Exceptional Flare of Mrk 421 at TeV energies

ATel #11184; Adrian Biland (ETH Zurich) for the FACT Collaboration
on 17 Jan 2018; 12:04 UT
Credential Certification: Daniela Dorner (dorner@astro.uni-wuerzburg.de)

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

Since December 2017, Mrk 421 has been showing already some activity at TeV energies, as reported in the ATels #11077 and #11086 by the FACT and HAWC collaborations.

The source was observed by FACT for a total of 5.5 hours between 1:15 UTC and 7:05 UTC (MJD 58135). At the beginning of the observation, the source showed a flux of about 6-7 times that of the Crab Nebula (CU) at TeV energies, then increasing to more than 9 CU. Over the whole observation, the flux was varying between 3 and 10 CU with a decrease towards the end of the night and an average flux of roughly 6 CU.

http://fact-project.org/monitoring/index.php?y=2018&m=01&d=16&source=1&timebin=12&plot=month shows the nightly-binned background subtraced light curve since then and http://fact-project.org/monitoring/index.php?y=2018&m=01&d=16&source=1&timebin=3&plot=night the 20-min-binned light curve of last night (MJD 58135).

FACT is regularly monitoring Mrk 421. Currently, observations of the source are carried out each night from 1:00 UTC to 7:00 UTC, if the observation 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 semi-conductor photo sensors (SiPM) 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.