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since the discovery of gamma - ray burst ( grb ) afterglows in the late 90 s  @xcite , these energetic phenomena have been targets of observational efforts at essentially all electromagnetic wavelengths . the wealth of available information put severe constraints on the various families of interpretative scenarios , showing an unexpected richness and complexity of possible behaviors  @xcite . the first observations at mev - gev energies , with the energetic gamma - ray experiment telescope ( egret ) on - board the compton gamma - ray observatory ( cgro )  @xcite , showed that the observations in the high energy ( he : 1 mev-30 gev ) and in the very high energy range ( vhe : 30 gev - 30 tev ) can be powerful diagnostic tools for the emission processes and physical conditions of grbs . the launch of _ fermi _ @xcite , with its large area telescope ( lat )  @xcite , showed that , at least for the brightest events , gev emission from grbs is a relatively common phenomenon  @xcite confirming that our comprehension of grb physics is still unsatisfactory . in this context , multiple efforts have been performed in grbs observations by the new generation of ground - based imaging atmospheric cherenkov telescopes ( iacts ) , such as magic , hess or veritas . in all cases only upper limits have been derived . the two most limiting factors are the heavily attenuation of the vhe signal by pair - production due to the interactions with the lower energetic photons of the extragalactic background light ( ebl )  @xcite and the delay of the observations . within this scenario , magic has the advantage , compared to other iacts , in its low energy sensitivity and pointing speed  @xcite . throughout the paper the convention @xmath1 has been adopted in cgs units . grb090102 was detected and located by the _ swift _ satellite  @xcite on january 02 , 2009 at 02:55:45 ut  @xcite . the prompt light curve is structured in four partially overlapping peaks for a total @xmath2 of @xmath3 s. since the burst was also detected by _ konus wind _ @xcite and _ integral _ @xcite , it has been possible to obtain a very good reconstruction of the prompt emission spectral parameters . the time - averaged spectrum can be modeled with the classical band function  @xcite with peak energy @xmath4 kev and a total fluence in the 20 kev - 2 mev range of @xmath5 erg @xmath6  @xcite . early optical follow - up measurements were performed by many groups like tarot  @xcite at @xmath7 + 40.8 s , the rem robotic telescope at @xmath7 + 53 s  @xcite and grond telescope  @xcite at @xmath7 + 2.5 h. the optical light - curve , monitored from several tens of seconds to slightly more than a day from the @xmath7 , shows a steep - to - shallow behavior with a break at about 1ks . before the break , the optical flux decay index is @xmath8 while the index became @xmath9 after the break , steeper and flatter respectively , when compared to the simultaneous x - ray emission . optical spectroscopy was rapidly carried out by the not telescope  @xcite deriving a redshift of z=1.547 , which allows to compute an isotropic energy value of @xmath10 erg and a rest frame peak energy of @xmath11=1149@xmath12 kev in good agreement with the amati relation . a detailed discussion of the follow up observations for this burst can be found in  @xcite . the magic telescope located at roque de los muchachos ( 28.75@xmath13n , 17.89@xmath13w , la palma , canary islands ) performed a follow - up measurement of grb090102 starting the observation at the position provided by _ swift _ ( ra : 08h 33 m 02s ; dec : 33@xmath13 05 29  ) at 03:14:52 ut at @xmath14 s. data presented in this paper have been taken before november 2009 , when magic was operating as a single telescope . first data runs were taken at very low zenith angles from 5@xmath13 reaching 52@xmath13 at the end of datataking at 06:54:01 ut after 13149 s of observation . magic upper limits above 80 gev have already been published for this grb  @xcite while results and scientific discussion about a successive dedicated analysis focused in the low energy band  @xcite will be presented here . to ensure the lowest energy threshold , only data taken with zenith distance @xmath15 25@xmath13 , corresponding to the first 5919 s of observation ( data sub - sample up to 04:53:32 ut ) has been taken into account during this analysis . together with the magic-1 sum trigger system  @xcite , it yields an analysis threshold around 30 gev evaluated using mc simulations . in order to accurately estimate the background from hadronic atmospheric showers , an off data sample has been taken