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the @xmath1 @xmath0-ray telescope detected around 300 sources at energies above 100 mev , the majority of which remain unidentified ( hartman et al .
large error boxes on the order of @xmath2 across make unambiguous identifications on the basis of source position difficult .
timing observations allow definitive identifications for sources showing aperiodic ( such as active galactic nuclei ) or periodic ( pulsars ) variability . between 6 - 8 pulsars
have been positively identified in the @xmath1 data by epoch folding on the known period determined from radio or , in the singular case of geminga , x - ray data ( thompson et al 1999 , kaspi et al . 2000 and references therein ) .
several other energetic pulsars were found to be coincident with unidentified @xmath1 sources , but folding the sparse @xmath0-ray data using contemporaneous radio ephemerides did not yield significant detections ( fierro , 1995 ) .
although @xmath3 , when it flies in 2006 , may be able to discover new pulsars just by searching the @xmath0-ray data , it is likely that a priori knowledge of a pulse period will in many cases prove crucial in determining which @xmath0-ray sources are pulsars .
since the effective end of the @xmath1 mission and the publishing of the third @xmath1 catalog ( hartman et al .
1999 ) , there have been both new wide scale pulsar surveys and searches of potential @xmath0-ray counterparts which have yielded several new , energetic young pulsars positionally coincident with unidentified @xmath1 sources . the most productive effort has been the parkes multibeam survey ( manchester et al .
manchester et al .
( 2002 ) list six energetic pulsars from the pm survey which are contained within @xmath1 positional error boxes . for two of these ,
psr j1016@xmath45857 and psr j1420@xmath46048 , previously obtained x - ray and radio data identified potential pulsar wind nebulae ( pwn ) ( camilo et al .
2001 , roberts , romani , & johnston 2001 ) .
hence , searches directed at the potential counterparts would have been successful . in a third case , psr j1015@xmath45719 ,
very little is known about any potential x - ray or radio pwn . in the last three cases ,
neither radio nor x - ray data show clear evidence for a potential source counterpart .
psr j1837@xmath40604 is on the edge of a bright molecular cloud , making detection of a radio pwn difficult ( damico et al .
a 10 ks _ chandra _ image of the region shows no evidence for a x - ray point source associated with the pulsar to extremely low flux levels ( roberts et al . in preparation ) .
the last two parkes multibeam pulsars , psr j1413@xmath46141 and psr j1412@xmath46145 , are located within the bright energetic supernova remnant g312.4@xmath40.4 .
_ chandra _ imaging and radio spectral tomography of the region show neither point sources nor potential pwn associated with either pulsar ( doherty et al . in press ) .
the low spin - down luminosity , large distance , and alternate potential source of @xmath0-rays ( the supernova remnant ) all argue against either of these latter two pulsars being the true counterpart of the coincident @xmath1 source .
deep pulse searches of potential x - ray and radio counterparts have also recently proven successful . the extremely energetic pulsar psr j2229
+ 6114 was discovered with the lovell telescope at jodrell bank in a 1.5  hr observation of a potential pwn identified by x - ray and radio imaging of the associated @xmath1 error box ( halpern et al .
an @xmath5 x - ray image of the unidentified source gev j2020 + 3658 , associated with the cos b source 2cg 075 + 00 , showed a point - like x - ray source at which the very small ( @xmath6 ) arecibo beam could be directed .
the young pulsar psr j2021 + 3651 , at a 20  cm flux of only 0.1 mjy , was discovered as a result ( roberts et al .
this flux is about a factor of two below the sensitivity limits of the parkes multibeam survey and similar to the sensitivity limit of a 5  hr observation with parkes .
thus , it is possible that deep searches of potential counterparts contained within the parkes multibeam survey region will yield further candidates .
however , three likely pwn identified by roberts , romani , and kawai ( 2001 ) were searched with 5  hr pointings at parkes and failed to yield any good candidates .
two of these , the rabbit and gev j1809@xmath42327 , are almost certainly pwn , based on their x - ray and radio properties ( roberts et al .
1999 , braje et al .
therefore , more sensitive radio searches might be necessary to detect the pulsars these contain .
the third source , gev j1825 - 1310 is less clear of a pwn candidate , due to its large size and the confused radio emission in the region , but other source classes for hard , extended x - ray sources seem less plausible .    between the parkes multibeam survey and directed searches , six new strong @xmath0-ray pulsar candidates
have been discovered as radio pulsars since the demise of @xmath1 .
since these are young pulsars , which tend to glitch , extrapolating ephemerides back to the @xmath1 era is uncertain making detection of @xmath0-ray pulsations difficult .
i list these pulsars in table 1 in boldface .
the dispersion measure ( dm ) distances , determined using the new cordes and lazio ( 2002 ) ne2001 model , are given .
