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after the discovery of the microwave background radiation ( mbr ) , that fills the universe as a sea of photons , with a mean temperature of 2.7 k at present epoch , greisen @xcite and zatsepin and kuzmin @xcite independently pointed out that the mbr would make the universe opaque to uhecr particles with energies approximately @xmath1 ev and above . this hypothesis is well accepted and is known as the gzk cutoff .
so far , observational data of several experiments about uhecr have report events with energies beyond the gzk cutoff @xcite .
in addition , the present data show strong discrepancies in the uhecr energy spectrum . on one side ,
the agasa ( extensive air shower ) experiment claims a spectrum beyond the gzk cutoff , without any evidence of the gzk cutoff , on the other side the hires ( air fluorescence ) experiment is in agreement with the expected universal distribution of sources and showing the gzk cutoff in the energy spectrum .
this means at least that there is a systematic error , probably in the energy determination , in one of the experiments .
however , this discrepancy can also be attributed to latitude effects , due to the limited coverage of the sky by experiments located in the northern hemisphere .
so far only experiments with a full ( sugar experiment @xcite ) or partial ( agasa experiment @xcite ) coverage of the galactic central region have reported a cosmic ray excess in the energy region of approximately @xmath0 ev in the direction of the galactic center .
the accumulating data from auger ( extensive air shower plus air fluorescence ) experiment @xcite , whose southern part is now in progress , on the basis of a larger data set will tell us which is the correct alternative in the near future .    in the past years
, there are some surveys about a statistical correlation between uhecr events and compact sources at high redshift . depending on the data set , in some cases
a positive correlation has been reported @xcite , but there are also negative results @xcite .
the identification of compact objects like bl lac or quasars at high redshift as uhecr sources reinforces a cosmological origin , and has taken to formulate new scenarios such as the violation of the lorentz invariance @xcite as responsible for the propagation of ultra high energy cosmic ray with energies above the gzk cutoff .
in addition to the top - down models @xcite , where the collapse or decay of super - massive particles like magnetic monopoles , superconducting strings , as well as the @xmath2 z burts @xcite inside a volume with a radius less than 50 mpc from the earth can explain the uhecr data above the gzk cutoff .
the hypothesis that gamma ray burst ( grb ) might be responsible for the origin of uhecr has been suggest earlier @xcite .
the grbs are probably the most powerfully events in the universe , and it is believed that protons can be accelerate in a grb by internal shocks taking place in a collimated jet direction .
the grbs have been observed in spacecraft experiments as short flashes of gamma rays that outshine the rest of the entire gamma - ray sky at a rate of around one event per day @xcite , mostly in the energy band of kev to mev .
the first measurements of the redshifts in grb afterglows @xcite , together with the highly isotropic distribution of their arrival directions , have established a cosmological origin for them .
the association of grbs and uhecrs is further supported by the similarity of energy generation rates .
the average rate of gamma - ray energy emitted by grbs is comparable to the energy generation rate of uhecrs .
in addition , the almost isotropic distribution of the arrival directions of uhecr and grbs is also an ingredient in favor of an association between them .
the main constraint on the association between grb and uhecr is that uhecr events above the gzk cutoff require only grbs inside a volume of radius less than 50 mpc from the earth can contribute to the uhecr flux .
what is the expected rate of grbs in this volume
? a pessimistic answer on the basis of only cosmological origin to the grbs gives an estimative of about one grb per 100 years . on the other hand , from an optimistic point of view
, statistical surveys have shown that it is not always possible to spectroscopically determine the red shift even in some long well located grbs accompanied by x - ray , optical and radio afterglow .
a plausible explanation for these results is the assumption of a local origin .
that is to say , they are close to our galaxy or inside it .
around 70 percent of the observed grbs are the long - soft type with approximately 30 s duration .
some of these bursts have their measured red shifts clustering in @xmath3 , consequently they have a cosmological origin and they are connected with supernovae events . there is also another category of grbs , the short - hard type with approximately 0.2 s duration , where no red shift and no afterglow have been detected from these bursts .
probably these short bursts result from a different engine than long bursts , they could be connected with compact binary mergers , like two neutron stars or with a black hole component , as well as binary pulsar .
they are expected at a high rate of @xmath4 per year in a galaxy like the milk way @xcite .
these characteristics suggest the two component model ( tcm ) , galactic and extra - galactic for the origin of the grbs .
another characteristic that strongly support the tcm for the grbs origin is their isotropic distribution of the arrival direction .
they are regarded to have a uniform distribution in galactic coordinates .
a more accurate analysis of the angular distribution of grb shows some deviations from a simple isotropic distribution @xcite . under certain assumptions such as an euclidean space and standard candle grbs ,
the formulation of the tcm is plausible . here
we make an extension of these assumptions to the uhecr events .
we point out here that a tcm for the uhecr is further supported by the experimental evidences of a uhecr particle excess around @xmath0 ev energies from the direction of the galactic central region .
a consequence of the seeliger s theorem is the following .
if sources are uniformly distributed in a spherical volume @xmath5 of radius @xmath6 , and have fixed brightness , then the flux @xmath7 obtained from a source is proportional to @xmath8 , while the number @xmath9 of sources observed down to a given flux limit is proportional to @xmath10 @xmath11 this relation has been observed in radioastronomy as the @xmath12 plot @xmath13 where @xmath14 provided that the sources are homogeneously distributed in an euclidian space .
