Detecting diffuse emission in the VHE domain

Diffuse emission is the most prominent observational signature from the sky at Gigaelectronvolt (GeV) energies. Galactic diffuse emission was established before individual gamma-ray sources started to emerge and constitute a prime source of knowledge about cosmic-ray particle interactions and radiation processes ever since. Diffuse GeV gamma-ray emission still constitutes the systematic limit of source detection near instrumental threshold. In contrast to the GeV domain the search for diffuse emission at Teraelectronvolt (TeV) energies is still in its infancy, largely due to the predominant charged particle background that constitutes a principal instrumental challenge of the atmospheric Cherenkov technique. Diffuse emission is expected in the VHE domain, too: on Galactic scale primarily from hadronic particle interactions with interstellar gas and Inverse Compton scattering of high energy electrons with interstellar radiation fields, but also when encountering intense radiation fields or dense molecular clouds in the local vicinity of cosmic accelerators. Both processes are indicative for particle escape from their acceleration regions. This last, most energetic window for astronomical investigation, the domain of Very High Energies (VHE) gamma-rays, was unveiled by the systematic observations with the H.E.S.S. telescope array, a breakthrough recognized by the award of the Descartes Prize in 2006 and Rossi Prize in 2010. One of the major achievements of H.E.S.S. was the survey of the inner regions of our Galaxy, which led to the discovery of more than 50 new energetic sources. The proposed project aims at establishing the existence, spatial and spectral signature of diffuse emission at TeV energies. H.E.S.S. observations are to be compared with predictions from a model of diffuse VHE emission that will be specifically developed in the project. On the instrumental side, the investigation will push the limits of atmospheric Cherenkov imaging in sensitivity and energy through the development of more precise reconstruction techniques, and more effective background subtraction methods. Advanced modelling of the isotropic charged particle background and development of a likelihood-based analysis technique is proposed, the latter being a novelty for investigating VHE data. Systematics induced by the geomagnetic field and inhomogeneities of the night sky background on the instrument response will be addressed with particular care. The construction of a model of diffuse emission at TeV energies appears to be demanding due to competing phenomena, such as the energy- dependent escape of charged particles from the acceleration region vs. particle transport on larger scales inside our Galaxy. Detection and study of diffuse VHE emission will constitute a major scientific breakthrough, allowing the community to further understand particle propagation in the Galaxy up to the knee (1015 eV) and how particles are released into the interstellar medium. It will allow a closer connection to GeV measurements, benefiting from orthogonal observational techniques satellite-based direct pair conversion vs. ground-based indirect air shower detections deployed on a large scale, non-source related investigation. Consequently, the intensity and energy dependence of different constituents of the diffuse emission will extend our understanding of common physics processes to the most energetic end of the electromagnetic spectrum. Through an assessment of the irreducible background it will prepare the advent of the Cherenkov Telescope Array by establishing the hard detection limit for gamma-ray sources and will allow investigation of a putative dark matter component in the suspected WIMP rest mass region. The project results will allow generalizing from single-source detection to source population studies, and, for the first time, estimating the unresolved source component in a comprehensive way.

Diffuse emission is the most prominent observational signature from the sky at Gigaelectronvolt (GeV) energies, and was established before individual gamma-ray sources started to emerge. It became a prime source of knowledge about cosmic-ray particle interactions and radiation processes at very high energies. In contrast to the GeV domain, the search for diffuse emission at Teraelectronvolt (TeV) energies is still in its infancy, largely due to the predominant charged particle background that constitutes the principal instrumental challenge for observational techniques. Diffuse emission is however expected in the TeV-domain too, on the Galactic scale primarily from the interaction of protons and nuclei with the interstellar gas, and by Inverse Compton scattering of high energy electron on the interstellar radiation field. Local emission is also predicted in the vicinity of cosmic accelerators in dense molecular clouds or locally enhanced radiation fields, the emission being indicative for particle escape from their acceleration region. The establishment and characterization of this diffuse emission is the core of the project, which requires the development of sophisticated modeling and analysis tools. Two complementary aspects were necessary to advance the project: The construction of an adequate model of diffuse emission at VHE energies (since characteristics scales of particle propagation are significantly smaller than at energies sampled at lower energies) and the development of more precise reconstruction and background subtraction techniques, which subsequently allows pushing forward the limits of the technique, both in terms of sensitivity and resolution. The project relies on a realistic simulation of the telescope behavior, significantly more advanced beyond what was done before in the domain, as well as a completely new analysis technique. The project resulted in major breakthroughs in its two main aspects. New modeling of particle propagation in the Milky Way, taking into account more accurately its actual geometry as well as the matter and radiation distribution, allowed for providing suitable predictions (diffuse emission templates). These predictions became already part of the science preparations for even next- generation instruments like CTA. On the instrumental side, the development and application of a new simulation paradigm allowed pushing forward the limits in the VHE domain, and yielded several, high impact scientific results, which were unreachable until now. The project resulted in several refereed publications, and presentations at renowned international conferences, some of them by invitation for plenary session. The PICARD code became known to the community. Our developed simulation, reconstruction and analysis software has been delivered to the HESS collaboration. Publications based on this analysis framework are now submitted to the journal (First measurement of the extension of the Crab Nebula), or being drafted for submission (First resolved analysis of the VHE emission of the Radio-Galaxy Centaurus A). The ultimate goal of the project - the detailed study of diffuse gamma-ray emission in the TeV domain - is presently investigated in a collaborative effort using data taken by the H.E.S.S. experiment and being analyzed with the framework and model developed within this joint Austrian- French research project.