Robust optical tracking of individual ejecta particles in hypervelocity impact experiments

New insights into the kinematics of ejecta clouds and the dynamics of crater formation are gained from the introduction of an approach to track individual particles ejected from a horizontal hypervelocity impact of a 2 mm aluminum sphere at 6.3km s−1 into vertically aligned Carrara marble. Particle trajectories are determined with 500 ns temporal resolution inside a 1–2 mm thick laser light sheet illuminating a single plane within the ejecta plume. In contrast to optical flow analysis, the methodology presented here enables us to track individual particles instead of relying on field-averaged information. This is realized by correlating particles not via their geometric shape but through their trajectory directly. It robustly identifies even partially obscured or strongly tumbling particles, allowing for a comprehensive physical description of the highly dynamical excavation process based on the precise determination of position, time, and ejection velocity of each individual particle. Specifically, we find ejecta particles launched in a short window of about 0–25 μs after impact and up to a radial distance of 10 mm from the impact location. During this time interval, the transient crater radius grows from 2 ±1 mm to 6 ± 2 mm. Velocities between 70 m s−1 and 1 km s−1 are observed and reveal a substantial steepening of the ejecta curtain within 15 μs after the impact. We additionally determine the particle size and find a μ-parameter of 0.6 for Carrara marble which is consistent with theoretical predictions for nonporous materials

FACT – Performance of the First Cherenkov Telescope Observing with SiPMs

The First G-APD Cherenkov Telescope (FACT) is pioneering the usage of silicon photo mul- tipliers (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 S I PMs on a nightly basis. Over the course of the last five years, FACT has been demonstrating their reliability and excel- lent 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

Higher Order Temperature Dependence of SiPM used in FACT

Solid state photosensors, usually called SiPM or G-APD, seem ideal devices to be used in Imaging Atmospheric Cherenkov Telescopes (IACT). Nevertheless, their temperature dependence poses questions about their suitability in the harsh environment intrinsic to the operation of IACTs. While detailed measurements in the laboratory are possible with some sample sensors, limited data about the performance and uniformity of large samples exist. The First G-APD Cherenkov Telescope (FACT) is pioneering the usage of SiPMs for IACTs. Its camera consists of 1440 SiPMs and it is operated since October 2011 each night when observation conditions permit. Using no temperature stabilization system for the sensors, their temperature is closely coupled to the outside temperature that can change by more than 20 ∘C. While the strong temperature dependence of the gain of the sensors was shown to be easily compensated by adapting the applied voltage, there could also be higher order temperature dependencies of parameters like optical cross-talk, after-pulsing and wavelength dependent photon-detection efficiency. While external calibration devices could be used, one would have to proof that these devices do not have their own temperature dependencies. Instead, we use the constant flux of high energetic cosmic ray particles as calibration device. Their measured flux can depend on variable absorption and scattering of Cherenkov light e.g. due to dust and clouds, as well as on seasonal variations of the atmosphere. Nevertheless, using data sets where the temperature drastically changed within short time periods, we show that temperature dependencies of FACT, including the SiPMs, are well under control.ISSN:1824-803

Single Photon Extraction for FACT’s SiPMs allows for Novel IACT Event Representation

Imaging Atmospheric Cherenkov Telescopes provide large gamma-ray collection areas > 10^4 m^2 and successfully probe the high energetic gamma-ray sky by observing extensive air showers during the night. The First G-APD Cherenkov Telescope (FACT) explores silicon based photoelectric converters (called G-APDs or SiPMs) which provide more observation time with strong moonlight, a more stable photon gain over years of observations, and mechanically simpler imaging cameras. So far, the signal extraction methods used for FACT originate from sensors with no intrinsic quantized responses like photomultiplier tubes. This standard signal extraction is successfully used for the long time monitoring of the gamma-ray flux of bright blazars. However, we now challenge our classic signal extraction and explore single photon extraction methods to take advantage of the highly stable and quantized single photon responses of FACT's SiPM sensors. Instead of having one main pulse with one arrival time and one photon equivalent extracted for each pixel, we extract the arrival times of all individual photons in a pixel’s time line which opens up a new dimension in time for representing extensive air showers with an IACT. In this contribution, we will introduce our novel IACT event representation which is a list of single photon arrival times for each pixel (Photon-Stream). We will discuss our single photon extractor, its performance and its limitations. We will present example events where we identify individual, single air shower Cherenkov photons in the pool of individual, single night sky background photons.ISSN:1824-803

