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Register a new userOn the Gamma-Ray Nebula of Vela Pulsar. I. Very Slow Diffusion of Energetic Electrons within the TeV Nebula
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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 $1 times 10^{26}$,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.
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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 $gamma$-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. Due to the disruption of the reverse shock, most of plasma of the PWN is driven into the ERN. During the subsequent reverberation phase, the ERN could be compressed by a large factor in radius, and the magnetic field in the ERN is thus significantly enhanced, burning off the high energy electrons. We thus obtain the electron spectrum of the ERN and the broadband spectrum of the ERN are explained satisfactorily.
Pulsars are known to power winds of relativistic particles that can produce bright nebulae by interacting with the surrounding medium. These pulsar wind nebulae (PWNe) are observed in the radio, optical, x-rays and, in some cases, also at TeV energies, but the lack of information in the gamma-ray band prevents from drawing a comprehensive multiwavelength picture of their phenomenology and emission mechanisms. Using data from the AGILE satellite, we detected the Vela pulsar wind nebula in the energy range from 100 MeV to 3 GeV. This result constrains the particle population responsible for the GeV emission, probing multivavelength PWN models, and establishes a class of gamma-ray emitters that could account for a fraction of the unidentified Galactic gamma-ray sources.
We have observed the Vela pulsar region at TeV energies using the 3.8 m imaging Cherenkov telescope near Woomera, South Australia every year since 1992. This is the first concerted search for pulsed and unpulsed emission from the Vela region, and the imaging technique also allows the location of the emission within the field of view to be examined. A significant excess of gamma-ray-like events is found offset from the Vela pulsar to the southeast by about 0.13deg. The excess shows the behavior expected of gamma-ray images when the asymmetry cut is applied to the data. There is no evidence for the emission being modulated with the pulsar period -- in contrast to earlier claims of signals from the Vela pulsar direction.
The pulsar emission mechanism in the gamma-ray energy band is poorly understood. Currently, there are several models under discussion in the pulsar community. These models can be constrained by studying the collective properties of a sample of pulsars, which became possible with the large sample of gamma-ray pulsars discovered by the Fermi Large Area Telescope (Fermi-LAT). In this paper we develop a new experimental multi-wavelength technique to determine the beaming factor $left( f_Omega right)$ dependance on spin-down luminosity of a set of GeV pulsars. This technique requires three input parameters: pulsar spin-down luminosity, pulsar phase-averaged GeV flux and TeV or X-ray flux from the associated Pulsar Wind Nebula (PWN). The analysis presented in this paper uses the PWN TeV flux measurements to study the correlation between $f_Omega$ and $dot{E}$. The measured correlation has some features that favor the Outer Gap model over the Polar Cap, Slot Gap and One Pole Caustic models for pulsar emission in the energy range of 0.1 to 100 GeV, but one must keep in mind that these simulated models failed to explain many of the most important pulsar population characteristics. A tight correlation between the pulsar GeV emission and PWN TeV emission was also observed, which suggests the possibility of a linear relationship between the two emission mechanisms. In this paper we also discuss a possible mechanism to explain this correlation.
The pulsar wind nebula (PWN) 3C 58 is one of the historical very-high-energy (VHE; E>100 GeV) gamma-ray source candidates. It is energized by one of the highest spin-down power pulsars known (5% of Crab pulsar) and it has been compared to the Crab Nebula due to their morphological similarities. This object was previously observed by imaging atmospheric Cherenkov telescopes (Whipple, VERITAS and MAGIC), although not detected, with an upper limit of 2.4% Crab Unit (C.U.) at VHE. It was detected by Fermi-LAT with a spectrum extending beyond 100 GeV. We analyzed 81 hours of 3C 58 data taken with the MAGIC telescopes and we detected VHE gamma-ray emission with a significance of 5.7 sigma and an integral flux of 0.65% C.U. above 1 TeV. The differential energy spectrum between 400 GeV and 10 TeV is well described by a power-law function dphi/dE=f_0(E/1TeV)^{-Gamma} with f_0=(2.0pm0.4_{stat}pm0.6_{sys})times10^{-13}cm^{-2}s^{-1}TeV^{-1} and Gamma=2.4pm0.2_{stat}pm0.2_{sys}. The skymap is compatible with an unresolved source. We report the first significant detection of PWN 3C 58 at TeV energies. According to our results 3C 58 is the least luminous VHE gamma-ray PWN ever detected at VHE and the one with the lowest flux at VHE to date. We compare our results with the expectations of time-dependent models in which electrons up-scatter photon fields. The best representation favors a distance to the PWN of 2 kpc and Far Infrared (FIR) comparable to CMB photon fields. If we consider an unexpectedly high FIR density, the data can also be reproduced by models assuming a 3.2 kpc distance. A low magnetic field, far from equipartition, is required to explain the VHE data. Hadronic contribution from the hosting supernova remnant (SNR) requires unrealistic energy budget given the density of the medium, disfavoring cosmic ray acceleration in the SNR as origin of the VHE gamma-ray emission.
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