Unraveling the Complex Behavior of Mrk 421 with Simultaneous X-Ray and VHE Observations during an Extreme Flaring Activity in 2013 April ^*

Abstract

We report on a multi-band variability and correlation study of the TeV blazar Mrk 421 during an exceptional flaring activity observed from 2013 April 11 to 2013 April 19. The study uses, among others, data from GASP-WEBT, Swift, NuSTAR, Fermi-LAT, VERITAS, and MAGIC. The large blazar activity, and the 43 hours of simultaneous NuSTAR and MAGIC/VERITAS observations, permitted variability studies on 15 minute time bins, and over three X-ray bands (3-7 keV, 7-30 keV and 30-80 keV) and three very-high-energy (>0.1 TeV, hereafter VHE) gamma-ray bands (0.2-0.4 TeV, 0.4-0.8 TeV and >0.8 TeV). We detected substantial flux variations on multi-hour and subhour timescales in all the X-ray and VHE gamma-ray bands. The characteristics of the sub-hour flux variations are essentially energy-independent, while the multi-hour flux variations can have a strong dependence on the energy of the X-ray and the VHE gamma rays. The three VHE bands and the three X-ray bands are positively correlated with no time-lag, but the strength and the characteristics of the correlation changes substantially over time and across energy bands. Our findings favour multi-zone scenarios for explaining the achromatic/chromatic variability of the fast/slow components of the light curves, as well as the changes in the flux-flux correlation on day-long timescales. We interpret these results within a magnetic reconnection scenario, where the multi-hour flux variations are dominated by the combined emission from various plasmoids of different sizes and velocities, while the sub-hour flux variations are dominated by the emission from a single small plasmoid moving across the magnetic reconnection layer.

The Gamma-ray Window to Intergalactic Magnetism

“…In general, AGNs are active over time scales of T ∼ 10 6 -10 8 years [217], which makes it hard to use temporal information for constraining IGMFs since they are, for all practical purposes, quasi-steady sources. However, some objects, such PKS 2155-304 [213], Mrk 421 [13,218] and Mrk 501 [210,219,220], display short-time variability [221].…”

Confronting observations of VHE gamma-ray blazar flares with reconnection models

“…Comparing blazar flares with reconnection models J. Jormanainen of Mrk 421, although the distributions are fairly wide. In [10], the variability of Mrk 421 was also studied using a similar reconnection model. They only used one set of simulation parameters where = 10 Γ = 14 , and = 2.1°.…”

“…In order to develop and introduce our method, we used only one particularly well-sampled light curve of Mrk 421 (Fig. 1) observed in a simultaneous campaign by MAGIC and VERITAS in VHE gamma-rays, and also by NuSTAR in X-rays in 2013 [10]. We use only the VHE data in our study.…”

“…The data set consists of about 200 hours of observations in three energy bands, 200-400 GeV, 400-800 GeV, and >800 GeV, obtained during nine consecutive nights. The data observed by VERITAS have been scaled to match the level of MAGIC data as described in [10]. The analysis presented here can also be applied to light curves of other sources as well as other energies and time scales.…”

X-ray Flux and Spectral Variability of the TeV Blazars Mrk 421 and PKS 2155-304

“…Substantial flux variations on multi-hour and sub-hour timescales were observed in X-ray and γ-ray bands. Various X-ray and γ-ray bands were found to be well correlated without any time lag [59]. A detailed X-ray IDV study was carried out for Mrk 421 with 72 pointed observations from Chandra taken from 2000-2015 and 3 pointed observations from Suzaku taken during its whole operational period.…”