The magnitudes of some general relativistic effects depend on the mass of gravitating objects; therefore, these effects can be used for the stellar mass measurement. Several methods have been proposed; for example, the microlensing effect (Paczyński 1986), and the parallactic variation of gravitational deflection of the light from the source (Hosokawa et al. 1993).

Recently some candidates for MACHOs – a kind of dark matter candidates in the Galactic halo – have been discovered by their microlensing effect, and their masses were estimated to be of the order of 0.1Mʘ, under many assumptions. Actual measurement of MACHO masses are important in specifying what kind of object the MACHO is.

When the radiation of a star passes near a massive object, its intensity is magnified due to the microlensing phenomenon. Similar effects should also apply to pulsar signals. However, because of the pulsating nature of pulsar radiation, the flux magnification will be accompanied by a time varying travel-time delay (Krauss and Small 1991, Larchenkova and Doroshenko 1995, Ohnishi et al. 1995, Wexet al. 1996).

Let us denote the epoch of observation by t⊕. A travel-time delay caused by a Schwarzschild-lens is characterized by four parameters: M, the mass of the lens, q, the ratio of the transverse velocity of the lens with respect to the line of sight and the minimum impact parameter, fm, the ratio of the minimum impact parameter and the Einstein radius, and T0, the time of maximum delay. If we denote the ‘intrinsic’ frequency and its derivative with respect to time (corresponding to the barycentric arrival time t = t⊕) by ν and then, we find for the observed phase of the pulsar at the barycentric time t:

A gravitational wave background (GWB) of sufficient strength, characterized by Ω, the energy density per logarithmic frequency interval in units of the closure density, would introduce timing residuals in the most stable millisecond pulsars. For a description pertaining to the observations of PSR’s 1937+21 and 1855+09 see Kaspi, Taylor and Ryba (1994), hereafter KTR, and references therein. Thorsett and Dewey (1996, see also this volume) present a method for placing a statistical upper limit on Ω. Their method however, cannot correctly account for the presence of a known level of white measurement noise in the timing residuals. We use a Bayesian approach which can best account for this white noise along with our lack of previous knowledge on the parameter Ω (McHugh, Zalamansky, Vernotte and Lantz, submitted).

Timing formula for data processing of observations of binary pulsars that accounts for the relativistic deflection of light in the gravitational field of the pulsar’s companion is presented, and the measurability of this effect along with its variance estimates is discussed. The deflection of the pulsar’s pulse trajectory in the gravitational field of its companion leads to variation in the pulsar’s rotational phase. This variation appears as a narrow sharp growth of the magnitude of the post-fit residuals in the vicinity of the moment of the superior conjunction of the pulsar with its companion. In contrast to the relativistic Shapiro effect, the amplitude of the effect of gravitational deflection of the pulsar radio beam has two peaks with opposite signs, which become sharper as the inclination i of the pulsar’s orbit approaches to the right angle. The effect under consideration influences the estimation of parameters of the relativistic Shapiro effect in the binary pulsars with nearly edgewise orbits. Its inclusion in the fitting procedure provides a more careful measurement of the sine of the orbital inclination i, as well as the masses of the pulsar and its companion. This permits an improved testing of alternative theories of gravity in the strong field regime. The effect of the gravitational deflection of light has been numerically investigated for binary pulsars with nearly edgewise orbits. It is shown that the effect is observed in general only when cosi is less than 0.003. This estimate becomes less restrictive as the pulsar’s spin axis approaches the line of sight.

A low-frequency, stochastic gravitational radiation background can be detected through the irregularities it induces in pulsar arrival times. In this poster we re-examine pulsar timing data presented in Kaspi, Taylor and Ryba (1994) [Ap.J., 428, p. 713] and present an optimal statistical framework for using timing data from a single pulsar to constrain the energy density in a gravitational wave background. Observations of PSRB1855+09 yield an upper limit (95% confidence) 1.0 × 10−8 or (90% confidence) 4.8 × 10−9 of the closure density at frequency 4.4 × 10−9 Hz. This result probably rules out cosmological models that use cosmic strings as seeds for galaxy formation. Using combined observations of the orbital decay of four binary pulsars we also derive weaker limits at frequencies as low as 10−12 Hz.

