Multiwavelength Observations of Markarian 421 in March 2001: an Unprecedented View on the X-ray/tev Correlated Variability

We present a detailed analysis of week-long simultaneous observations of the blazar Mrk 421 at 2–60 keV X-rays (RossiXTE) and TeV γ-rays (Whipple and HEGRA) in 2001. Accompanying optical monitoring was performed with the Mt. Hopkins 48" telescope. The unprecedented quality of this dataset 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 γ-ray bands, which are highly correlated. No evidence of a X-ray/γ-ray interband lag τ is found on the full week dataset, with τ . 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 2–4 keV X-ray and TeV band being simultaneous. We estimate a 2.1±0.7 ks TeV lag. The amplitudes of the X-ray and γ-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 electrons population produces the X-rays by synchrotron radiation and the γ-ray component by inverse Compton scattering. However, the finding that for the individual best observed flares the γ-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. Subject headings: galaxies: active — galaxies: jets — BL Lacertae objects: individual (Mrk 421) — gammarays: observations — X–rays: individual (Mrk 421) — radiation mechanisms: non-thermal Electronic address: Corresponding authors: Giovanni Fossati and Jim Buckley 1 Department of Physics & Astronomy, Rice University, Houston, TX 77005, USA 2 Department of Physics, Washington University, St. Louis, MO 63130, USA 3 School of Physics & Astronomy, University of Leeds, Leeds, LS2 9JT, UK 4 Department of Physics & Astronomy, Iowa State University, Ames, IA 50011, USA 5 Department of Physics & Astronomy, University of California, Los Angeles, CA 90095, USA 6 Department of Physics, Purdue University, West Lafayette, IN 47907, USA 7 Department of Astronomy & Astrophysics, Pennsylvania State University, University Park, PA 16802, USA 8 Department of Physics & Astronomy, University of Delaware, Newark, DE 19716, USA 9 Fred Lawrence Whipple Observatory, Harvard–Smithsonian CfA, P.O. Box 97, Amado, AZ 85645, USA 10 Argonne National Lab, Argonne, IL 60439, USA 11 Institut für Experimentalphysik Univ. Hamburg, Germany 12 Raytheon, Tucson, AZ 85706, USA 13 Department of Physics, University of Utah, Salt Lake City, UT 84112, USA 14 Department of Physics, National University of Ireland, Galway, Ireland 15 School of Science, Galway-Mayo Institute of Technology, Galway, Ireland