Interpreting Crab Nebula’s synchrotron spectrum: two acceleration mechanisms

Journal Article · 05 August 2019 · Monthly Notices of the Royal Astronomical Society

DOI: https://doi.org/10.1093/mnras/stz2023 · OSTI ID:1560709

Lyutikov, Maxim ^[1]; Temim, Tea ^[2]; Komissarov, Sergey  ^[3]; Slane, Patrick ^[4]; Sironi, Lorenzo ^[5]; Comisso, Luca ^[5]

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1. Department of Physics and Astronomy, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907, USA
2. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
3. School of Mathematics, University of Leeds, LS29JT Leeds, UK
4. Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
5. Department of Astronomy, Columbia University, 550 W 120th St, New York, NY 10027, USA

ABSTRACT We outline a model of the Crab pulsar wind nebula with two different populations of synchrotron emitting particles, arising from two different acceleration mechanisms: (i) Component-I due to Fermi-I acceleration at the equatorial portion of the termination shock, with particle spectral index pI ≈ 2.2 above the injection break corresponding to γwindσwind ∼ 105, peaking in the ultraviolet (UV, γwind ∼ 102 is the bulk Lorentz factor of the wind, σwind ∼ 103 is wind magnetization); and (ii) Component-II due to acceleration at reconnection layers in the bulk of the turbulent Nebula, with particle index pII ≈ 1.6. The model requires relatively slow but highly magnetized wind. For both components, the overall cooling break is in the infrared at ∼0.01 eV, so that the Component-I is in the fast cooling regime (cooling frequency below the peak frequency). In the optical band, Component-I produces emission with the cooling spectral index of αo ≈ 0.5, softening towards the edges due to radiative losses. Above the cooling break, in the optical, UV, and X-rays, Component-I mostly overwhelms Component-II. We hypothesize that acceleration at large-scale current sheets in the turbulent nebula (Component-II) extends to the synchrotron burn-off limit of ϵs ∼ 100 MeV. Thus in our model acceleration in turbulent reconnection (Component-II) can produce both hard radio spectra and occasional gamma-ray flares. This model may be applicable to a broader class of high-energy astrophysical objects, like active galactic nuclei and gamma-ray burst jets, where often radio electrons form a different population from the high-energy electrons.

• Sponsoring Organization: USDOE
• Grant/Contract Number: SC0016369; SC0016542
• OSTI ID: 1560709
• Journal Information: Monthly Notices of the Royal Astronomical Society, Journal Name: Monthly Notices of the Royal Astronomical Society Journal Issue: 2 Vol. 489; ISSN 0035-8711
• Publisher: Oxford University PressCopyright Statement
• Country of Publication: United Kingdom
• Language: English