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Mrk 421 exceptionally bright at TeV energies

ATel #11086; Adrian Biland (ETH Zurich) for the FACT Collaboration
on 24 Dec 2017; 11:28 UT
Credential Certification: Daniela Dorner (dorner@astro.uni-wuerzburg.de)

Subjects: Gamma Ray, TeV, VHE, AGN, Blazar

Referred to by ATel #: 11184, 11194, 11195

The FACT collaboration reports the measurement of an enhanced gamma-ray flux at TeV energies from the high-energy peaked BL Lac type object Mrk 421 (z=0.031). This measurement shows one of the highest fluxes of the source recorded in six years of monitoring with FACT.

Mrk 421 showed already an enhanced flux and two bright flares at TeV energies on Dec 19 (MJD 58106), as reported in ATel #11077 by the FACT and HAWC collaborations. The average nightly flux on Dec 19 was around 2-2.5 times the flux of the Crab Nebula at TeV energies. After a slight decrease, the mean nightly flux further increased during the last week to roughly 4 times the Crab Nebula flux on Dec 24., as it can be seen from FACT's background subtracted light curve publicly available at http://fact-project.org/monitoring/index.php?y=2017&m=12&d=23&source=1&timebin=12&plot=week

On Dec 24 (MJD 58111), FACT observed Mrk 421 for 3.8 hours between 2:50 UTC and 7 UTC. During that time, the flux was varying between roughly 3 times and roughly 6 times the flux of the Crab Nebula at TeV energies. The results of a preliminary, automatic quick look analysis are publicly available. http://fact-project.org/monitoring/index.php?y=2017&m=12&d=23&source=1&timebin=3&plot=night shows the 20-minute-binned background subtracted light curve.

FACT is regularly monitoring Mrk 421. Currently, it is observing the source each night from 2:50 UTC to 7:00 UTC, if the weather conditions permit. The FACT contact person for this source is Daniela Dorner (dorner@astro.uni-wuerzburg.de)

The First G-APD Cherenkov Telescope (FACT) is an Imaging Atmospheric 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 with an analysis threshold of 750 GeV. The Collaboration includes ETH Zurich and the Universities of Dortmund, Geneva and Wuerzburg.