Environment Studies of Pulsar Wind Nebulae and Their Interactions with the Interstellar Medium

Abstract

Pulsars, rapidly rotating neutron star born from the core-collapse of massive stars, convert a fraction of their rotational energy to accelerate electrons up to high energies. The generated pulsar wind eventually reaches the termination shock and creates a pulsar wind nebula (PWN). There, the particles’ trajectories become randomized, and they produce radio to X-ray emission via synchrotron radiation; and TeV γ-ray emission from the interaction of high energy electrons with the Cosmic Microwave Background; and the infra-red emission from Galactic dust. Although progress has been made towards the understanding of the structure of the pulsar environment, several issues, such as the composition of the pulsar winds, still need to be addressed. Indeed, no direct evidence of hadronic components have yet been discovered inside the PWN. However, nearby dense molecular clouds could provide sufficient target particles for the potential hadrons from the PWN and its progenitor supernova remnant (SNR) to produce significant TeV emission via proton-proton (p-p) interaction. My work thus first consists of conducting interstellar matter (ISM) studies towards several PWNe using the 22-metre Mopra and the 4-metre Nanten radio telescopes. Among the studied PWNe, I particularly focus on HESS J1825−137 and its plausible association with the nearby unidentified TeV source HESS J1826−130. I have mapped the HESS J1826−130 region with Mopra in the 7 and 12 mm bands which, combined with the Nanten CO(1–0) survey and the GRS ¹³CO(1–0), enable an accurate analysis of the morphological and physical properties of several dense molecular clouds found in the line of sight. Interestingly, I have found a massive molecular cloud adjacent to the PWN HESS J1825−137 and overlapping the HESS J1826−130 TeV emission. From our mass estimates, I suggest that the cosmic-rays originating from the progenitor SNR of the pulsar PSRJ1826−1334 can significantly contribute to the TeV emission. We then attempt to model and predict spectral and morphological properties of the TeV emission from the propagation of high energy CRs and electrons, originating from the progenitor SNR and potentially from the PWN.We find that the resulting spectral shape of the TeV γ-ray emission is very sensitive to the diffusion coefficient of high energy particles inside molecular clouds. I also find that only a ‘slow’ diffusion’ of CRs (diffusion coefficient D(E) ∼ 10²⁶ √E/10GeV cm² s⁻¹) results in a significant contribution of the gamma-ray emission towards HESS J1826−130 at all energies. We finally notice that the contribution from hypothetical CRs escaping the PWN HESS J1825−137 is expected to be overshadowed by the contribution of CRs escaping the progenitor SNR. As one expects the latter’s contribution to decrease as time evolves while the former’s contribution to remain somewhat constant, I thus argue that older PWNe may be more suitable candidates to obtain direct evidence of CRs inside PWNe. Among the studied PWNe, I find that, based on simplistic diffusion studies, the ISM surrounding HESS J1809−193 may be a good laboratory to detect CRs escaping the pulsar PSRJ1809−1917.