The nebula around pulsar B1853+01 in X-rays

We report on the results of a comprehensive analysis of X-ray observations with, and of the pulsar wind nebula (PWN) associated with PSR B1853+01, located inside the W44 supernova remnant (SNR). Previous X-ray observations unveiled the presence of a fast-moving pulsar, PSR B1853+01, at the southern edge of the W44 thermal X-ray emission region, as well as an elongated tail structure trailing the pulsar. Our analysis reveals, in addition, an ``outflow'' feature ahead of the pulsar extending for about 1 Chandra XMM-Newton NuSTAR (∼1. 0 pc at a distance of 3. 2 kpc). On larger scales, the entire PWN seems to be surrounded by a faint, diffuse X-ray emission structure. The southern part of this structure displays the same unusual morphology as the outflow feature and extends along ∼6 ^ (∼5 pc) in the direction of the pulsar proper motion. In this work, a spatially resolved spectral analysis for different extended regions around PSR B1853+01 was carried out. For an updated value of the column density of 0. 65_ -0. 42 +0. 46 10^ 22 cm ^ -2, a power-law fit to the outflow region yields a spectral index of Γ ≈ 1. 24_ -0. 24 ^ +0. 23, which is significantly harder than that of the pulsar (Γ ≈ 1. 87_ -0. 43 ^ +0. 48) and the pulsar tail (Γ ≈ 2. 01_ -0. 38 ^ +0. 39). We argue that both the outflow structure and the surrounding halo-like X-ray emission might be produced by high-energy particles escaping the PWN around PSR B1853+01, a scenario recently also suggested for other bow-shock PWNe with jet-like structures and/or TeV halos.