Research Division Seminar
Clouds vs Cherenkov telescopes: problems and solutions.

Abstract

Very-high-energy (VHE >~100 GeV) gamma rays are absorbed in Earth's
atmosphere and thus cannot be detected directly on Earth. Their fluxes
are also typically too low to efficiently study them with satellite
instruments. A VHE gamma ray entering the atmosphere initiates an
electromagnetic cascade that induces faint flashes of blueish
Cherenkov light. Such flashes can be then detected by Imaging
Atmospheric Cherenkov Telescopes registering images of passage of
individual gamma rays through the atmosphere.
The usage of atmosphere as a part of the detector allows us to achieve
a collection area of gamma rays over two orders of magnitude higher
than the physical size of the detector. But it also introduces
systematic errors connected with the atmosphere's transparency. In
particular, cloud presence during the observations can significantly
affect the data. In this seminar I will cover different methods used
to correct the influence of the clouds. I will show how lack of such a
correction introduces bias in the energy estimation of gamma rays. I
will present how the affected images of showers are degraded and thus
can be confused with background events, lowering the collection area
of the telescope. Finally, I will show a novel method of correcting
the influence of the clouds already at the image level, and discuss
the possibility of measuring the parameters of a cloud directly with
the observations by the Cherenkov telescopes.