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

Xiying Zhang, Pol Bordas, Samar Safi-Harb, Kazushi Iwasawa

TL;DR

This study investigates the X-ray PWN around PSR B1853+01 in SNR W44 using archival Chandra, XMM-Newton and NuSTAR data to map morphology and perform spatially resolved spectroscopy. It uncovers an elongated tail trailing the pulsar, an ahead-oriented outflow with a notably hard spectrum, and a faint halo enveloping the PWN, with NuSTAR detecting emission up to about 20 keV. Spectral fits across regions yield non-thermal continua, with the outflow having a photon index $\\Gamma \approx 1.24$ while the tail and pulsar exhibit softer indices (~$\\Gamma \sim 2.0$ and ~$1.9$–$1.7$, respectively), underscoring spectral gradients tied to particle transport. The authors argue that the outflow and halo arise from energetic particles escaping the PWN bow shock, a scenario consistent with particle leakage and diffusion seen in other bow-shock PWNe and with the growing concept of TeV halos around pulsars, thereby providing new constraints on particle acceleration and escape in evolved PWNe.

Abstract

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.

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

TL;DR

This study investigates the X-ray PWN around PSR B1853+01 in SNR W44 using archival Chandra, XMM-Newton and NuSTAR data to map morphology and perform spatially resolved spectroscopy. It uncovers an elongated tail trailing the pulsar, an ahead-oriented outflow with a notably hard spectrum, and a faint halo enveloping the PWN, with NuSTAR detecting emission up to about 20 keV. Spectral fits across regions yield non-thermal continua, with the outflow having a photon index while the tail and pulsar exhibit softer indices (~ and ~–, respectively), underscoring spectral gradients tied to particle transport. The authors argue that the outflow and halo arise from energetic particles escaping the PWN bow shock, a scenario consistent with particle leakage and diffusion seen in other bow-shock PWNe and with the growing concept of TeV halos around pulsars, thereby providing new constraints on particle acceleration and escape in evolved PWNe.

Abstract

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~(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 6\arcmin~(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 , a power-law fit to the ``outflow'' region yields a spectral index , which is significantly harder than that of the pulsar () and the pulsar tail (). 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.

Paper Structure

This paper contains 19 sections, 6 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Observations and Data Reduction
  3. XMM-Newton
  4. Chandra
  5. NuSTAR
  6. Analysis and Results
  7. Imaging
  8. The Pulsar B1853+01
  9. The PWN Tail
  10. The Outflow
  11. An X-ray Halo
  12. Spectral Results
  13. The Pulsar B1853+01
  14. Spatially Resolved Spectral Analysis
  15. Discussion
  16. ...and 4 more sections

Figures (6)

  • Figure 1: XMM-Newton exposure-corrected mosaic images of SNR W44 region in hard (4.0-8.0 keV) X-ray bands. Left: In the FoV of W44 with the SNR radio contours in yellow (radio fits file obtained from SNRCat at http://snrcat.physics.umanitoba.ca/SNRrecord.php?id=G034.7m00.4) and the soft (0.5-4.0 keV) X-ray contour of W44 in cyan. The position of PSR B1853+01 is marked by a black cross with a green arrow indicating the direction of the pulsar's proper motion inferred from the pulsar's position relative to the SNR center and the axis of symmetry of the PWN. Right: Extended X-ray emission coinciding with the position of PSR B1853+01 inside of W44. The position of PSR B1853+01 is marked by a black cross. The inner part of the dashed white lines corresponds to the PWN surrounding PSR B1853+01 while the faint extended emission surrounding the PWN is a possible X-ray "halo". Note that the edge of the thermal emission from SNR cut the halo into two parts.
  • Figure 2: Images of PSR B1853+01 (position marked by a black cross) and its surrounding emission. Left: Exposure-corrected merged Chandra ACIS image in the 4-8 keV energy band, created by combining all three archived observations. The green contours show the radio emission from the PWN (radio FITS file obtained from SNRCat). The image is binned by a factor 5 (pixel size=2.46"). The dashed yellow line shows the approximate symmetry axis of the PWN tail, which can be divide into two subregions: a compact nebula (CN) surrounding the pulsar and a trail emission further out. The dashed magenta line indicates the N-S direction which roughly coincides with the direction of the outflow just leaving the PWN bow shock. The position of a nearby source 2CXO J185613.6+011207 is marked by a black X. Middle: Merged (MOS1+MOS2) mosaic on-axis (Obs. 0551060101) XMM-Newton counts image in the 4-8 keV energy band (pixel size=2.5"). Right: Merged (FPMA+FPMB) mosaic NuSTAR image in the 4-10 keV band (pixel size=2.46"), together with the contour map (in cyan) extracted from the Chandra image shown in the left panel. All images are smoothed with a Gaussian kernel of $r$=3 pixels and $\sigma$=1.5 pixels
  • Figure 3: Merged (FPMA+FPMB) NuSTAR images obtained in increasing energy bands. Images are smoothed with a Gaussian kernel of $r$=8 pixels and $\sigma$=4 pixels. Position of PSR B1853+01 is marked by a black cross while the position of a nearby source 2CXO J185613.6+011207 is marked by a black X. Left: The 4.0-10.0 keV image shows an extended emission coincides with the position of PSR B1853+01, exhibiting a spindle-like shape tapering at both ends and a faint extended emission in the south. Regions used for spatially resolved spectral analysis are shown here. Pulsar spectra were tried to extract with magenta circles of two different radii 18"/29". Region A is similar to the outflow region, Region C is similar to the trail region and D is similar to part of the southern halo where one of the large antenna feature lies. Background region is shown in dashed green circle. Right: The 10.0-20.0 keV image. The PSR+PWN system is detected up to energies of $\sim 20$ keV.
  • Figure 4: Chandra 2-7 keV on-axis counts image, unbinned and unsmoothed. Circle and annulus are centered at the pulsar position detected by Chandra. The magenta circle as a radius of 1.8 and is used for the extraction of the pulsar spectra; the green dashed annulus, with inner/outer radii of 3 and 6, is used as the background region.
  • Figure 5: Merged Chandra broad band (0.5-7.0 keV) flux image with extraction regions used for spectral extraction overlaid. The image is unbinned yet smoothed with a Gaussian function with radius r=3 pixel and sigma $\sigma$=1.5 image pixels. Weighted spectra have been extracted from tail (solid cyan) region with pulsar (r=3") excluded, using bkg1 region (dashed green) as background region, and from outflow region (solid yellow) using bkg2 region (dashed green) as background region.
  • ...and 1 more figures