one night later with the telescope pointing close to the burst location and in the same observational conditions and instrument setup . in spite of the excellent magic low energy analysis threshold , no significant excess of gamma - ray photons has been detected from a position consistent with grb090102 . differential upper limits with 95@xmath16 cl were evaluated . telescope efficiency was evaluated with a 30@xmath16 estimation of systematic uncertainties . however , the excellent low energy threshold for ground - based observations with cherenkov telescopes makes these observations particularly interesting in spite of the high redshift of grb090102 . .magic - i 95@xmath16 confidence level upper limits for the afterglow emission of grb090102 . the values correspond to the first 5919 s of observation from 03:14:52 ut to 04:53:32 ut for a zenith distance ranging between 5@xmath13 and 25@xmath13 . [ cols="^,^,^ " , ] in the so - called standard scenario , known as the fireball model  @xcite , grb dynamics during the prompt phase are governed by relativistic collisions between shells of plasma emitted by a central engine ( internal shocks ) . similarly , the emission during the afterglow is thought to be connected to the shocks between these ejecta with the external medium ( external shocks ) . in both cases , the observed photons are radiation from particles accelerated to ultrarelativistic energies by successive collisions with magnetized medium ( fermi mechanism ) . even if the details of the acceleration process are not firmly known , several non - thermal mechanisms have been suggested to be sources of vhe photons . possible processes comprise both leptonic ( e.g. electron synchrotron , synchrotron self compton - ssc ) and hadronic models ( e.g. proton synchrotron )  @xcite . in the most plausible scenario , electron synchrotron radiation is the dominant process in the low energy regime ( @xmath15 mev ) while ssc emission overwhelm other processes in the vhe range . as first approximation , a broken power - law can be used in modeling the expected emission . the relevant break energies of the spectrum will be the minimum injection @xmath17 and the cooling @xmath18 . the first one refers to emission frequency of the bulk of the electrons population while the cooling frequency identifies where electrons effectively cool . both are strongly dependent on the microphysical parameters used to describe the fireball and for the ssc are ( following  @xcite ) :    @xmath19\end{aligned}\ ] ]    @xmath20\end{aligned}\ ] ]    where @xmath21 the medium particle density , @xmath22 is the energy per unit solid angle , @xmath23 the time delay after the grb onset ( in hours ) and @xmath24 is the source redshift . the slope of the electron energy distribution ( @xmath25 ) can be evaluated using the optical spectral index light - curve  @xcite and has been found to be @xmath26 in good agreement with numerical simulations which suggest a value of @xmath25 ranging between 2.2 - 2.3  @xcite . on the contrary , we can only constrain the values for the micro - physical parameters @xmath27 , the fraction of total energy going to electrons , and @xmath28 , the fraction of total energy going to magnetic fields . assuming that the optical light curve time break  @xcite is less then the start time of the shallow decay phase ( @xmath29 s ) , we obtain @xmath30 and @xmath31 which only barely fix the @xmath28 , @xmath27 values . we thus assume @xmath32 and @xmath33 which correspond to typical values for the late afterglow  @xcite . +              gamma ray absorption by pair production with extragalactic background light ( ebl ) limits the iacts capability in detecting sources at redshift @xmath341 . we adopted the model in  @xcite to evaluate the ebl absorption . the ebl intensity evaluated using this model matches the minimum level allowed by galaxy counts , which leads to the highest transparency of the universe to vhe @xmath35-rays . this is indeed an optimistic yet realistic scenario for grbs studies in the vhe regime . as a matter of fact , we have obtained a value for the optical depth @xmath36 of @xmath37 at about 40 gev . this gives an attenuation of the flux at the same energy of the order of @xmath38 , a value that does not significantly compromise the detection capability of magic . we have used the above derived equations in order to predict the expected emission in the magic energy range . from numerical results it is evident that , for both the chosen parameters and at the magic observation time , leptonic components are usually the dominant mechanisms from the low ( electron synchrotron emission ) to the very