Observations over an eight month period from July 1997 to March 1998 show echoes following the radio pulses, and unusual changes in dispersion measure. We interpret this remarkable and complex event as refraction and dispersion in an ionised shell in the outer part of the Crab Nebula. The shell is remarkably compact; it has a electron density of about 103 – 104 cm−3 and a thickness of about 3 × 1011m. Similar events have been noticed on several other occasions during the continuous monitoring of the Crab pulsar over the past 25 years.

We obtained narrow-band continuum images of the Crab Nebula at 16, 6.4 and 2.2 µm. From these we derive spectral index maps, extending the optical work of Véron-Cetty & Woltjer (1993). In the central parts of the Nebula the IR spectral index is close to the radio one, indicating the region where the radio-emitting electrons are accelerated.

The plasma mechanism of synchrotron knots’ formation discovered in the Crab Nebula at the distances 0″.65 (1016 cm) and 3″.8 (6 × 1016 cm) from the Crab pulsar is presented. The mechanism is based on exitation of cyclotron and Cherenkov-drift instabilities in the relativist s electron-positron plasma of the nebula. The higher luminosity of the knots is supposed due to the orientation of the direction of motion of the synchrotron radiation sources (Larmor circles) relative to the observer.

We present results of time-dependent numerical modeling of the internal structure of the collisionless shock terminating the pulsar wind in Crab Nebula. We treat the equatorial relativistic wind as composed of ions and electron-positron plasma with an embedded toroidal magnetic field. Relativistic cyclotron instability of the ion ring downstream from the shock is found to launch outward propagating magnetosonic waves. Due to the fresh supply of ions crossing the shock, the time-dependent process achieves a limit-cycle pattern, in which the waves are launched with periodicity on the order of the ion Larmor time. Compressions in magnetic field and pair density associated with these waves as well as their propagation speed qualitatively reproduce the features observed in the wisps.

The 39.5 ms pulsar PSR 1951+32 and the remnant with which it is associated, CTB 80, have both been observed with the recently upgraded Westerbork synthesis array (WSRT). Parts of CTB 80 have been observed over the frequency coverage from 0.3 to 8 GHz, while PSR 1951+32 has been observed from 0.3 to 1.6 GHz. The results are described and discussed in the context of the pulsar-remnant association.

Shock waves in young supernova remnants (SNR) are generally considered to be the places where production and acceleration of charged particles (relativistic electrons and cosmic rays) take place. Older remnants can be re-energised if an active pulsar catches up with the shell of the remnant (Shull, Fesen, & Saken 1989). In that case a pulsar-driven wind can inject energetic particles into the shell, resulting into a rejuvenation of the radio emission of the old remnant due to the presence of additional relativistic electrons.

Radio observations of CTB80 (Angerhofer et al. 1981) and G5.4-1.2 (Frail & Kulkarni 1991) give evidence for the importance of the presence of an active pulsar close to the old shell of the remnants. In the first case the pulsar is believed to be inside the SNR. In the second case the pulsar is thought to have penetrated the shell of the SNR, and resides in the interstellar medium (ISM). We intend to investigate the physics which are connected with these kind of systems. One expects new effects resulting from the interaction of the three different shocks; the SNR shock, the bowshock bounding the pulsar wind nebula (PWN) and the (pulsar) wind termination shock. The dynamics of the system is described by a hydrodynamics code. We use the results from the hydrodynamics code to investigate the process of acceleration and transport of particles which are advected by the flow and diffuse with respect to the flow. We have applied the latter to a simple problem, the case of a spherically expanding SNR.

Twenty consecutive VLBI images of supernova 1993J in M81 from the time of explosion to the present show the dynamic evolution of the expanding radio shell of an exploded star. No clear sign of a pulsar nebula, expected to have a spectral luminosity 10 to 1,000 times larger than that of the Crab, has yet been seen. The upper limit on the brightness at 8.4 GHz in the center of the shell in one of the latest images is 0.15 mJy per beam of 0.4 mas2, corresponding to a spectral luminosity of that of the Crab. Any nebula that may have formed in the center is probably still obscured by the surrounding thermal matter with no substantial filamentation having yet occurred in the latter.

