X-Ray/TeV Variability of Mrk 421

doi:10.1086/527311

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

G. Fossati et al 2008 ApJ 677 906-925 doi: 10.1086/527311

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G. Fossati¹, J. H. Buckley², I. H. Bond³, S. M. Bradbury³, D. A. Carter-Lewis⁴, Y. C. K. Chow⁵, W. Cui⁶, A. D. Falcone⁷, J. P. Finley⁶, J. A. Gaidos⁶, J. Grube³, J. Holder⁸, D. Horan^9,10, D. Horns¹¹, M. M. Jordan¹², D. B. Kieda¹³, J. Kildea⁹, H. Krawczynski², F. Krennrich⁴, M. J. Lang¹⁴, S. LeBohec¹³, K. Lee², P. Moriarty¹⁵, R. A. Ong⁵, D. Petry^4,16, J. Quinn¹⁷, G. H. Sembroski⁶, S. P. Wakely¹⁸ and T. C. Weekes⁹
¹ Department of Physics and Astronomy, Rice University, Houston, TX 77005
² Department of Physics, Washington University, St. Louis, MO 63130
³ School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
⁴ Department of Physics and Astronomy, Iowa State University, Ames, IA 50011
⁵ Department of Physics and Astronomy, University of California, Los Angeles, CA 90095
⁶ Department of Physics, Purdue University, West Lafayette, IN 47907
⁷ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802
⁸ Department of Physics and Astronomy, University of Delaware, Newark, DE 19716
⁹ Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, P.O. Box 97, Amado, AZ 85645
¹⁰ Argonne National Laboratory, Argonne, IL 60439
¹¹ Institut für Experimentalphysik, Universität Hamburg, Germany
¹² Raytheon Company, Tucson, AZ 85706
¹³ Department of Physics, University of Utah, Salt Lake City, UT 84112
¹⁴ Department of Physics, National University of Ireland, Galway, Ireland
¹⁵ School of Science, Galway-Mayo Institute of Technology, Galway, Ireland
¹⁶ Max-Planck-Institut für Extraterrestrische Physik, D-85741 Garching, Germany
¹⁷ School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
¹⁸ Enrico Fermi Institute, University of Chicago, Chicago, IL 60637
E-mail: gfossati@rice.edu and buckley@wuphys.wustl.edu

ABSTRACT. We present a detailed analysis of week-long simultaneous observations of the blazar Mrk 421 at 2-60 keV X-rays (RXTE) and TeV γ-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 γ-ray bands, which are highly correlated. No evidence of an X-ray/γ-ray interband lag τ is found on the full week data set, with τ lesssim 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 electron 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: BL Lacertae objects: individual (Mrk 421); galaxies: active; galaxies: jets; gamma rays: observations; radiation mechanisms: non-thermal; X-rays: individual (Mrk 421)

Print publication: Issue 2 (2008 April 20)
Received 2007 July 12, accepted for publication 2007 October 18

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

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