A novel camera type for very high energy gamma-ray astronomy based on Geiger-mode avalanche photodiodes

Geiger-mode avalanche photodiodes (G-APD) are promising new sensors for light detection in atmospheric Cherenkov telescopes. In this paper, the design and commissioning of a 36-pixel G-APD prototype camera is presented. The data acquisition is based on the Domino Ring Sampling (DRS2) chip. A sub-nanosecond time resolution has been achieved. Cosmic-ray induced air showers have been recorded using an imaging mirror setup, in a self-triggered mode. This is the first time that such measurements have been carried out with a complete G-APD camera.Comment: 9 pages with 11 figure

A Site Evaluation Campaign for a Ground Based Atmospheric Cherenkov Telescope in Romania

Around the world, several scientific projects share the interest of a global network of small Cherenkov telescopes for monitoring observations of the brightest blazars - the DWARF network. A small, ground based, imaging atmospheric Cherenkov telescope of last generation is intended to be installed and operated in Romania as a component of the DWARF network. To prepare the construction of the observatory, two support projects have been initiated. Within the framework of these projects, we have assessed a number of possible sites where to settle the observatory. In this paper we submit a brief report on the general characteristics of the best four sites selected after the local infrastructure, the nearby facilities and the social impact criteria have been applied.Comment: 6 pages, 5 Postscript figure

Hox-controlled reorganisation of intrasegmental patterning cues underlies Drosophila posterior spiracle organogenesis

10 páginas, 8 figuras. Material complementario del artículo esta disponible en http://dev.biologists.org/cgi/content/full/132/13/3093/DC1Hox proteins provide axial positional information and control segment morphology in development and evolution. Yet how they specify morphological traits that confer segment identity and how axial positional information interferes with intrasegmental patterning cues during organogenesis remain poorly understood. We have investigated the control of Drosophila posterior spiracle morphogenesis, a segment-specific structure that forms under Abdominal-B (AbdB) Hox control in the eighth abdominal segment (A8). We show that the Hedgehog (Hh), Wingless (Wg) and Epidermal Growth Factor Receptor (Egfr) pathways provide specific inputs for posterior spiracle morphogenesis and act in a genetic network made of multiple and rapidly evolving Hox/signalling interplays. A major function of AbdB during posterior spiracle organogenesis is to reset A8 intrasegmental patterning cues, first by reshaping wg and rhomboid expression patterns, then by reallocating the Hh signal and later by initiating de novo expression of the posterior compartment gene engrailed in anterior compartment cells. These changes in expression patterns confer axial specificity to otherwise reiteratively used segmental patterning cues, linking intrasegmental polarity and acquisition of segment identity.This work was supported by the `Centre National de la Recherche Scientifique' (CNRS), grants from `la Ligue Nationale Contre Le Cancer (équipe labellisée La Ligue)', `l'Association pour la Recherche contre le Cancer' (ARC), The Royal Society, The Welcome Trust, the `Minesterio de education y ciencia (BFU 2004 0 96) and ARC and EMBO long term fellowships to S. Merabet.Peer reviewe

Calibration and performance of the photon sensor response of FACT -- The First G-APD Cherenkov telescope

The First G-APD Cherenkov Telescope (FACT) is the first in-operation test of the performance of silicon photo detectors in Cherenkov Astronomy. For more than two years it is operated on La Palma, Canary Islands (Spain), for the purpose of long-term monitoring of astrophysical sources. For this, the performance of the photo detectors is crucial and therefore has been studied in great detail. Special care has been taken for their temperature and voltage dependence implementing a correction method to keep their properties stable. Several measurements have been carried out to monitor the performance. The measurements and their results are shown, demonstrating the stability of the gain below the percent level. The resulting stability of the whole system is discussed, nicely demonstrating that silicon photo detectors are perfectly suited for the usage in Cherenkov telescopes, especially for long-term monitoring purpose

An empirical analysis of the Swiss generalized system of preferences

The progressively introduced DFQFMA for LDCs has a positive effect on the size of LDCs preferential exports to Switzerland. Consequently, the DFQFMA has considerably improved market access for the worlds poorest countries. Eliminating tariffs (progressively) causes preferential agro-food and textile exports of LDCs to rise substantially. However, it has to be remarked that the success of the DFQFMA is limited to the agro-food and textile sectors and to a few countries. The descriptive analysis of agro-food exports indicates that trade liberalization is a success story merely for a few LDCs, namely Tanzania, Ethiopia, Côte dIvoire, Mozambique, Malawi, Senegal and Uganda. Those seven countries capture a total share of nearly 80 percent of LDCs agro-food exports to Switzerland. In the textile sector we observe an even higher degree of market concentration concerning LDCs preferential exports. Here, three countries, namely Bangladesh, Cambodia and Nepal, account for 98 percent of LDCs preferential textile exports. It also can be noted that the GSP is a useful supplement to duty-free tariffs (duty-free market access) under the WTO regime. For instance, 100 percent of LDCs agro-food exports from 2002 to 2011 entered Switzerland under reduced or duty-free tariffs. However, the share of preferential exports under the GSP was on average only 36 percent. In this context, the preference margin, which represents the main incentive to export under preferential conditions, compensates the costs of compliance associated with the GSP and yields an additional benefit for the importer has a consistent and positive effect on the level of the utilization rate. In particular, the application of the Heckman´selection model in article no. 1 makes clear that once trade contracts are established and an exporter has overcome bureaucratic obstacles in the form of proof of origin and proof of direct shipment, the preference margin appears as the main incentive to export under preferential conditions granted by the GSP. While the effect of the size of GSP eligible trade has a positive and significant effect in the case of the PPML estimations, the effect turned negative when the sample was restricted to positive values of the utilization rate in the case of the outcome equation of the Heckman selection model. This finding encourages our confidence that the preference margin acts as the main incentive for exporting under preferential conditions. However, to benefit from these preferential tariffs, the institutional quality of a given DC or LDC is of crucial importance. Additionally, we address the question of whether reciprocal trade preferences are more beneficial for DCs compared to non-reciprocal trade preferences. Because trade preferences under the Swiss GSP are offered to the country group of DCs as a whole, non-reciprocal trade preferences are not tailored to the export structure of a particular DC. Consequently, by switching from non-reciprocal to negotiated reciprocal trade preferences, DCs such as Tunisia expect to negotiate terms which are tailored to their export structure and better conditions than competitors from countries which are still beneficiaries of the GSP. The Tunisian case study reveals that the switch from the GSP to an FTA causes no significant advantage in most of the export sectors. This implies that switching from non-reciprocal to reciprocal trade preferences yields advantages in export sectors where Tunisia has comparative cost advantages. This is especially true for the textile sector and partly so for the agro-food sector

