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

Mahlke, Max; Adam, Jan; Ahnen, M. L.; Baack, Dominik; Balbo, Matteo; Biland, A.; Blank, Michael; Bretz, T.; Bruegge, Kai; Buß, Jens; Dmytriiev, A.; Dorner, D.; Einecke, S.; Elsäesser, D.; Hempfling, Christina; Herbst, Thomas; Hildebrand, D.; Kortmann, L.; Linhoff, Lena; Mannheim, K.; Mueller, Susanne A.; Neise, Dominik; Neronov, A.; Noethe, Maximilian; Oberkirch, J.; Paravac, Aleksander; Pauß, F.; Rhode, W.; Schleicher, Bernd; Schulz, F.; Shukla, Amit; Sliusar, Vitalii; Thaele, J.; Walter, R.

doi:10.22323/1.301.0612

Cited by 16 publications

(17 citation statements)

References 5 publications

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“…The FACT collaboration has presented a preliminary study on the zenith angle dependence of the trigger rate of an imaging air-Cherenkov telescope [8] which was the basis for this study. For the presented study, the cosmic ray rates obtained with an artificial software trigger and the gamma-ray excess rates of data taken from the Crab Nebula were presented.…”

Section: Fitting the Datamentioning

confidence: 99%

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Zenith angle dependence of the cosmic ray rate as measured with imaging air-Cherenkov telescopes

Bretz

2019

Astroparticle Physics

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A: The rate of extensive air-showers observed with imaging air-Cherenkov telescopes is zenith angle dependent. This effect originates from the increasing geometrical distance of the observed shower to the telescope with increasing zenith distance. This paper investigates how this alters the observed image and how this affects the trigger rate as a function of zenith angle. The discussed effects include the change of Cherenkov light yield, of absorption in the atmosphere, of photon density at the aperture and of the image size at the focal plane of the telescope. Based on a simple model for the atmosphere and well-known first principles on the development of extensive air-showers, the zenith angle dependence is expressed analytically. The assumption that most light is emitted from the shower core and mathematical approximations allow to derive an analytical expression describing the zenith angle dependence well with only three free parameters which are directly linked with the underlying physics. This suggests further investigations about how these fit parameters are linked to the properties of the atmosphere and the instrument. Using data published by the First G-APD Cherenkov Telescope, a good match of the fit functions with the data is obtained. For the trigger rate of cosmic rays, the obtained parameters are consistent with the naive expectation.

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Section: Fitting the Datamentioning

confidence: 99%

“…For the Crab Nebula, only good quality data and data with a low trigger-threshold had been used. For the cosmic ray rate, the maximum of the kernel density [8]. Shown is the fit of the calculation using the exact solution for the effective area but the approximations from Eqs.…”

Section: Fitting the Datamentioning

confidence: 99%

Zenith angle dependence of the cosmic ray rate as measured with imaging air-Cherenkov telescopes

Bretz

2019

Astroparticle Physics

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“…In addition, a data quality selection based on the cosmic-ray rate [31] has been applied resulting in a data sample of about 215 hours. The excess rates have been converted to fluxes using the Crab Nebula data as described in [32]. The flaring episode can in addition be studied in more details thanks to the H.E.S.S.…”

Section: Variability and Flux Distributions For Mrk 501 In 2014mentioning

confidence: 99%

Flux Distribution of Gamma-Ray Emission in Blazars: The Example of Mrk 501

et al. 2018

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Flux distribution is an important tool to understand the variability processes in active galactic nuclei. We now have available a great deal of observational evidences pointing towards the presence of log-normal components in the high energy light curves, and different models have been proposed to explain these data. Here, we collect some of the recent developments on this topic using the well-known blazar Mrk 501 as example of complex and interesting aspects coming from its flux distribution in different energy ranges and at different timescales. The observational data we refer to are those collected in a complementary manner by Fermi-LAT over multiple years, and by the First G-APD Cherenkov Telescope (FACT) telescope and the H.E.S.S. array in correspondence of the bright flare of June 2014.

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“…There could be an additional contribution from the seasonal variation, but (at least for 2015 and 2016) this effect has the tendency to result in lower rates measured in summer. The reason for the observed further rate reduction is under investigation 7 . Nevertheless, this measurement and [10] indicate that even with a small IACT like FACT, a reduction of the Cherenkov yield on the percentage level can be seen within few minutes.…”

Section: Sensitivity Of the Methodsmentioning

confidence: 93%

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

Hildebrand¹,

Ahnen²,

Balbo³

et al. 2017

Proceedings of 35th International Cosmic Ray Conference — PoS(ICRC2017)

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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.

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

Cited by 16 publications

References 5 publications

Zenith angle dependence of the cosmic ray rate as measured with imaging air-Cherenkov telescopes

Zenith angle dependence of the cosmic ray rate as measured with imaging air-Cherenkov telescopes

Flux Distribution of Gamma-Ray Emission in Blazars: The Example of Mrk 501

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

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