then , if a true deviation from this value eventually takes place , we will have the evidence that the space is non - euclidian or / and the sources are inhomogeneously distributed .
the black line in fig.1 shows the @xmath12 plot obtained for the grbs on the basis of 2704 burst from batse catalog .
we can see from this figure that a simple @xmath15 ( blue line ) does not fit the data .
this means that the spatial distribution of grbs is not consistent with a homogeneous case .
as already has been commented , a possible interpretation for this result is to invoke a non - euclidian space .
however , another alternative is to introduce a second population of grbs with a gaussian distribution that reflects the radial distribution of matter in the galaxy and its surrounding halo .
the fluencies of the observed grbs between @xmath16 to @xmath17 @xmath18 imply isotropic burst energies up to approximately @xmath19 erg . while taking beaming angle corrections of about 5 degrees into account , it is expected a narrow distribution around @xmath20 erg . here
, we assume the same value as in @xcite that gives @xmath21 erg to the gaussian peak which corresponds to a mean flux of 2.8 .
the best fit is obtained for an amplitude of about 7.5 percent of the first component and a r.m.s .
value of 13.4 . in short ,
the tcm spatial burst distribution is given by the following cumulutive function @xmath22 ^ 2}.\ ] ] _ _ _ _ the red line in fig.xx represent this function .
under the assumption of an association between grbs and uhecr , the hypothesis of a second population of grbs of galactic origin implies also the existence of uhecr of galactic origin . due to the small statistics , especially in the energy region above the gzk cutoff , it is still not possible to mount the log n @xmath23 log s(e ) plot , only on the basis of uhecr data .
consequently , we make a calibration to obtain the tcm uhecr distribution on the basis of the tcm grbs distribution .
the detection of an excess of cosmic ray from the direction of the galactic center region observed by agasa and sugar in the energy range of @xmath0 ev opens the door to the possibility that uhecr above the gzk might be created also in our galaxy .
we assume that the excess of cosmic ray from the direction of the galactic center in the energy range of @xmath0 ev corresponds to the peak of a gaussian distribution and represent the second population of the uhecr .
this assumption permits us to make a calibration between the grb flux and the energy of the uhecr .
the other parameters of this second population distribution of uhecr are the same as the grbs distribution .
figure 2 summarizes the situation where the cumulative distributions for uhecr and for three different energies in the peak of the gaussian distribution are shown .
it is possible to see that the contribution of the second population to the uhecr flux around the gzk cutoff is sensitive to the value of the gaussian peak .
the comparison of the predictions of the tcm and the experimental data is shown in fig.3 .
under the assumption that grbs might be responsible for the origin of uhecr , we have presented here an extension of the two component model of grbs ( galactic , and extra - galactic ) to explain the origin of uhecr .
the inclusion of a local galactic source for the uhecr origin is further supported by the experimental evidences of an uhecr excess around @xmath0 ev from the direction of the galactic center region .
this excess can be considered as the peak of a gaussian distribution of the second component and permits us to make a calibration among the grb flux and the uhecr energies .
perhaps the main constraint to the tcm is , if the uhecr sources are close , why the arrival direction of the events does not point toward their sources ?
so far , the uhecr distribution does not follow the galactic star distribution .
this means that probably its surrounding diffuse halo and even the local distribution of galaxies can lodge uhecr sources .
uhecr event rates beyond the gzk cutoff as observed by agasa experiment can be linked at least in part to grbs of short - hard type , because these grbs neither have afterglows nor redshift suggesting that some of these grbs sources are close to or inside of our galaxy .
these results are in agreement with others surveys @xcite , showing that grbs with time duration below @xmath24 ms appear to form a separate class of grbs , because from their asymmetry plot the events appear to originate nearby within the galaxy .
the short - hard grbs probably are formed for instance by coalescence of corotating binary neutron star systems .
this process is dominated by a strong magnetic field .
thus , magnetic deflections of the uhecr in the first stages of their propagation can be responsible of a arrival direction of events without pointing toward their sources .
we are waiting for the next round of the auger experiment . certainly only after a large set of data that we can confirm or refute the tcm for the origin of uhecr .
this work was partially supported by faperj ( the research fostering foundation of the state of rio de janeiro ) and cnpq ( the national council of research and development ) of brazil .
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under the hypothesis that gamma ray burst ( grb ) might be responsible for the origin of ultra high energy cosmic ray ( uhecr ) , we propose a two component ( galactic and extra - galactic ) model for the uhecr origin . 
 the model is based on two facts . 
 the first is the anisotropies found in the angular distribution of grbs from batse catalog . 
 second is that , of all the located long - grbs , only approximately 15 percent of them have their spectroscopic redshift determined , and some 38 percent of them have a x - ray , optical , or radio afterglow . 
 so far , in short - grbs , no afterglow and no red shift have been detected , suggesting that these grb sources are inside or close to our galaxy . 
 this two component model for the uhecr is further supported by the experimental evidences of an uhecr excess around @xmath0 ev from the direction of the galactic central region . 
 the model offers in a natural way an explanation for the presence of cosmic rays with energies beyond the greisen - zatsepin - kuzmin ( gzk ) cutoff .