SUPERNOVA REMNANTS, PULSARS AND THE INTERSTELLAR MEDIUM - SUMMARY OF A WORKSHOP HELD AT U SYDNEY, MARCH 1999
Vikram Dwarkadas , Lewis Ball , James Caswell , Anne Green , Simon Johnston , Brian Schmidt , Mark Wardle, PASA, 17 (1), 83.
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Pulsars and the Interstellar Medium I

The first session on Thursday afternoon opened with Simon Johnston (SRCfTA) reviewing pulsar wind nebulae (PWN). Typically 1% of the spin-down luminosity of pulsars appears as pulsed emission, the remaining energy presumably coming off in the form of a relativistic particle wind, which is eventually stopped and shocked by the pressure of the surrounding interstellar medium (ISM), producing nonthermal radio emission. If the space velocity of the pulsar is low, the wind produces a bubble, or plerion, which can be imaged in radio/optical or X rays. On the other hand, if the pulsar has a high space velocity (and many do - see the next talk), the rapid motion through the ISM produces a bow shock, which can be seen in $H_{\alpha}$ or the radio continuum. Clearly what will be seen in individual cases will vary depending on the properties of the pulsar, its space motion, and the nature of the surrounding ISM. A search for PWN associated with 35 pulsars at 8.4 GHz with the VLA turned up 14 examples. There appear to be two classes of pulsars - young systems with $L_{\mathrm{radio}}/\dot{E} \sim 10^{-4}$ and middle-aged pulsars where this ratio is below 10^-6. Several hypotheses could account for this difference - perhaps the ratio of Poynting flux to particle flux decreases, or the energy spectrum steepens, or more pulse energy appears as gamma-rays. Of course, more observations are needed: a survey of 50 pulsars with ATCA is underway at 1.4 GHz using pulsar gating to increase the sensitivity 200-fold. The survey to date has turned up one bow shock in 5 pulsars examined - the Speedboat Nebula associated with PSR 0906-49.

Matthew Bailes (Swinburne) gave a talk on the distribution of pulsar velocities. The first part promoted the new supercomputer centre at Swinburne, extolling the processing power of the planned network of 64 linked workstations. This is impressive, but will be limited to problems that can be broken into many fairly-independent parallel modules. The second part discussed pulsar velocities, a topic that is important in understanding the Galaxy's pulsar population as a whole. He briefly reviewed the methods for measuring them. Generally the old millisecond pulsars have $v\leq 300$ km/s - they are likely bound to the Galaxy as one would expect. However, younger pulsars overall have a higher velocities, which indicates that their progenitor supernova explosions have a substantial asymmetry.

Lewis Ball (SRCfTA) spoke about inverse Compton scattering by relativistic pulsar winds. Electrons in the wind can upscatter ambient photons - starlight or the cosmic microwave background - to TeV energies for the expected Lorentz factors $\sim 10^6$. Generally this effect is small except for pulsars embedded in strong radiation fields, such as those in close binary systems. The pulsar B1259-63, which is in an eccentric orbit about a Be star, is of particular interest: conversion of 0.1% of the pulsar's spin-down luminosity into 100 GeV photons would give a flux detectable by the CANGAROO II Cerenkov telescope. The scattering is a strong function of geometry and distance from the star - the pulsar must be rather close to the star for the effect to be significant. The distance of B1259-63 to the Be star ranges from 20 to 300 R_*, so the gamma ray luminosity at periastron is large, with predictable variations around the well-determined orbit (Kirk, Ball & Skjæraasen 1999; Ball & Kirk 1999). Thus CANGAROO II observations will potentially be able to probe the properties of the pulsar wind, which is otherwise difficult to detect.

Kinwah Wu (SRCfTA) presented observations of optical and infrared lines in the spectrum of the X-ray binary Cir X-1. The emission lines are asymmetric, with a narrow component at +350 km/s and a broader blue-shifted component. Previously it had been suggested that the narrow component arises from rotation of an accretion disc, the corresponding blue-shifted component being absent because of a shadowing effect at that particular orbital phase. However, the new observations and archival data show that the profiles have varied systematically over the last 20 years: the narrow component always lies in the range 200-400 km/s, while the blue component varies somewhat both in shape and redshift (Johnston, Fender & Wu 1999). Kinwah offered a new model in which the narrow component is interpreted as arising from the heated surface of the $3-5 {\rm M}_{\odot}$ companion star, and the broad component arises in an optically thick outflow driven by super-Eddington accretion from the neutron star. The variability in the blue component reflects the eccentricity of the orbit: at periastron, the companion overfills its Roche lobe and dumps matter onto the star, producing the outflow. The overflow shuts off after periastron; near apastron the remaining overflow material settles into a quasi-steady accretion disc. This model explains the variability of the blueshifted component of the spectrum and the X-ray behaviour. One implication of this picture is that the system has a radial velocity of +430 km/s, which makes Cir X-1 one of the fastest binaries known. Even so, a sufficiently asymmetric supernova explosion can impart the required kick without unbinding the system (Tauris et al. 1999).

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