FACT – Performance of the First Cherenkov Telescope Observing with SiPMs

Abstract

The First G-APD Cherenkov Telescope (FACT) is pioneering the usage of silicon photo multipliers (SIPMs also known as G-APDs) for the imaging atmospheric Cherenkov technique. It is located at the Observatorio Roque de los Muchachos on the Canary island of La Palma. Since first light in October 2011, it is monitoring bright TeV blazars in the northern sky. By now, FACT is the only imaging atmospheric Cherenkov telescope operating with SIPMs on a nightly basis. Over the course of the last five years, FACT has been demonstrating their reliability and excellent performance. Moreover, their robustness allowed for an increase of the duty cycle including nights with strong moon light without the need for UV-filters. In this contribution, we will present the performance of the first Cherenkov telescope using solid state photo sensors, which was determined in analysis of data from Crab Nebula, the so called standard candle in gamma-ray astronomy. The presented analysis chain utilizes modern data mining methods and unfolding techniques to obtain the energy spectrum of this source. The characteristical results of such an analysis will be reported providing, e. g., the angular and energy resolution of FACT, as well as, the energy spectrum of the Crab Nebula. Furthermore, these results are discussed in the context of the performance of coexisting Cherenkov telescopes.

Ordinal Quantification Through Regularization

“…Regularized Unfolding (RUN) [6,5] is used by physicists for decades [27,2]. It estimates the vector p of class prevalence values by minimizing a loss function L : R n → R over the estimate p; L consists of two terms, i.e., a negative loglikelihood term to model the error of p, and a regularization term to model the plausibility of p.…”

“…We focus on these two methods due to their long-standing popularity within physics research. In fact, they are among the first methods that have been proposed in this field, and are still widely adopted today, in astro-particle physics [27,2], high-energy physics [1], and more recently in quantum computing [24]. Moreover, RUN and IBU already cover the most important aspects of unfolding methods with respect to ordinal quantification.…”

Silicon Photo-Multipliers

“…However, it was also realised that different applications need different optimisations as discussed in [1]. Today SiPMs are used in fields as diverse as nuclear and particle physics, use for IACTs has been pioneered by the construction of FACT [6]. The FACT camera consisting of 1440 SiPMs with 3660 SPADs each has a diameter of 40 cm.…”

FACT - Time-Resolved Blazar SEDs

“…It pioneered the usage of silicon based photosensors (SiPMs, aka Geiger-Mode Avalanche Photo Diodes, G-APDs) in ground-based gammaray astronomy. The excellent performance of SiPMs is highlighted in [1] and the performance of the telescope is discussed in [2]. As discussed in [3], SiPMs are ideally suited for the long-term monitoring and the observation strategy of the FACT project provides an unprecedented, unbiased data sample of blazars at TeV energies.…”

“…different background suppression). Calculating fluxes using Monte Carlo simulations as described in [2] requires a huge amount of simulations taking into account different zenith distances and night sky background conditions. To facilitate a quick estimate of the fluxes for the QLA, the excess rates of the Crab Nebula have been studied for periods of constant performance of the whole system.…”