Properties of Young Pulsar Wind Nebulae: TeV Detectability and Their Pulsar Properties

Abstract

Among dozens young pulsar wind nebulae, some have been detected in TeV γ-rays (TeV PWNe), while others have not (non-TeV PWNe). The TeV emission detectability is not correlated either with the spin-down power or with the characteristic age of their central pulsars, and it is an open problem what determines the detectability. To study this problem, we investigate spectral evolution of five young non-TeV PWNe, 3C58, G310.6-1.6, G292.0+1.8, G11.2-0.3 and SNR B0540-69.3. We use a spectral evolution model which has been developed to be applied to young TeV PWNe in our previous works. TeV γ-ray flux upper limits of non-TeV PWNe give upper or lower limits on parameters, such as the age of the PWN and the fraction of the spin-down power going to the magnetic energy injection (the fraction parameter). Combined with other independent observational and theoretical studies, we can guess a plausible value of the parameters for each object. For 3C58, we prefer the parameters with an age of 2.5 kyr old and the fraction parameter of 3.0x10^{-3}, although the spectral modeling alone does not rule out a shorter age and a higher fraction parameter. The fraction parameter of 3.0x10^{-3} is also consistent for other non-TeV PWNe and then the value is regarded as common to young PWNe including TeV PWNe. Moreover, we find that the intrinsic properties of the central pulsars are similar, 10^{48-50}erg for the initial rotational energy and 10^{42-44}erg for the magnetic energy (2x10^{12} - 3x10^{13}G for the dipole magnetic field strength at their surfaces). The TeV detectability is correlated with the total injected energy and the energy density of the interstellar radiation field around PWNe. Except for G292.0+1.8, a broken power-law injection of the particles well reproduces the broadband emission from non-TeV PWNe.