Atmospheric remote sensing for Cherenkov Telescope Array and its impact on science from large sky survey observations J1-3011

Cosmic very high-energy (VHE, E ≥ 30 GeV) gamma rays carry crucial and unique information

about the most energetic phenomena in the Universe. The most sensitive experimental approach in

VHE gamma-ray astronomy is based on simultaneous imaging of Cherenkov flashes from the VHE

gamma ray induced air showers using multiple telescopes (imaging air Cherenkov telescopes or

IACTs), and reconstruction of the primary gamma-ray properties from those images. The

construction of a new generation IACT facility, the Cherenkov Telescope Array Observatory (CTAO) with design sensitivity improved by at least an order of magnitude compared to existing

VHE instruments and extended energy coverage, will start in 2022 in the legal form of a European

Research Infrastructure Consortium (CTAO ERIC), where Slovenia will be one of the founding

members. CTA is expected to enable the detection of more than 1000 new VHE gamma-ray

sources over the whole sky.

Since the atmosphere is used as a CTA calorimeter, atmospheric monitoring and the understanding

of atmospheric processes is of paramount importance for its operation and the achievement of its

science goals. The evolution of the main aerosol characteristics is extremely complex and requires

the use of remote-sensing instruments, for example a Raman lidar. Laser light emitted by the lidar

directly interacts with the atmospheric constituents and provides in its backscattering return the

footprint of atmospheric ingredients as a function of altitude. A Raman lidar is foreseen in the CTA

baseline configuration, to determine atmospheric transmission from each point along its line-of-sight

to ground up to distances of 30 km, and to provide the required improvement in reducing systematic

uncertainties and increasing the duty cycle for the CTA observatory, compared to the current

generation of Cherenkov telescopes.

Development and implementation of a Raman lidar for CTA is the primary aim of the proposed

project, which directly contributes to the official “CTA Pathfinder” for the CTA North observatory

at La Palma, Spain. The lidar prototype is being developed by an international team consisting

researchers from Institut de Física d’Altes Energies, Spain, Universitat Autònoma de Barcelona,

Spain, University of Nova Gorica, Slovenia and INFN sezione di Padova and Università degli studi

di Padova, Italy. Our lidar device is regarded as the prime candidate for the future in-kindcontribution

of a “Raman lidar” as foreseen in the CTAO technical documentation. The Barcelona

Raman lidar system is therefore a crucial part of the CTA architecture.

In addition to lidar development and its integration into the CTA Observatory the second aim of the

project is to study the atmosphere-induced systematic uncertainties, which are expected to be

particularly important for the all sky surveys with the CTA. These surveys, providing an unbiased

view of the Universe at energies above tens of GeV, will be crucial in the search for the

extragalactic ‘‘dark accelerators’’. To optimize the time needed to perform these surveys, a socalled

”divergent mode” of the CTA was proposed as an alternative observation strategy to the

traditional parallel pointing in order to increase its instantaneous field of view. The project will focus

on performance and systematic uncertainty estimation for possible CTA divergent mode setups.

Both project goals address necessary steps for the construction and early-operation of the CTA and

represent a critical part of Slovenian contribution to CTAO.