Probing the Spatial and Spectral Distribution of Galactic Cosmic Rays with High-Energy Gamma-Rays

Gamma-ray astronomy provides a unique tool to study Galactic cosmic rays far from Earth. Cosmic rays in fact, during their propagation, interact with the interstellar medium, producing high-energy gamma rays that can be detected from Earth. These gamma rays carry the information on the origin and the energetics of the parent cosmic rays at the location of interaction. The detection is facilitated in regions with enhanced gas density, like giant molecular clouds, or in regions with enhanced cosmic-ray flux, for example near an accelerator. In this thesis, I studied both scenarios by analyzing the data accumulated by the Fermi Large Area Telescope between a few tens of MeV up to almost 1 TeV. Firstly,the gamma-ray emission arising from giant molecular clouds, distributed all over the Milky Way, has been investigated, obtaining an unprecedented mapping of the cosmic-ray distribution in the Galaxy. Secondly, the emission that originates in the medium surrounding the supernova remnant W44 has been studied, confirming the presence of recently accelerated particles and unveiling evidence for anisotropic escape. Finally, the potential of the current and future gamma-ray instruments in detecting the radiation emitted from the interstellar medium, both in correspondence of clouds and in the vicinity of accelerators, is evaluated and discussed

On the gamma-ray emission of W44 and its surroundings

We present the analysis of 9.7 years Fermi-LAT data of the middle-aged supernova remnant W44 and the massive molecular gas complex that surrounds it. We derived a high-quality spectral energy distribution of gamma-radiation of the shell over three decades. The very hard spectrum below 1 GeV supports the earlier claims regarding the hadronic origin of radiation. We also confirm the presence of two extended $\gamma$-ray structures located at two opposite edges of the remnant along its major axis. Based on the high-resolution gas maps, we demonstrate that the gamma-ray structures are caused by the enhanced cosmic-ray density rather than the gradient of the gas distribution. We argue that the revealed cosmic-ray "clouds" suggest an anisotropic character of the escape of high-energy particles from the shell along the magnetic field of the remnant

Probing the Cosmic Ray density in the inner Galaxy

The galactic diffuse $\gamma$-ray emission, as seen by Fermi Large Area Telescope (LAT), shows a sharp peak in the region around 4 kpc from the Galactic center, which can be interpreted either as due to an enhanced density of cosmic-ray accelerators or to a modification of the particle diffusion in that region. Observations of $\gamma$-rays originating in molecular clouds are a unique tool to infer the cosmic-ray density point by point, in distant regions of the Galaxy. We report here the analysis of 11 yr Fermi-LAT data, obtained in the direction of nine molecular clouds located in the 1.5--4.5 kpc region. The cosmic-ray density measured at the locations of these clouds is compatible with the locally measured one. We demonstrate that the cosmic-ray density gradient inferred from the diffuse gamma-ray emission is the result of the presence of cosmic-ray accelerators rather than a global change of the sea of Galactic cosmic rays due to their propagation

The contribution of unresolved sources on the gamma-ray spectrum of molecular clouds

Fermi-LAT observations of the large-scale diffuse gamma-ray emission unveiled that in some locations, towards the inner Galaxy, the spectrum of this component is harder than the local one, measured at the Earth position. If this signal is associated with the “truly” diffuse emission produced by the interaction of cosmic rays (CRs) with the interstellar medium (ISM) then its observed spectral features can be interpreted as indirect evidence of a CR spectral hardening toward the Galactic center. However, in order to correctly interpret the data, the contribution from unresolved sources has to be taken into account. Newly developed theoretical models showed that the cumulative flux produced by unresolved pulsar wind nebulae (PWNe) added to the “truly” diffuse emission significantly shapes the spectrum of the large-scale diffuse emission, challenging the CR spectral hardening hypothesis. In light of the recent results, we discuss the effect of unresolved PWNe on the observed spectra of the diffuse emission with a particular focus on molecular clouds (MCs). We analyze the influence of unresolved sources on clouds of different sizes and locations. Finally, we provide a prescription on how to choose the regions to target in order to have an unbiased determination of the “truly” diffuse emission

