The pulsar wind nebula around B1853+01 in X-rays

We report on the results of a comprehensive analysis of X-ray observations with \textit{Chandra}, \textit{XMM-Newton} and \textit{NuSTAR} 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\arcmin~($\sim$1.0 pc at a distance of 3.2 kpc). At 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 $\sim$6\arcmin~($\sim$5 pc) in the direction of the pulsar proper motion. In this report, a spatially-resolved spectral analysis for different extended regions around PSR B1853+01 is carried out. For an updated value of the column density of $0.65_{-0.42}^{+0.46} \times 10^{22} ~\textrm{cm}^{-2}$, a power-law fit to the ``outflow'' region yields a spectral index $\Gamma \approx 1.24_{-0.24}^{+0.23}$, which is significantly harder than that of the pulsar ($\Gamma \approx 1.87_{-0.43}^{+0.48}$) and the pulsar tail ($\Gamma \approx 2.01_{-0.38}^{+0.39}$). 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 suggested also for other bow-shock PWNe with jet-like structures and/or TeV halos.