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

Abstract

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.