high energy ( ssc ) . magic observations ( @xmath0 40 gev ) were carried out in the sed region where @xmath39f@xmath40 so that it is possible to evaluate the expected ssc emission : @xmath41 which gives ( for our first energy bin and taking into account the ebl absorption ) @xmath42(40 gev)@xmath43 erg @xmath6 s@xmath44 . this result is below of about one order of magnitude ( in the first energy bins ) to the reported upper limits . however , a change in the microphysical fireball values can influence the vhe emission giving scenarios with substantially higher flux . we also explored the possibility of a strong and prevalent proton synchrotron component in the gev range . the cooling frequency for protons can in fact easily reach the gev regime since @xmath45 . however , the required energy budget ( @xmath46 erg ) and medium density ( @xmath47 @xmath48 ) needed to match the expected low energy flux with observed data is rather incompatible with the measured total energy release . this makes the possibility to observe the hadronic emission component with the magic telescope unrealistic , at least for a canonical fireball . other hadronic - induced emissions such as @xmath49 decay @xcite can have a non negligible effect with new features in the spectrum in a higher energies range . moreover , at these energies , @xmath50 , implying that lowering the temporal delay of the observations can make the expected emission higher by one order of magnitude . our estimates show that magic follow - up observations within few hundreds of seconds from the t@xmath51 would have the potential to detect this source . this demonstrates both the capabilities of the system and the necessity of a fast - response observations . although we could not yet set firm constraints on the main theoretical scenarios , our results show the interesting perspectives for an afterglow detection in very high energy domain . this energy range will have very high impact on the understanding of the grb phenomena and the unique opportunity of having simultaneous follow - up with _ fermi_/lat and the magic telescope will have an important role in constraining different emission mechanisms and the space parameters . + the above results will be discussed in a forthcoming dedicated publication together with the fermi / lat data . we would like to thank the instituto de astrofisica de canarias for the excellent working conditions at the observatorio del roque de los muchachos in la palma . the support of the german bmbf and mpg , the italian infn and spanish micinn is gratefully acknowledged . this work was also supported by eth research grant th 34/043 , by the polish mniszw grant n n203 390834 , and by the yip of the helmholtz gemeinschaft . +    9 costa , e. et al . , 1997 , nature , 387 , 783 gehrels , n. , ramirez - 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hints for a gev component in the emission from grbs are known since the egret observations during the 90s and they have been recently confirmed by the data of the fermi satellite . 
 these results have , however , shown that a fully satisfactory interpretative framework of the grb phenomena is still lacking . 
 the magic telescope opens the possibility to extend the measurement of grbs in the several tens up to hundreds of gev energy range . from the theoretical point of view 
 , both leptonic and hadronic processes have been suggested to explain the possible gev / tev counterpart of grbs . 
 observations with ground - based telescopes of very high energy photons ( e@xmath030 gev ) from these sources are going to play a key role in discriminating among the different proposed emission mechanisms which are barely distinguishable at lower energies . 
 magic telescope observations of the grb090102 ( z=1.547 ) field from 03:14:52 ut to 06:54:01 ut are analyzed to derive upper limits to the gev / tev emission . 
 we compare these results to the expected emissions evaluated for different processes in the framework of the standard fireball model . 
 the results we obtained are compatible with the expected emission but can not yet set further constraints on the theoretical scenario . 
 however , the difficulty in modeling the low energy data for this event makes it difficult to fix in an unambiguous way the physical parameters which describe the fireball . 
 nonetheless , the magic telescope , thanks to its low energy threshold and fast repositioning , is opening for the first time the possibility to fill the energy gap between space - based gamma detectors and the ground - based measurements . 
 this will makes possible grbs multiwavelength studies in the very high energy domain .