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 ISM II
Extreme Scattering Events (ESEs) from pulsars were the topic of Mark Walker's (SRCfTA) presentation. ESEs were first discovered in extra-galactic sources, the symptoms being a rapid change in flux density of the observed source. These flux density changes are attributed to ionised gas clouds in our own Galaxy. From the observational data, Mark Walker and Mark Wardle have determined the parameters of these clouds: they have a size of roughly 2 AU, an electron density of 
cm-3 and a filling factor of about
5 x 10-3. They postulate that these clouds may solve the `missing mass' problem, at least in our Galaxy (Walker & Wardle 1998; 1999).
If a pulsar undergoes an ESE, one can in principle measure three different quantities. These are the deflection of the image (which can be measured by VLBI techniques), the delay of the signal (which can be obtained from pulsar timing) and the magnification of the image. Pulsars are exceedingly small, and this implies both a large peak magnification and a large coherent path length. Pulsars are also bright at low frequencies where the effects should be strongest. Previous work on ESEs on pulsars include the time delay and flux changes in the millisecond pulsar PSR B1937+21 and the fringe patterns in the dynamic spectrum of PSR B1237+25. However, there has been no systematic observational program carried out and this is needed as a matter of some urgency.
The nature of pulsars means that more information can be gleaned from ESEs than from say quasars. This in turn will lead to a better understanding of the structures in the interstellar medium which cause ESEs.
Jean-Pierre Macquart (USydney) continued the theme of scintillations with his presentation on scintillation and density fluctuations in the ISM. In scintillation theory it is thought that energy is deposited at very large scales (kpc or more), that it then `cascades' down to lower scales before finally dissipating at some small scale. However, although this sounds good, the questions of what provides the energy, how exactly it cascades down and what the dissipation mechanism is are all unanswered! (see, for example, Cordes, Weisberg & Boriakoff 1985)
If supernovae are providing the energy at the large scales then perhaps one might expect to see more turbulence in the vicinity of supernova remnants. Also, one might expect the power-law index of the turbulence, 
, to be 
4 rather than the canonical (Kolmogorov) value of 11/3. Is there any observational evidence for this? In or near the Vela supernova remnant there is some evidence for 
. Two surveys of extra-galactic point sources located behind supernova remnants have been ambiguous with no clear evidence for a higher power law index although one group do claim an enhancement behind the Cygnus Loop (Dennison et al. 1984). In summary, although supernova explosions are the popular choice for the energy input there is no unambiguous evidence for this (Spangler et al. 1986).
Jianke Li (ANU) gave his talk on the topic of the spin-up mechanism for millisecond pulsars (MSPs). It is widely believed that MSPs are formed from low-mass X-ray binaries in which a neutron star accretes matter from its low mass companion. Along with the mass transfer, the neutron star `accretes' angular momentum causing it to spin up. Typically, to end up with a 1 millisecond rotation rate requires the accretion of
/yr over 107 years.
Li argued that even a low magnetic field (say 104 Tesla) is enough to truncate the inner edge of the accretion disk and thus one has to have a magnetic boundary layer. This magnetic boundary may impede angular momentum accretion on to the star, so that the angular momentum accretion could be far less efficient as compared to the standard model. This casts doubt on whether a low-mass X-ray binary system such as J1808-369, with a binary period of only two hours, is indeed spun up by accretion.
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