Swift Observations of Mrk 421 in Selected Epochs. III. Extreme X-Ray Timing/Spectral Properties and Multiwavelength Lognormality during 2015 December–2018 April

Abstract

We present the results from the timing and spectral study of Mrk 421 based mainly on the Swift data in the X-ray energy range obtained during the time interval 2015 December-2018 April. The most extreme X-ray flaring activity on the long-term, daily and intraday timescales was observed during the 2-month period which started in 2017 December when the 0.3-10 keV flux exceeded a level of 5×10 −9 erg cm −2 s −1 , recorded only twice previously. While the TeV-band and X-ray variabilities mostly were correlated, the source often varied in a complex manner in the MeV-GeV and radio-UV energy ranges, indicating that the multifrequency emission of Mrk 421 could not be always generated in a single zone. The longer-term flares at X-rays and γ-rays showed a lognormal character, possibly indicating a variability imprint of the accretion disk onto the jet. A vast majority of the 0.3-10 keV spectra were consistent with the log-parabolic model, showing relatively low spectral curvature and correlations between the different spectral parameters, predicted in the case of the first and secondorder Fermi processes. The position of the synchrotron spectral energy distribution (SED) peak showed an extreme variability on diverse timescales between the energies E p <0.1 keV and E p >15 keV, with 15% of the spectra peaking at hard X-rays and was related to the peak height as S p ∝E α p with α∼0.6, which is expected for the transition from Kraichnan-type turbulence into the "hard-sphere" one. The 0.3-300 GeV spectra showed the features of the hadronic contribution, jet-star interaction and upscatter in the Klein-Nishina regime in different time intervals.

Relativistic Jet Of Markarian 421: Observational Evidences Of Particle Acceleration Mechanisms

“…Our intensive X-ray spectral study ([15], [26]- [29]) has shown that Mrk 421 mostly exhibits a log-parabolic spectral energy distribution, expected in the framework of the efficient first-and second-order Fermi accelerations of electrons (as discussed in the previous section).…”

“…Within our intensive X-ay spectral study, we also have proved the liability of these two hypotheses and, consequently, the importance of the second-order Fermi mechanism is checked. Figure 3: The S p -E p relation in different periods (adopted from [27] and [29]). is characterized by a wide synchrotron SED with 99% of the spectra exhibiting b∼0.3 or even smaller curvature (i.e.…”

“…The anti-correlation E p − b in different periods (adopted from[15],[26]-[29]). Dashed lines represent linear fits to the scatter plots.…”

“…): a transition from the Kraichnan spectrum of the turbulence with PoS(HEPRO VII)015 The a-b correlation in different periods (adopted from[26]-[29]). Dashed lines represent linear fits to the scatter plots.…”

“…Dashed lines represent linear fits to the scatter plots. The 0.3-300 GeV SEDs from the time intervals showing a soft gamma-ay excess (adopted from[29]). The blue and red dashed lines represent linear fits to the 0.3-2 GeV and 2-300 GeV SEDs, respectively.…”