Gamma-ray emission from molecular clouds with the Cherenkov Telescope Array and the cosmic ray spectrum in the Galaxy

In this work we calculate the diffuse emission from molecular clouds (MCs) in the Galaxy at very high energy (from hundreds of GeV to hundreds of TeV).We present the results from the analysis of the simulated emission of these sources as it will be detected by the incoming imaging Cherenkov telescope detector, the Cherenkov telescope array (CTA) and we estimate the capabilities of such measurements to constrain the cosmic ray spectrum in the cloud. Molecular clouds are regions of the Galaxy, typically a few tens of parsec in size, where the density of cold molecular gas is often orders of magnitude higher than that of the diffuse interstellar medium. The MCs high gas density enhances the gamma-ray emission produced through the hadronic channel by cosmic ray nuclei interacting with the ambient gas. The gamma-ray emission from MCs depends only upon the total mass and the distance of the cloud. Assuming to know them from radio observations, one can thus test the cosmic ray spectrum far away from the Earth, in regions of the Galaxy, where no direct measurements of the cosmic ray spectrum can be carried out. MCs serve then as cosmic ray barometers

The contribution of winds of star clusters to the Galactic cosmic-ray population

Spectral energy distribution and relative uncertainties in the GeV waveband, of the star clusters in the Vela molecular cloud ridge

Detection prospects of Very and Ultra High-Energy gamma rays from extended sources with ASTRI, CTA, and LHAASO

International audienceContext. The recent discovery of several ultra high-energy gamma-ray emitters in our Galaxy constitutes a significant advancement towards unveiling its most powerful accelerators and their properties. Nonetheless, in order to unambiguously locate the regions where the highest energy particles are produced and understand the responsible physical mechanisms, detailed spectral and morphological studies are required, especially given that most of the observed sources were found to be significantly extended. Aims. In these regards, pointing observations with the next-generation Imaging Atmospheric Cherenkov Telescopes, like the Cherenkov Telescope Array (CTA) Observatory and the ASTRI Mini-Array (ASTRI), are expected to provide significant improvements. Here we aim at identifying the most promising sources to target in future observations. Methods. To this purpose, we performed a comparative analysis of the expected performance of ASTRI and CTA, computing their differential sensitivities towards extended sources, and further explored their capabilities with respect to specific case studies, including follow-ups of existing gamma-ray source catalogs. Results. We find that almost all of the sources so far detected by LHAASO-WCDA and HGPS will be in the reach of ASTRI and CTA with 300 and 50 hours of exposure, respectively. For the highest energy emitters detected by LHAASO-KM2A, in turn, we provide the list of the most promising objects to be investigated. We further examined specific classes of sources in order to identify potentially detectable gamma-ray emitters, such as passive molecular clouds (i.e. illuminated by the cosmic-ray sea) and pulsars surrounded by a halo of runaway particles

The contribution of unresolved sources on the gamma-ray spectrum of molecular clouds

International audienceFermi-LAT observations of the large-scale diffuse gamma-ray emission unveiled that in some locations, towards the inner Galaxy, the spectrum of this component is harder than the local one, measured at the Earth position. If this signal is associated with the “truly” diffuse emission produced by the interaction of cosmic rays (CRs) with the interstellar medium (ISM) then its observed spectral features can be interpreted as indirect evidence of a CR spectral hardening toward the Galactic center. However, in order to correctly interpret the data, the contribution from unresolved sources has to be taken into account. Newly developed theoretical models showed that the cumulative flux produced by unresolved pulsar wind nebulae (PWNe) added to the “truly” diffuse emission significantly shapes the spectrum of the large-scale diffuse emission, challenging the CR spectral hardening hypothesis. In light of the recent results, we discuss the effect of unresolved PWNe on the observed spectra of the diffuse emission with a particular focus on molecular clouds (MCs). We analyze the influence of unresolved sources on clouds of different sizes and locations. Finally, we provide a prescription on how to choose the regions to target in order to have an unbiased determination of the “truly” diffuse emission