Abstract
Pulsar wind nebulae are formed when outflows of relativistic electrons and positrons hit the surrounding supernova remnant or interstellar medium at a shock front. The Vela pulsar wind nebula is powered by a young pulsar (B0833-45, aged 11,000 years)1 and located inside an extended structure called Vela X, which is itself inside the supernova remnant2. Previous X-ray observations revealed two prominent arcs that are bisected by a jet and counter jet3,4. Radio maps have shown high linear polarization of 60% in the outer regions of the nebula5. Here we report an X-ray observation of the inner part of the nebula, where polarization can exceed 60% at the leading edge—approaching the theoretical limit of what can be produced by synchrotron emission. We infer that, in contrast with the case of the supernova remnant, the electrons in the pulsar wind nebula are accelerated with little or no turbulence in a highly uniform magnetic field.
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Data availability
IXPE data are available through the NASA’s HEASARC data archive (https://heasarc.gsfc.nasa.gov). Other derived data supporting the findings of this study are available from the corresponding author on request. Source data are provided with this paper.
Code availability
The High Energy Astrophysics Science Archive Research Center (HEASARC) developed the HEASOFT (HEASARC Software). We used the HEASOFT v.6.30.1 package for the spectropolarimetric IXPE data analysis, which is available online (https://heasarc.gsfc.nasa.gov/docs/software/heasoft/). The proper instrument response functions are provided by the IXPE Team as a part of the IXPE calibration database released on 14 March 2022 are and available in the HEASARC Calibration Database (https://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/caldb_supported_missions.html). The software developed by IXPE collaboration is available publicly online (https://ixpeobssim.readthedocs.io/en/latest/?badge=latest). Information supporting the findings of this study is available from the corresponding author on request.
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Acknowledgements
The IXPE is a joint US and Italian mission. The US contribution is supported by the National Aeronautics and Space Administration (NASA) and is led and managed by the Marshall Space Flight Center (MSFC), with industry partner Ball Aerospace (contract NNM15AA18C). The Italian contribution is supported by the Italian Space Agency (Agenzia Spaziale Italiana, ASI) through contract ASI-OHBI-2017-12-I.0, agreements ASI-INAF-2017-12-H0 and ASI-INFN-2017.13-H0, and its Space Science Data Center (SSDC) with agreements ASI-INAF-2022-14-HH.0 and ASI-INFN 2021-43-HH.0, and by the Istituto Nazionale di Astrofisica (INAF) and the Istituto Nazionale di Fisica Nucleare (INFN) in Italy. This research used data products provided by the IXPE Team (MSFC, SSDC, INAF and INFN) that are distributed with additional software tools by the High-Energy Astrophysics Science Archive Research Center (HEASARC), at NASA Goddard Space Flight Center (GSFC). The research at Guangxi University was supported in part by National Natural Science Foundation of China (grant no. 12133003). The research at Boston University was supported in part by National Science Foundation grant AST-2108622. I.A. acknowledges financial support from the Spanish ‘Ministerio de Ciencia e Innovación’ (MCINN) through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía-CSIC (SEV-2017-0709) and through grants AYA2016-80889-P and PID2019-107847RB-C44.
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F.X. led the data analysis and the writing of the paper. A.D.M. and F.L.M. performed the spectropolarimetric analysis and contributed to the paper. F. Muleri and J.R. contributed to the data analysis and energy calibration of the dataset. K.L. and N.B. contributed to the data analysis and result interpretation. R.W.R. and L.L. helped to revise the manuscript. E. Costa, P. Soffitta, M.B., S.F., R.F., M. Pilia and A.T. contributed to the interpretation of the results and to text revisions. N.D.L., S.G., N.O., M.N. and E.W. performed independent analysis of the data. The other authors contributed to the design of the mission, to the calibration of the instrument, to the definition of its scientific case and to the planning of the observations. All of the authors provided inputs and comments on the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Light curve of the Vela PWN observed by IXPE DU1 before and after data filtering (top and bottom panels, respectively).
Filtering the high count-rate excursions from the event_l2 file removes approximately 1.47% of the exposure time.
Extended Data Fig. 2 Total nebula source (white 1′ radius circle) and background (blue annulus, inner radius 2′, outer radius 4.7′) regions shown on images from DU1 (left) to DU3 (right).
Intensity is on a logarithmic scale to bring out the faint background.
Extended Data Fig. 3 Spectral joint fitting for the Stokes parameters in the 2–8 keV energy band for the three DUs using model TBABS*POWERLAW*POLCONST with previously fit spectral parameters fixed.
The average background count I spectrum for the three DUs is shown in black. Fit residuals are shown at the bottom.
Extended Data Fig. 4 Polar plot showing the polarization degree (PD) and polarization angle (PA) fit with data from the three DUs, for different energy bands.
Ellipses show the 68.3% confidence level errors obtained with XSPEC.
Extended Data Fig. 5
An alternative spatial partition of the PWN with regions aligned with the PWN symmetry axis on image observed by Chandra. L, R, F, B label the left, right, front, back regions with respect to the C centre region respectively, corresponding to the analysis tabulated in Extended Data Table 4. The inner circle has a radius of 15′′ and the outer circle radius is 45′′.
Extended Data Fig. 6 Q/I (top) and U/I (bottom) Stokes parameters of the Vela PWN as functions of energy in 2–8 keV range derived with ixpeobssim and XSPEC independently.
The results are from the joint analysis of three DUs.
Source data
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Xie, F., Di Marco, A., La Monaca, F. et al. Vela pulsar wind nebula X-rays are polarized to near the synchrotron limit. Nature 612, 658–660 (2022). https://doi.org/10.1038/s41586-022-05476-5
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DOI: https://doi.org/10.1038/s41586-022-05476-5
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