We have used the catalogue of (Taylor et al., 1993) to investigate the problem of a classification of pulsars. Five samples have been analysed by the main components method and by methods of the cluster analysis.

1. i) 15 pulsars with 14 known parameters P, Ṗ, W50, L (Malov et al., 1994), B, z,spectral index a, frequencies of maximum VM and minimum Vc in a spectrum (Malofeev et al., 1994), an angle between magnetic moment and rotation axis β (Malov, 1994), the transformation coefficient η = L/Ė, magnetic field near the light cylinder Blc and a maximum derivative of a position angle C = (dψ/dφ)max (Malov, 1990).

2. ii) 130 objects with 10 parameters (the spectral characteristics α, VM, VC and the parameter C were excluded from the consideration).

3. iii) 218 pulsars with 4 known parameters P, Ṗ, L and z. These parameters can be considered as independent variables.

4. iv) 89 sources with the same 4 parameters. This sample consists of two subsamples: 65 pulsars with P > 1 s and 24 ones with P < 0.1 s.

5. v) 24 pulsars with P < 0.1 s from the previous sample.

We have calculated accurate integral radio luminosities L for 232 pulsars (Malov et al., 1994) using new average spectra of these objects. Histogram of L-distribution is characterized by the mean value < log L >= 28.45 and by the mean-square-root deviation S = 1.0. We have analysed also data for short-periodic pulsars (P < 0.1 s) and long-periodic ones (P > 1 s) separately.The main goal of such separation was to test the hypothesis on two types of pulsars (Malov, 1987): i) for the first group of objects radiation is emitted from the neighbourhood of the light cylinder (r = rLC = cP/2π, P is the pulsar period), ii) for the second one emission is generated at distances r ≪ rLC. In the second case the main mechanism of radiation is curvature radiation. For the first group of pulsars the radiation is connected with the cyclotron mechanism. The difference between two basic mechanisms and the locations of the emission generation regions must cause some differences in the observable features for these two classes of pulsars.

We have organized a system to provide pulsar profiles and time-of-arrivals (TOAs) via anonymous ftp and the World Wide Web (WWW) at the address http://orion.usno.navy.mil/archive.html.

Timing measurements of pulsars have yielded a number of important results over the years, and their value is generally a strong function of measurement accuracy. I briefly summarize the present state of affairs in pulsar timing, and then offer some judgments about what may help to produce the best results in coming years.

In general, radio pulsars are superb clocks. However a few years after their discovery, it became clear that some pulsars show significant departures from their regular slow-down which make their rotation unpredictable. Two main forms of irregularities have been identified – glitches and timing noise – which are most marked in, but not confined to, young pulsars. Both are probably related to the internal structure of the neutron star and the properties of the internal neutron superfluid which prevent the smooth outward flow of angular momentum as the star slows down. In this review, the observational status and statistical aspects of the phenomena are described. We do not discuss in any detail their implications for neutron star structure. Rather, we consider them in so far as they limit studies in which the rotational stability is paramount and may limit the use of pulsars as astronomical chronometers. Glitch activity, the amount of post-glitch relaxation and the amplitude of timing noise all depend roughly linearly on the frequency derivative, implying that even the millisecond pulsars are prone to these effects.

Although 13 years have passed since the first millisecond pulsar (MSP) was discovered (Backer et al. 1983), it still has the shortest known rotational period (1.56 ms). MSPs are nature’s most stable clocks (Taylor 1991), with timing stabilities that rival atomic clocks on time scales beyond six months (Matsakis & Foster 1996). Specifically, the instantaneous measurement of the period of the original MSP is determined to a precision of ~ 20 attoseconds (10−18 s). With such precision, we are able to predict the pulsar pulse arrival times to a small fraction of the rotational period years into the future. MSPs are powerful sources for use in fundamental astrometry and time keeping applications. Collectively, a population of MSPs distributed around the sky can be used to establish a nearly inertial space-time reference frame (e.g. Foster & Backer 1990). Such a pulsar timing array (PTA) could be used to study drifts in Earth based atomic time scales, perturbation in the planetary ephemerides, relativistic corrections in the solar gravitational potential, and limit the energy density of a stochastic background of gravitational waves from the early universe.

We report on six months of dual-frequency, precision timing of PSR B1937+21 and the Crab pulsar. Variations in the column density of electrons – interstellar weather – are studied. The column is not unique in the presence of frequency-dependent multipath scattering, and this may explain the non-square law dependence of systematic timing residuals on B1937+21. The electron column density variations of the Crab pulsar are summarized in a structure function that ranges from 1012 cm to 1015 cm. These density fluctuations are located in the thermal shell surrounding the synchrotron nebula.

Recent results of timing observations of the planets pulsar, PSR B1257+12, are discussed. The current timing model includes significant contributions from the second and the third–order period derivatives. If these contributions are related to dynamical effects induced by a fourth distant planet, it would be a Saturn–mass object in a ~170 year orbit around the pulsar.

Millisecond pulsar as clocks are excellent tools for studying a variety of phenomena in physics and astrophysics (e.g. Foster & Backer 1990). We have been observing millisecond pulsars with the 100–m Effelsberg radiotelescope since April 1994. Initially, the goal of this program was to help continuing the timing of Arecibo pulsars during the upgrade–related shutdown period of the 305–m radiotelescope. Gradually, the program has evolved to time and study the emission physics of all short period pulsars detectable from Effelsberg. In particular, polarization measurements are used to make inferences about the emission physics of millisecond pulsars (cf. Xilouris & Kramer, this proceeding). At present, the programme involves approximately monthly observations of a set of 22 sources.

