Aims. We aim to characterize the multiwavelength emission from Markarian 501 (Mrk 501), quantify the energy-dependent variability, study the potential multiband correlations, and describe the temporal evolution of the broadband emission within leptonic theoretical scenarios. Methods. We organized a multiwavelength campaign to take place between March and July of 2012. Excellent temporal coverage was obtained with more than 25 instruments, including the MAGIC, FACT and VERITAS Cherenkov telescopes, the instruments on board the Swift and Fermi spacecraft, and the telescopes operated by the GASP-WEBT collaboration. Results. Mrk 501 showed a very high energy (VHE) gamma-ray flux above 0.2 TeV of ∼0.5 times the Crab Nebula flux (CU) for most of the campaign. The highest activity occurred on 2012 June 9, when the VHE flux was ∼3 CU, and the peak of the high-energy spectral component was found to be at ∼2 TeV. Both the X-ray and VHE gamma-ray spectral slopes were measured to be extremely hard, with spectral indices < 2 during most of the observing campaign, regardless of the X-ray and VHE flux. This study reports the hardest Mrk 501 VHE spectra measured to date. The fractional variability was found to increase with energy, with the highest variability occurring at VHE. Using the complete data set, we found correlation between the X-ray and VHE bands; however, if the June 9 flare is excluded, the correlation disappears (significance < 3σ) despite the existence of substantial variability in the X-ray and VHE bands throughout the campaign. Conclusions. The unprecedentedly hard X-ray and VHE spectra measured imply that their low- and high-energy components peaked above 5 keV and 0.5 TeV, respectively, during a large fraction of the observing campaign, and hence that Mrk 501 behaved like an extreme high-frequency-peaked blazar (EHBL) throughout the 2012 observing season. This suggests that being an EHBL may not be a permanent characteristic of a blazar, but rather a state which may change over time. The data set acquired shows that the broadband spectral energy distribution (SED) of Mrk 501, and its transient evolution, is very complex, requiring, within the framework of synchrotron self-Compton (SSC) models, various emission regions for a satisfactory description. Nevertheless the one-zone SSC scenario can successfully describe the segments of the SED where most energy is emitted, with a significant correlation between the electron energy density and the VHE gamma-ray activity, suggesting that most of the variability may be explained by the injection of high-energy electrons. The one-zone SSC scenario used reproduces the behavior seen between the measured X-ray and VHE gamma-ray fluxes, and predicts that the correlation becomes stronger with increasing energy of the X-rays.
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.
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.
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.
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
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