FACT - Searching for periodicity in five-year light-curves of Active Galactic Nuclei

The First G-APD Cherenkov Telescope (FACT) has been monitoring Active Galactic Nuclei (AGN) for the past five years. The use of robust silicon photomultipliers (SiPMs) allows for a continuous, unbiased sampling even during bright-light conditions. This dataset promises insights into the core regions of AGN by investigating the periodicity of the sources. Periodic changes in the flux could indicate a binary nature of the supermassive black holes. A study using the Lomb-Scargle periodogram to find periodicity in monitored AGN is presented. Repeating patterns in the observation times, like moon periods and seasonal effects, affect the analysis by introducing spurious peaks into the periodogram. The zenith-dependence of the observed γ-ray rates further complicate the interpretation. Showing no variability at TeV energies, the γ-ray flux of the Crab Nebula is used to characterize this latter effect, before applying the Lomb-Scargle algorithm.ISSN:1824-803

Using Charged Cosmic Ray Particles to Monitor the Data Quality of FACT

Imaging Air Cherenkov Telescopes (IACT) measure the faint flashes of Cherenkov light emitted by air-showers that are produced when charged particles or gamma rays hit the atmosphere. Therefore, the atmosphere above the IACT is an integral part of the detector. Variations in the performance of the IACT itself, but also changes in the absorption and scattering of Cherenkov light due to clouds or dust affect the interpretation of measured signals. Therefore, information about the status of the full system is crucial to combine measurements from different time periods. The First G-APD Cherenkov Telescope (FACT) is using for the first time solid state photosensors (so-called G-APDs or SiPM) to measure the flashes of Cherenkov light. Based on the stability of these sensors, we showed in the past that it is possible to identify the existence of strong clouds or calima when measuring the intrinsically constant flux of cosmic ray particles at different trigger levels. This necessitated dedicated measurements, preventing normal data taking in parallel. We have now improved the method to use instead those cosmic ray events that are recorded during normal data taking as dominant background. By applying a fixed virtual trigger threshold in software, we measure the rate of charged cosmic ray particles. A deviation from the expected flux allows to identify data sets with reduced performance of the complete system in quasi real-time, without the need for any additional device. Applying the method to a data set when one of the 30 mirror tiles of FACT was missing, we show that a change of total yield of the Cherenkov light by few percent can be identified within few minutes of standard data taking. This nicely demonstrates that the hadron rate determined from standard data taking with FACT can be used for monitoring of the data quality.ISSN:1824-803

FACT - Highlights from more than Five Years of Unbiased Monitoring at TeV Energies

The First G-APD Cherenkov Telescope (FACT) is monitoring blazars at TeV energies. Thanks to the observing strategy, the automatic operation and the usage of solid state photosensors (SiPM, aka G-APDs), the duty cycle of the instrument has been maximized and the observational gaps minimized. This provides a unprecedented, unbiased data sample of almost 9000~hours of data of which 2375 hours were taken in 2016. An automatic quick look analysis provides results with low latency on a public website. More than 40 alerts have been sent in the last three years based on this. To study the origin of the very high energy emission from blazars simultaneous multi-wavelength and multi-messenger observations are crucial to draw conclusions on the underlying emission mechanisms, e.g. to distinguish between leptonic and hadronic models. FACT not only participates in multi-wavelength studies, correlation studies with other instruments and multi-messenger studies, but also collects time-resolved spectral energy distributions using a target-of-opportunity program with X-ray satellites. At TeV energies, FACT provides an unprecedented, unbiased data sample. Using up to 1850 hours per source, the duty cycle of the sources and the characteristics of flares at TeV energies are studied. In the presentation, the highlights from more than five years of monitoring will be summarized including several flaring activities of Mrk 421, Mrk 501 and 1ES 1959+650.ISSN:1824-803