Daily timing observations of the Vela pulsar have been made over the last 14 years. During this time there has been seven large period jumps, or glitches, four of these have been observed as they occurred. We examine the time scale of these glitches and several other events which appear to be mini-glitches.

We present dual-frequency measurements on the Vela pulsar with a view to study the slow variations in the pulsed flux and the apparent differences in the pulse arrival times. We examine the data for correlated variations between the pulse intensities and arrival times at the two frequencies and discuss two main possibilities in order to explain the observed behaviour.

The data presented here consists of a) Pulse intensities, S635 & S950, at S635 & S950 MHz respectively and b) the ‘residual’ differences in the time of arrival of the pulse at the lower frequency, ΔTOA, with respect to that at the higher frequency. These data, over a span of ~1300 days (during 1988-92), were obtained as a part of the routine monitoring of the Vela pulsar from Mt. Pleasant Observatory of University of Tasmania, Hobart (see McCulloch et al. 1990).

The Vela pulsar (PSR B0833–45) has been observed from Hartebeesthoek RAO since 1984, as part of an ongoing monitoring project. During this time, the pulsar has undergone five sudden and relatively large spin-ups (“glitches”). Good observational coverage was obtained for the four most recent of these events.

The post-glitch behaviour of (t) is modelled by:

We study the expectancy of large glitches (ΔΩ/Ω > 10−7) from a sample of 472 pulsars other than the Vela pulsar. The pulsars in this sample have exhibited 20 large glitches. In the sample the total observation span is larger than 2000 pulsar years. We assume that all pulsars experience such glitches, with rates that depend on the pulsars’ rotation rate and spin-down rate, and on the glitch model. The superfluid vortex unpinning model gives good agreement with the observed distribution of glitches and with the parameter values deduced for the Vela pulsar glitches.

A simple model of a combined magnetic dipole, gravitational and/or magnetic quadrupole and particle accelerator is applied to simulate the time behaviour of the timing parameters of PSR 0531+21, PSR 1509-58 and PSR0540-69, as well as their energy budget. The model restricts the allowed values for the second braking index and is therefore testable through measurements of .

We determined with VLBI the coordinates of PSR B1937+21 and compare them with other determinations obtained with interferometry and timing. All the position estimates agree within one combined standard error, after the timing positions have each been rotated to the IERS extragalactic reference frame.

Recent improvements in cesium and hydrogen terrestrial frequency standards have brought the frequency precision of International Atomic Time (TAI) to a value of 2.5E-15 s/s over an averaging time of a month. In this paper we illustrate the improvement graphically, and discuss the state of the art for frame ties between the radio, dynamical, and optical frames. In a larger paper, available via the World Wide Web, we illustrate the measured accuracy curves of the frequency standards, show their effect on the ensemble time scales, explain the reasons for the confusing array of available time scales, and discuss the inverse problem of using pulsar data to correct the terrestrial time scale.

In this paper we present the preliminary results of phase-referenced VLBA observations of two pulsars PSRB1937+21 and PSRB0329+54. Since pulsar observations usually require a lower observing frequency than that used for other astrometric applications, ionospheric delays are a significant source of systematic error. For the observations presented here we used total-electron-content (TEC) measurements derived from dual-frequency GPS observations (Wilson, Mannucci & Edwards, 1995) to calibrate ionospheric delays.

Using this calibration technique were able to obtain positions for these two pulsars which we believe to be accurate to approximately 2 mas in each coordinate. With this level of accuracy we expect astrometric pulsar observations to be able to address a variety of interesting issues ranging from reference frame alignment and fundamental astrometry to the origin of pulsar velocities.

Interferometric measurement of a pulsar is one of the most effective techniques to measure accurate source position. Precise measurement of pulsar position will give us the information of proper motion and parallax. Interferometric measurements on pulsars have been carried out, but the number of the pulsars whose position were measured is still smaller than the total number of pulsars. Because newly discovered pulsars are rapidly increasing due to recent intensive systematic surveys. We have started measuring their position by using VLBI between Kashima and Kalyazin. This report shows the results of the first experiment.

A complete theoretical description of the parallactic terms in the timing formula for a wide orbit binary pulsar is presented. It is shown that the terms depend not only on the annual motion of Earth but also on the orbital motion of the binary pulsar as well. It is proved that measurements of the effect of orbital parallax open a new way in determination of pulsar distances, and can be used for measurement of companion masses as well. Estimates of expected magnitudes of the corresponding timing signals for the most appropriate binary pulsar candidates are tabulated, and prospects for the measurement of orbital parallaxes are discussed.

A possible functional dependence of the ratio of ‘gravitational’ mass mG and ‘inertial’ mass mI on the gravitational self-energy EG,

is called a violation of the Strong Equivalence Principle (SEP).

Weakly self-gravitating bodies are found in the solar system where lunarlaser-ranging data restrict the Nordtvedt parameter η to absolute values smaller than 0.001, (Dickey et al. 1994, Müller et al. 1995). To test higher order contributions one needs to consider strongly self-gravitating bodies such as neutron-stars.

Small-eccentricity binary-star systems consisting of a neutron star (|EG|/mc2 ~ 0.15) and a white dwarf (|EG|/mc2 ~ 10−4) are excellent ‘laboratories’ to test the SEP in a strong-field regime. As shown by Damour and Schäfer (1991) a violation of the SEP would lead to a periodic change in the eccentricity of the orbit of the binary pulsar caused by the galactic acceleration. Thus the observation of old small-eccentricity long-orbital-period neutron-star white-dwarf binary systems put (with a certain confidence level) a limit on the violation of the SEP.