On the Gamma-Ray Nebula of Vela Pulsar. I. Very Slow Diffusion of Energetic Electrons within the TeV Nebula

Abstract

High-energy particle transport in pulsar wind nebulae (PWNe) plays an essential role in explaining the characteristics revealed in multiwavelength observations. In this paper, the TeV-gamma-ray-emitting electrons in the Vela X PWN are approximated to be injected impulsively when the cocoon is formed due to the interaction between the SNR reverse shock and the PWN. By solving the diffusion-loss equation analytically, we reproduce the broadband spectral energy distribution and surface brightness profile simultaneously. The diffusion coefficient of TeV electrons and positrons, which is well constrained by the spectral and spatial properties of the TeV nebula, is thus determined to be 1 × 10 26 cm 2 s −1 for 10 TeV electrons and positrons. This coefficient is more than three orders of magnitude lower than that in the interstellar medium, in agreement with a constraint recently obtained from HAWC observations of a TeV nebula associated with the Geminga pulsar. These results suggest that slow diffusion of high-energy particles might be common in PWNe.

Hadronic vs leptonic models for $γ$-ray emission from VER J2227+608

“…Then we have E d = 160 , and 39 TeV for δ = 1/2, and 1/3, respectively. We note that the diffusion coefficients we have here are about one order of magnitude lower than mean values around TeV PWNe (Di Mauro et al 2020) but are comparable to that derived for the Vela X (Bao et al 2019). To increase the diffusion coefficient, one needs to increase the size of the emission zone or decrease the age T of the system so that the γ-ray spectrum does not change significantly.…”

“…The center of the TeV source is also displaced from the pulsar. The TeV spectrum of Vela X cuts off at a bit above 10 TeV, which has been attributed to radiative energy loss of TeV electrons displaced by reverse shocks of the SNR a few thousand years ago (Bao et al 2019). In our model, we have a constant injection rate of high-energy electrons into the cooling region, implying a very long spin down age for PSR J2229+6114.…”

Extended Very-High-Energy Gamma-Ray Emission Surrounding PSR J0622 + 3749 Observed by LHAASO-KM2A

“…Note that different definitions of pulsar halos exist in the literature[10,11]. For example, some young pulsars that exhibit particle escape are also called pulsar halos[12,13].…”

On the Gamma-Ray Nebula of Vela Pulsar. II. The Soft Spectrum of the Extended Radio Nebula

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The Vela X pulsar wind nebula (PWN) is characterized by the extended radio nebula (ERN) and the central X-ray "cocoon". We have interpreted the γ-ray spectral properties of the cocoon in the sibling paper (Bao et al 2019); here, we account for the broadband photon spectrum of the ERN. Since the diffusive escape of the electrons from the TeV emitting region is expected to play an insignificant role in shaping the spectrum of the ERN, we attribute the GeV cutoff of the ERN to the reverse shock-PWN interaction.

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