, the observed flux density falls off as the inverse square of
the distance. This results in the observed sample being dominated
by nearby and/or bright objects. Beyond distances of a few kpc
from the Sun, the apparent flux density of most pulsars falls
below the flux thresholds
of the a survey. Following [56], we may parameterise the survey threshold by:
In this expression,
is a correction factor
which reflects losses to hardware limitations, SNR
is the threshold signal-to-noise ratio (typically 7-10),
and
are the receiver and sky noise temperatures,
G
is the antenna gain,
is the number of polarisations observed,
is the observing bandwidth,
is the integration time,
W
is the observed pulse width and
P
is the pulse period.
It follows from this equation that the sensitivity decreases
as
W
/(P
-
W) increases. Also note that if
W
>
P, the pulsed signal is smeared into the background emission and
is no longer detectable. The observed pulse width
W
is in fact broader than the intrinsic value for a number of
reasons: finite sampling effects; pulse dispersion, as well as
scattering due to the presence of free electrons in the
interstellar medium. As discussed above, the dispersive smearing
scales as
, where
is the observing frequency. This can largely be removed by
dividing the pass-band into a number of channels and applying
successively longer time delays to higher frequency channels
before
summing over all channels to produce a sharp ``de-dispersed''
profile
. The smearing across the individual channels, however, still
remains and becomes significant at high dispersions when
searching for short-period pulsars. Multi-path scattering results
in a one-sided broadening due to the delay in arrival times which
scales roughly as
, which can not be removed by instrumental means.
Dispersion and scattering become most severe for distant
pulsars in the inner Galaxy as the number of free electrons along
the line of sight becomes large. The strong frequency dependence
of both effects means that they are considerably less of a
problem for surveys at observing frequencies > 1400 MHz [46
,
82] compared to the usual 400 MHz search frequency. An added bonus
for such observations is the reduction in
, since the spectral index of the non-thermal emission is about
-2.8 [91]. Pulsars themselves have steep radio spectra. Typical spectral
indices are -1.6 [110,
99], so that flux densities are an order of magnitude lower at 1400
MHz compared to 400 MHz. Fortunately, this can usually be
compensated somewhat by the use of larger receiver bandwidths at
higher radio frequencies. For example, the 1380 MHz system at
Parkes has a bandwidth of 270 MHz compared to their 430 MHz
system, where 32 MHz is available.
In the past, the main disadvantage in surveying at high
frequencies has been the sky coverage rate which scales with the
solid angle of the telescope beam. The current generation of
high-frequency pulsar searches at Parkes and Jodrell Bank tackles
this problem by installing multi-beam receivers in these
telescopes. At Parkes, a 13 beam system [7] has been installed for use in neutral hydrogen surveys. The
system is also being used for pulsar searches and can cover the
sky at the same rate as the recent Parkes low-frequency 430 MHz
survey [111,
107]. Together with a 4-beam system being installed at Jodrell Bank,
the sensitivity of these systems is about 7 times better than
previous surveys at 1400 and 1520 MHz [46,
82] and should thus discover several hundred new pulsars. Indeed,
the Parkes survey has recently begun and has already discovered
over 100 new pulsars. For an update, and more information, see
Fernando Camilo's multibeam page [8].
|
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 |
