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pulsars are steep - spectrum radio sources , with a `` typical '' spectral index of @xmath3 ( lorimer et al .
for this reason , most large - area pulsar surveys have been done at relatively low frequencies , @xmath4mhz . however , at low frequencies and low galactic latitudes the contribution from synchrotron radiation ( with spectral index @xmath5 ) dominates the system temperature of a radio telescope , greatly reducing the sensitivity to most pulsars .    to search for pulsars along the disk of the galaxy , one should therefore consider using a relatively high frequency , @xmath6mhz ( @xmath7 cm ) . to search for _ distant _ pulsars along the galactic plane , one
is virtually compelled to use high frequencies , because multi - path propagation of radio pulses through the inhomogeneous interstellar medium results in broadening ( `` scattering '' ) of intrinsically sharp pulses ( see fig .
this effect , greatly reducing the detectability of pulsars , varies with frequency approximately as @xmath8 ( cordes , weisberg , & boriakoff 1985 ) .
the obvious drawback of a survey at 1400mhz is that , in addition to the long individual integration times required to maintain high sensitivity due to the reduced pulsar fluxes , the number of independent telescope pointings needed increases as @xmath9 .
llll & jodrell bank & parkes & parkes + latitude range , @xmath10 & @xmath11 & @xmath12 & @xmath13 + longitude range , @xmath14 & @xmath15  @xmath16 & @xmath17  @xmath18 & @xmath19 
@xmath20 + center frequency ( mhz ) & 1400 & 1520 & 1374 + number of beams & 1 & 1 & 13 + integration time ( min ) & 10 & 2.5 & 35 + sample interval ( ms ) & 2.0 & 1.2 & 0.25 + bandwidth ( mhz ) & @xmath21 & @xmath22 & @xmath23 + @xmath24 ( jy ) & 60 & 70 & 36 + @xmath25 ( mjy ) & 1.2 & 1.0 & 0.15 + pulsars found / detected & 40/61 & 46/100 & 440+/630 + + reference & clifton et al .
& johnston et al . &
this work +    despite this hindrance there are very good reasons for wanting to search the galactic plane : young pulsars will naturally be found close to their places of birth , viz . the galactic disk . while relatively rare , they are interesting for a variety of reasons , including the study of pulsar 
supernova remnant interactions , and the preferential display of rotational `` glitches '' , and increased likelihood of emission at x- and @xmath26-ray energies , which are of interest for studies of the internal dynamics , and cooling and emission mechanisms of magnetized neutron stars , respectively .
also , to obtain an unbiased picture of the intrinsic galactic distribution of pulsars , rather than just of the local population , one must penetrate deep into the galaxy , i.e. , use high frequencies .
only two large - area pulsar surveys had been carried out at 20 cm prior to the one described here .
the surveys of clifton et al .
( 1992 ) and johnston et al .
( 1992 ) in the 1980s ( see table  1 ) were very successful at finding many pulsars , preferentially young and relatively distant .    in early 1997
a 13-element receiver package with very good system noise characteristics was installed on the parkes telescope .
developed for surveying hi in the local universe ( staveley - smith et al .
1996 ) , it is also ideally suited for pulsar searching .
we began collecting data for the survey in august 1997 .
receivers for each of 13 beams are sensitive to two orthogonal linear polarizations .
signals from each polarization of each beam are detected in 96 filters , each 3mhz wide , upon which they are added in polarization pairs , high - pass filtered with a cutoff of 0.2hz , integrated for 0.25ms , and 1-bit sampled before being written to magnetic tape with relevant telescope information , for off - line processing .
the sensitivity of the survey to long - period pulsars , about 0.15mjy , is a factor of seven better than the previous parkes 20 cm survey ( johnston et al .
1992 ) , and we are even more sensitive to short - period pulsars , owing to the faster sample interval and narrower filters used ( see table  1 ) . figure  1_a _ shows the calculated sensitivity of the survey .
note that the figure does not take into account scattering : according to it a pulsar like the crab , with period 33ms , but located across the galaxy , with @xmath27@xmath2pc , will be detectable with a minimum flux density of about 0.6mjy .
as figure  4_b _ shows , such a pulsar will most likely not be detectable as a pulsed source almost regardless of strength .
the 13 beam patterns ( each subtending a @xmath28 diameter ) are not adjacent on the sky ; rather , one central beam ( with the best sensitivity ) is surrounded by a ring of six beams separated by two beams - widths , surrounded in turn by a second ring separated by a further two beam - widths .
we collect data by interleaving pointings on a hexagonal grid , resulting in complete sky coverage with adjacent beams overlapping at half - power points .
each pointing covers an area about 0.6deg@xmath29 , resulting in sky coverage at a rate of 1deg@xmath29/hr , and the total survey area requires about 2700 individual pointings .    as of september 1999 we have collected and analyzed about 1200 independent telescope pointings , some 700deg@xmath29 , or 45% of the total .
data reduction takes place in workstations in a manner similar to previous surveys ( e.g. , manchester et al .
