Direct measurements of the distances to pulsars are notoriously difficult to obtain. There are three basic techniques: neutral hydrogen absorption, trigonometric parallax (measured either with an interferometer or through pulse time-of-arrival techniques) and from associations with objects of known distance (i.e. supernova remnants, globular clusters and the Magellanic Clouds). Together, these provide measurements of (or limits on) the distances to over 100 pulsars. For an excellent review of these measurements and their implications, see [163].
In the absence of a direct measurement, the distances to most
pulsars can be estimated from an effect known as
pulse dispersion
which arises from the fact that the group velocity of the pulsed
radiation through the ionised interstellar medium is frequency
dependent: Pulses emitted at higher radio frequencies travel
faster through the interstellar medium, arriving earlier than
those emitted at lower frequencies. The delay
in arrival times between a high frequency
(MHz) and a low one
(MHz), can be shown [109
] to be
where the dispersion measure DM (
pc) is the integrated column density of free electrons along the
line of sight:
Here,
d
is the distance to the pulsar (pc) and
is the free electron density (). Pulsars at large distances have higher column densities and
therefore larger DMs than those pulsars closer to Earth so that,
from equation
1
, the dispersive delay across the bandwidth is greater. Thus,
from a measurement of the delay across a finite bandwidth we
infer the DM. Hence, the distance can be estimated from a model
of the Galactic distribution of free electrons,
. Taylor & Cordes [149] have developed such a model calibrated from the sample of
pulsars with independently known distances plus measurements of
interstellar scattering for various Galactic and extragalactic
sources. The model appears to be free of large systematic trends
and can be used to provide distance estimates with an uncertainty
of
%.
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Binary and Millisecond Pulsars
D. R. Lorimer (dunc@mpifr-bonn.mpg.de) http://www.livingreviews.org/lrr-1998-10 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |
