Possible evidence of a universal radio/X-ray correlation in a near-complete sample of hard X-ray selected seyfert galaxies

Abstract

ABSTRACT Because the disc–jet coupling likely depends on various properties of sources probed, the sample control is always an important but challenging task. In this work, we re-analysed the INTEGRAL hard X-ray-selected sample of Seyfert galaxies. We only consider sources that have measurements in black hole (BH) mass, and luminosities in radio and X-rays. Our final sample includes 64 (out of the original 79) sources, consists of both bright active galactic nucleus and low-luminosity ones. The 2–10 keV X-ray Eddington ratio LX/LEdd locates in the range between ∼10−4.5 and ∼10−0.5. We first find that, because of the similarity in the LHX/LX distribution, the X-ray origin of radio-loud Seyferts may be the same to that of radio-quiet (RQ) ones, where we attribute to the hot accretion flow (or similarly, the corona). We then investigate the connections between luminosities in radio and X-rays. Since our sample suffers a selection bias of a BH mass MBH dependence on LX/LEdd, we focus on the correlation slope ξX between the radio (at 1.4 GHz) and X-ray luminosities in Eddington unit, i.e. $({L_{\rm R}}/{L_{\rm Edd}}) \propto ({L_{\rm X}}/{L_{\rm Edd}})^{\xi _{\rm X}}$. We classify the sources according to various properties, i.e. 1) Seyfert classification, 2) radio loudness, and 3) radio morphology. We find that, despite these differences in classification, all the sources in our sample are consistent with a universal correlation slope ξX (note that the normalization may be different), with ξX = 0.77 ± 0.10. This is unexpected, considering various possible radio emitters in RQ systems. For the jet (either relativistic and well collimated, or sub-relativistic and weakly collimated) interpretation, our result may suggest a common/universal but to be identified jet launching mechanism among all the Seyfert galaxies, while properties like BH spin and magnetic field strength only play secondary roles. We further estimate the jet production efficiency ηjet of Seyfert galaxies, which is $\eta _{\rm jet}\approx 1.9^{+0.9}_{-1.5}\times 10^{-4}$ on average. We also find that ηjet increases as the system goes fainter. Alternative scenarios for the radio emission in RQ systems are also discussed.