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 Very Large Array observations available of the RQ optically selected Palomar–Green (PG) quasars at z < 0.5. We derive the 5–8.4 GHz spectral slope, α_R, for 25 of the complete and well-studied sample of 71 RQ PG quasars. We find a highly significant correlation of α_R with L/L_Edd, where high L/L_Edd(>0.3) quasars have a steep slope (α_R < −0.5), indicative of an optically thin synchrotron source. In contrast, lower L/L_Edd(<0.3) quasars generally have a flat slope (α_R > −0.5), indicative of a compact optically thick synchrotron source. Flat α_R quasars also have a low Fe II/Hβ 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 α_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 disc 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 allows the exploration of its relation with the coronal X-ray emission.

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