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 $Γ\approx 1.24_{-0.24}^{+0.23}$, which is significantly harder than that of the pulsar ($Γ\approx 1.87_{-0.43}^{+0.48}$) and the pulsar tail ($Γ\approx 2.01_{-0.38}^{+0.39}$). 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.

💡 Research Summary

The authors present a comprehensive X‑ray study of the pulsar wind nebula (PWN) associated with PSR B1853+01, which resides inside the mixed‑morphology supernova remnant W44. Using archival observations from Chandra (three ACIS‑S pointings, total exposure ≈ 137 ks), XMM‑Newton (four EPIC observations, total exposure ≈ 350 ks) and NuSTAR (one FPMA/FPMB pointing, ≈ 105 ks), they construct high‑resolution images and perform spatially resolved spectroscopy of the pulsar, its cometary tail, and newly identified extended features.

The most striking new discovery is an “outflow” structure located ahead of the pulsar’s direction of motion. In the hard‑band Chandra images this feature appears as a narrow, linear extension ≈ 1 arcmin (∼ 1 pc at 3.2 kpc) pointing away from the pulsar. At larger scales, a faint, diffuse X‑ray halo surrounds the entire PWN and stretches ≈ 6 arcmin (∼ 5 pc) in the same direction as the pulsar’s proper motion. The outflow and halo are also visible in the XMM‑Newton mosaics and in the NuSTAR 3–20 keV band, confirming that they emit up to at least tens of keV.

Spectral fitting was performed with an absorbed power‑law model. The column density was re‑estimated as N_H = 0.65 × 10²² cm⁻² (−0.42 + 0.46 × 10²² cm⁻²). The photon indices are: Γ = 1.87 (−0.43 + 0.48) for the pulsar itself, Γ = 2.01 (−0.38 + 0.39) for the cometary tail, and Γ = 1.24 (−0.24 + 0.23) for the outflow. The outflow is therefore significantly harder than both the pulsar and its tail, indicating a population of higher‑energy electrons (or positrons) that have not yet suffered substantial synchrotron cooling. The halo’s spectrum, though faint, is consistent with a similarly hard index, suggesting it is the same escaping particle population diffusing into the surrounding medium.

The authors argue that the outflow cannot be readily explained by a ballistic jet or by simple hydrodynamic deflection of the wind. Instead, they favor a scenario in which particles accelerated at the termination shock escape preferentially in the direction of the pulsar’s motion, forming a collimated “leak” that we see as the outflow. The diffuse halo would then be the broader diffusion front of these escaped particles. This picture mirrors recent interpretations of TeV halos observed around middle‑aged pulsars such as Geminga and Monogem, where inverse‑Compton scattering of escaped electrons on the cosmic‑microwave background produces extended γ‑ray emission. In the case of B1853+01, the escaped particles still radiate synchrotron X‑rays because of the relatively high magnetic field in the immediate surroundings of W44.

Comparisons with other bow‑shock PWNe (BSPWNe) that show misaligned outflows or jet‑like features (e.g., PSR J1509‑5850, PSR B0355+54) reveal that B1853+01 adds a new class: a “head‑outflow‑halo” morphology. The work highlights that high‑velocity pulsars can generate not only the classic cometary tail but also forward‑directed particle streams and large‑scale halos, emphasizing the importance of particle escape and diffusion in shaping PWN X‑ray morphologies.

The paper concludes that the detection of a hard‑spectrum forward outflow and an associated diffuse halo around PSR B1853+01 provides strong observational support for particle‑escape driven models of bow‑shock PWNe. It suggests that such escape processes may be common among middle‑aged, fast‑moving pulsars and that they can produce both X‑ray and TeV‑scale halos. Future multi‑wavelength observations (radio, optical, GeV/TeV γ‑rays) combined with 3‑D magnetohydrodynamic simulations will be essential to quantify diffusion coefficients, magnetic field configurations, and the impact of the surrounding supernova remnant on particle propagation.