FACT -- The G-APD revolution in Cherenkov astronomy

Since two years, the FACT telescope is operating on the Canary Island of La Palma. Apart from its purpose to serve as a monitoring facility for the brightest TeV blazars, it was built as a major step to establish solid state photon counters as detectors in Cherenkov astronomy. The camera of the First G-APD Cherenkov Telesope comprises 1440 Geiger-mode avalanche photo diodes (G-APD), equipped with solid light guides to increase the effective light collection area of each sensor. Since no sense-line is available, a special challenge is to keep the applied voltage stable although the current drawn by the G-APD depends on the flux of night-sky background photons significantly varying with ambient light conditions. Methods have been developed to keep the temperature and voltage dependent response of the G-APDs stable during operation. As a cross-check, dark count spectra with high statistics have been taken under different environmental conditions. In this presentation, the project, the developed methods and the experience from two years of operation of the first G-APD based camera in Cherenkov astronomy under changing environmental conditions will be presented.Comment: Proceedings of the Nuclear Science Symposium and Medical Imaging Conference (IEEE-NSS/MIC), 201

FACT - The First G-APD Cherenkov Telescope: Status and Results

The First G-APD Cherenkov telescope (FACT) is the first telescope using silicon photon detectors (G-APD aka. SiPM). It is built on the mount of the HEGRA CT3 telescope, still located at the Observatorio del Roque de los Muchachos, and it is successfully in operation since Oct. 2011. The use of Silicon devices promises a higher photon detection efficiency, more robustness and higher precision than photo-multiplier tubes. The FACT collaboration is investigating with which precision these devices can be operated on the long-term. Currently, the telescope is successfully operated from remote and robotic operation is under development. During the past months of operation, the foreseen monitoring program of the brightest known TeV blazars has been carried out, and first physics results have been obtained including a strong flare of Mrk501. An instantaneous flare alert system is already in a testing phase. This presentation will give an overview of the project and summarize its goals, status and first results

Herbivore-induced terpenoid emission in Medicago truncatula: concerted action of jasmonate, ethylene and calcium signaling

Plant volatiles emitted by Medicago truncatula in response to feeding larvae of Spodoptera exigua are composed of a complex blend of terpenoids. The cDNAs of three terpene synthases (TPSs), which contribute to the blend of terpenoids, were cloned from M. truncatula. Their functional characterization proved MtTPS1 to be a β-caryophyllene synthase and MtTPS5 to be a multi-product sesquiterpene synthase. MtTPS3 encodes a bifunctional enzyme producing (E)-nerolidol and geranyllinalool (precursors of C11 and C16 homoterpenes) from different prenyl diphosphates serving as substrates. The addition of jasmonic acid (JA) induced expression of the TPS genes, but terpenoid emission was higher from plants treated with JA and the ethylene precursor 1-amino-cyclopropyl-1-carboxylic acid. Compared to infested wild-type M. truncatula plants, lower amounts of various sesquiterpenes and a C11–homoterpene were released from an ethylene-insensitive mutant skl. This difference coincided with lower transcript levels of MtTPS5 and of 1-deoxy-d-xylulose-5-phosphate synthase (MtDXS2) in the damaged skl leaves. Moreover, ethephon, an ethylene-releasing compound, modified the extent and mode of the herbivore-stimulated Ca2+ variations in the cytoplasm that is necessary for both JA and terpene biosynthesis. Thus, ethylene contributes to the herbivory-induced terpenoid biosynthesis at least twice: by modulating both early signaling events such as cytoplasmic Ca2+-influx and the downstream JA-dependent biosynthesis of terpenoids