Time Variability Detected in the Gamma-Ray Emission of 3C 273 by OSSE

Abstract

We report results of a 3 week observation of the quasar 3C 273 with the Oriented Scintillation Spectrometer Experiment (OSSE) instrument on the Compton Gamma Ray Observatory (CGRO). During the observation between 1994 August 31 and September 20, 3C 273 exhibited variability on timescales as short as 3-4 days in the 0.05-0.15 MeV energy range. This is the shortest measured variability of 3C 273 in this energy range. The measured flux at 100 keV is 2.40 ± 0.06 (×10-2 ph cm-2 s-1 MeV-1), which is the highest recorded flux for 3C 273 by OSSE in 5 years of monitoring and nearly twice as great as the previously recorded peak flux measured in 1991 June. While the flux varied by a factor of 1.7, no significant gamma-ray spectral evolution in the OSSE band was observed throughout the viewing period. The data for the full viewing period are described better with a broken power law than a single power law, with photon spectral index Γ = 1.62 ± 0.08 below a break at 300 ± 100 keV, and with a spectral index change of ΔΓ = 0.9+ 1.6−0.6. This break energy is significantly lower than the 1 MeV break reported by Johnson et al. in 1991 June.