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We report on a multiwavelength campaign on the TeV gamma-ray blazar Markarian(Mrk) 421 performed during December 2002 and January 2003. These target ofopportunity observations were initiated by the detection of X-ray and TeVgamma-ray flares with the All Sky Monitor (ASM) on board the Rossi X-ray TimingExplorer (RXTE) and the 10 m Whipple gamma-ray telescope.The campaign includedobservational coverage in the radio (University of Michigan Radio AstronomyObservatory), optical (Boltwood, La Palma KVA 0.6m, WIYN 0.9m), X-ray (RXTEpointed telescopes), and TeV gamma-ray (Whipple and HEGRA) bands.  At TeV energies, the observations revealed several flares at intermediateflux levels, peaking between 1 and 1.5 times the flux from the Crab Nebula.While the time averaged spectrum can be fitted with a single power law ofphoton index Gamma =2.8, we find some evidence for spectral variability.Confirming earlier results, the campaign reveals a rather loose correlationbetween the X-ray and TeV gamma-ray fluxes. In one case, a very strong X-rayflare is not accompanied by a comparable TeV gamma-ray flare. Although thesource flux was variable in the optical and radio bands, the sparse sampling ofthe optical and radio light curves does not allow us to study the correlationproperties in detail.  We present a simple analysis of the data with a synchrotron-self Comptonmodel, emphasizing that models with very high Doppler factors and low magneticfields can describe the data.Comment: Accepted for publication in the Astrophysical Journa

Multiwavelength Observations of the Blazar Markarian 421 in 2002 December and 2003 January