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the egret telescope on the compton gamma ray observatory ( cgro ) identified at least six young spin - powered @xmath0-ray pulsars @xcite at energies up to 20  gev . a search for pulsed emission above 20  gev remains to be successful for ground - based cherenkov instruments .
despite a reduction of the energy threshold down to 60  gev @xcite , achieved with the the non - imaging telescopes , no evidence of a pulsed signal has been seen so far .
it is widely accepted that this unexplored energy region is vital to address the long - standing question of how and where high energy emission emerges from the pulsar .    _
polar cap _ models ( for review of the models see @xcite ) assume that particles are accelerated right above the neutron star surface and that @xmath0 rays emanate from curvature radiation or inverse compton induced pair cascades in a strong magnetic field . on the other hand _
outer gap _ models consider particle acceleration which occurs along null charge surfaces in the outer magnetosphere where @xmath0-rays result from photon - photon pair production - induced cascades .
these two types of basic models make rather different predictions , particularly of the spectral characteristics of pulsed @xmath0-ray emission .
detection and study of this high energy emission , which is closely tied to the primary population of radiating particles , seems to be a good discriminant between pulsar models .
the _ high energy stereoscopic system _ ( h.e.s.s . ) of four imaging atmospheric cherenkov telescopes with a low energy threshold of 100  gev at zenith and below 1% crab flux sensitivity for long exposures @xcite was used for observations of three young pulsars : the crab and vela pulsars , and psr  b1706 - 44 , characterized by their high - ranking position on a list of pulsars ordered by the parameter @xmath1 , which ultimately determines pulsar luminosity at high energies for a given pulsar age . here
@xmath2 is the spin - down energy loss rate , and @xmath3 is the distance determined from the dispersion measure to the object .
the spectra of the egret - detected pulsars can be described quite well by simple power laws with spectral indices in the range 1.39 - 2.07 @xcite .
some of the spectra show clear evidence for a drop in flux at high energy , above few gev , whereas others have rather large uncertainties in the 4 - 10  gev band , which prevent the clear identification of a similar cutoff .
the measured crab spectrum is a straight power law .
extrapolation of egret spectra into the dynamic range of h.e.s.s . suggested that high energy pulsed emission might be observable from these objects within a reasonable exposure time @xcite .
the outer gap model also supplements pulsed tev emission via inverse compton scattering by gap - accelerated particles .
the most recent outer gap models @xcite have tev @xmath0-ray fluxes for the vela pulsar which should be detectable with h.e.s.s .
is an array of four atmospheric imaging cherenkov telescopes , each with @xmath4 of mirror area and equipped with a 960 photo - multiplier tube camera @xcite .
telescopes are operated in a stereoscopic mode with a system trigger , requiring at least two telescopes to provide images of each individual atmospheric shower in cherenkov light .
h.e.s.s . has large field of view of @xmath5 diameter .
the angular resolution for individual @xmath0 rays is better than @xmath6 .
it allows a very good source localization accuracy of 30@xmath7 for relatively faint sources , which is important for point - like source ( e.g. pulsars ) identification .
a substantial fraction of the data on young pulsars was taken during the construction phase of the h.e.s.s .
array , when only two or three telescopes were available .
short summary of data is given in table  [ data ] .
the crab pulsar , located in the northern sky , can be observed with h.e.s.s . only at a rather large average zenith angle ( @xmath8 ) and consequently above rather a high energy threshold ( @xmath9 ) , whereas vela and psr b1706 - 44 can be seen with h.e.s.s .
at much higher elevations .
most of the data were taken in so - called _ wobble _ source - tracking mode , which is optimal for observations of a point - like source .
a few additional hours of observations of psr  b1706 - 44 have been extracted from the long scan of the galactic plane @xcite .       derived cutoff energies for three young pulsars
observed with h.e.s.s .
are given in table  [ results ] .
the stringent integral upper limits above 200 - 300  gev reported here for three young pulsars after rather limited exposures with h.e.s.s .
appear to be still above the predictions by the polar cap and outer gap models at these energies .
therefore , these upper limits can not be used to discriminate between two competing models .
however , they can severely restrain the luminosity of pulsed @xmath0-ray emission .
in addition , for the vela pulsar the outer gap model predicts a rather flux emission via inverse compton scattering at tev energies @xcite , which is inconsistent with the model - independent h.e.s.s .
upper limits reported here .
the inverse compton flux level depends on the emission spectrum mainly in the infra - red band , which is difficult to measure in most pulsars .
thus the h.e.s.s .
upper limits in particular for vela pulsar constrain the density of local soft photon field in the gap .
* acknowledgement .
* the support of the namibian authorities and of the university of namibia in facilitating the construction and operation of h.e.s.s . is gratefully acknowledged , as is the support by the german ministry for education and research ( bmbf ) , the max planck society , the french ministry for research , the cnrs - in2p3 and the astroparticle interdisciplinary programme of the cnrs , the u.k . particle physics and astronomy research council ( pparc ) , the ipnp of the charles university , the south african department of science and technology and national research foundation , and by the university of namibia .
we appreciate the excellent work of the technical support staff in berlin , durham , hamburg , heidelberg , palaiseau , paris , saclay , and in namibia in the construction and operation of the equipment .     the energy spectrum of the pulsed emission from the vela pulsar as measured by egret together with the h.e.s.s .
upper limits .
predictions from the polar cap ( solid curve ) @xcite and outer gap model ( histogram ) @xcite are also shown .
other tev upper limits are given for durham @xcite and cangaroo @xcite experiments.,scaledwidth=58.0% ]
pulsars are commonly regarded as highly magnetized neutron stars , rotating up to several hundred times per second . 
 over 1,500 radio pulsars have been found so far , about 70 of which are x - ray pulsars , but only a handful have been observed in @xmath0 rays . 
 this high - energy emission is believed to be produced by the electrons accelerated to tev energies in the pulsar magnetosphere , resulting in cascades of secondary particles . 
 the h.e.s.s . 
 experiment is a system of four imaging atmospheric cherenkov telescopes in namibia , which is sensitive to @xmath0 rays above 100 gev . 
 three young pulsars , crab , vela , and psr b1706 - 44 have been observed with h.e.s.s . 
 the results of the search for pulsed emission for these targets , and the constraints on the theories of pulsed very high energy @xmath0-ray emission , are summarized here .