Abstract
Aims. We present a detailed characterisation and theoretical interpretation of the broadband emission of the paradigmatic TeV blazar Mrk 421, with a special focus on the multi-band flux correlations. Methods. The dataset has been collected through an extensive multi-wavelength campaign organised between 2016 December and 2017 June. The instruments involved are MAGIC, FACT, Fermi-LAT, Swift, GASP-WEBT, OVRO, Medicina, and Metsähovi. Additionally, four deep exposures (several hours long) with simultaneous MAGIC and NuSTAR observations allowed a precise measurement of the falling segments of the two spectral components. Results. The very-high-energy (VHE; E > 100 GeV) gamma rays and X-rays are positively correlated at zero time lag, but the strength and characteristics of the correlation change substantially across the various energy bands probed. The VHE versus X-ray fluxes follow different patterns, partly due to substantial changes in the Compton dominance for a few days without a simultaneous increase in the X-ray flux (i.e., orphan gamma-ray activity). Studying the broadband spectral energy distribution (SED) during the days including NuSTAR observations, we show that these changes can be explained within a one-zone leptonic model with a blob that increases its size over time. The peak frequency of the synchrotron bump varies by two orders of magnitude throughout the campaign. Our multi-band correlation study also hints at an anti-correlation between UV-optical and X-ray at a significance higher than 3σ. A VHE flare observed on MJD 57788 (2017 February 4) shows gamma-ray variability on multi-hour timescales, with a factor ten increase in the TeV flux but only a moderate increase in the keV flux. The related broadband SED is better described by a two-zone leptonic scenario rather than by a one-zone scenario. We find that the flare can be produced by the appearance of a compact second blob populated by high energetic electrons spanning a narrow range of Lorentz factors, from γ′min=2×104 to γ′max=6×105.
Highlights
Blazars belong to the group of jetted active galactic nuclei (AGNs) and constitute the most populated class of sources in the extragalactic very-high-energy (VHE; E > 100 GeV) sky1
We have reported on a dense MWL observing campaign of Mrk 421 carried out between 2016 December and 2017 June
The MWL dataset comprises more than ten instruments, providing information from radio to VHE gamma rays, and including various instruments covering the optical and UV bands (e.g. GASP-WEBT and Swift-UVOT), X-ray bands (Swift-X-ray Telescope (XRT) and Swift-BAT and NuSTAR), and GeV gamma rays
Summary
Blazars belong to the group of jetted active galactic nuclei (AGNs) and constitute the most populated class of sources in the extragalactic very-high-energy (VHE; E > 100 GeV) sky. The simplest emission models of blazars (called one-zone models) assume a cospatial particle population responsible for the SED above the infrared ( 1013 Hz) In this scenario, a correlation between UV-optical and X-ray photons is generally expected. The observed synchrotron flux is proportional to the product δ4 ne B 2, where δ is the Doppler factor, ne is the number of electrons, and B is the magnetic field (here and in the following, primed quantities refer to quantities in the plasma reference frame) Any change in the latter parameters would simultaneously affect the UV-optical and X-ray emissions. Together with the Swift schedulers team, we coordinated many of these observations to happen simultaneously (or close in time) to the VHE gamma-ray observations performed with MAGIC and FACT in order to be able to properly characterise the temporal evolution of the lowand high-energy SED bumps of Mrk 421. We study the X-ray versus UV-optical correlated variability
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