What drives the radio slopes in radio-quiet quasars?

Ari Laor, Ranieri D. Baldi, Ehud Behar

Research output: Contribution to journalArticlepeer-review


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, αR, for 25 of the complete and well-studied sample of 71 RQ PG quasars. We find a highly significant correlation of αR with L/LEdd, where high L/LEdd(>0.3) quasars have a steep slope (αR < −0.5), indicative of an optically thin synchrotron source. In contrast, lower L/LEdd(<0.3) quasars generally have a flat slope (αR > −0.5), indicative of a compact optically thick synchrotron source. Flat αR 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 αR is set by MBH, suggesting that the radio emission mechanisms in RL and RQ quasars are different. A possible interpretation is that high L/LEdd RQ quasars produce a strong outflow and an associated optically thin synchrotron emission. In lower L/LEdd 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.

Original languageEnglish
Pages (from-to)5513-5523
Number of pages11
Issue number4
StatePublished - 1 Feb 2019


  • quasars: general

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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