What drives the radio slopes in radio-quiet quasars?

Abstract

The origin of the radio emission in Radio Quiet (RQ) quasars is not established yet. Important hints can be provided by the spectral slope, and its relation to other emission properties. We compiled the highest resolution 5 and 8.4 GHz VLA observations available of the RQ optically selected PG quasars at z<0.5. We derive the 5-8.4 GHz spectral slope, alpha_R, for 25 of the complete and well studied sample of 71 RQ PG quasars. We find a highly significant correlation of alpha_R with L/L_Edd, where high L/L_Edd (>0.3) quasars have a steep slope (alpha_R<-0.5), indicative of an optically thin synchrotron source. In contrast, lower L/L_Edd (<0.3) quasars generally have a flat slope (alpha_R>-0.5), indicative of a compact optically thick synchrotron source. Flat alpha_R quasars also have a low Fe II/Hbeta line ratio, and a flat soft X-ray slope. The 16 Radio Loud (RL) PGs do not follow the RQ quasar set of correlations, and their alpha_R is set by M_BH, suggesting that the radio emission mechanisms in RL and RQ quasars are different. A possible interpretation is that high L/L_Edd RQ quasars produce a strong outflow and an associated optically thin synchrotron emission. In lower L/L_Edd RQ quasars, the strong outflow is missing, and only a compact optically thick radio source remains, possibly associated with the accretion disk coronal emission. A systematic study of RQ quasars at higher frequencies, and higher resolution, can test whether a compact flat source indeed resides in the cores of all RQ quasars, and allow the exploration of its relation with the coronal X-ray emission.