A method to correct IACT data for atmospheric absorption due to the Saharan Air Layer

¹ Universität Würzburg, 97074 Würzburg, Germany e-mail: dorner@astro.uni-wuerzburg.de
² Tuorla Observatory, University of Turku, 21500 Piikkiö, Finland

Received: 20 February 2008
Accepted: 29 July 2008

Abstract

Context. Using the atmosphere as a detector volume, Imaging Air Cherenkov Telescopes (IACTs) depend highly on the properties and the condition of the air mass above the telescope. On the Canary Island of La Palma, where the Major Atmospheric Gamma-ray Imaging Cherenkov telescope (MAGIC) is situated, the Saharan Air Layer (SAL) can cause strong atmospheric absorption affecting the data quality and resulting in a reduced gamma flux.

Aims. To correlate IACT data with other measurements, e.g. long-term monitoring or Multi-Wavelength (MWL) studies, an accurate flux determination is mandatory. Therefore, a method to correct the data for the effect of the SAL is required.

Methods. Three different measurements of the atmospheric absorption are performed on La Palma. From the determined transmission, a correction factor is calculated and applied to the MAGIC data.

Results. The different transmission measurements from optical and IACT data provide comparable results. MAGIC data of PG 1553+113, taken during a MWL campaign in July 2006, were analyzed using the presented method, providing a corrected flux measurement for the study of the spectral energy distribution of the source.