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Increased TeV gamma-ray activity from Mrk 421 on January 1-4

ATel #9936; J. Wood (University of Wisconsin), I. Martinez (University of Maryland), R. Lauer (University of New Mexico)
on 4 Jan 2017; 17:20 UT
Credential Certification: Robert Lauer (rjlauer@unm.edu)

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

Referred to by ATel #: 9946

The HAWC Observatory measured increased gamma-ray fluxes from the direction of BL Lac Markarian 421 (z=0.031) over four successive nights starting on January 1, 2017:

Transit Starting 07:40:55 UTC on Jan 1, 2017 (MJD 57754.32):
Flux = (4.8 +/- 1.1) x10^-11 photons/cm2/s [2.5 Crab Units]

Transit Starting 07:36:59 UTC on Jan 2, 2017 (MJD 57755.32):
Flux = (3.6 +/- 1.0) x10^-11 photons/cm2/s [1.9 Crab Units]

Transit Starting 07:33:04 UTC on Jan 3, 2017 (MJD 57756.31):
Flux = (4.0 +/- 1.0) x10^-11 photons/cm2/s [2.1 Crab Units]

Transit Starting 07:29:08 UTC on Jan 4, 2017 (MJD 57757.31):
Flux = (4.1 +/- 1.0) x10^-11 photons/cm2/s [2.2 Crab Units]

All fluxes reported here are the integral flux above 1 TeV averaged over the 6 hour source transit 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. This shape is the current best fit for HAWC data from Markarian 421. The highest flux occurred on Jan 1, 2017. HAWC is a TeV gamma ray water Cherenkov array located in the state of Puebla, Mexico that monitors 2/3 of the sky every day with an instantaneous field of view of ~2 sr. The HAWC contact people for this analysis are Joshua Wood (jwood22@wisc.edu), Israel Martinez (imc@umd.edu), and Robert Lauer (rjlauer@unm.edu).

Additionally, public data from the FACT experiment shows an increase in the nightly background corrected light curve of Markarian 421 which peaks just prior to the fourth night of HAWC observations (MJD 57757.1). A link to the FACT data is provided below. 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.
http://fact-project.org/monitoring/index.php?y=2017&m=01&d=03&source=1&timebin=12&plot=week