Characterization of the extreme blazar PG1553+113 with the early data from the Large Size Telescope (LST-1) of the Cherenkov Telescope Array (CTA)

The aim of this work is to characterize the extreme blazar PG1553+113 with the early data from the prototype Large Size Telescope (LST-1), part of the Cherenkov Telescope Array (CTA). The source is a well known high-frequency peaked BL Lac object, deeply studied at all wavelengths from radio to gamma-ray, and with this work also in the very high energy domain. The very luminous source is located at redshift z=0.433 and displays a complex behaviour with different states and variability. Data collected by Fermi-LAT since 2008 showed a signature of a periodic modulation of 2.18 ± 0.08 years in the high energy band, and in addition an intra-day variability of 2.4 ± 0.7ks has recently been found in X-ray data from XMM-Newton. Periodic behaviour or short-time variability has not been observed yet in the Very High Energy band, maybe due to the limited duration of the observation window employed in previous studies. In this work, data collected from January 2021 up to April 2023 are analyzed, with a particular attention for the night of 26th April 2023 during which LST-1 did a deep observation of PG1553+113 lasted 4 hours, since the Fermi-LAT periodicity indicated that the source reached the high emission state during that period. The identification of these rapid fluctuations will provide an essential tool for constraining the emission model and determining the dimensions of the emitting region. Moreover, they are also crucial to investigate compact structures responsible of the emission in the jet, the cooling process and the acceleration mechanisms. Considering the high tecnhological capabilities of the CTA experiment, we expect to obtain significant results in the near future, enabling us to investigate previously unexplored or poorly known regions of the VHE spectrum and further enhance our understanding of VHE astrophysical phenomena.