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

Bretz, T.

doi:10.1016/j.astropartphys.2019.02.004

Cited by 16 publications

(16 citation statements)

References 17 publications

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“…This leads to lower excess rates R. Note that this process is energy dependent and therefore leads to the appearance of the spectral index γ in the phenomenological model. The following fit function, with the free parameters χ, κ and ξ , is taken from a detailed study in [10]. As the Crab Nebula has not been observed at large zenith distance since winter 2014/2015, the model fit is obtained from data taken only in preceding seasons.…”

Section: Zenith Distancementioning

confidence: 99%

FACT - Probing the Periodicity of Mrk 421 and Mrk 501

Beck¹,

Arbet-Engels²,

Baack³

et al. 2019

Proceedings of 36th International Cosmic Ray Conference — PoS(ICRC2019)

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Markarian 421 and Markarian 501 are Active Galactic Nuclei and two of the most prominent sources of very high energetic gamma rays. In contrast to the predominantly stable Crab Nebula, the two blazars have extremely variable gamma-ray fluxes. Those could be caused by erratic accretion flow instabilities, but also by processes leading to quasi periodic oscillations (QPOs). Recently, hints of such a QPO of Mrk 501 with a period of about 322 days have been observed with the Large Area Telescope on-board of the Fermi satellite at GeV energies featuring global and local significances of the signal above 99%. The First G-APD Cherenkov Telescope (FACT) has monitored a small sample of known TeV gamma-ray sources including Mrk 421 and Mrk 501 over the last seven years. Therefore, it is ideally suited for periodicity studies of Active Galactic Nuclei, as they require unbiased and continuous long-term observations. Based on measurements of the well-known gamma-ray flux originating from the Crab Nebula, corrections to the measured light curves are applied. In total, 2500 hours of data for each of the two blazars have been analysed using the Lomb-Scargle algorithm. The resulting periodogram of Mrk 501 shows a hint of QPO with a similar period as the one observed by Fermi/LAT.

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Section: Zenith Distancementioning

confidence: 99%

FACT - Probing the Periodicity of Mrk 421 and Mrk 501

Beck¹,

Arbet-Engels²,

Baack³

et al. 2019

Proceedings of 36th International Cosmic Ray Conference — PoS(ICRC2019)

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“…The data quality selection was performed based on the cosmic-ray rate (Hildebrand et al 2017). The effect of the zenith distance on the cosmic-ray rate was taken into account following Mahlke et al (2017) and Bretz (2019). A total of 279 h of good-quality data spread over 86 nights was obtained.…”

Section: Factmentioning

confidence: 99%

Investigation of the correlation patterns and the Compton dominance variability of Mrk 421 in 2017

Acciari¹,

Ansoldi²,

Antonelli³

et al. 2021

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Aims. We present a detailed characterisation and theoretical interpretation of the broadband emission of the paradigmatic TeV blazar Mrk 421, with a special focus on the multi-band flux correlations. Methods. The dataset has been collected through an extensive multi-wavelength campaign organised between 2016 December and 2017 June. The instruments involved are MAGIC, FACT, Fermi-LAT, Swift, GASP-WEBT, OVRO, Medicina, and Metsähovi. Additionally, four deep exposures (several hours long) with simultaneous MAGIC and NuSTAR observations allowed a precise measurement of the falling segments of the two spectral components. Results. The very-high-energy (VHE; E > 100 GeV) gamma rays and X-rays are positively correlated at zero time lag, but the strength and characteristics of the correlation change substantially across the various energy bands probed. The VHE versus X-ray fluxes follow different patterns, partly due to substantial changes in the Compton dominance for a few days without a simultaneous increase in the X-ray flux (i.e., orphan gamma-ray activity). Studying the broadband spectral energy distribution (SED) during the days including NuSTAR observations, we show that these changes can be explained within a one-zone leptonic model with a blob that increases its size over time. The peak frequency of the synchrotron bump varies by two orders of magnitude throughout the campaign. Our multi-band correlation study also hints at an anti-correlation between UV-optical and X-ray at a significance higher than 3σ. A VHE flare observed on MJD 57788 (2017 February 4) shows gamma-ray variability on multi-hour timescales, with a factor ten increase in the TeV flux but only a moderate increase in the keV flux. The related broadband SED is better described by a two-zone leptonic scenario rather than by a one-zone scenario. We find that the flare can be produced by the appearance of a compact second blob populated by high energetic electrons spanning a narrow range of Lorentz factors, from γ′min=2×104 to γ′max=6×105.

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“…Description of the events reconstruction and data reduction technique is presented in [7,14]. The quality checks and background suppression techniques are described in [15], [16] and [17]. The energy threshold based on simulated data is determined to be ∼700 GeV for Mrk 421 and Mrk 501 [7].…”

Section: Multi-instrument Data-setmentioning

confidence: 99%

Relentless multi-wavelength variability of Markarian 421 and Markarian 501

Sliusar¹,

Arbet-Engels²,

Baack³

et al. 2021

Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021)

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Mrk 421 and Mrk 501 are two close bright and well-studied high-synchrotron-peaked blazars, which feature bright and persistent GeV and TeV emission. We use the longest and densest dataset of unbiased observations of these two sources, obtained at TeV and GeV energies during 5 years with the FACT telescope and Fermi-LAT. To characterize the variability and derive constraints on the emission mechanism, we augment the dataset with contemporaneous multi-wavelength observations from radio to X-rays. We correlate the light curves, identify individual flares in TeV and X-rays, and look for inter-band connections, which are expected from the shock propagations within the jet. For Mrk 421 we find that the X-rays and TeV are well correlated with close to zero lag, supporting the SSC emission scenario. The timing between the TeV, X-ray flares in Mrk 421 is consistent with periods expected in the case of Lense-Thirring precession of the accretion disc. Mrk 501 variability on long-term periods is also consistent with SSC, with a sub-day lag between X-rays and TeVs. Fractional variability for both blazars shows two bump structure with the highest variability in X-ray and TeV bands.

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

Cited by 16 publications

References 17 publications

FACT - Probing the Periodicity of Mrk 421 and Mrk 501

FACT - Probing the Periodicity of Mrk 421 and Mrk 501

Investigation of the correlation patterns and the Compton dominance variability of Mrk 421 in 2017

Relentless multi-wavelength variability of Markarian 421 and Markarian 501

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