We present optical observations and radio non-detections of the bow shock nebula associated with the pulsar B2224+65 (the “Guitar Nebula”), and fit an analytic model to the observed bow shock to estimate its inclination and constrain other parameters (distance, pulsar velocity, ISM density). We also test scaling laws for bow shock parameters.

The spin-down luminosity together with the morphology of pulsar nebula, such a the Crab, have long been known to imply that the pulsar wind be practically unmagnetized immediately before it impinges onto the nebula. In fact, the condition of low wind magnetization can be shown to further imply that the pulsar outer magnetosphere be strongly distorted, where the toroidal field exceeds the conventionally-assumed dipole field by nearly two orders of magnitude in the outer magnetosphere; the current needed requires the plasma density to exceed the Goldreich-Julian charge density by several orders of magnitude. Such a physical condition can certainly impose a constraint on the candidate mechanisms of non-thermal radio emission, and may also place a constraint on the high-energy, electrode-gap emission mechanisms.

PSR B1259–63 is a unique system consisting of a young radio pulsar in orbit about a 10 M⊙ Be star. Its orbital period is 3.4 yr and the eccentricity close to 0.9. Around periastron, the system is a radio and X-ray transient and shows changes in flux density, dispersion measure, rotation measure and scattering parameters on very short timescales. Timing observations show the equatorial plane of the Be star and the plane of the orbit are not aligned and the orbit precesses as a result. I briefly summarise 10 years of radio observations of PSR B1259–63 and the implications of these observations. Future work includes the determination of the orbital size through VLBI and observations at TeV energies.

We investigate the modifications to the calculations of inverse Compton scattering by the wind of pulsar B1259–63 due to the inclusion of a realistic spectrum for the target photons from the pulsar’s Be-star companion.

The unique binary pulsar system PSR B1259–63, comprising a radio pulsar in an eccentric orbit about a hot, massive companion star, has been extensively monitored at radio frequencies between 843 MHz and 8.4 GHz during its 1994 and 1997 periastron passages. Unpulsed radio emission was detected for about 100 days around each periastron, varying on time scales as short as a day. We argue that the emission is synchrotron radiation generated as the pulsar passes through the Be-star’s excretion disk just before and just after periastron.

Unpulsed high energy (TeV) emission has been detected from several isolated pulsars (Aharonian 1999) and presumably results from relativistic electrons accelerated at the termination shock of an MHD wind driven by the pulsar itself. These electrons inverse Compton scatter target photons from either the cosmic microwave background, or from their own synchrotron radiation.

The rotation-powered binary pulsar PSR B1259–63 (Johnston et al. 1996) is also thought to drive an MHD wind, and the synchrotron radiation of electrons accelerated at its termination shock is probably the source of the unpulsed X-rays seen from this object by ROSAT, OSSE and ASCA (Tavani & Arons 1997). Compared to the isolated pulsars, however, the the pulsar’s Be-star companion provides an energy density of target photons available for inverse Compton scattering which is some 11 orders of magnitude larger. Using delta-function approximations to the emissivities and a monochromatic approximation to the spectrum of the target photons, we modelled the observed X-ray synchrotron emission and predicted the TeV emission in a recent paper (Kirk et al. 1999). In this contribution we improve these calculations in two respects – by treating the target spectrum more precisely, as described in the companion paper (Ball & Kirk 1999), and by relaxing the approximations made in the emissivities.

It is shown that the phenomena of radio eclipsing can be explained by the linear mechanism of transformation of the transverse electromagnetic wave, propagating in the pulsar wind, into the plasma waves in the region of interaction of wind with a companion star atmosphere. The coefficient of the passing η depends on the wave frequency ω by the exponential manner η = exp{–const · ω−1}. The estimated scale for the pulsar wind and star’s atmosphere density gradients are of the order of 100 meters. Such gradient can be obtained in the bow shock forming when the pulsar wind enters into the companion star atmosphere. Annihilation of the part of the wind’s positrons with the star’s electrons produces the electric current. This current generates the magnetic field from which the pulsar wind’s particles are reflected. The magnitude of the magnetic field in this shock of about several Gauss.

