Enhanced and increasing activity in gamma rays and X-rays from the HBL Mrk421

ATel #9137; A. Biland (ETH Zurich) and D. Dorner (University of Wuerzburg, FAU Erlangen) for the FACT Collaboration, R. Lauer (University of New Mexico) and J. Wood (University of Maryland) for the HAWC Collaobration, B. Kapanadze (Abastumani Astrophysical Observatory, Ilia State University), A. Kreikenbohm (University of Wuerzburg)
on 10 Jun 2016; 19:00 UT
Credential Certification: Daniela Dorner (dorner@astro.uni-wuerzburg.de)

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FACT, HAWC and Swift-XRT report an enhanced flux from the high-energy peaked BL Lac type object Mrk 421 both in gamma rays above 750 GeV and in the 0.3-10 keV band in X-rays respectively.

FACT is monitoring Mrk 421 regularly. From 2016 June 5 to 2016 June 9, the flux increased constantly from less than 0.5 to more than 2 Crab units. Each night, the source was observed for about 1 hour from 21 UTC to 22 UTC, and the statistical significance of the detection increased with the flux from 3 to 8 standard deviations.
The results of an automatic quick look analysis are publicly available. http://fact-project.org/monitoring/index.php?y=2016&m=06&d=09&source=1&timebin=12&plot=week shows the flux increase over the last few nights. Contact person for this source is Daniela Dorner (dorner@astro.uni-wuerzburg.de)

The HAWC Observatory is monitoring Mrk 421 regularly. For the transit starting June 9, 21:10 UTC, overlapping with and extending beyond the last FACT observation, HAWC measured an increased gamma­-ray flux at the level of (7.0 +/­ 1.3) x 10e­-11 photons cm^­-2 s^­-1, integrated above 1 TeV, when averaged over 6­ hours (MJD 57548.88 - 57549.15). This is approximately 3.1 times the average Crab flux observed by HAWC.
The significance of this observation is 6.5 standard deviations compared to the null hypothesis, and the flux level is approximately a factor 4 above the average flux measured for Markarian 421 by HAWC during the previous year.
The HAWC contact people for this analysis are Robert Lauer (University of New Mexico, rjlauer@unm.edu) and Joshua Wood (University of Maryland, jwood@umdgrb.umd.edu).

Between June 5 and June 9, Mrk 421 was observed three times with the Swift-XRT X-ray telescope, revealing a strong X-ray flare. The observation-binned 0.3-10 keV count rate for June 9 reaches 94.5+/-0.33 cts/s which is by a factor of 4.4 larger than that from the June 5 snapshot, and by a factor of 2.7 larger compared to the mean rate from all Swift-XRT pointings to this source. The spectrum is hard with a photon index at 1 keV of a=1.79+/-0.01, and the unabsorbed 0.3-10 keV flux amounts to (2.51+/-0.02)\times10^{-9} erg/cm^2/s. Note that higher or comparable X-ray fluxes were observed only four times so far (in June 2008, February 2010, April 2013, and March-April 2014).

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.