Multiwavelength Observations of Markarian 421 in 2001 March: An Unprecedented View on the X‐Ray/TeV Correlated Variability

Abstract

We present a detailed analysis of week-long simultaneous observations of the blazar Mrk 421 at 2Y60 keV X-rays (RXTE ) and TeV -rays ( Whipple and HEGRA) in 2001. Accompanying optical monitoring was performed with the Mt. Hopkins 48 inch telescope. The unprecedented quality of this data set enables us to establish the existence of the correlation between the TeV and X-ray luminosities, and also to start unveiling some of its characteristics, in particular its energy dependence and time variability. The source shows strong variations in both X-ray and -ray bands, which are highly correlated. No evidence of an X-ray/-ray interband lag is found on the full week data set, with P 3 ks. A detailed analysis of the March 19 flare, however, reveals that data are not consistent with the peak of the outburst in the 2Y4 keV X-ray and TeV band being simultaneous. We estimate a 2:1 AE 0:7 ks TeV lag. The amplitudes of the X-ray and -ray variations are also highly correlated, and the TeV luminosity increases more than linearly with respect to the X-ray one. The high degree of correlation lends further support to the standard model in which a unique electron population produces the X-rays by synchrotron radiation and the -ray component by inverse Compton scattering. However, the finding that for the individual best observed flares the -ray flux scales approximately quadratically with respect to the X-ray flux poses a serious challenge to emission models for TeV blazars, as it requires rather special conditions and/or fine tuning of the temporal evolution of the physical parameters of the emission region. We briefly discuss the astrophysical consequences of these new findings in the context of the competing models for the jet emission in blazars.

MULTIWAVELENGTH STUDY OF QUIESCENT STATES OF Mrk 421 WITH UNPRECEDENTED HARD X-RAY COVERAGE PROVIDED BY NuSTAR IN 2013

“…The nonthermal and polarized continuum observed from the radio band to the soft X-ray band suggests that this part of the SED is due to a distribution of relativistic electrons radiating via the synchrotron process. The radiation in the γ-ray band is likely due to inverse-Compton scattering by energetic electrons responsible for the synchrotron radiation, as confirmed by simultaneous, correlated variations in the low-and high-energy SED components (e.g., Giebels et al 2007;Fossati et al 2008;Aleksić et al 2015b). The seed photons are most likely the synchrotron photons internal to the jet.…”

“…The existence of a correlation between the X-ray and VHE fluxes is well established on certain timescales and in certain activity states of Mrk 421: claims of correlated variability stem from long-term monitoring of fluxes in these bands that include high-activity states (Bartoli et al 2011;Acciari et al 2014), as well as observations of particular flaring events that probe correlated variability on timescales as short as 1 hr (Giebels et al 2007;Fossati et al 2008;Acciari et al 2009). The detection of such a correlation in a low state was reported for the first time in Aleksić et al (2015b), using the X-ray (Swift-XRT, RXTE-PCA) and VHE (MAGIC, VERITAS) data obtained during the 4.5-month multiwavelength campaign in 2009, when Mrk 421 did not show any flaring activity and varied around its typical Swift-XRT 0.3-10keV count rates of ∼25s −1 and VHE flux of 0.5Crab.…”

“…The seed photons are most likely the synchrotron photons internal to the jet. Such "synchrotron selfCompton" (SSC) models, developed by many authors (for early examples see, e.g., Jones et al 1974;Ghisellini et al 1985;Marscher & Gear 1985), have been successfully invoked to describe the full SED of HBL objects (e.g., Ulrich et al 1997;Fossati et al 2008;Tavecchio et al 2010).…”

“…The range of measured flux of Mrk 421 spans up to two orders of magnitude in some spectral bands, with flares occurring on very short timescales (a day or less; e.g., Gaidos et al 1996;Tanihata et al 2003;Fossati et al 2008). Possibly the best bands to study such variability are the X-ray and VHE γ-ray bands: in the context of the SSC model, they represent radiation from the most energetic electrons, which have the shortest timescales for radiative losses.…”

“…Less is known about the hard X-ray (10 keV) properties of blazar jet emission: the data are far fewer and available mostly for flaring episodes, or averaged over relatively long timescales (e.g., Guainazzi et al 1999;Giebels et al 2007;Fossati et al 2008;Ushio et al 2009;Abdo et al 2011). This energy band probes the most energetic and fastest-varying tail of the distribution of synchrotron-radiating particles and therefore represents an important diagnostic of the content of the jet and the processes responsible for the acceleration of particles to the highest energies.…”