We suggest that companions of two binary eclipsing pulsars, B1957+20 and J2051-0827, are degenerate magnetic dwarfs. The magnetospheres of the white dwarfs are infused with relativistic particles supplied by the pulsar wind. We demonstrate that the radio waves emitted by the pulsars are strongly damped due to cyclotron resonance with the plasma particles, in the magnetospheres of companions. Large stable continuos and frequency-dependent radio eclipses should occur as a result of this process. This agrees well with observations.

In this paper, I review our current understanding of interstellar scintillations (ISS) of pulsars. The emphasis is on new results that have appeared during the last five years. The topics covered include (i) review of the understanding of refractive ISS (ii) the shape of the spectrum of electron density fluctuations in the interstellar medium (iii) the distribution of scattering plasma in the Galaxy (iv) resolving pulsar emission regions using ISS and (v) ISS and pulsar velocities.

Results from new observations of pulsars using the Ooty Radio Telescope (ORT) are used for investigating the structure of the Local Interstellar Medium (LISM). The observations show anomalous scintillation effects towards several nearby pulsars, and these are modeled in terms of large-scale spatial inhomogeneities in the distribution of plasma density fluctuations in the LISM. A 3-component model, where the Solar neighbourhood is surrounded by a shell of enhanced plasma turbulence, is proposed for the LISM. The inferred scattering structure is strikingly similar to the Local Bubble. Further, analysis based on recent scintillation measurements show evidence for enhanced scattering towards pulsars located in the general direction of the Loop I Superbubble. The model for the LISM has been further extended by incorporating the scattering due to turbulent plasma associated with Loop I.

Bhat et al. (1998) presented a model in which they explain the increase in scintillation bandwidth with decreasing pulsar distance as being due to a shell of scattering material associated with the Local Bubble. However, Britton et al. (1998) concluded that the scattering material for local pulsars is nearer to the pulsar (i.e. possibly in a bow shock or shell near the pulsar). We have investigated the effects of the local bubble shell and pulsar bow shocks on the scintillation bandwidth, the scintillation timescales and the angular broadening of pulsars. We find that a) there is no evidence for a shell of scattering material associated with the Local Bubble because the scintillation timescale data do not fit the Bhat et al. model and the angular broadening measurements do not suggest a scattering screen near the Sun, and b) that pulsar bow shocks cannot produce any enhanced scintillations.

Refractive Interstellar Scintillation (RISS) effects on pulsar signals are powerful techniques for discriminating between different models that have been proposed for the power spectrum of plasma density fluctuations in the Interstellar Medium (ISM; e.g. Rickett 1990). The nature of the spectrum is considered to be a major input for understanding the underlying mechanism of interstellar plasma turbulence. Data from our long-term pulsar scintillation observations using the Ooty Radio Telescope (ORT) at 327 MHz are used to investigate the nature of the spectrum in the Local Interstellar Medium (LISM; region within ∼ 1 kpc of the Sun). Dynamic scintillation spectra were obtained for 18 pulsars in the DM range 3–35 pc cm−3 at ∼10–100 epochs spanning ∼100–1000 days during 1993–1995 (Bhat et al. 1999). From these observations, various scintillation properties and the ISM parameters are estimated with accuracies much better than that which has been possible from most earlier data. The time series of parameters, viz., decorrelation bandwidth (vd), scintillation time scale (τd) and the drift slope of intensity scintillation patterns, and pulsar flux density are used to study various observable effects of Interstellar Scintillation, based on which the spectral form is inferred over the spatial scale range ∼ 107 m to ∼ 1013 m.

Preliminary results of 1.7 GHz monitoring of the interstellar scintillation of low-DM pulsars with the TCfA 32-m radiotelescope are presented. The dynamic spectra of pulsar intensity variations have been measured with the PSPM-2 which is a 2 × 64 × 3 MHz - channel spectrometer. The 192 MHz total bandwidth of this device provides an adequate frequency coverage to fully parametrize the observed scintillation. A distinct multiple imaging event has been identified in the dynamic spectra of PSR B1133+16.