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 (email@example.com)
Subjects: Gamma Ray, TeV, VHE, AGN, Blazar
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
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.