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presently , few testable theoretical predictions or observational diagnostics exist to robustly constrain whether grb outflows are initially dominated by particles or poynting flux , and what fraction of the particles are baryons vs. pair plasma @xcite .
whatever the case close to the engine , at large distances observations imply that particles are accelerated to highly relativistic velocities and dominate the energy budget .
magnetic reconnection is a plausible mechanism of particle acceleration in grb jet plasma if the free energy is carried by magnetic fields .
reconnection has mainly been studied in the lab , the solar corona , and particularly the earth magnetosphere , which produced the ` geospace environmental modeling ( gem ) reconnection challenge ' : here a simplified reconnection problem was studied and compared using several mhd , hybrid and pic codes @xcite .
much less attention has been paid either analytical or numerical to the fundamental reconnection mechanisms in either the relativistic regime or for a pair plasma .
reconnection in a ( relativistic ) pair plasma is very different from an electron - ion plasma . for the latter , the difference in skin depth between the electrons and ions and the corresponding scale lengths of the current sheet and reconnection region play an important role ;
reconnection is facilitated by whistler modes , which can be related to the hall current in the non - ideal generalized ohm s law . in a pair plasma , however , the contributions from the electron and positron equations of motion to the hall current cancel and whistler waves do nt exist @xcite .
the non - ideal term in ohm s law that allows field lines to reconnect is an effective enhanced resistivity related to the electron and positron pressure tensors and the tearing mode is the dominant instability .    to improve our understanding of magnetic reconnection for conditions applicable to grb outflows we are performing simulations with a fully explicit and highly parallelized relativistic pic code , _ osiris _ @xcite . by tuning the plasma parameters from non to highly relativistic values and comparing results for a pure pair plasma and a plasma with an increasing proton fraction ,
we can obtain different particle acceleration spectra to be used to construct observational diagnostics of the jet s nature .
this complements the existing pic simulation work in the grb outflow context which has focused on shock acceleration via the weibel instability in ion - electron plasmas @xcite , or magnetic pulse acceleration without magnetic reconnection @xcite .
spectra from each of these classes of simulation should ultimately be compared .
a simple geometry for studying reconnection is the harris current sheet , where two regions of nearly uniform magnetic field of opposite sign are driven into a thin current sheet by their own magnetic pressure .
if the plasma can deviate from ideal mhd , the field lines can slip and reconnect at one or more @xmath0-points , after which the reconnected field lines are strongly bent .
when those field lines straigten to minimise their tension , particles in the outflow region are accelerated .
free magnetic energy is thus converted to particle energy in the process . using the harris configuration
, we can compare to the results in the gem reconnection challenge @xcite and its extension to pair plasma@xcite .
the simulations presented here are for a pair plasma and have two current sheets to facilitate use of periodic boundary conditions on all walls . in each half - plane
we have an initial magnetic field normalized as : @xmath1 \tanh(x_{2}/a)\hat{x}_{1}$ ] , where @xmath2 is the thickness of the current sheet and @xmath3 is the skin depth in terms of the plasma frequency @xmath4 and we choose a non - relativistic ) alfvn velocity @xmath5 . in a steady state
( @xmath6 ) we have @xmath7 and @xmath8 with thermal velocity spread @xmath9 , such that @xmath10 . a small uniform particle density @xmath11 is added to replenish the accelerated particles leaving the reconnection region and the total number of particles is @xmath12
. the spatial and time resolutions are @xmath13 and @xmath14 with typical runs exceeding @xmath15 .
most simulations use a @xmath16 grid of dimension @xmath17 , but for some we double the length along the current - sheet to reduce the effect of periodic boundaries . to quickly force the system into the non - linear regime , sometimes we impose an initial perturbation , @xmath18 , @xmath19 .
figure  [ f1 ] ( left ) shows the magnetic field at @xmath20 when the reconnection has reached a quasi steady state , and the reconnection electric field is well developed .
this simulation was started from a perturbed equilibrium magnetic field .
the electric field at the @xmath0-points is @xmath21 , which is slightly lower than in @xcite and @xcite .
when we start from numerical noise , the secondary tearing mode quickly develops , and the magnetic field fragments into a series of small @xmath0- and @xmath22-points that move under the magnetic pressure and finally merge into one dominant @xmath0-point on each current - sheet . the right panel shows the in- and outflow velocities ( @xmath23-@xmath24 ) around the @xmath0-points .
one can see the particles flowing in with inflow velocity from @xmath25 and being accelerated along the @xmath26 axis . in this figure
the effect of the periodic boundary conditions is already becoming visible with accelerated particles re - entering the box , but they re not yet influencing the reconnection region .
in all simulations of non - relativistic reconnection we ve seen in the literature , the fluctuating electric field is either neglected by not evolving the displacement current ( common in mhd codes ) , or artificially eliminated at each time - step @xcite .
_ osiris _ does neither and evolves the full fields , including all electromagnetic wave modes .
doing so reveals weak plane - wave like fluctuations in @xmath27 ( due partly to numerical noise , and partly to particles being allowed to cross the current sheet that do nt cancel out exactly ) .
when this oscillating component has the ` wrong ' sign , the flow of plasma into the current sheet is impeded , and initially this component is strong enough to reverse the bulk plasma motion away from the current sheet . only during the wave - phase where @xmath27 has the proper sign do the particles flow into the current sheet .
eventually , the @xmath0-points and a mean steady reconnection electric field develop and fast non - linear reconnection is achieved .
the final in- and outflow velocities reached in our simulations are somewhat slower than in previous work , and the evolution to a steady - state takes a bit longer .
currently , we are analysing larger runs on the nersc clusters using more particles , higher resolution , and larger aspect ratio along the sheet . furthermore , the implemented distribution functions in _ osiris _ are only valid in the mildly relativistic regime , so we re implementing the proper relativistic jttner - synge distribution
. then we can proceed to vary the alfvn velocity arbitrarily , adding a small fraction of baryons , and 3d generalizations where possible .
finally , we will calculate the radiation spectra . if the synchrotron cooling time is long compared to the reconnection rate , this can be done post - procees .
if the cooling time is fast , but the plasma is optically thin to the produced radiation , we must calculate the radiation self - consistently at each time - step .
differences in the spectra produced by reconnection in a magnetically dominated wind ( ac model @xcite ) , a magnetic pulse model , or shock - acceleration in a matter dominated model , will provide a useful observational diagnostic to distinguish between the three .    999 g.  drenkhahn and h.c .
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we present preliminary results of particle - in - cell ( pic ) simulations of collisionless magnetic reconnection for conditions that apply to magnetically dominated pair plasma jets such as those in gamma - ray bursts ( grb ) . 
 we find similar behaviour to previous authors but with reduced reconnection efficiency . 
 this results because we include the full electromagnetic field dynamically and allow electric field fluctuations to grow . although weak , these fluctuations impede early @xmath0-point formation by periodically decelerating and even reversing the inflow .    
 pic simulations , magnetic reconnection , grb , poynting flux , relativistic jets