From December 10, 1996 to January 28, 1997 the CGRO instruments EGRET and COMPTEL observed the Virgo sky region continuously for 7 weeks, detecting 3C 273 in an active -ray state. EGRET (100 MeV) observed a time-variable flux, peaking during a 2-week flaring period at its highest level observed during the CGRO-era. COMPTEL, however, does not observe any contemporaneous -ray flare at energies below 30 MeV, showing that this outburst is restricted to -ray energies above 30 MeV. This is consistent with the spectral hardening observed in the 3 MeV to 10 GeV energy band during the flaring period.
The peculiar variability properties of the flare may be explained in terms of a two-component spectral model with the emission in the EGRET energy range produced by Comptonization of reprocessed accretion disk emission. The different variability behaviour in -rays is inconsistent with the synchrotron-reflection model being the cause of the -ray flare.
This observation covers an opportune sequence of low pre-flare, high flare, and again low post-flare emission in -rays. In general, this -ray observation could turn out to be important for further modelling of blazar emission processes because the flare is well located in time and therefore can possibly be correlated to flux measurements of monitoring observations in other wavelength regions.
© European Southern Observatory (ESO) 2000
Online publication: February 9, 2000