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Energetic pulsars power winds of relativistic leptons which produce photonnebulae (so- called pulsar wind nebulae, PWNe). Their spectral energydistribution has a double-humped structure: the first hump lies in the X-rayregime, the second in the gamma-ray range. The X-ray emission is generallyunderstood as synchrotron radiation by highly energetic leptons, the gamma-rayemission as Inverse Compton scattering of energetic leptons with ambient photonfields. The spectral evolution is influenced by the time-dependent spin-down ofthe pulsar and the decrease of the magnetic field strength with time. Thus, thepresent spectral appearance of a PWN depends on the age of the pulsar: whileyoung PWNe are bright in X-rays and gamma-rays, the X-ray emission of evolvedPWNe is suppressed. Hence, evolved pulsar wind nebulae may offer an explanationof the nature of some of the unidentified VHE (very high-energy, E > 100GeV)gamma-ray sources not yet associated with a counterpart at other wavelengths.The purpose of this work is to develop a model which allows to calculate theexpected X-ray fluxes of unidentified VHE gamma-ray sources considered to bePWN candidates. Such an estimate may help to evaluate the prospects ofdetecting the X-ray signal in deep observations with current X-rayobservatories in future studies. We present a time-dependent leptonic modelwhich predicts the broad-band emission of a PWN according to thecharacteristics of its pulsar. For a sample of representative PWNe, theresulting model predictions in the X-ray and gamma-ray range are compared toobservations. The comparison shows that the energy flux of the X-ray emissionof identified PWNe from different states of evolution can be roughly predictedby the model. This implies the possibility of an estimate of the non-thermalX-ray emission of unidentified VHE gamma-ray sources in case of an evolved PWNscenario.

Predicting the X-ray flux of evolved pulsar wind nebulae based on VHE gamma-ray observations