency. We estimated the mean flux of the MP, IP and the full pulse region of PSR B0826- 34 at different frequencies of observation. Significant correlation in the total intensity of the individual pulses between 303 and 610 MHz is reported from the simultaneous dual frequency observations, which is indicative of the broad-band nature of the emission. The intensity correlations are positive for large lags, indicating that there is some kind of memory in the underlying structure. This memory is the longest for PSR B0826- 34, amongst all known cases. Our study of this pulsar brings out insight into simultaneous behavior of the single pulses from PSR B0826- 34 at 303 and 610 MHz, which has not been examined so far. We see about 6 -7 drift bands in the MP region at 303 and at 610 MHz. At 610 MHz we see about 2 -3 drift bands in the IP region. We observe wide variations in the drift rates, including positive and negative drift rates and curved drift bands, which are simultaneous for both frequencies. We have noticed coherence between simultaneous multiple drift bands - at some given instant of time all the drift bands (6 -7 drift bands) under the MP window show similar kind of drift. Though we find the drift pattern to be very similar in the simultaneous 303 and 610 MHz data, we observe that the drift band separation (Pm2) evolves significantly between these two frequencies, and in a manner opposite to the average profile evolution. In addition, we confirm the dependence of Pm2 on pulse longitude at 303 MHz and find indications for the same at 610 MHz. Significant linear polarization is observed in the MP region which drops abruptly at the edges of the pulse profile. Two orthogonal mode jumps are seen at the edges of the MP for both 325 and 610 MHz. We observe somewhat non orthogonal mode jump at the edges of IP for 610 MHz. Significant circular polarization in the MP along with the sense reversal near the center is observed for both the frequencies. The PA curve shows typical ”S” shaped swing (though there is some hint of a kink in the central part of the PA curve). RVM fit (Radhakrishnan & Cooke, 1969) to the PA curve is obtained with α~ 9.8 deg, β~3.2 deg, at both 325 and 610 MHz. The detailed study of two unique wide profile pulsars, PSR B0818-41 and PSR B0826 -34, was very rewarding and provided fair amount of insight towards the emission properties of pulsars. We broadly conclude that the emission from simultaneous multiple drift bands are coherent. In other words the emission mechanism responsible for generation of the drift bands is heavily correlated in the whole on pulse window. Also the equispaced sparks argues for a more isotropic arrangement of sparks which is favored by the conal model (Rankin, 1983). Drifting from more than one rings are observed only for two pulsars, PSR B0818-41 and PSR B0826-34. For PSR B081841 we observe that the emission from different rings are always locked in phase. This constant phase relation is maintained even during sequences of irregular drifting as well as after nulling. PSR B0826 -34 is another wide profile pulsar for which presence of simultaneous multiple drift regions are observed. For this pulsar the MP and the IP emission are interpreted to be coming from two concentric rings of emission. The drift bands in these regions are locked in phase implying that the emission from the inner and the outer rings are in phase. For PSR B0826- 34 we observe frequent nulling and changes of drift rates which are simultaneous for both the inner and outer rings. Hence for all pulsars for which we know drifting from more than one ring, the drift pattern in the inner and outer rings are always phase locked. No counter example is observed. This requires common drift rate in the inner and outer rings, implying that emission in the two rings are not independent, and the conditions responsible for drifting are similar in both rings. Our finding of PLR between the emission from the inner and the outer rings puts constraints on the theoretical models of pulsar emission mechanism and favors a pan magnetospeheric radiation mechanism. Preliminary study of single pulse properties of six other pulsars Inspired by the success of our study of PSR B0818- 41 and PSR B0826-34 we carried out single pulse study of few other pulsars with diverse profile. Preliminary results from this study are presented in Chapter 6. However, some of the new results form this work are highlighted in the following. We report occasional nulling for PSR B0540+23 which is important in the sense that nulls are not commonly seen in the core components. We observe simultaneous two drift bands for B1819- 22 at 325 and 610 MHz. We observe some kind of mode changing between stronger and weaker modes with changes of drift rates, which are probably associated with occasional nulling observed in this pulsar. For PSR B1839 -04 subpulse drifting is observed under the two peaks of the profile. The emission under the leading and trailing peaks appear to be in phase. Determination of the orbital parameters of binary pulsars Apart from the above work, I got interested in determination of the orbital parameters of the binary pulsars. This work was triggered by the discovery of a binary pulsar PSR J0514- 4002 (the first known pulsar in the globular cluster NGC 1851) at the GMRT in 2004 (Freire et al., 2004). We present a novel method for determination of the orbital parameters of binary pulsars, using data on the pulsar period at multiple observing epochs in contrast to the method described by Freire, Kramer & Lyne (2001) which requires both pulsar period and period derivatives at particular observing epochs. This method uses the circular nature of the velocity space orbit of Keplerian motion and produces preliminary values based on two one dimensional searches. Preliminary orbital parameter values are then refined using a computationally efficient linear least square fit. This method works for random and sparse sampling of the binary orbit. Unlike the method used by Freire, Kramer & Lyne (2001), which works for nearly circular binary orbits, this method works for binary orbit with any eccentricity. We demonstrate the technique on (a) the highly eccentric binary pulsar PSR J0514- 4002 (the first known pulsar in the globular cluster NGC 1851) and (b) 47 Tuc T, a binary pulsar with a nearly circular orbit. Our result agrees with the earlier determination of the orbital parameters of the binary pulsars done with coherent multi-epoch timing (Freire, Kramer & Lyne (2001) and Freire et al. (2007)). In our method the computation involves only one dimensional searches and linear least square fits. This study is reported in Chapter 7. The main conclusions and the possible future works are presented in Chapter 8.