  Propagation of large amplitude ion-acoustic solitary waves (IASWs) in a fully
relativistic plasma consisting of cold ions and ultrarelativistic hot electrons
and positrons is investigated using the Sagdeev's pseudopotential method in a
relativistic hydrodynamics model. Effects of streaming speed of plasma fluid,
thermal energy, positron density and positron temperature on large amplitude
IASWs are studied by analysis of the pseudopotential structure. It is found
that in regions that the streaming speed of plasma fluid is larger than that of
solitary wave, by increasing the streaming speed of plasma fluid the depth and
width of potential well increases and resulting in narrower solitons with
larger amplitude. This behavior is opposite for the case where the streaming
speed of plasma fluid is smaller than that of solitary wave. On the other hand,
increase of the thermal energy results in wider solitons with smaller
amplitude, because the depth and width of potential well decreases in that
case. Additionally, the maximum soliton amplitude increases and the width
becomes narrower as a result of increase in positron density. It is shown that
varying the positron temperature does not have considerable effect on the width
and amplitude of IASWs. The existence of stationary soliton-like arbitary
amplitude waves is also predicted in fully relativistic electron-positron-ion
(EPI) plasmas. Effects of streaming speed of plasma fluid, thermal energy,
positron density and positron temperature on these kinds of solitons are the
same as that for large amplitude IASWs.
