Small-eccentricity binary pulsars with white dwarf companions provide excellent test laboratories for various effects predicted by alternative theories of gravity, in particular tests for the emission of gravitational dipole radiation and the existence of gravitational Stark effects. We will present new limits to these effects. The statistical analysis presented here, for the first time, takes appropriately care of selection effects.

The influence of the low-frequency timing noise on the precision of measurements of the Keplerian and post-Keplerian orbital parameters in binary pulsars is studied. Fundamental limits on the accuracy of tests of alternative theories of gravity in the strong-field regime are established. The gravitational low-frequency timing noise formed by an ensemble of binary stars is briefly discussed.

We present Arecibo observations of PSR B1534+12 which confirm previous suggestions that the pulse profile is evolving secularly. This effect is similar to that seen in PSR B1913+16, and is almost certainly due to general relativistic precession of the pulsar’s spin axis.

A change of the component separation in the profiles of the binary pulsar PSR B1913+16 has been observed for the first time (Kramer 1998) as expected by geodetic precession. In this work we extend the previous work by accounting for recent data from the Effelsberg 100-m telescope and Arecibo Observatory and testing model predictions. We demonstrate how the new information will provide additional information on the solutions of the system geometry.

Relativistic spin-orbit coupling should cause the spin axis of Binary Pulsar B1913+16 to precess at a rate of about 1 deg / yr. As a result, the pulse profile is expected to exhibit secular evolution. Weisberg, Romani, & Taylor (1989), and Weisberg & Taylor (1992) found that the intensity ratio of the two conal components changed at a one percent per year rate from 1981 to 1989, but that their spacing did not measurably change. They attributed these observations to our line of sight passing across the middle of a patchy cone of emission. Recently Kramer (1998) found that the intensity ratio continued its secular change for another decade, and also detected a narrowing of the conal component separation.

We present our analysis of eighteen years of 21 cm pulse profile data. We confirm that the profile is narrowing as precession finally carries our line of sight away from the emission beam axis, and we map the beam in two dimensions. The beam is elongated in the latitude direction, and the degree of elongation grows with radius.

The motion of the binary pulsar PSR J0751+1807 is assumed to satisfy the gravitational two-body equation (Barker and O’Connell 1975). Then by studying the motion of the binary system from initial state which determined by 3 precession phases of angular momenta (the orbit, the pulsar and the companion star) to the current state which determined by the observations on the orbital inclination. The spin angular momenta of the pulsar and the companion star are obtained.

I briefly review the means by which VLBI observations can determine the position, proper motion, and parallax of a pulsar, and consider a small subset of the applications of such results.

We outline some new techniques being used and present initial results from a pulsar astrometry program at the NRAO Very Long Baseline Array (VLBA). We find a proper motion of 89.0±0.4 mas/yr and a preliminary parallax of 1.05±0.25 mas for B0919+06.

A preliminary determination of the proper motion and parallax of the Vela pulsar was made using phase-referencing VLBI. The measured proper motion in right ascension and declination are respectively, µα = −49.8 ± 0.2 mas yr−1 and µδ = 30.5 ± 0.1 mas yr−1. The parallax of Vela was calculated to be πveia = 3.16 ± 0.33 mas, which corresponds to a distance of pc.

We report on a project to determine the proper motions of a sample of 13 young pulsars. Sky positions are measured with phase referencing using two elements of the MERLIN array, giving a baseline of 198km. This technique involves the concurrent observation of each pulsar with one or more extra-galactic reference sources located in the same primary beam. At an observing wavelength of 21 cm, this allows the measurement of pulsar positions to an accuracy greater than 1 mas.

Pulsar positions obtained by VLBI and timing are compared. The difference between these positions is proven to be mainly due to timing noise. A new method which significantly reduces the difference have been developed.

Pulsar radio emission shows remarkably rich, but complex behavior in both intensity and polarization when considered on a pulse-to-pulse basis. A large number of pulses, when averaged together, tend to approach & define stable shapes that can be considered as distinct signatures of different pulsars. Such average profiles have shapes ranging from that describable as a simple one-component profile to those suggesting as many as 9 components. The components are understood as resulting from an average of many, often narrower, intities — the subpulses —that appear within the longitude range of a given component. The pulse components are thus formed and represent statistically an intensity-weighted average pattern of the radiation received as a function of longitude. The profile mode changes recognized in many pulsars suggest that the emission profile of a given pulsar may have two quasi-stable states, with one (primary) state more probable/brighter than the other (secondary) state. There are also (often associated) polarization modes that represent polarization states that are orthogonal to each other. The complex nature of orthogonal jumps observed in polarization position-angle sweeps may be attributable to possible superposition of two profile/polarization modes with orthogonal polarizations.

