FACT - Performance of the first SiPM camera

The First G-APD Cherenkov Telescope (FACT) is the first operational test of the performance of silicon photomultipliers (SiPM) in Cherenkov Astronomy. These novel photo detectors promised to be an inexpensive and robust alternative for vacuum photomultiplier tubes, but had, up to now, never been applied in an Imaging Air shower Cherenkov Telescope (IACT). For more than three years FACT has operated on La Palma, Canary Islands (Spain), with the primary aim of long-term monitoring of astrophysical sources. Stable performance of the photo detectors is crucial and therefore has been studied in great detail. Special care has been taken with regards to their temperature and over voltage dependence through implementation of a feedback method for keeping their properties stable. Several independent long term measurements were conducted to analyze and verify SiPM gain stability. Dark count spectra, which also make for an excellent self calibration mechanism, were used to study and correct for temperature dependencies. Rate scans make it possible to derive a method, for quickly finding appropriate trigger thresholds by measuring pixel currents, and thus allow for a consistent data acquisition rate. Dedicated measurements using an LED flasher are used to study the correct application of SiPM bias voltages. In this paper, the results of the long term studies will be presented and the applicability of SiPMs in IACTs for long term monitoring will be shown.SCOPUS: cp.pinfo:eu-repo/semantics/publishe

FACT - TeV flare alerts triggering multi-wavelength observations

Active galactic nuclei show variability on time scales ranging from minutes to decades. The radiation from these extreme objects spans many orders of magnitude along the whole electromagnetic spectrum. The spectral energy distribution shows two peaks, where for the subgroup of blazars the first peak is in the radio to X-ray regime, while the second peak is in the gamma-ray regime. Due to the extreme variability and the wide spectral range, simultaneous multi-wavelength observations are vital to understand the underlying physics. Furthermore, long-term monitoring is crucial to obtain an unbiased data sample. While for the measurements of the low-energy peak, many instruments are available, the data at TeV energies are sparse. The First G-APD Cherenkov Telescope (FACT) is a gamma-ray telescope dedicated to the long-term monitoring of bright TeV blazars. Operational since October 2011, it has collected more than three years of data from a dedicated sample of sources. The results of an automatic quick look analysis are publicly available on a website the same night. Based on this, other instruments are informed in case of a high flux state and target-of-opportunity observations are carried out. In the previous year, seven flare alerts have been sent to the community and several periods of strong variability have been observed for the blazars Mrk 421 and Mrk 501.SCOPUS: cp.pinfo:eu-repo/semantics/publishe

FACT - Status and experience from three years operation of the first SiPM camera

The First G-APD Cherenkov Telescope (FACT) is pioneering the usage of solid state photosensors (G-APD, also known as SiPM). The 1440 pixel camera is installed in a 9.5 m2 refurbished HEGRA telescope on the Canary Island La Palma. Physics data-taking with FACT started in October 2011, a few hours after installation of the camera. Since Summer 2012, FACT is operated remotely without the need of a data-taking crew on site. During more than three years of operation of FACT, G-APDs have proven to be very reliable. Despite operating them regularly also under very strong moonlight conditions, the GAPDs show no change in their properties or any indication for aging. This allows FACT to have a successful monitoring program of the brightest TeV blazars in the Northern hemisphere and several flare-alerts have been sent to the community. This proceeding summarizes the history and status of FACT as well as reporting the lessons learned about the usage of SiPM in a Cherenkov telescope from the construction and operation of FACT.SCOPUS: cp.pinfo:eu-repo/semantics/publishe

FACT - Novel mirror alignment using bokeh and enhancement of the VERITAS SCCAN alignment method

Imaging Air Cherenkov Telescopes, including the First G-APD Cherenkov Telescope (FACT), use segmented reflectors. These offer large and fast apertures for little resources. However, one challenge is the alignment of the mirrors to gain a sharp image. For Cherenkov telescopes, high spatial and temporal resolution is crucial to reconstruct air shower events. Therefore one has to align the individual mirror positions and orientations precisely. Alignment is difficult due to the large number of degrees of freedom and, because most techniques involve a star, has to be done during good weather nights which overlaps with observation time. We present the mirror alignment of FACT, done using two methods. Firstly, we show a new method which we call Bokeh alignment. This method is simple, cheap and can even be done during daytime. Secondly, we demonstrate an enhancement of the SCCAN method by F. Arqueros et al. and first implemented by the McGill VERITAS group. Using a second camera, our enhanced SCCAN is optimized for changing weather, changing zenith distance, and changing reference stars. Developed off site in the lab on a 1/10th scale model of FACT, both our methods resulted in a highly telescope independent procedure, e.g. both our methods run without communication to the telescope's drive. We compare alignment results by using the point spread function of star images, ray tracing simulations, and overall muon rates before and after the alignment.SCOPUS: cp.pinfo:eu-repo/semantics/publishe

