On the X-ray feature associated with the Guitar Nebula

Abstract

Context: A mysterious X-ray nebula, showing a remarkably linear geometry, was recently discovered close to the Guitar Nebula, the bow-shock nebula associated with B2224+65, which is the fastest pulsar known. The nature of this X-ray feature is unknown, and even its association with pulsar B2224+65 is unclear. Aims: We attempt to develop a self-consistent scenario to explain the complex phenomenology of this object. Methods: We assume that the highest energy electrons accelerated at the termination shock escape from the bow shock and diffuse into the ambient medium, where they emit synchrotron X-rays. The linear geometry should reflect the plane-parallel geometry of its ambient field. Results: We estimate the Lorentz factor of the X-ray emitting electrons and the strength of the magnetic field. The former (~10^8) is close to its maximum possible value, while the latter, at ~45 uG, is higher than typical interstellar values and must have been amplified in some way. The magnetic field must also be turbulent to some degree to trap the electrons sufficiently for synchrotron X-ray emission to occur effectively. We propose a self-consistent scenario in which, by some streaming instability, the electrons themselves generate a turbulent field in which they then diffuse. Some numerical coincidences are explained, and tests are proposed to verify our scenario. Conclusions: Electron leaking may be common in the majority of pulsar bow-shock nebulae, even though the X-ray nebulosity in general is too diffuse to be detectable.