Interpreting Crab Nebula’s synchrotron spectrum: two acceleration mechanisms
Nov 5, 2018
14 pages
Published in:
- Mon.Not.Roy.Astron.Soc. 489 (2019) 2, 2403-2416
- Published: Oct 21, 2019
e-Print:
- 1811.01767 [astro-ph.HE]
DOI:
- 10.1093/mnras/stz2023 (publication)
View in:
Citations per year
Abstract: (Oxford University Press)
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 p_I ≈ 2.2 above the injection break corresponding to γ_windσ_wind ∼ 10^5, peaking in the ultraviolet (UV, γ_wind ∼ 10^2 is the bulk Lorentz factor of the wind, σ_wind ∼ 10^3 is wind magnetization); and (ii) Component-II due to acceleration at reconnection layers in the bulk of the turbulent Nebula, with particle index p_II ≈ 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 α_o ≈ 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 ϵ_s ∼ 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.Note:
- shortened version of the initial submission, accepted by MNRAS
- pulsars: Individual: Crab
- radiation mechanisms: nonthermal
References(124)
Figures(25)