1202.1455 (Michael J. Mayer et al.)

Implications on the X-ray emission of evolved pulsar wind nebulae based on VHE gamma-ray observations    [PDF]

Michael J. Mayer, Julia Brucker, Ira Jung, Kathrin Valerius, Christian Stegmann

Energetic pulsars power winds of relativistic leptons which produce photon nebulae (so-called pulsar wind nebulae, PWNe) detectable across the electromagnetic spectrum up to energies of several TeV. The spectral energy distribution has a double-humped structure: the first hump lies in the X-ray regime, the second in the gamma-ray range. The X-ray emission is generally understood as synchrotron radiation by highly energetic electrons, the gamma-ray emission as Inverse Compton scattering of energetic electrons with ambient photon fields. The evolution of the spectral energy distribution is influenced by the time-dependent spin-down of the pulsar and the decrease of the magnetic field strength with time. Thus, the present spectral appearance of a PWN depends on the age of the pulsar: while young PWNe are bright in X-rays and gamma-rays, the X-ray emission of evolved PWNe is suppressed. Hence, evolved PWNe may offer an explanation of the nature of some of the unidentified VHE gamma-ray sources not yet associated with a counterpart in other high-energy ranges. The purpose of this work is to develop a model which allows to calculate the expected X-ray fluxes of unidentified VHE gamma-ray sources considered to be PWN candidates. Such an estimate helps to evaluate the prospects of detecting the X-ray signal in deep observations with current X-ray observatories in future studies. We present a time-dependent leptonic model which predicts the broad-band emission of a PWN according to the characteristics of its pulsar. The values of the free parameters of the model are determined by a fit to observational VHE gamma-ray data. For a sample of representative PWNe, the resulting model predictions in the X-ray and gamma-ray range are compared to observations. The comparison shows that the high-energy emission of identified PWNe from different states of evolution is predicted correctly by the model.

View original: http://arxiv.org/abs/1202.1455