A large high-energy gamma-ray flare from the blazar 3C 273

The Compton Gamma-Ray Observatory (CGRO) experiments EGRET and COMPTEL obse rved the Virgo sky region continuously for 7 weeks between December 10, 199 6 and January 28, 1997. The prominent quasar 3C 273 was found to be the bri ghtest source in gamma-rays and was significantly detected by EGRET and COM PTEL. The EGRET experiment observed a time-variable flux at energies above 100 MeV, which reached in a 2-week flaring period (December 30, 1996 to Jan uary 14, 1997) its highest flux level observed during the CGRO-era. COMPTEL , however, does not observe obvious time variability at energies below simi lar to 30 MeV contemporaneous to EGRET. In particular, no flare was observe d, indicating that this outburst is solely a high-energy (>100 MeV) phenome non. The energy spectrum between 3 MeV and 10 GeV is well represented by a simple power-law model. Below 3 MeV a spectral turnover is indicated. Perfo rming spectral analysis for different time periods, we found evidence for a spectral hardening during the flaring period, which is consistent with the flare occurring mainly at the higher energies and with its absence at COMP TEL energies of a few MeV. This may be interpreted as an indication that th e emission in the EGRET energy range is dominated by a different radiation mechanism than the MeV emission. We argue that the most likely mechanism fo r the high-energy flare is inverse-Compton scattering of reprocessed accret ion-disk radiation.