IMPLICATIONS OF THE VERY HIGH ENERGY GAMMA-RAY DETECTION OF THE QUASAR 3C279

Abstract

The Major Atmospheric Gamma-Ray Imaging Cherenkov Telescope very high energy (VHE) γ-ray astronomy collaboration recently reported the detection of the quasar 3C279 at >100 GeV γ-ray energies. Here, we present simultaneous optical (BVRI) and X-ray (RXTE PCA) data from the day of the VHE detection and discuss the implications of the snapshot spectral energy distribution for jet models of blazars. A one-zone synchrotron-self-Compton (SSC) origin of the entire spectral energy distribution (SED), including the VHE γ-ray emission is highly problematic as it would require an unrealistically low magnetic field. The measured level of VHE emission could, in principle, be interpreted as Compton upscattering of external radiation (e.g., from the broad-line regions, BLRs). However, such an interpretation would require either an unusually low magnetic field of B ∼ 0.03 G, or (in order to achieve approximate equipartition between magnetic field at B ∼ 0.25 G and relativistic electrons) an unrealistically high Doppler factor of Γ ∼ 140. In addition, such a model fails to reproduce the observed X-ray flux. Furthermore, both versions of leptonic one-zone models produce intrinsic VHE γ-ray spectra steeper than measured, even in the case of the lowest plausible extragalactic γγ absorption. We therefore conclude that a simple one-zone, homogeneous leptonic jet model is not able to plausibly reproduce the SED of 3C279 including the recently detected VHE γ-ray emission. This as well as the lack of correlated variability in the optical with the VHE γ-ray emission and the substantial γγ opacity of the BLR radiation field to VHE γ-rays suggests a multi-zone model in which the optical emission is produced in a different region than the VHE γ-ray emission. In particular, an SSC model with an emission region far outside the BLR reproduces the simultaneous X-ray–VHE γ-ray spectrum of 3C279. Alternatively, a hadronic model is capable of reproducing the observed SED of 3C279 reasonably well, both in scenarios in which only the internal synchrotron field serves as targets for pγ pion production, and with a substantial contribution from external photons, e.g., from the BLR. However, either version of the hadronic model requires a rather extreme jet power of up to Lj ∼ 1049 erg s−1, compared to a requirement of Lj ∼ 2 × 1047 erg s−1 for a multi-zone leptonic model.