Abrupt acceleration of a ‘cold’ ultrarelativistic wind from the Crab pulsar


Pulsars are thought to eject electron–positron winds that energize the surrounding environment, with the formation of a pulsar wind nebula1. The pulsar wind originates close to the light cylinder, the surface at which the pulsar co-rotation velocity equals the speed of light, and carries away much of the rotational energy lost by the pulsar. Initially the wind is dominated by electromagnetic energy (Poynting flux) but later this is converted to the kinetic energy of bulk motion2. It is unclear exactly where this takes place and to what speed the wind is accelerated. Although some preferred models imply a gradual acceleration over the entire distance from the magnetosphere to the point at which the wind terminates3,4, a rapid acceleration close to the light cylinder cannot be excluded5,6. Here we report that the recent observations of pulsed, very high-energy γ-ray emission from the Crab pulsar7,8,9 are explained by the presence of a cold (in the sense of the low energy of the electrons in the frame of the moving plasma) ultrarelativistic wind dominated by kinetic energy. The conversion of the Poynting flux to kinetic energy should take place abruptly in the narrow cylindrical zone of radius between 20 and 50 light-cylinder radii centred on the axis of rotation of the pulsar, and should accelerate the wind to a Lorentz factor of (0.5–1.0) × 106. Although the ultrarelativistic nature of the wind does support the general model of pulsars, the requirement of the very high acceleration of the wind in a narrow zone not far from the light cylinder challenges current models.

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Figure 1: Spectral energy distribution of γ-ray radiation produced by the pulsar magnetosphere and by the pulsar wind.
Figure 2: Complex comprising the pulsar magnetosphere, the ultrarelativistic wind and the pulsar wind nebula.
Figure 3: Formation of the pulsed VHE inverse-Compton γ-ray signal in the wind of the Crab pulsar.


  1. 1

    Rees, M. J. & Gunn, J. E. The origin of the magnetic field and relativistic particles in the Crab Nebula. Mon. Not. R. Astron. Soc. 167, 1–12 (1974)

    ADS  Article  Google Scholar 

  2. 2

    Kennel, C. F. & Coroniti, F. V. Magnetohydrodynamic model of Crab nebula radiation. Astrophys. J. 283, 710–730 (1984)

    CAS  ADS  Article  Google Scholar 

  3. 3

    Coroniti, F. V. Magnetically striped relativistic magnetohydrodynamic winds: the Crab Nebula revisited. Astrophys. J. 349, 538–545 (1990)

    ADS  Article  Google Scholar 

  4. 4

    Lyubarsky, Y. & Kirk, J. G. Reconnection in the striped pulsar wind. Astrophys. J. 547, 437–448 (2001)

    ADS  Article  Google Scholar 

  5. 5

    Vlahakis, N. Ideal magnetohydrodynamic solution to the σ problem in Crab-like pulsar winds and general asymptotic analysis of magnetized outflows. Astrophys. J. 600, 324–337 (2004)

    ADS  Article  Google Scholar 

  6. 6

    Beskin, V. S. & Nokhrina, E. E. The effective acceleration of plasma outflow in the paraboloidal magnetic field. Mon. Not. R. Astron. Soc. 367, 375–386 (2006)

    ADS  Article  Google Scholar 

  7. 7

    Aliu, E. et al. Detection of pulsed gamma rays above 100 GeV from the Crab pulsar. Science 334, 69–72 (2011)

    CAS  ADS  Article  Google Scholar 

  8. 8

    Aleksić, J. et al. Observations of the Crab pulsar between 25 and 100 GeV with the MAGIC I telescope. Astrophys. J. 742, 43 (2011)

    ADS  Article  Google Scholar 

  9. 9

    Aleksić, J. et al. Phase-resolved energy spectra of the Crab Pulsar in the range of 50-400 GeV measured with the MAGIC Telescopes. Astron. Astrophys.. (submitted); preprint at 〈http://arxiv.org/abs/1109.6124〉 (2011)

