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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., PoS: Proceedings of Science, 395, ISSN:1824-8039, 37th International Cosmic Ray Conference (ICRC 2021)

The blazar Mrk 421 shows frequent, short flares in the TeV energy regime. Due to the fast nature of such episodes, we often fail to obtain sufficient simultaneous information about flux variations in several energy bands. To overcome this lack of multi-wavelength (MWL) coverage, especially for the pre- and post-flare periods, we have set up a monitoring program with the FACT telescope (TeV energies) and the Neil Gehrels Swift Observatory (X-rays). On 2019 June 9, Mrk 421 showed a TeV outburst reaching a flux level of more than two times the flux of the Crab Nebula at TeV energies. We acquired simultaneous data in the X-rays with additional observations by XMM-Newton and INTEGRAL. For the first time, we can study a TeV blazar in outburst taking advantage of highly sensitive X-ray data from XMM-Newton and INTEGRAL combined. Our data set is complemented by pointed radio observations by Effelsberg at GHz frequencies. We present our first results, including the gamma-ray and X-ray light curves, a timing analysis of the X-ray data obtained with XMM-Newton, as well as the radio spectra before, during and after the flare., PoS: Proceedings of Science, 395, ISSN:1824-8039, 37th International Cosmic Ray Conference (ICRC 2021)

Mrk 421 is a high-synchrotron-peaked blazar featuring bright and persistent GeV and TeV emission. We use multi-wavelength light curves of Mrk 421 spanning 5.5 years with FACT (TeV) and Fermi LAT (GeV) in the gamma rays, Swift BAT, Swift XRT and MAXI in the X-rays, together with optical and radio data and investigate the physical processes driving the emission and variability. Observations by FACT are continuous and not triggered, so the source was found in a wide range of flux states and more than 30 flares were identified from X-rays to TeV. The light curves in TeV and X-rays feature very similar flares with rise and decay times of a few days and zero lag, characteristic for electron processes. At least two parameters per flare, the amplitude and the cut-off energy, are required to explain the observed variability. In addition, the GeV light curve leads and is strongly correlated with the optical and radio light curves as expected from SSC emitting shock propagating in a conical jet., PoS: Proceedings of Science, 358, ISSN:1824-8039, Proceedings of th 36th International Cosmic Ray Conference (ICRC 2019)

The First G-APD Cherenkov Telescope (FACT) has been monitoring TeV-blazars since October 2011. Thanks to its unbiased observing strategy, the robotic operation and the usage of solid state photosensors (SiPM, aka G-APDs), the instrument's duty cycle has been maximized and the observational gaps minimized, providing an unprecedented data sample of more than 13'800 hours of physics data, of which more than 2600 hours were taken in the past 12 months. Results of an automatic quick-look analysis are published with low latency on an open-access website. Based on this, 98 alerts and 11 astronomer's telegrams have been issued in more than five years, triggering a variety of multi-wavelength studies. Target-of-opportunity programs with X-ray satellites are in place. For example, combined gamma-ray observations (FACT, Fermi-LAT, HESS) on Mrk 501 in 2014 have been used to constrain the underlying physics processes analysing the shape of flux distributions on different time scales. A trigger to MAGIC on a high state of 1ES 2344+51.4 in 2016 reveals a very hard spectrum. The combined observations are used to study the source's extreme behaviour. Studying the long-term multi-wavelength behaviour of Mrk 421, more than 30 flares have been found in 5.5 years of data. Correlating different wavelengths, delays are determined and models are constrained. In addition to multi-wavelength studies, the unprecedented, unbiased data sample at TeV energies provides a unique chance to study the duty cycle and variability behaviour of the sources. Based on this, a search for a periodic signal has been performed., PoS: Proceedings of Science, 358, ISSN:1824-8039, 36th International Cosmic Ray Conference (ICRC 2019) - GRI - Gamma Ray Indirect