Arcetri High Energy Group

Pulsar Wind Nebulae

PWNe are bubbles of relativistic plasma arising from the confinement of a pulsar's wind by the surrounding medium (usually the SNR), that shine in non thermal emission from radio to X-ray. For the first time we apply an RMHD code to the study of the evolution (1D spherically symmetric) of PWNe inside SNR, to verify the accuracy of previous simplified analytic solution. However more important results came from multidimensional studies. Recent high resolution observations with Chandra and Hubble have in fact shown that many such objects are characterized by an evident axisymmetric feature know as ``jet-torus structure''. A main torus is often observed with a brighter inner ring, and jets comin g from the vicinity of the pulsar. Such structure can not be explained in the 1D radial model, expecially the jet, given the difficulties in self-collimation of ultra relativistic flows. Our results show that if the energy flux in the pulsar wind is higher at the equator than at the pole (as in the split monopole model), magnetic hoop stresses in the post shock region can divert part of the equatorial flow toward the axis, collimating and accelerating it. The anisotropic energy flux is also important in shaping the wind termination shock and producing high velocity flow channels in the post shock region. We find that the formation of the jet requires an higher wind magnetization than what previous 1D models assumed. Moreover the shape of the termination shock and the flow pattern in the downstream region lead to very characteristic signature in the high energy emission. By building synchrotron maps based on our numerical results we were able to recover the main observed features such as the inner ring, the torus, the jet and the inner knot. We also found that the best agreement between observations and synthetic maps requires a wind with magnetic field vanishing at the equator (as in the striped wind model). The work on PWNe modeling, is still proceeding, we are now trying to sample more in detail the parameter space in terms of wind latitudinal dependence to derive some criteria in order to correlate the observed structure, their morphology and variability to the conditions in the wind, which in turn can be used to derive some insight into pulsar magnetospheres. Recently we have presented new models of the spectral and polarization properties of such nebulae. We have also investigated the Gamma-ray comptonized emission, and shown that previous estimate based on very simplified models for the nebular evolution and internal structure, could lead to discrepancy of a factor 5 to 10. Very recently in collaboartion with Serguei Komissarov, we have investigated the time variability, and shown that MHD processes can reproduce with great accuracy also the short term temporal properties of these nebulae.