The 2009 multiwavelength campaign on Mrk 421: Variability and correlation studies

Abstract

Aims: We perform an extensive characterization of the broadbandemission of Mrk 421, as well as its temporal evolution, during thenon-flaring (low) state. The high brightness and nearby location (z =0.031) of Mrk 421 make it an excellent laboratory to study blazaremission. The goal is to learn about the physical processes responsiblefor the typical emission of Mrk 421, which might also be extended toother blazars that are located farther away and hence are more difficultto study. Methods: We performed a 4.5-month multi-instrumentcampaign on Mrk 421 between January 2009 and June 2009, which includedVLBA, F-GAMMA, GASP-WEBT, Swift, RXTE, Fermi-LAT, MAGIC, and Whipple,among other instruments and collaborations. This extensive radio tovery-high-energy (VHE; E> 100 GeV) γ-ray dataset providesexcellent temporal and energy coverage, which allows detailed studies ofthe evolution of the broadband spectral energy distribution. Results: Mrk421 was found in its typical (non-flaring) activity state,with a VHE flux of about half that of the Crab Nebula, yet the lightcurves show significant variability at all wavelengths, the highestvariability being in the X-rays. We determined the power spectraldensities (PSD) at most wavelengths and found that all PSDs can bedescribed by power-laws without a break, and with indices consistentwith pink/red-noise behavior. We observed a harder-when-brighterbehavior in the X-ray spectra and measured a positive correlationbetween VHE and X-ray fluxes with zero time lag. Such characteristicshave been reported many times during flaring activity, but here they arereported for the first time in the non-flaring state. We also observedan overall anti-correlation between optical/UV and X-rays extending overthe duration of the campaign. Conclusions: Theharder-when-brighter behavior in the X-ray spectra and the measuredpositive X-ray/VHE correlation during the 2009 multi-wavelength campaignsuggests that the physical processes dominating the emission duringnon-flaring states have similarities with those occurring during flaringactivity. In particular, this observation supports leptonic scenarios asbeing responsible for the emission of Mrk 421 during non-flaringactivity. Such a temporally extended X-ray/VHE correlation is not drivenby any single flaring event, and hence is difficult to explain withinthe standard hadronic scenarios. The highest variability is observed inthe X-ray band, which, within the one-zone synchrotron self-Comptonscenario, indicates that the electron energy distribution is mostvariable at the highest energies.Appendix A is available in electronic form at http://www.aanda.orgThecomplete data set shown in Fig. 1 is only available at the CDS viaanonymous ftp to http://cdsarc.u-strasbg.fr(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/576/A126