1996 ) . figure  1_b _ shows the search - code output for the discovery of a binary pulsar . to date we have discovered 440 new pulsars , and have re - detected 190 known pulsars . because of the long integrations , some binary pulsars ( in particular millisecond pulsars ) have signal - to - noise ratios reduced , owing to doppler - induced varying spin periods .
we therefore complement our standard search analysis with `` acceleration search '' reduction , recently begun .
while we have discovered 10 new pulsars for every 1% of the survey area searched so far , it should not be concluded that we will amass a total of 1000 new pulsars .
we began by surveying the regions closest to the galactic plane , which are richest in pulsars . in figure  2_a
_ we see that we have already searched essentially the entire @xmath30 region , with a new - pulsar density , averaged over the entire longitude range , of slightly over 1/deg@xmath29 .
clearly the density of pulsars drops dramatically for @xmath31 , which comprises much of the region yet to be searched .
conversely in figure  2_b _ we see that , as a function of longitude , the region we have preferentially searched so far ( @xmath32 ) has the lowest pulsar density . accounting for these selection effects in some detail , we predict that the number of new discoveries for the entire survey should be somewhat over 600 . another estimate is derived from the number of pulsars previously known in the overall search area , 255 , and the number of ( re-)detections : these would suggest an eventual total of @xmath33 new objects .
in fact we expect a total of about 700 pulsars to be discovered , after accounting for some further selection effects ( e.g. , so far we have not been complete in confirming new pulsar candidates down to the sensitivity level implicit in the calculations underlying figure  1_a _ ) .
we have therefore already detected about 2/3 of all pulsars we will discover .
manchester et al .
( this volume ) describe in some detail what is already known about many of the new pulsars .
figure  3_a _ shows the distribution of dm for the newly discovered pulsars , and for comparison also shows that for previously known pulsars .
qualitatively the distribution is as expected : we find pulsars predominantly with large dm , since our survey is by far the most sensitive ever to distant pulsars along the galactic plane ; on the other hand we find very few `` nearby '' ( @xmath34@xmath2pc ) pulsars , in part because selection effects ( e.g. , scattering ) preventing the detection of such pulsars in past surveys were far less important than for high - dm pulsars .
the median dm for the new pulsars is about 400@xmath2pc , and we also see that there is a marked decrease in the number of objects with @xmath35@xmath2pc , with a maximum of about 1300@xmath2pc . naturally it is more difficult to find such highly dispersed pulsars , both because of the attending level of scattering , and reduced flux density due to large distance . but
as we can see from figure  4_a _ , we do not in any case expect large numbers of pulsars in the galaxy to have @xmath36@xmath2pc .    a comparison of the period distributions for newly discovered and previously known pulsars ( fig .
3_b _ ) yields some surprises .
the most obvious discrepancy is that we have not discovered a significant number of short - period pulsars ( only one with @xmath37ms ) .
the parkes 70 cm survey ( manchester et al . 1996 ) , with a long - period sensitivity of @xmath38mjy and poorer time- and frequency - resolution than the multibeam survey , yielded 17 millisecond pulsars . assuming a spectral index of @xmath39 , 3mjy at 70 cm are equivalent to 0.3mjy at 20 cm  i.e.
, we should be more sensitive to millisecond pulsars than that survey .
one can not reasonably appeal to hypothetical larger spectral indices : edwards et al .
( this volume ) report on the very successful use of the multibeam data - acquisition system for finding millisecond pulsars in a survey at intermediate galactic latitudes and with short integration times .
it is conceivable that strong scattering prevents us from detecting many millisecond pulsars with @xmath40@xmath2pc ; in any case we are still investigating this dearth of millisecond pulsar discoveries .
a second surprise concerns the discovery of relatively many pulsars with long periods , @xmath41s .
this may be due to surveys with shorter integration times not collecting data for many such pulse periods , coupled with the prevalence of `` nulling '' pulsars among those with long periods ; and to the effectiveness with which much local radio - frequency interference gets `` dispersed away '' in our search for high - dm pulsars .    in figure  4_a
_ we plot the positions of the pulsars detected in the multibeam survey projected onto the galactic plane , according to the distance model of taylor & cordes ( 1993 ) .
two things are immediately apparent : the previously known re - detected pulsars are located relatively nearby ( @xmath42kpc ) , while many of the newly discovered pulsars are very distant , with many beyond the inner - most spiral arm ; and many of the newly discovered pulsars seem to be located along spiral arms , particularly the two inner - most ones . regarding this last point , we do expect pulsars to be correlated with spiral arm locations , but we should be careful to consider potential biases in the distance model that will place pulsars preferentially in regions of high electron density .
the multibeam survey is being remarkably successful at uncovering very distant pulsars .
the dm - distribution of these pulsars , @xmath43 , together with measured scattering parameters ( cf .
4_b _ ) , and eventually faraday - rotation parameters , probing the interstellar magnetic field , should give us a more unbiased picture of the galactic distribution of pulsars , and add considerably to our knowledge of the interstellar medium .
the parkes multibeam pulsar survey uses a 13-element receiver operating at a wavelength of 20 cm to survey the inner galactic plane with remarkable sensitivity . to date 
 we have collected and analyzed data from 45% of the survey region ( @xmath0 ; @xmath1 ) , and have discovered 440 pulsars , in addition to re - detecting 190 previously known ones . 
 most of the newly discovered pulsars are at great distances , as inferred from a median dispersion measure ( dm ) of 400@xmath2pc .    1.25 in .125 in .25 in