Thesis (Ph. D. (Physics and Electronics))--Rhodes University, 2005.

The high time-resolution radio sky represents unexplored astronomical territory where the discovery potential is high. In this thesis I have studied the transient radio sky, focusing on millisecond scales. As such, this work is concerned primarily with neutron stars, the most populous member of the radio transient parameter space. In particular, I have studied the well known radio pulsars and the recently identified group of neutron stars which show erratic radio emission, known as RRATs, which show radio bursts every few minutes to every few hours. When RRATs burst onto the scene in 2006, it was thought that they represented a previously unknown, distinct class of sporadically emitting sources. The difficulty in their identification implies a large underlying population, perhaps larger than the radio pulsars. The first question investigated in this thesis was whether the large projected population of RRATs posed a problem, i.e. could the observed supernova rate account for so many sources. In addition to pulsars and RRATs, the various other known neutron star manifestations were considered, leading to the conclusion that distinct populations would result in a 'birthrate problem'. Evolution between the classes could solve this problem - the RRATs are not a distinct population of neutron stars. Alternatively, perhaps the large projected population of RRATs is an overestimate. To obtain an improved estimate, the best approach is to find more sources. The Parkes Multi-beam Pulsar Survey, wherein the RRATs were initially identified, offered an opportunity to do just this. About half of the RRATs showing bursts during the survey were thought to have been missed, due to the deleterious effects of impulsive terrestrial interference signals. To remove these unwanted signals, so that we could identify the previously shrouded RRATs, we developed new interference mitigation software and processing techniques. Having done this, the survey was completely re-processed, resulting in the discovery of 19 new sources. Of these, 12 have been re-detected on multiple occasions, whereas the others have not been seen to re-emit since the initial discovery observations, and may be very low burst-rate RRATs, or, isolated burst events. These discoveries suggest that the initial population estimate was not over-estimated - RRATs, though not a distinct population, are indeed numerous. In addition to finding new sources, characterisation of their properties is vital. To this end, a campaign of regular radio observations of the newly discovered sources, was mounted, at the Parkes Observatory, in Australia. In addition, some of the initially identified RRATs were observed with the Lovell Telescope at Jodrell Bank. These have revealed glitches in J1819-1458, with anomalous post-glitch recovery of the spin-down rate. If such glitches were common, it would imply that the source was once a magnetar, neutron stars with the strongest known magnetic fields of up to 10¹⁵ gauss. The observations have also been used to perform 'timing' observations of RRATs, i.e. determination of their spin-down characteristics. At the beginning of this thesis, 3 of the original sources had 'timing solutions' determined. This has since risen to 7, and furthermore, 7 of the newly discovered sources now also have timing solutions. With this knowledge, we can see where RRATs lie in period-period derivative space. The Parkes RRATs seem to be roughly classifiable into three groupings, with high observed nulling fractions - normal pulsars, high magnetic field pulsars and old, 'dying' pulsars. It seems that RRATs and pulsars are one and the same. When a pulsar is more easily detected in searches for single bright pulses, as opposed to in periodicity searches, we label it a RRAT. Such searches impart a selection effect on the parameter space of possible sources, in both nulling fraction and rotation period. In this sense, an observational setup could be designed to make any pulsar appear as a RRAT. For realistic survey parameters however, this is not the case, and the groups mentioned above seem to be the most likely to appear as RRATs. In fact, we can utilise RRAT searches to identify neutron stars, difficult to find by other means, in particular high-magnetic field pulsars, and pulsars approaching the pulsar "death valley". Some of the RRATs are well explained as being distant/weak pulsars with a high modulation index, others seem to be nulling pulsars. This highlights the incomplete knowledge of nulling behaviour in the pulsar population. It seems that there may be a continuum of nulling durations, under a number of guises, from 'nulling pulsars' to 'RRATs' to 'intermittent pulsars'. In fact this nulling may fit into the emerging picture, whereby pulsar magnetospheres switch between stable configurations.

On étudie, du point de vue théorique ainsi que par des simulations numériques, l'instabilité vis-à-vis du rayonnement de courbure d'un faisceau de particules chargées guidées par un très fort champ magnétique courbé, qui est intéressante en tant que mécanisme de rayonnement radio des pulsars. Les conditions de croissance sont un gradient de densité assez raide sur la frontière extérieure du faisceau et des fréquences élevées, satisfaisant une condition non-WKB.

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.

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.