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).
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.
© Copyright Astronomical Society of Australia 1997