Investigation of the correlation patterns and the Compton dominance variability of Mrk 421 in 2017

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.

The X-Ray Polarization View of Mrk 421 in an Average Flux State as Observed by the Imaging X-Ray Polarimetry Explorer

“…The data reduction procedure for all of the telescopes is described in Appendix. At the epoch of the IXPE observation, the 0.3-10 keV X-ray flux of Mrk 421 was ∼3.5 × 10 −10 erg s −1 cm −2 , which is near its typical flux state (Liodakis et al 2019). As a comparison, during occasional flaring events reaching γ-ray energies, the X-ray flux of Mrk 421 can be 10 times higher (Donnarumma et al 2009).…”

Probing Photon-ALP Oscillations from the Flat Spectrum Radio Quasar 4C+21.35

“…In Refs. [42,43], the VHE γ-ray observations of Mrk 421 measured by Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) and Fermi -LAT in 2013 − 2014 (with ten phases [50]) and 2017 (with four phases [51]) show the almost similar combined results at 95% C.L. for the ALP mass [ 1 × 10 −8 eV m a 2 × 10 −7 eV ].…”

Relevance of VHE blazar spectra models with axion-like particles

“…In ref. [58], they report the Mrk 421 observations from the multi-wavelength campaign measured from December 2016 to June 2017. The observations are provided by different instruments that covering the energy range from radio to VHE γ-ray.…”