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 . 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  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 .
|Binary and Millisecond Pulsars
D. R. Lorimer (firstname.lastname@example.org)
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