Building on the analysis of the previous paper by one of us, we here turn first to a discussion of B0943+ 10’s chaotic “Q” mode and then to the polarisation properties of its “B”-mode radiation. Here again, we see evidence that the plasma processes responsible for pulsar emission may be organized into a system of columns. In short, 0943+10 provides unprecedented insight into the phenomena comprising what might be termed pulsar polar-fluxtube “weather”.

A few bright pulsars were observed at 35 MHz for ≳ 1000s using the Gauribidanur Radio Telescope, and the data were analysed to study their single-pulse fluctuation properties (Asgekar & Deshpande, 1999a; & ref.s therein).

The well-known drifter B0943+10 shows a well resolved two-component profile at 35 MHz. The longitude-resolved fluctuation spectrum (Fig 1) shows a stable phase modulation feature (aliased) at 0.459 c/P1, consistent with its drifting pattern seen at higher radio-frequencies. Using the “Cartographic Transform” technique (Deshpande & Rankin, 1999; hereafter DR), we have mapped the pattern of its polar emission at 35 MHz (Asgekar & Deshpande, 1999b) as shown in fig 1. Helped by the larger cone radius at lower frequencies, the subbeams can be sampled in their full radial extent at 35 MHz. These results combined with those at higher frequencies (DR) suggest a steadily rotating system consisting of 20 emission-columns in the polar region as that responsible for the observed fluctuations over the entire range of observed frequencies.

To study the structure of emission beam, we have analysed the single pulse data of PSR B0329+54 at 325 and 606 MHz. In order to unambiguously detect the weak emission components, we have developed a new data analysis technique, which we term “window-thresholding”. By applying this technique to the data, we have detected three new emission components, and also confirmed the presence of a component which was proposed earlier. Hence our analysis indicates that PSR B0329+54 has nine components, which is among the highest of all the known pulsars. The symmetric distribution of pulse components about the pulse centre, indicates that the emission beam is conal.

Recent observations of the “giant” radio pulses received from the Crab pulsar are reviewed. The frequency range over which they have been detected spans at least two decades, and individual pulses appear to have a frequency span of 3:1 or more. The pulse structure is dominated by scattering at frequencies below about 2 GHz; at higher frequencies the intrinsic pulse width is about 1 µs with nanostructure extending down to 10 ns. The polarization structure may be contaminated by scattering at low frequencies and the polarization is weak and highly variable at high frequencies. No definitive emission mechanism with testable hypotheses for the giant pulses has been put forth. One of the most promising, based on the collapse of plasma solitons, predicts nanostructure bandwidths narrower than what is seen for the giant pulses themselves, though the observations do not show wideband correlation of the shortest resolvable intensity structure.

We present 150 MHz observations of the bright millisecond pulsar J0437-4715, and pulsars J1453-6413 and J1752-2806, made with the Mauritius Radio Telescope (MRT). We use single-pulse sequences to derive some preliminary results on the pulse morphologies.

High sensitivity data were obtained on PSR B0031-07 using Ooty Radio Telescope (ORT), which has a single polarization. As this pulsar lies outside the range of other sensitive instruments, such observations are possible only with ORT. Some earlier results, such as the different drift modes, were verified. Our analysis suggests competition between the energies of pairs of drifting sub-pulses and a phase memory across short nulls. The constraints these results put on the theories of pulse emission are enumerated.

Single pulse studies of pulsar radio emission provide a window into the time dependent behavior of the radio loud region. I have analyzed a series of precision polarimetric observations of pulsar B0611+22 to determine the geometry of the emission region. The observations are consistent with a central core emission region, and a periodically present conal component. This identification leads to the surprising result that all emission is from the leading half of the polar cap.

We have used power spectra analysis on high frequency (1.4 GHz) data, obtained with the Effelsberg 100m radio-telescope in order to investigate the emission characteristics of pulsars. This analysis has not only revealed the prominent periodicities of the fluctuating components, but has also proven a powerful tool for identifying weak and often unresolved components and their longitudinal location within the integrated pulsar profile. Furthermore, close examination of the properties of the fluctuating components indicates physical connections between them, giving new insight to the structure of the emitting region.