If strange quark plasma is the real ground state of baryonic matter (Witten 1984), then some of neutron stars (NS) could actually be strange stars (SS). It is difficult to distinguish SS from NS observationally. They have similar radii and masses and their crusts are built of the same matter. It seems that a good method for testing the existence of SS would be the studies of phenomena related to the stellar pulsations. In 1976 Boriakoff proposed that radial oscillations of NS could be observed within radio subpulses of pulsars. While various modes of pulsations of NS were studied by a number of authors, little attention was paid to seismological signatures of SS. The radial oscilations of bare SS were studied by Väth & Chanmugam (1992). Recently Weber (this volume) studied properties of stars made of matter described by BPS equation of state (EOS) (Baym et al. 1971) with a ball of strange matter inside, but they mainly concentrated on stability of white-dwarf-like SS. In this work I present fully relativistic calculations of the radial oscillation frequencies of SS. I determined the fundamental frequency for bare SS and SS with two different types of crusts depending on origin (Alcock et al. 1986) of SS and showed differences between them.

The hypothesis that strange quark matter may be the absolute ground state of the strong interaction (not 56Fe) has been raised independently by Boder and Witten. If the hypothesis is true, then a separate class of compact stars could exist, which are called strange matter stars. The properties of the complete sequence of such stars, which range from compact neutron-star-like strange stars to strange dwarfs to strange planets. The latter two constitute the strange counterparts of ordinary white dwarfs and planets, respectively. The properties of these objects are discussed in this paper.

Radio emission mechanisms for pulsars are reviewed with emphasis on five possible sites for the emission: the pair production front (PPF), two regions in the relativistically outflowing plasma (denoted ROP1 and ROP2) between the PPF and the light cylinder (LC), and two sited near the LC. Several maser emission mechanism are viable for ROP1, where radius-to-frequency mapping applies. Suggested Schott radiation from outside the LC is discussed critically.

The mechanism for the generation of pulsar radio emission has not yet been identified. Several coherent emission processes, linked to the motion of relativistic particles in the extremely strong pulsar magnetic field, have been proposed as possible candidates. Essential improvements, based on fundamental concepts of plasma physics, prove that collective plasma effects can provide the necessary degree of coherence. Progress in the 1990s, which is reported here, relates to curvature maser emission processes and relativistic plasma emission mechanisms.

The radio emission process in pulsars is still a mystery. It will help us all if the “theorist’s freedom” in allowed models can be reduced. In order to do this, high quality data must be considered carefully in the light of existing or new theories, and the theories must be extended to address the data.

Inverse Compton Scattering (ICS) is a very important process not only in inner gap physics, but also for radio emission. ICS of high energy particles with thermal photons is the dominant and a very efficient mechanism of the particle energy loss above the neutron star surface, and is an important process in causing gap breakdown. The pulsar distribution in the P − P diagram and the observed mode changing phenomenon of some pulsars can be expained by the sparking conditions due to ICS. ICS of the secondary particles with the low frequency wave from the inner gap sparking can be responsible for radio emission. In this ICS model, many observational features of pulsar radio emission can be explained, such as: one core and two conal emission components, their different emission altitudes and relative time delay effects; spectral behavior of pulse profiles; the behavior of the linear polarization and position angle.

We present data which indicates that micropulses from the bright millisecond pulsar PSR J0437–4715 are observed at equally-spaced preferred phases within the pulse window. We find the pattern similar to that expected if each micropulse is radiated from a source with a coherence scale of ~ 100 m.

Considering the geometry of the hollow-cone model of radio pulsar emission in detail, one is led to the conclusion that several well-known observed effects may be explained quite easily purely on geometrical terms. It is shown that a steepening of the spectrum (cut-off) may be expected for certain geometrical situations as well as variations in component spectra, e.g. steeper spectra for the central component. These effects appear in addition to the already existing physical variations with frequency.

The nonlinear propagation of pulsar radiation in an electronpositron plasma is considered. Accounting for the pulsar radiation ponderomotive force, the relativistic mass variation, and the plasma magnetization, we obtain a pair of nonlinear equations which exhibit coupling of pulsar radiation with the background plasma slow motions in magnetized plasmas. The modulational/filamentation instability as well as the localization of pulsar radiation are investigated. We conclude that our investigation is relevant to the microstructures in the pulsar magnetosphere.