the @xmath0-ray efficiencies , @xmath7 , are determined from the 100  mev to 10  gev flux , assuming the dm distance and 1 steradian beaming .
all of the new pulsars have high dms and have 20  cm fluxes @xmath8 mjy .
the inferred @xmath0-ray efficiencies are also quite high . for comparison , table 1
lists the values for the known @xmath0-ray pulsars , using the ne2001 distances for consistency and adapting the efficiencies from the values given by kaspi et al .
if the @xmath0-ray sources associated with these new pulsars are nearly 100% pulsed , as are the known sources , then the oft - noted inverse correlation of efficiency with spin - down power ( @xmath9 , eg .
zhang & harding 2000 ) and characteristic age , @xmath10 , is not well supported .
lcccccc pulsar & log @xmath11 & log @xmath10 & dm & @xmath12 & @xmath13 & @xmath7 + & @xmath14 & yr & pc@xmath15 & kpc & mjy & + crab & 38.7 & 3.1 & 57 & 1.7 & 14 & 0.0001 + * j2229 + 6114 * & 37.3 & 4.0 & 200 & 7.3 & 0.3 & 0.007 + * j1420@xmath46048 * & 37.0 & 4.1 & 360 & 5.6 & 0.9 & 0.01 + vela & 36.8 & 4.1 & 68 & 0.24 & 1000 & 0.0004 + b1951 + 32 & 36.6 & 5.0 & 45 & 3.2 & 1.0 & 0.004 + b1706@xmath444 & 36.5 & 4.2 & 76 & 2.3 & 10 & 0.01 + * j2021 + 3651 * & 36.5 & 4.2 & 369 & 12 & 0.1 & 0.2 + * j1016@xmath45857 * & 36.4 & 4.3 & 394 & 8.0 & 0.5 & 0.03 + b1046@xmath458 & 36.3 & 4.3 & 129 & 2.7 & 8 & 0.009 + * j1837@xmath40604 * & 36.3 & 4.5 & 462 & 6.4 & 0.4 & 0.07 + * j1015@xmath45719 * & 35.9 & 4.6 & 283 & 5.1 & 0.9 & 0.1 + b0656 + 14 & 34.6 & 5.0 & 14 & 0.67 & 3.7 & 0.0007 + geminga & 34.5 & 5.5 & - & 0.16 & - & 0.02 + b1055 - 52 & 34.5 & 5.7 & 30 & 0.72 & - & 0.03 +
in addition to the galactic plane population , there is at least one distinct population of unidentified @xmath0-ray sources at mid - galactic latitudes .
some authors have modelled these as a gould belt ( a local region of recent star formation ) population plus a galactic halo population ( grenier 2001 ) .
it has also been noted that the distribution is similar to that of the millisecond pulsars ( romani 2001 ) . a population of older , geminga - like pulsars has been postulated , as well as a population of @xmath0-ray emitting millisecond pulsars .
indeed , there has been a recent claim of a marginal detection of pulsed @xmath0-rays in the @xmath1 data set from the millisecond pulsar psr j0218 + 4232 ( kuiper et al . 2000 ) .
practically speaking , because the mid - latitude sources are generally weaker and have steeper spectra than the galactic plane sources , they will be difficult to identify if they are not pulsed or have some other characteristic variability . if some fraction are pulsars , then a priori knowledge of the pulse period may be crucial to finding the pulsations at @xmath0-ray energies even with the large effective area of the @xmath3 mission .
we have begun a survey of the unidentified mid - latitude @xmath1 sources that are observable by the parkes telescope . with the multibeam system ,
a series of four pointings in the standard tesselation pattern can completely cover most @xmath0-ray error boxes out to the 95% confidence contour .
alist of 56 sources from the 3rd @xmath1 catalog at latitudes @xmath16 have been selected on the basis of their error box size and lack of probable agn counterpart .
four 35  min pointings using a time resolution of 0.125  ms cover each of the error boxes .
in order to be sensitive to millisecond pulsars in tight binaries , a full acceleration search will be performed using the presto software package ( ransom 2001 ) .
the sensitivity of this survey will be similar to that of the pm survey , and much greater than the highly successful mid - latitude surveys with parkes , which generally use only 4 min .
pointings ( edwards et al .
2001 ) .
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the majority of galactic high - energy @xmath0-ray sources continue to elude identification . 
 currently , we have a handful of firm pulsar identifications , one of which is radio quiet , and a few marginal detections , including one millisecond pulsar . 
 recently , both blind searches of @xmath1 error boxes and targeted searches of x - ray counterpart candidates have had some success in finding new pulsars 
 . i review these results , and discuss our current program of searching mid - galactic latitude @xmath1 error boxes using the parkes multi - beam system .    
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