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The blazar 3C 279, one of the brightest identified extragalactic objects inthe gamma-ray sky, underwent a flare of a factor 10 amplitude in gamma-raystowards the end of a 3-week pointing by CGRO, in 1996 January-February. Theflare peak represents the highest gamma-ray intensity ever recorded for thisobject. During the high state, extremely rapid gamma-ray variability was seen.Coordinated multifrequency observations were carried out with RXTE, ASCA, ROSATand IUE and from many ground-based observatories, covering most accessiblewavelengths. The well-sampled, simultaneous RXTE light curve shows an outburstof lower amplitude (factor of ~3) well correlated with the gamma-ray flarewithout any apparent lag. The optical-UV light curves, which are not wellsampled during the high energy flare, exhibit more modest variations (factor of~2) and a lower degree of correlation. The flux at millimetric wavelengths wasnear an historical maximum during the gamma-ray flare peak, with suggestion ofa correlated decay. We present simultaneous spectral energy distributions of 3C279 prior to and near to the flare peak. The gamma-rays vary by more than thesquare of the observed IR-optical flux change, which poses some problems forspecific blazar emission models. The synchrotron-self Compton model wouldrequire that the largest synchrotron variability occurred in the mostlyunobserved sub-mm/far-infrared region. Alternatively, a large variation in theexternal photon field could occur over a time scale of few days. This occursnaturally in the ``mirror'' model, wherein the flaring region in the jetphotoionizes nearby broad-emission-line clouds, which in turn provide softphotons that are Comptonized to gamma-ray energies.Comment: 18 pages, Latex, 2 tables, 3 postscript figures, to appear in The  Astrophysical Journa

Multiwavelength Observations of a Dramatic High‐Energy Flare in the Blazar 3C 279