We found two nebulae associated with J2055, with angular dimensions of 12′ × 20″ and 250″ × 30″, respectively, for the brightest and faintest. The observed extensions of the nebulae, at a distance of 600 pc, would correspond to physical dimensions of ∼2.1 × 0.05 pc and ∼0.7 × 0.09 pc, respectively (assuming no inclination with respect to the plane of the sky). The luminosities of the nebulae in the 0.3–10 keV energy range (assuming d = 600 pc) are 1.2 × 1031 erg s−1 and 2.4 × 1030 erg s−1, corresponding to fractions of 2.4 × 10−3 and 3.0 × 10−4 of the pulsar spin-down luminosity. A few elongated tails of X-ray emission associated with rotation-powered pulsars have been discovered in the past (Gaensler et al. 2004; Becker et al. 2006; McGowan et al. 2006; Kargaltsev & Pavlov 2008b). Although for our pulsar we have no information about the proper motion, the bow-shock PWN scenario would seem the most natural explanation for one of the nebulae. Luminosity values are fully compatible with that measured for other synchrotron nebulae, for which the pulsars channel into their tails 10−2 to 10−4 of their rotational energy loss. In the case of synchrotron emission tails, if we assume the optimistic maximum Lorentz factor of ∼108 for electron acceleration in such a low-energetic pulsar magnetosphere and the high value of the ambient magnetic field of ∼50 μG (following considerations in De Luca et al. 2011), it is possible to estimate the synchrotron cooling time of the emitting electrons as 





 (B/50 μG)





 (E/1 keV)





 yr. Coupling this value with the estimated physical length of the feature yields an estimate of the bulk flow speed of the emitting particles of ∼20,000 and 3000 km s−1, assuming no inclination with respect to the plane of the sky. The first value is only marginally consistent with results in the literature, while the second one is fully consistent (Kargaltsev & Pavlov 2008b). Taking into account the energetics, both nebulae are at least marginally consistent with a classical synchrotron nebula explanation. For classical synchrotron nebulae, we expect a relatively bright diffuse emission surrounding the pulsar, where the emission from the wind termination shock is brightest. The model of Gaensler & Slane (2006) predicts a low-scale termination shock of ∼06 fo J2055 (at 600 pc), thus not resolved by XMM-Newton, even in the case of typical ambient densities (0.01 atoms cm−3) and pulsar velocities (some hundreds of kilometers per second). In that case, part of the flux we assign to the pulsar comes instead from the termination shock (Kargaltsev & Pavlov 2008a). Along their main axis, the brightness profiles of both  nebulae are consistent with an almost flat behavior, with a sudden decrease at the end. This is acceptable for synchrotron emission nebulae: in fact, the pulsar wind is more energetic in the surroundings of the pulsar, where the loss of energy through synchrotron emission is higher, decreasing in flux and energy along the pulsar trail. Taking also into account an inhomogeneous medium and/or magnetic field, this behavior is expected, and it is observed for many objects in the literature (see, e.g., Kargaltsev & Pavlov 2008b). Such a synchrotron cooling of the particles injected at the termination shock induces a significant softening of the emission spectrum as a function of the distance from the pulsar in bow-shock PWNe. Taking into account the upper limit variation of the photon index we found, for the brightest tail of J2055 we do not have the predicted spectral variation. Classical theories of ram pressure particle confinement in pulsar tails are hardly put to the test by parsec-long nebulae, which would require much higher efficiencies than in all the other cases. The tightness of the main feature of J2055, if confirmed to be a synchrotron nebula from its shape and the pulsar motion, cannot be accounted for by any model in the literature. We conclude that the low energetics of the pulsar, the lack of any spatial-spectral variability, and the tightness of the brighter nebula disfavor the classical synchrotron emission nebula. This model can explain the characteristics of the secondary feature.
Citation: De Luca et al. 2011
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