Göttingen 2025 – wissenschaftliches Programm

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T: Fachverband Teilchenphysik

T 57: Gamma Astronomy I

T 57.4: Vortrag

Mittwoch, 2. April 2025, 17:00–17:15, VG 3.101

Revealing FACTs about the Harder-when-Brighter Behaviour of Mrk 421 in an Unbiased Long-Term Study — •Daniela Dorner¹, Bernd Schleicher², and FACT Collaboration³ — ¹Universität Würzburg, Deutschland — ²ETH Zürich — ³www.fact-project.org

Featuring two peaks in their spectral energy distribution, blazars exhibit a strong variability both in X-rays and very-high-energy gamma rays. Many studies find a harder-when-brighter behaviour of the spectral index in correlation with the flux.

Within the FACT monitoring program, the blazar Mrk 421 has been observed for more than 3200 hours at TeV energies. Thanks to the unbiased observing strategy, the data sample is ideally suited for systematic long-term studies. Owing to the stable photosensors that allow for observations during bright moon and the automatic and remote operation, the 10-year data sample covers more than 1100 nights.

Results from an unprecedented study of Mrk 421 are presented, focussing on the correlation of the spectral index with the flux at very high energies and probing the often observed hard-when-brighter behaviour.

Keywords: Blazars; Cherenkov Astronomy; Unbiased Monitoring; AGN; FACT