Pulsar Wind Nebulae: observations and models of 3C58 and discovery of superefficiency

This thesis focuses on the study of Pulsar wind Nebulae (PWNe), which form as a result of the bulk of the pulsar rotational energy begin lost via the emission of a relativistic wind of particles. The winds, being supersonic with respect to the interstellar medium, produce a termination shock where particles are accelerated. Since the nebula is threaded with photon and magnetic fields, particles are able to emit at all frequencies, from radio to TeV energies, via non-thermal processes such as synchrotron and inverse Compton. This thesis zooms into studying the evolution of this non-thermal radiation along the pulsar lifetime, analyzing the changes produced to the spectral energy distribution as a result of the expansion and contraction of the PWN due to pressure balances and interaction with the environment. The thesis first considers the case of the complex formed by the pulsar/PWN, PSR J0205+6449/3C 58, which is especial due to its young age, significant power, and similarity to the Crab Nebula (the best studied PWN). The thesis presents the results of the analysis of 8 years of Fermi-LAT data. The main aspect is that using a contemporaneous ephemeris for the pulsation, we could significantly detect 3C 58 during the off-peak phase interval of PSR J0205+6449. I analyzed the observed data with a time-dependent model of PWNe based on the code TIDE, developed by the group in which I worked over the last 8 years. My model provides a reasonable fit to data; one in which the PWN 3C 58 is not yet reverberating. Reverberation is the period of PWN evolution when the reverse shock created by the supernova explosion travels back towards the pulsar, compressing the wind bubble. It is a relatively short but significant period, barely studied. The rest of the thesis studies older PWNe, or younger ones like 3C 58 but evolved into the future so as to grasp the behavior of reverberation when they pass through it. This study has led to the discovery and characterization of superefficiency. Superefficiency happens when, due to its compression because of the returning reverse shock of the supernova explosion, the nebula is subject to significant adiabatic heating. To what extent this heating affected the luminosities of the PWN at different energy ranges was not clear. The thesis describes in detail how due to the compression, the magnetic field of the PWN also increases, as well as there are more particles at higher energies than there were previously. I found that such process can produce PWNe that for a short time emit more in X-rays and other frequencies than what they have as rotational energy at the time. The former is not a paradox, but the consequence of the fact that the rotational spinning down of the pulsar is no longer the energy reservoir of the system. This period ends when the magnetic field pressure, increased because of the magnetic field, significantly risen up, is able to detain the kinematic pressure provided by the reverse shock. I took on both, several well-characterized PWNe and a broad range of PWN models representative of the observed pulsars to study their reverberation and superefficiency properties. Having attained such modelling, I estimated via Monte Carlo simulations how many Galactic PWNe are expected to be reverberating or in a superefficiency stage at any given time and realized predictions for possible future detections with the next generation of instruments. This thesis is presented as a compendium of published results. Three papers published in The Astrophysical Journal, The Astrophysical Journal Letters, and Monthly Notices of the Royal Astronomical Society correspondingly conform Chapters 2, 3 and 4 of the thesis.