One of the essential ingredients in existing polar gap models (e.g. Ruderman & Sutherland 1975; Arons & Scharlemann 1979, hereafter AS) is a pair cascade above polar caps. In these models, pair cascade is initiated by curvature photons radiated by primary particles and produced pairs screen out the electric field, forming a pair-production-limited acceleration zone (called the polar gap). For pulsars with strong magnetic fields (B ≳ 1012 G) and hot polar caps, resonant inverse Compton scattering (RICS) can be important (e.g. Dermer 1990; Sturner 1995, and references therein). The gap height is significantly reduced by the RICS effect (Luo 1996), and this may reduce the energetics of the polar gap as the maximum energy of primary particles is constrained by the gap height. The minimum temperature for RICS to be important is (Luo 1996)

Not long ago Rankin (1990) presented strong evidence in favor of a low altitude (r ≈ R*) dipole geometry for the site of the core component of pulsar radio emission. Arons (1993) gave evidence that spun up millisecond pulsars must have a substantially dipolar large scale field at low altitute. Electron-positron pair creation at low altitude above the polar caps has long been hypothesized to be an essential ingredient of pulsar radio emission. If so, all observed pulsars must lie in the region of P − Ṗ space where polar cap acceleration has sufficient vigor to lead to the copious pair production. Yet, to date, all internally consistent theories of polar cap pair creation have required hypothesizing a large scale (eg, quadrupole) component of the magnetic field with strength comparable to that of the dipole, in contradiction with the evidence in favor of an apparently dipolar low altitude geometry. The internally consistent theories also violate other observational constraints. The discharge models of Ruderman and Sutherland (1975), Gurevich and Istomin (1985), and Jones (1977, 1978, 1979) all accelerate equal, counterstreaming flows of electrons and positrons, thus putting one half of the particle acceleration energy into high energy particle and photon bombardment of the polar caps. The heating causes pulsed thermal X-ray emission from hot spots in excess of what is seen (Ogelman 1993). While the Arons and Scharlemann (1979) model does not have this problem, since the space charge in the starvation zone above the polar cap is made up almost entirely of the outbound beam, in star centered dipole geometry it dramatically fails to account for pulsar emission over most of the P − Ṗ diagram and predicts radio polarization variations in contradiction to the observations (Narayan and Vivekanand 1982).

The statistical distributions of certain giant pulse (GP) properties appear to be well described by power laws. This suggests that the emission mechanism that produces giant pulses is a scale-invariant one. In turn this may indicate that the source of the GPs is in a state of self-organized criticality (SOC). For a recent discussion of SOC see Sornette et al. (1995).

Prior to this conference, the only pulsars reported to exhibit GPs were the Crab pulsar, PSR B0531+21 (Lundgren et al. 1995), and the millisecond pulsar PSR B1937+21 (Cognard et al. 1996). However, at the conference it was reported that giant micropulses had recently been observed from PSR J0437–4715 (Ables and McConnell, this volume). In all cases the statistical distributions of observed GP heights and/or fluxes are found to be well described by simple power laws. The arguments in this note apply to all these pulsars.

Radiation from a charge accelerated along its path or Linear Acceleration Emission (LAE) involves a number of subtleties (Pauli 1921; Ginzburg 1970, 1989). Potential interest of the mechanism for astrophysics has been pointed out by Wagoner (1969). Melrose (1978) and Rowe (1995) have studied amplified LAE from time-varying electric fields for radio pulsars. In contrast with the latter work our calculations are for static electric field structures or double layers (DLs) as are thought to occur in magnetospheres of neutron stars. In ordinary stellar atmospheres a LAE maser can operate in non-relativistic DLs (Kuijpers 1990) at a frequency ω ≈ kDLυ ≈ 2π/ttr, and a wave vector with kDL = 2π/L (L is the DL length, υ is the particle speed, and ttr is the transit time of the DL by the particle). The emission process can be considered as scattering of the electrostatic electric field on fast electrons into electromagnetic radiation satisfying the resonance condition: , when the frequency of the radiated mode in the frame of the emitting electron equals the Doppler shifted frequency of the electric field of the DL (DL wave frequency ωDL ≈ 0). For relativistic DLs, as are applicable to pulsar magnetospheres, the emission is expected to be beamed under an angle θ ≈ γ−1 and the frequency of emission boosted (ω ≈ kDLυ(1 − υcosθ/c)−1 ≈ γ2kDLυ).

Primary results of our recent work show that radio, infrared and optical, X-ray and γ-ray emission of Crab pulsar might be explained by Inverse Compton Scattering (ICS) mechanism.

Pulsars have typical magnetic field strengths between 108 and 1012 G. Therefore, even if the particles are produced with nonzero perpendicular component of velocity at the pole, they radiate out this component quickly due to synchrotron radiation. Therefore, the motion of a particle along a field line becomes one dimensional in regions where filed lines are straight. However, when they move along the curved magnetic field lines, they get accelerated due to the centrifugal force. They gain again the perpendicular energy at the expense of parallel energy, but can not gyrate in the curved field lines and hence do not experience any curvature or gradient drifts.