Continued High Activity of Mrk 421 at TeV Energies

ATel #12680; Adrian Biland (ETHZ) for the FACT Collaboration
on 23 Apr 2019; 13:12 UT
Credential Certification: Daniela Dorner (dorner@astro.uni-wuerzburg.de)

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FACT reports an enhanced flux from the high-energy peaked BL Lac type object Mrk 421 in gamma rays above 750 GeV ongoing since about two months.

Within the FACT monitoring program, Mrk 421 (z=0.031) is observed regularly. While its usual flux level is around 30% of the flux of the Crab Nebula at TeV energies, the source shows an enhanced flux of more than 50% of that of the Crab Nebula since more than two months with peak fluxes of more than 3 times the flux of the Crab Nebula at TeV energies in the nights of 28./29.3.2019, 30./31.3.2019, 31.3./1.4.2019, 11./12.4.2019, 12./13.4.2019. The highest flux was measured in the night of 11./12.4.2019 with a peak flux of 4.5 times the flux of the Crab Nebula at TeV energies.

The results of an automatic quick-look analysis can be found at https://fact-project.org/monitoring/index.php?y=2019&m=04&d=22&source=1&timebin=3&plot=night for the night of 22./23.4.2019
The evolution of the nightly average of the flux for the last month is available at https://fact-project.org/monitoring/index.php?y=2019&m=04&d=22&source=1&timebin=12&plot=month
The FACT contact person for this source is Daniela Dorner (dorner@astro.uni-wuerzburg.de)

FACT is regularly monitoring Mrk 421. Currently, observations of the source are carried out each night from 20:35 UTC to 1:00 UTC, if the observation conditions permit.
Based on the flux of 3.7 times the flux of the Crab Nebula from the night of 22./23.4.2019 also observations with INTEGRAL and Swift have been triggered.
Further follow-up observations are encouraged.

The First G-APD Cherenkov Telescope (FACT) is an imaging air Cherenkov telescope with 9.5 sqm mirror area located in the Observatorio del Roque de los Muchachos on the Canary Island La Palma. It is pioneering the usage of silicon photosensors and monitoring bright variable sources at energies above 750 GeV. The Collaboration includes ETH Zurich and the Universities of Dortmund, Geneva and Wuerzburg.