Unconfined relativistic outflows from rotating , magnetized compact objects are often well-modeled by assuming the field geometry is approximately a split-monopole at large radii .
Earlier work has indicated that such an unconfined flow has an inefficient conversion of magnetic energy to kinetic energy .
This has led to the conclusion that ideal magnetohydrodynamical ( MHD ) processes fail to explain observations of , e.g. , the Crab pulsar wind at large radii where energy conversion appears efficient .
In addition , as a model for astrophysical jets , the monopole field geometry has been abandoned in favor of externally confined jets since the latter appeared to be generically more efficient jet accelerators .
We perform time-dependent axisymmetric relativistic MHD simulations in order to find steady state solutions for a wind from a compact object endowed with a monopole field geometry .
Our simulations follow the outflow for 10 orders of magnitude in distance from the compact object , which is large enough to study both the initial ‘ ‘ acceleration zone ’ ’ of the magnetized wind as well as the asymptotic ‘ ‘ coasting zone. ’ ’ We obtain the surprising result that acceleration is actually efficient in the polar region , which develops a jet despite not being confined by an external medium .
Our models contain jets that have sufficient energy to account for moderately energetic long and short gamma-ray burst ( GRB ) events ( \sim 10 ^ { 51 } – 10 ^ { 52 } erg ) , collimate into narrow opening angles ( opening half-angle \theta _ { j } \approx 0.03 rad ) , become matter-dominated at large radii ( electromagnetic energy flux per unit matter energy flux \sigma < 1 ) , and move at ultrarelativistic Lorentz factors ( \gamma _ { j } \sim 200 for our fiducial model ) .
The simulated jets have \gamma _ { j } \theta _ { j } \sim 5 – 15 , so they are in principle capable of generating ‘ ‘ achromatic jet breaks ’ ’ in GRB afterglow light curves .
By defining a ‘ ‘ causality surface ’ ’ beyond which the jet can not communicate with a generalized ‘ ‘ magnetic nozzle ’ ’ near the axis of rotation , we obtain approximate analytical solutions for the Lorentz factor that fit the numerical solutions well .
This allows us to extend our results to monopole wind models with arbitrary magnetization .
Overall , our results demonstrate that the production of ultrarelativistic jets is a more robust process than previously thought .