  10. 10

    Abdo, A. A. et al. Fermi Large Area Telescope observations of the Crab pulsar and nebula. Astrophys. J. 708, 1254–1267 (2010)

    ADS  Article  Google Scholar 

  11. 11

    Tang, A. P. S., Takata, J., Jia, J. J. & Cheng, K. S. A revisit of the phase-resolved X-ray and γ-ray spectra of the Crab pulsar. Astrophys. J. 676, 562–572 (2008)

    CAS  ADS  Article  Google Scholar 

  12. 12

    Lyutikov, M., Otte, N. & McCann, A. The very-high energy emission from pulsars: a case for inverse Compton scattering. Astrophys. J.. (submitted); preprint at 〈http://arxiv.org/abs/1108.3824〉 (2011)

  13. 13

    Bogovalov, S. V. & Aharonian, F. A. Very-high-energy γ radiation associated with the unshocked wind of the Crab pulsar. Mon. Not. R. Astron. Soc. 313, 504–514 (2000)

    CAS  ADS  Article  Google Scholar 

  14. 14

    Kuiper, L. et al. The Crab pulsar in the 0.75–30 MeV range as seen by the CGRO COMPTEL. A coherent high-energy picture from soft X-rays to high-energy γ-rays. Astron. Astrophys. 378, 918–935 (2001)

    ADS  Article  Google Scholar 

  15. 15

    Rots, A. H. et al. Absolute timing of the Crab pulsar with the Rossi X-Ray Timing Explorer. Astrophys. J. 605, L129–L132 (2004)

    CAS  ADS  Article  Google Scholar 

  16. 16

    Hirotani, K. Outer-gap versus slot-gap models for pulsar high-energy emissions: the case of the Crab pulsar. Astrophys. J. 688, 1254–1267 (2010)

    Google Scholar 

  17. 17

    Osmanov, Z. & Rieger, F. M. On particle acceleration and very high energy γ-ray emission in Crab-like pulsars. Astron. Astrophys. 502, L25–L28 (2008)

    Google Scholar 

  18. 18

    Chkheidze, N., Machabeli, G. & Osmanov, Z. On the very high energy spectrum of the Crab pulsar. Astrophys. J. 730, 62 (2011)

    ADS  Article  Google Scholar 

  19. 19

    Sturrock, P. A. A model of pulsars. Astrophys. J. 164, 529–556 (1971)

    CAS  ADS  Article  Google Scholar 

  20. 20

    Wilson, D. B. & Rees, M. J. Induced Compton scattering in pulsar winds. Mon. Not. R. Astron. Soc. 185, 297–304 (1978)

    ADS  Article  Google Scholar 

  21. 21

    Gould, R. J. High-energy photons from the Compton-synchrotron process in the Crab nebula. Phys. Rev. Lett. 15, 577–579 (1965)

    ADS  Article  Google Scholar 

  22. 22

    de Jager, O. C. & Harding, A. K. The expected high-energy to ultra-high-energy γ-ray spectrum of the Crab nebula. Astrophys. J. 283, 710–730 (1992)

    Google Scholar 

  23. 23

    Atoyan, A. M. & Aharonian, F. A. On the mechanisms of γ radiation in the Crab nebula. Mon. Not. R. Astron. Soc. 278, 161–172 (1992)

    Google Scholar 

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We would like to thank J. Cortina, E. de Ona Wilhemi, S. Klepser and N. Otte for information about high-energy γ-ray observations. We also appreciate discussions with F. Rieger, D. Jones and P. Gandhi.

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F.A.A., S.V.B. and D.K. jointly contributed in comparable proportions to all aspects of the work, including the calculations and preparation of the manuscript.

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Correspondence to F. A. Aharonian.

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Aharonian, F., Bogovalov, S. & Khangulyan, D. Abrupt acceleration of a ‘cold’ ultrarelativistic wind from the Crab pulsar. Nature 482, 507–509 (2012). https://doi.org/10.1038/nature10793

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