FACT-tools - Streamed real-time data analysis

The First G-APD Cherenkov telescope (FACT) is dedicated to monitor bright TeV blazars in the northern sky. The use of silicon photon detectors allows for a larger duty cycle, which results in a huge amount of collected data (800 GB/night). In order to satisfy its monitoring purpose, changes in the flux of the observed sources have to be registered without delay. This requires a data analysis that provides physical results at a rate that is comparable to the trigger rate of 80Hz. The recently developed data analysis software FACT-Tools aims to accomplish these requirements in real-time. It is implemented based on of the streams-framework, which was developed at Dortmund's collaborative research center for resource-constrained data analysis (SFB 876). Streams delivers an easy-to-use abstraction layer to design analysis processes by use of human readable XML files, which also make the analysis reproducible. Multi-source processes (e.g. simultaneous analyses of data from several telescopes) and multi-core processes (parallelization) are already included in the streams-framework. Therefore, Streams is an ideal framework for use in gamma-ray astronomy. The FACT-Tools are an extension library for the streams-framework with analysis methods for Cherenkov telescopes. The collection of methods is ranging from RAWdata handling and calibration up to image parameter extraction and Gamma-Proton classification. The latter is performed by an online application of a random forest classifier, which in turn, allows for an adaptation in other tasks e.g. image cleaning or online estimation of the energy spectrum. In this contribution we want to present the features of FACT-Tools and the streams-framework alongside with their performance measured on the data from the FACT Cherenkov telescope.SCOPUS: cp.pinfo:eu-repo/semantics/publishe

FACT - First energy spectrum from a SiPM cherenkov telescope

The First G-APD Cherenkov Telescope (FACT) is an Imaging Air Cherenkov Telescope located on the Canary Island of La Palma. It is the first of its kind which uses Geiger-mode Avalanche Photo Diodes (G-APDs) as photosensors to detect the Cherenkov radiation emitted from secondary particles in a high-energy gamma-ray air shower. A new analysis chain has been developed using modern data mining methods and unfolding techniques to obtain the energy spectrum of an observed source. This analysis chain has been applied to data of the Crab Nebula, the so called 'standard candle' in Cherenkov astronomy. Here, the individual steps are described and results of this application are reported providing the energy spectrum and light curve.SCOPUS: cp.pinfo:eu-repo/semantics/publishe

FACT - Calibration of imaging atmospheric cherenkov telescopes with muon rings

The First G-APD Cherenkov Telescope (FACT) is an Imaging Air Cherenkov Telescope (IACT) located on the Canary Island of La Palma. Its goal is to provide long term monitoring of cosmic objects like Active Galactic Nuclei. FACT is the first IACT to use Silicon Photomultipliers instead of conventional Photo Multiplier Tubes. Therefore studying the detector properties is especially important. Muon Rings are an event class that can offer valuable insight into the detector. These images are created by single muons that hit the mirror of the telescope. Due to their unique shape, one can use pattern recognition techniques like the Hough-Transform to identify these events. The well-known properties of muon rings and their small photon arrival time distribution allow to estimate the time resolution, photon detection efficiency and optical Point-Spread-Function of the telescope. In this contribution, the multivariate method using a random forest to extract muons from FACT data will be presented. Also the first results, including the time resolution of FACT and the impact of the mirror alignment in May 2014 on the ring width of muon events, are shown.SCOPUS: cp.pinfo:eu-repo/semantics/publishe

FACT - Influence of sipm crosstalk on the performance of an operating cherenkov telescope

The First G-APD Cherenkov Telescope (FACT) is the first operational telescope of its kind, employing a camera equipped with silicon photon detectors (G-APD aka. SiPM). SiPMs have a high photon detection efficiency (PDE), while being more robust to bright light conditions than the commonly used photo-multiplier tubes. This technology has allowed us to increase the duty cycle beyond that of the current generation of imaging air Cherenkov telescopes. During the last four years, the operation of FACT has proven that SiPMs are a suitable photon detectors for an application in the field of ground-based gamma-ray astronomy. Nevertheless, it has been argued that crosstalk, after-pulses and dark counts are the main drawback of SiPMs, as these effects produce photon-like signals that would add up the signal background. Consequently, it is necessary to understand their impact on the analysis of data from FACT. In this contribution, we will show the current status of a study about the influence of different settings of crosstalk and dark counts on the performance of FACT. For that purpose, Monte Carlo simulations are used and compared to the actual data from the SiPM camera of FACT.SCOPUS: cp.pinfo:eu-repo/semantics/publishe