The X-ray and radio connection in low-luminosity active nuclei

Abstract

We present the results of the correlation between the nuclear 2-10 keV X-ray and radio (at 2 cm, 6 cm, and 20 cm) luminosities for a well-defined sample of local Seyfert galaxies. We use a sample of low luminosity radio galaxies (LLRGs) for comparison. In both Seyfert and LLRG samples, X-ray and radio luminosities are significantly correlated over 8 orders of magnitude, indicating that the X-ray and radio emission sources are strongly coupled. Moreover, both samples show a similar regression slope, L X ∝ L 0.97 R , but Seyfert galaxies are three orders of magnitude less luminous in the radio band than LLRGs. This suggests that either similar physical mechanisms are responsible for the observed emission or a combination of different mechanisms ends up producing a similar correlation slope. Indeed, the common belief for LLRG is that both the X-ray and radio emission are likely dominated by a relativistic jet component, while in Seyfert galaxies the X-ray emission probably arises from a disk-corona system and the radio emission is attributed to a jet/outflow component. We investigate the radio loudness issue in the two samples and find that the Seyfert galaxies and the LLRGs show a different distribution of the radio loudness parameters. No correlation is found between the luminosity and the radio loudness; however, the latter is related to the black hole mass and anti-correlated with the Eddington ratio. The dichotomy in the radio loudness between Seyfert and LLRG observed down to low Eddington ratios, L 2−10 keV /L Edd ∼ 10 −8 , does not support the idea that the origin of the radio loudness is due to a switch in the accretion mode.

Discovery of millimetre-wave excess emission in radio-quiet active galactic nuclei

“…We determined the RQ sources from Doi et al (2011) based on their core 5 GHz luminosity and the associated log LR/LX < −2.8 criterion (Panessa et al 2007) 1 . In the figure, we plot only 95 GHz flux densities, and average over multi-epoch data when present.…”

“…A connection between the radio and X-ray emission in RQ AGN is suggested by the correlation of the radio luminosity LR at 5 GHz and the X-ray luminosity LX (Brinkmann et al 2000;Salvato et al 2004; Wang et al 2006; Panessa et al 2007). Laor & Behar (2008) used the PG quasar sample (Schmidt & Green 1983) to demonstrate that not only are LR(≡ νLν at 5 GHz) and LX (0.2 -20 keV) correlated over a large range of AGN luminosity, but that the correlation follows the well established relation for coronally active cool stars LR/LX ∼ 10 −5 (Güdel & Benz 1993).…”

X-ray spectral parameters for a sample of 95 active galactic nuclei

“…According to this criterion, a galaxy is radio quite if P 5 GHz ≤ 10 32 erg s -1 Hz -1 (or P 5 GHz~1 0 25 W Hz -1 sr -1 ) and 5 GHz / 10 B band FF -< , respectively. For several objects (in the absence of data for radio-optical criterion) we also used the X-ray criterion, as ratio between 2-10 keV and 20-100 keV intrinsic luminosities and radio luminosity at the above frequencies (see, e.g., Terashima & Wilson 2003;Panessa et al 2007) …”

No AGN evidence in NGC 1614 from deep radio VLBI observations

“…Using our 5 GHz luminosity upper limit and assuming that most of the hard X-ray emission (1.4 × 10 41 erg s −1 within a region of 1.5 kpc; Pereira-Santaella et al 2015b) is due to the AGN, NGC 1614 would be compatible with a LLAGN in Terashima & Wilson (2003) diagnostic plots. However, according to the X-ray and radio 6 cm relation by Panessa et al (2007), the expected 6 cm luminosity would be 1.1 +4.0 −0.8 × 10 37 erg s −1 , possibly but unlikely missed by our observations. Anyhow, since most of the hard X-ray luminosity can be explained with star formation (Pereira-Santaella et al 2011), we find more plausible that NGC 1614 is being essentially powered by stellar activity and/or supernova shocks.…”