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Previous observations of the middle-aged pulsar Geminga with XMM-Newton andChandra have shown an unusual pulsar wind nebula (PWN), with a 20" long central(axial) tail directed opposite to the pulsar's proper motion and two 2' long,bent lateral (outer) tails. Here we report on a deeper (78 ks) Chandraobservation and a few additional XMM-Newton observations of the Geminga PWN.The new Chandra observation has shown that the axial tail, which includes up tothree brighter blobs, extends at least 50" (i.e., 0.06 d_{250} pc) from thepulsar. It also allowed us to image the patchy outer tails and the emission inthe immediate vicinity of the pulsar with high resolution. The PWN luminosity,L_{0.3-8 keV} ~ 3\times 10^{29} d_{250}^2 erg/s, is lower than the pulsar'smagnetospheric luminosity by a factor of 10. The spectra of the PWN elementsare rather hard (photon index ~ 1). Comparing the two Chandra images, we foundevidence of PWN variability, including possible motion of the blobs along theaxial tail. The X-ray PWN is the synchrotron radiation from relativisticparticles of the pulsar wind; its morphology is connected with the supersonicmotion of Geminga. We speculate that the outer tails are either (1) a skyprojection of the limb-brightened boundary of a shell formed in the region ofcontact discontinuity, where the wind bulk flow is decelerated by shearinstability, or (2) polar outflows from the pulsar bent by the ram pressurefrom the ISM. In the former case, the axial tail may be a jet emanating alongthe pulsar's spin axis, perhaps aligned with the direction of motion. In thelatter case, the axial tail may be the shocked pulsar wind collimated by theram pressure.

New X-ray observations of the Geminga pulsar wind nebula