Mrk 421 showing enhanced activity at TeV energies with two short bright outbursts

ATel #11077; Adrian Biland (ETH Zurich), Daniela Dorner (University of Wuerzburg) for the FACT Collaboration, Jose Andres Garcia-Gonzalez (IF-UNAM), Israel Martinez (UMD) for the HAWC Collaboration
on 20 Dec 2017; 22:58 UT
Credential Certification: Daniela Dorner (dorner@astro.uni-wuerzburg.de)

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The FACT and HAWC collaborations report enhanced flux from the high-energy peaked BL Lac type object Mrk 421 at TeV energies with two short bright outbursts on top of it.

FACT is monitoring Mrk 421 regularly at TeV energies. In the night of Dec 19, an increase in the flux was measured. FACT observed the source for a total of 3.24 hours between 3:31 UTC and 6:58 UTC. In the first half of the observation, the flux was between one and two times that of the Crab Nebula above 750 GeV. A bright, very short flare was detected reaching a flux of 6-7 times the Crab Nebula flux between 5:32 UTC and 5:48 UTC. Results of an automatic quick look analysis are publicly available http://fact-project.org/monitoring/index.php?y=2017&m=12&d=18&source=1&timebin=3&plot=night In the following night, a flux of roughly 1.3 times the flux of the Crab Nebula was measured showing also an increase of the flux towards the end of the night http://fact-project.org/monitoring/index.php?y=2017&m=12&d=19&source=1&timebin=3&plot=night
FACT will continue to monitor Mrk 421 each night roughly between 3 UTC and 7 UTC. Contact person for this source is Daniela Dorner (dorner@astro.uni-wuerzburg.de)

The HAWC Observatory measured an increased gamma-ray flux from Mrk 421 at the level of (6.10 +/- 1.23) x 10^-11 photons cm^-2 s^-1 above 1 TeV when averaged during the 6 hour transit over HAWC from 2017-12-19 08:33:38 to 2017-12-19 14:49:34 UTC, which is 3.08 times the average Crab flux observed by HAWC. When split into two time intervals, before and after culmination, only the second interval (from 2017-12-19 11:41:60 to 2017-12-19 14:49:34 UTC) shows a significant increase above average. The flux after culmination is (8.97 +/- 1.60) x 10^-11 photons cm^-2 s^-1 integrated above 1 TeV (~ 5.0 x HAWC measured Crab flux).
For the following transit on Dec 20, 2017 from 2017-12-20 08:29:42 to 2017-12-20 14:45:38 UTC, a decreased but still above-average flux of (3.64 +/- 1.03) x 10^-11 photons cm^-2 s^-1 was observed, 1.8 times the Crab flux seen by HAWC. For the interval before culmination (2017-12-20 08:29:42 to 2017-12-20 11:37:40), the flux is (4.14 +/- 1.5) x 10^-11 photons cm^-2 s^-1(~ 2.08 x HAWC measured Crab flux), and for the interval after culmination (2017-12-20 11:37:40 to 2017-12-20 14:45:38) the flux is (3.26 +/- 1.37) x 10^-11 photons cm^-2 s^-1 (~ 1.64 x HAWC measured Crab flux).
The flux in Dec 19 reached ~10 times the average flux that was measured by HAWC during the previous year. All flux values are obtained from a maximum likelihood fit under the assumption of a fixed spectral shape with power law index of 2.2 and exponential cut-off at 5 TeV. The HAWC contact people for this analysis are Jose Andres Garcia-Gonzalez (IF-UNAM, jagarcia@fisica.unam.mx) and Israel Martinez (UMD,imc@umd.edu)

Swift follow-up observations 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.

HAWC is a TeV gamma ray water Cherenkov array located in the state of Puebla, Mexico, that monitors two thirds of the sky every day with an instantaneous field of view of ~2 sr.