Multiwavelength variability and correlation studies of Mrk 421 during historically low X-ray and γ-ray activity in 2015–2016
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2020-12-08
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Lien, A.Y.; et al.; Multiwavelength variability and correlation studies of Mrk 421 during historically low X-ray and γ-ray activity in 2015–2016; Monthly Notices of the Royal Astronomical Society (2020); https://academic.oup.com/mnras/advance-article-abstract/doi/10.1093/mnras/staa3727/6027705?redirectedFrom=fulltext
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This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record Lien, A.Y.; et al.; Multiwavelength variability and correlation studies of Mrk 421 during historically low X-ray and γ-ray activity in 2015–2016; Monthly Notices of the Royal Astronomical Society (2020); https://academic.oup.com/mnras/advance-articleabstract/doi/10.1093/mnras/staa3727/6027705?redirectedFrom=fulltext is available online at: https://academic.oup.com/mnras/advance-articleabstract/doi/10.1093/mnras/staa3727/6027705?redirectedFrom=fulltext.
This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record Lien, A.Y.; et al.; Multiwavelength variability and correlation studies of Mrk 421 during historically low X-ray and γ-ray activity in 2015–2016; Monthly Notices of the Royal Astronomical Society (2020); https://academic.oup.com/mnras/advance-articleabstract/doi/10.1093/mnras/staa3727/6027705?redirectedFrom=fulltext is available online at: https://academic.oup.com/mnras/advance-articleabstract/doi/10.1093/mnras/staa3727/6027705?redirectedFrom=fulltext.
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Abstract
We report a characterization of the multi-band flux variability and correlations of the nearby (z=0.031) blazar Markarian 421 (Mrk 421) using data from Metsähovi, Swift, Fermi-LAT, MAGIC, FACT and other collaborations and instruments from November 2014 till June 2016. Mrk 421 did not show any prominent flaring activity, but exhibited periods of historically low activity above 1 TeV (F>₁ₜₑᵥ < 1.7× 10⁻¹² ph cm⁻² s⁻¹) and in the 2-10 keV (X-ray) band (F2 ₋₁₀ₖₑᵥ < 3.6 × 10⁻¹¹ erg cm⁻² s⁻¹), during which the Swift-BAT data suggests an additional spectral component beyond the regular synchrotron emission.The highest flux variability occurs in X-rays and very-high-energy (E>0.1 TeV) γ-rays, which, despite the low activity, show a significant positive correlation with no time lag. The HRₖₑᵥ and HRₜₑᵥshow the harder-when-brighter trend observed in many blazars, but the trend flattens at the highest fluxes, which suggests a change in the processes dominating the blazar variability. Enlarging our data set with data from years 2007 to 2014, we measured a positive correlation between the optical and the GeV emission over a range of about 60 days centered at time lag zero, and a positive correlation between the optical/GeV and the radio emission over a range of about 60 days centered at a time lag of 43⁺⁹₋₆ days.This observation is consistent with the radio-bright zone being located about 0.2 parsec downstream from the optical/GeV emission regions of the jet. The flux distributions are better described with a LogNormal function in most of the energy bands probed, indicating that the variability in Mrk 421 is likely produced by a multiplicative process.