| dc.description.abstract |
Between end of 2019 and 2022, the seven eROSITA telescopes onboard the Russian-German Spectrum-Roentgen-Gamma satellite (SRG) were used to perform an All-Sky X-ray survey in the soft to medium energy X-ray band (0.2-10.0 keV). The eROSITA All-Sky Survey (eRASS) surpasses the ROSAT All-Sky Survey (its predecessor) sensitivity by more than an order of magnitude. At the same time, current X-ray instruments that are used for pointed observations, such as XMM-Newton, Chandra, Suzaku, and NuSTAR are unable to cover the entire sky, even after decades of operation, and can therefore not be used to conduct a comprehensive Galactic Supernova remnant (SNR) population study. In addition, SNRs in close proximity to Earth (hundreds of parsecs), that can reach degree-scale sizes in X-rays, are difficult to be studied with pointed X-ray instruments. Proposing blind observations aiming to cover the whole sky for potential new remnants is not an option. As it appears, a large fraction of the missing number between the expected Galactic SNRs and detected Galactic SNRs is a result of not having properly covered the whole sky in all distinct energy bands that SNRs are detectable. Given eROSITA’s CCD-type sensitivity and energy coverage well beyond the ROSAT XRT’s upper energy range (2 keV), eRASS is ideally suited to discover and investigate the X-ray emission from a variety of astrophysical objects including SNRs which are highly absorbed and/or exhibit non-thermal spectral components. The corresponding X-ray data, therefore, permit to study the known population of both thermal and non-thermal sources and to search for new such sources, especially the latter ones (i.e., non-thermal) which are potential accelerators of cosmic-rays (CRs). The thorough screening of the first four eRASS surveys (eRASS:4), accessible to the German eROSITA consortium, that we conducted has revealed several tens of new SNR candidates and new X-ray counterparts to known SNRs, out of which we have identified compelling candidates for being previously unknown accelerators of ultra-relativistic particles.
The first part of this dissertation is a compilation of five publications resulting from a detailed and deep multiwavelength analysis of three SNRs (Michailidis et al. 2024a, Michailidis et al. 2024b, Khabibullin et al. 2024, Michailidis et al. 2024c)
that we have detected for the first time in X-rays with eROSITA (G279.0+01.1 SNR, the Spaghetti nebula (S147 or G180.0-01.7 SNR), and the G309.8+00.0 SNR) and the identification of the SNR nature of the HESS J1614-518 SNR candidate (Pühlhofer et al. 2024), as suggested by H.E.S.S. data, using eROSITA and GLEAM radio data. For all the above SNRs/targets we have conducted a detailed analysis to probe the individual remnant's properties such as (if detected in the respective source): X-ray thermal plasma temperature, ionization age, chemical composition of both the local ISM and the progenitor star (elemental abundances), thermal plasma state (equilibrium or not), powerlaw shape (indices) that characterizes the spectrum of non-thermal particle populations (both in X-rays and gamma-rays), plasma density, distance, and age determination. The goal of this study is to inspect the properties of those SNRs that can efficiently accelerate particles to GeV/TeV energies (non-thermal SNRs). Its results also contribute towards the reduction and explanation of the gap between the total number of detected Galactic SNRs and the number of Galactic SNRs detected in X-rays and ultimately close the gap between the expected and the detected number of Galactic SNRs.
The second part of this dissertation is a compilation of two publications in the field of Indirect detection of Dark Matter (DM) using gamma-ray instruments (The Cherenkov Telescope Array (CTA) and Fermi-LAT). Specifically, we focus on the Weakly Interacting Massive Particles (WIMPs) annihilation signal in the diffuse halo of M31 and M33 neighboring spiral galaxies (not that frequently selected targets for DM studies), for which a large variety of DM profiles and a detailed baryonic mass characterization are reported in the literature mainly due to their close proximity to Earth that permits extensive studies in the context of their astrophysical nature. In particular, our study (Michailidis et al. 2023) provides the expected sensitivity of CTA to an annihilation signal from WIMPs from M31 and M33. We show that a 100 h long observation campaign will allow CTA to probe annihilation cross-sections up to ⟨συ⟩ ≈ 5 · 10^(−25) cm^3 s^(−1) for the τ +τ − annihilation channel (for M31, at a DM mass of 0.3 TeV), improving the current limits derived by HAWC by up to an order of magnitude. For the derivation of the expected CTA sensitivity to the annihilating DM signal we analysed uncertainties connected to (i): the potential astrophysical background contamination within the Field of View (FoV) of CTA, (ii) the presence of DM substructures, (iii): the imperfect knowledge of the instrument itself and/or misidentification of CRs, i.e. systematic uncertainties, and (iv): the lack of knowledge of the actual DM density distribution. Thus, we argue that our study provides an excellent basis for the specifics (i.e., exposure time) of the upcoming observation of M31 with CTA that has already been planned. We suggest that taking into account the possible effects of the highly uncertain astrophysical background and DM density distribution, the observations of the selected targets could provide important constraints on the WIMP DM parameter space. Driven by our result that the uncertainties on the DM density profiles result in the highest uncertainty (among the four aforementioned causes of uncertainty) in the derived prospects, we provide an excellent review of the DM distribution to commonly studied astrophysical objects (1095 objects and 5659 DM density profiles in total plus seventy-four thousand DM density profiles from thirty-two thousand nearby galaxies provided by two recent studies that employed Sloan Digital Sky Survey (SDSS) and ALFALFA survey data) spanning many orders of magnitude in mass (from dSphs to Galaxy Clusters) that can provide a valuable guide for colleagues seeking to inspect the unseen mass component on individual targets (Michailidis et al. 2024d). The latter study also provides evidence for a new universal property of DM at all observed masses that we are addressing in detail in an upcoming publication where we put all different DM density profiles collected into context (in the form of the DM column density (S) as a function of the halo mass) to introduce a new scaling relation that allows direct comparison of observations with theoretical predictions/numerical simulations. |
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