FACT -- Operation of the First G-APD Cherenkov Telescope

Since more than two years, the First G-APD Cherenkov Telescope (FACT) is operating successfully at 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 aka. MPPC or SiPM) for photon detection. Since properties as the gain of G-APDs depend on temperature and the applied voltage, a real-time feedback system has been developed and implemented. To correct for the change introduced by temperature, several sensors have been placed close to the photon detectors. Their read out is used to calculate a corresponding voltage offset. In addition to temperature changes, changing current introduces a voltage drop in the supporting resistor network. To correct changes in the voltage drop introduced by varying photon flux from the night-sky background, the current is measured and the voltage drop calculated. To check the stability of the G-APD properties, dark count spectra with high statistics have been taken under different environmental conditions and been evaluated. The maximum data rate delivered by the camera is about 240 MB/s. The recorded data, which can exceed 1 TB in a moonless night, is compressed in real-time with a proprietary loss-less algorithm. The performance is better than gzip by almost a factor of two in compression ratio and speed. In total, two to three CPU cores are needed for data taking. In parallel, a quick-look analysis of the recently recorded data is executed on a second machine. Its result is publicly available within a few minutes after the data were taken. [...]Comment: 19th IEEE Real-Time Conference, Nara, Japan (2014

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

First bounds on the very high energy gamma-ray emission from Arp 220

Using the Major Atmospheric Gamma Imaging Cherenkov Telescope (MAGIC), we have observed the nearest ultra-luminous infrared galaxy Arp 220 for about 15 hours. No significant signal was detected within the dedicated amount of observation time. The first upper limits to the very high energy $\gamma$-ray flux of Arp 220 are herein reported and compared with theoretical expectations.Comment: Accepted for publication in Ap

Constraints on the steady and pulsed very high energy gamma-ray emission from observations of PSR B1951+32/CTB 80 with the MAGIC Telescope

We report on very high energy gamma-observations with the MAGIC Telescope of the pulsar PSR B1951+32 and its associated nebula, CTB 80. Our data constrain the cutoff energy of the pulsar to be less than 32 GeV, assuming the pulsed gamma-ray emission to be exponentially cut off. The upper limit on the flux of pulsed gamma-ray emission above 75 GeV is 4.3*10^-11 photons cm^-2 sec^-1, and the upper limit on the flux of steady emission above 140 GeV is 1.5*10^-11 photons cm^-2 sec^-1. We discuss our results in the framework of recent model predictions and other studies.Comment: 7 pages, 7 figures, replaced with published versio

Systematic search for VHE gamma-ray emission from X-ray bright high-frequency BL Lac objects

All but three (M87, BL Lac and 3C 279) extragalactic sources detected so far at very high energy (VHE) gamma-rays belong to the class of high-frequency peaked BL Lac (HBL) objects. This suggested to us a systematic scan of candidate sources with the MAGIC telescope, based on the compilation of X-ray blazars by Donato et al. (2001). The observations took place from December 2004 to March 2006 and cover sources on the northern sky visible under small zenith distances zd < 30 degrees at culmination. The sensitivity of the search was planned for detecting X-ray bright F(1 keV) > 2 uJy) sources emitting at least the same energy flux at 200 GeV as at 1 keV. In order to avoid strong gamma-ray attenuation close to the energy threshold, the redshift of the sources was constrained to values z<0.3. Of the fourteen sources observed, 1ES 1218+304 and 1ES 2344+514 have been detected in addition to the known bright TeV blazars Mrk 421 and Mrk 501. A marginal excess of 3.5 sigma from the position of 1ES 1011+496 was observed and has been confirmed as a source of VHE gamma-rays by a second MAGIC observation triggered by a high optical state (Albert et al. 2007). For the remaining sources, we present here the 99% confidence level upper limits on the integral flux above ~200 GeV. We characterize the sample of HBLs (including all HBLs detected at VHE so far) by looking for correlations between their multi-frequency spectral indices determined from simultaneous optical, archival X-ray, and radio luminosities, finding that the VHE emitting HBLs do not seem to constitute a unique subclass. The absorption corrected gamma-ray luminosities at 200 GeV of the HBLs are generally not higher than their X-ray luminosities at 1 keV.Comment: 15 pages, 7 figures, 5 tables, submitted to ApJ (revised version