Münster 2017 – scientific programme

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T: Fachverband Teilchenphysik

T 14: Gammaastronomie 1

T 14.7: Talk

Monday, March 27, 2017, 18:20–18:35, H 2

FACT - Towards 100% Observation Efficiency — •Dominik Neise and Thomas Bretz for the FACT collaboration — ETH Zürich, Schweiz

The First G-APD Cherenkov Telescope (FACT) is monitoring the brightest gamma-ray sources since 2012. Due to automatic operation and if weather permits FACT's observation efficiency reaches 93%.

Imaging Atmospheric Cherenkov Telescopes (IACT) typically observe a source displaced from camera center to estimate the hadronic background from locations away from the source position. In order to mitigate effects of camera inhomogeneities, the telescope is periodically repositioned and the trigger thresholds are adjusted to the modified star field. This is called wobble mode.

FACT's camera trigger is devided into patches, whose threshold is regulated to keep the rate per patch at about 0.1Hz. Measuring such low rates with high enough precision needs some time before optimal observation conditions are reached, and can hardly deal with stars moving fast through the field-of-view.

We investigate methods to further increase an IACT's ontime. The orbit observation mode allows to let the source position continuously revolve around the camera center and thus avoids repositioning time. But this causes stars to move faster through the field-of-view than with the wobble mode. A novel threshold regulation entirely based on the supply current of each photon detector can react faster on varying ambient light conditions and allows to omit dedicated threshold adjustments. In this talk, the concept and first tests will be presented.