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it has been believed for long time that supernova ( sn ) explosions and their remnants ( snrs ) are the places where the acceleration of galactic cosmic rays takes place .
energetic arguments as well as the diffuse shock acceleration mechanism applied to young snrs support this idea . a sn explosion every 30 years is enough to balance the escape losses of cosmic rays in the galaxy . during the past years
, iacts have confirmed that snrs are sources of vhe gamma - rays .
two scenarios can explain the production of the gamma - rays . in the electromagnetic scenario , accelerated electrons
can up - scatter low energy photons through the inverse compton mechanism
. in the hadronic scenario , accelerated nuclei can interact with matter or radiation producing neutral mesons that will decay into gamma - rays .
* magic * ( _ major atmospheric gamma - ray imaging cherenkov _ ) is a ground based gamma - ray telescope located on the canary island of la palma .
it is a new generation iact with a trigger threshold of 55  gev , and an energy detection range from 60  gev to 10  tev , overlapping with the upper energy threshold of satellites such as fermi .
the design of the magic telescope was a technological challenge which took the existing technology to its limit  @xcite .
one of its main characteristics is its octagonal parabolic reflector of 17  m of diameter resulting in an area of 240  m@xmath0 , capable of obtaining three times more light than a conventional iact of 10  m. for energies above 150  gev , the telescope angular and energy resolutions are @xmath10.1@xmath2 and @xmath125% respectively @xcite . besides this , in april 2007
its data acquisition system was upgraded with multiplexed 2  ghz fadcs which improved the timing resolution of the recorded shower images .
accordingly , the integral sensitivity of magic improved significantly from 2.2% to 1.6% of the crab nebula flux above 270  gev for 50 hours of observation @xcite .
the magic collaboration has finished the construction of a second telescope , magic - ii , similar to magic with improved technology .
located at a distance of eighty five metres from the first telescope , it will be fully operational in the second half of 2009 .
the operation of both telescopes in stereoscopic mode will allow us to improve the spatial resolution and sensitivity of the magic experiment .
during 2007 and 2008 the magic telescope has performed observations of different snrs .
the data include a deep observation of the tycho snr and shorter observations of various radio selected snrs .
the tycho snr is one of the best known and most studied snrs .
it is a shell - type snr ( well defined in radio and x - ray ) which was formed from , most likely , a ia supernova explosion in 1572 .
this young snr is a bright x - ray source with a similar diameter of 8 in both x - rays and radio .
the spatial structure of tycho has been studied by chandra and xmm - newton , providing constraints to the allowed explosion models .
although the age of the snr is known with precision , the distance is not well known .
distance estimates vary between 2.2  kpc  @xcite and 4.4  kpc  @xcite .
most accepted values are in the range 2.3 - 2.8  kpc .
however , recent measurements based on the light echo  @xcite suggest that a larger distance of 3.8  kpc could also be possible .
vhe gamma - ray emission from tycho is predicted by the non linear kinetic theory of cosmic ray acceleration in snrs  @xcite  @xcite . in such models ,
the dominating mechanism for gamma - ray emission is @xmath3 decay rather than inverse compton .
however , spatial correlations in various snrs between hard x - rays and vhe gamma - rays favour the idea that energetic electrons are responsible for the gamma - ray emission  @xcite , @xcite . the detection of vhe gamma - rays from the tycho snr above several tevs would help to clarify the nature of the mechanism responsible for the vhe gamma - ray emission .
tycho has been observed at vhe gamma - rays by the whipple  @xcite telescope and the hegra telescope array  @xcite , however none of them found vhe gamma - ray emission . in the case of hegra , a search for tev gamma - radiation from tycho snr was performed over 2 years ( 1997/98 ) .
no evidence for such emission was found and a @xmath4 level upper limit was estimated : 3.3% crab at 1 tev ( @xmath5ph@xmath6@xmath7s@xmath8 ) .      the green catalog of snrs @xcite contains 265 galactic snrs plus some possible candidates .
it includes information about the flux and spectral index at 1  ghz , summarizing the information scattered over hundreds of publications and catalogs .
this information has been used to select a list of snrs whose parameters are similar to those of 6 well identified snrs that are also known to be vhe gamma - ray emitters : ic443 , rxj0852 - 4622 , rcw86 , rxj1713 - 3946 , w28 and cas a. from the known snrs with vhe emission , we defined the following selection criteria to find targets for our observations :    * * flux at 1 ghz @xmath9 49  jy * * * radio spectral index ( @xmath10 ) @xmath11 0.6 * * * distance @xmath12 7  kpc * * * age @xmath12 50000  yr *    from all the sources contained in the green catalog , a total of 37 candidates met the selection criteria
. only 25 of these sources are observable by magic with a zenith angle below 50@xmath2 . from these 25 sources some where removed because they have already been detected in gamma - rays ( crab , ic443 , ... ) , were previously observed ( tycho ) or were covered by the h.e.s.s .
galactic scans .
finally we selected the 9 sources shown in table  [ selection_criteria ] as interesting targets to perform observations with magic .
.parameters of the selected snrs . [ cols="^,^,^,^,^,^ " , ]     the skymaps obtained for every source are shown in figure  [ fig : multiskymap ] where no significant point - like gamma - ray excess has been found . for extended sources , the significance is reduced in our observations because of the small trigger area of magic which makes difficult the detection of sources of various tenths of degree . in the case of w51 , the recently reported vhe emission from j1923.0 + 1411 by milagro  @xcite is coincident with an area of higher significance in the rim of the shell ( see figure  [ fig : multiskymap ] ) . h.e.s.s .
reported  @xcite a 3% crab flux from this source which is well below our upper limit ( 9% crab ) . the small observation time ( 7.4  hours ) , the size of the source and its low flux made it not possible to have a significant signal in our observations and only a hint of the source can be seen as the @xmath13 significance area in the skymap .
no significant vhe gamma - ray emission has been found for tycho and the radio selected snrs . in the case of tycho ,
vhe gamma - ray emission is predicted at a level that should have been detected in our observations if the distance is below @xmath14  kpc .
the distance to the snr is still uncertain although most measurements lie below 3.6  kpc .
our result may indicate that the distance could be larger than it is usually believed . in the case of the radio selected snrs
, the computed upper limits range from 5% to 25% crab , assuming a point - like source at the center of the observations .
in addition , the skymaps show no significant ( @xmath15 ) evidence of vhe emission coming from point - like or small size sources located inside the shell of the snrs .
supernova remnants ( snrs ) are believed to be the acceleration sites of galactic cosmic rays . as such 
 they are expected to produce very high energy ( vhe ) gamma - rays through hadronic and/or electromagnetic scenarios , hence they are natural targets for observations with ground - based imaging atmospheric cherenkov telescopes ( iacts ) . currently , vhe emission has been detected from several snrs , making them one of the most abundant types of established galactic vhe sources . 
 the magic telescope , located in the canary island of la palma , has been performing observations of several snrs over the last two years . 
 parameters like age , distance , radio flux , or possible egret association ( i.e. , criteria matching those already used for previous successful iact snr detections ) , were used to select candidate targets . here 
 we summarize the results of the past two years of observations .    
 supernova remnants , vhe gamma - rays