Studies of the interstellar medium towards dark TeV gamma-ray sources

Abstract

Recent advances involving ground based observations of the very-high-energy sky have led to the discovery of a growing number of Galactic tera-electron volt (TeV) γ-ray sources. Many of these sources have been associated with other high energy phenomena, such as supernova remnants (SNRs) and pulsar wind nebulae. However, a number of TeV γ-ray sources are `dark', with no strong counterparts seen at other wavelengths. Astrophysical TeV γ-rays are produced via two channels; leptonic and hadronic. The leptonic process involves the upscattering of background photons by high energy electrons through the inverse-Compton effect. The hadronic process involves the decay of neutral pions produced by interactions between highly accelerated cosmic-rays (CRs) and the ambient interstellar medium (ISM). Dark TeV γ-ray sources may result from CR and ISM interactions, and studying these sources will shed light on the nature of the parent particle accelerators. Some of the sources selected for study in this thesis are considered candidate PeVatrons, an emerging class of extreme accelerators which produce CRs with energies in the peta-electron volt (PeV) range. A detailed understanding of the ISM towards dark TeV γ-ray sources is vital in order to investigate possible origin scenarios. In addition to providing contextual clues, morphological similarities between TeV γ-rays and the ISM can provide strong evidence of hadronic CR interactions. This may lead to positive associations between TeV sources and other sources observed at different wavelengths. The work in this thesis primarily involves the use of radio telescopes to investigate the distribution of the ISM towards dark TeV γ-ray sources. The data has been used to constrain the mechanisms behind these objects, and underpins the investigations into their mysterious origins. Five TeV γ-ray sources were studied as part of the work in this thesis. HESS J1640 - 465 and the PeVatron candidate HESS J1641 - 463 are a pair of γ-ray sources each coincident with a SNR bridged by bright Hii regions. The study of the ISM suggests that the TeV emission from both sources may be hadronic in origin, and provides evidence to support the PeVatron nature of HESS J1641 - 463. HESS J1614 - 518 has no obvious counterparts seen in other wavelengths. The investigation of the ISM reveals an overlapping ring of gas potentially associated with an undiscovered SNR or the stellar cluster Pismis 22, which may power the TeV emission. The nearby TeV source HESS J1616 - 508 has several potential counterparts, including two SNRs and three pulsars, though there was no evidence in the study of the ISM to support any association. Spatially matching gas, however, is found coincident with the TeV source, suggesting a possible link with an undiscovered accelerator. HESS J1702 - 420 is both a dark TeV γ-ray source and a PeVatron candidate. Though a SNR and pulsar are seen near the outskirts of this source, no evidence in the study of the ISM is found to support any association. Morphologically favourable gas overlapping the TeV source, however, suggests possible scenarios involving nearby acclerators, such as stellar winds from massive stars or an undetected SNR.