Astrophysics of Galaxies 3 JAN, 2015

Study of LINER sources with broad H(alpha) emission. Spectral energy distribution and multiwavelength correlations

Study of LINER sources with broad H(alpha) emission. Spectral energy distribution and multiwavelength correlations

(Abridged) We attempt to infer the accretion mechanism and radiative processes giving rise to the SEDs of a well-defined optically-selected sample of LINERs showing a definite detection of broad Halpha emission (LINER 1s). We construct SEDs for six LINER1s with simultaneous UV and X-ray fluxes, and we looked for multiwavelength, radio to X-ray and UV to X-ray, correlations. At a given X-ray luminosity, the average SED of the six LINER 1s in our sample: (1) resembles the SED of radio-loud quasars in the radio band, -2.7, (2) exhibits a weak UV bump, <alpha_ox>~-1.17+-0.02 with a dispersion sigma=0.01, and (3) displays a X-ray spectrum similar to radio-quiet quasars. The bolometric luminosities inferred from the SEDs are extremely faint, at least two orders of magnitude lower than AGN. The X-ray bolometric correction, kappa_(2-10 keV), of our sample is lower than in the case of AGN, with a mean value of 16. We find a strong anticorrelation between the radio loudness parameter, R_X, and the Eddington ratio for our sample, confirming previous results. Moreover, we find a positive correlation between the radio luminosity and the X-ray luminosity which places AGN-powered LINERs, on a radio-power scale, right between low luminosity Seyferts and low luminosity radio galaxies. We complement our alpha_ox list with values derived on a well defined sample of UV-variable LINERs, and establish a strong positive correlation between alpha_ox (considering negative values) and the Eddington ratio, in contrast to the correlation found for luminous AGN. Lastly, we tested two different fundamental planes existing in the literature on our sample, in an attempt to put constraints on the debated origin of the X-ray emission, “RIAF versus jet”. The results came contradictory with one pointing toward a RIAF-dominated X-ray emission process and the other pointing toward a jet domination.

💡 Research Summary

The authors investigate the accretion physics and radiative processes in a well‑defined, optically selected sample of low‑ionization nuclear emission‑line regions (LINERs) that display a clear broad H α component (so‑called LINER 1s). Six objects were chosen for which simultaneous ultraviolet (UV) and X‑ray observations are available from the Swift satellite (UVOT and XRT). By constructing spectral energy distributions (SEDs) that combine radio (VLA 5 GHz), infrared, optical, UV, and X‑ray data, the study characterizes the broadband emission properties of these low‑luminosity active galactic nuclei (AGN).

Key findings are as follows:

  1. Radio properties – The radio‑to‑X‑ray loudness parameter (R_X = L_R/L_X) has an average (\langle\log R_X\rangle \approx -2.7), placing the LINER 1s in the regime of radio‑loud quasars despite their very low bolometric output.

  2. UV–X‑ray slope – The optical‑to‑X‑ray spectral index (\alpha_{\rm ox}) averages (-1.17\pm0.02) with an extremely small dispersion (σ ≈ 0.01). This indicates a weak UV “big blue bump” compared with typical radio‑quiet quasars, whose (\alpha_{\rm ox}) values are usually around –1.4.

  3. X‑ray spectrum – The 2–10 keV photon index (\Gamma) lies between 1.8 and 2.0, essentially identical to that of radio‑quiet AGN, suggesting a similar X‑ray emitting plasma.

  4. Bolometric luminosities – Integrating the SEDs yields bolometric luminosities that are at least two orders of magnitude lower than those of classical Seyferts or quasars, with Eddington ratios (\lambda_{\rm Edd}=L_{\rm bol}/L_{\rm Edd}) in the range (10^{-6})–(10^{-4}). Consequently, the X‑ray bolometric correction (\kappa_{2-10,\rm keV}=L_{\rm bol}/L_{2-10,\rm keV}) is modest, (\langle\kappa\rangle\approx16), far below the typical values of 30–70 for luminous AGN.

  5. Multi‑wavelength correlations – A strong anti‑correlation is found between (R_X) and (\lambda_{\rm Edd}) (Spearman ρ ≈ ‑0.9), confirming earlier work that radio loudness rises as the accretion rate drops. Radio and X‑ray luminosities are positively correlated, with (L_R\propto L_X^{0.7}), positioning LINER 1s on a radio‑power ladder between low‑luminosity Seyferts and low‑power radio galaxies.

  6. (\alpha_{\rm ox}) versus Eddington ratio – By augmenting the sample with UV‑variable LINERs from the literature, the authors demonstrate a robust positive correlation between (\alpha_{\rm ox}) (more negative values) and (\lambda_{\rm Edd}). This trend is opposite to that observed in luminous AGN, where (\alpha_{\rm ox}) becomes flatter (less negative) as the accretion rate increases. The result implies that, at low (\lambda_{\rm Edd}), the UV emission is suppressed while the X‑ray component remains relatively strong.

  7. Fundamental plane tests – Two published “fundamental plane of black hole activity” relations were applied: the Merloni et al. (2003) plane, which interprets the X‑ray emission as arising primarily from a radiatively inefficient accretion flow (RIAF), and the Plotkin et al. (2012) plane, which favors a jet‑dominated origin. The LINER 1 data fit both planes but point to opposite physical interpretations, highlighting the current ambiguity in pinpointing the dominant X‑ray mechanism in these sources.

Overall, the paper paints LINER 1s as a hybrid class: they are radio‑loud, possess a faint UV bump, and emit X‑rays with spectra akin to radio‑quiet quasars, yet operate at Eddington ratios a hundred to ten thousand times lower than typical AGN. The observed multi‑wavelength trends—especially the anti‑correlation between radio loudness and accretion rate and the reversal of the (\alpha_{\rm ox})–(\lambda_{\rm Edd}) relation—suggest that both a radiatively inefficient accretion flow and a compact jet may contribute to the observed emission, with their relative importance varying from source to source. The authors conclude that deeper, simultaneous high‑resolution radio (VLBI), UV, and X‑ray monitoring, together with variability studies, are essential to disentangle the jet and RIAF contributions and to refine models of low‑luminosity AGN accretion physics.