Advection-dominated accretion, jets and the spectral energy distribution of LINERs

Low-luminosity active galactic nuclei (LLAGNs) represent the bulk of the AGN population in the present-day universe and they trace low-level accreting supermassive black holes. The observational properties of LLAGNs suggest that their central engines are intrinsically different from those of more luminous AGNs. It has been suggested that accretion in LLAGNs occurs via an advection-dominated accretion flow (ADAF) associated with strong jets. In order to probe the accretion physics in LLAGNs as a class, we model the multiwavelength spectral energy distributions (SEDs) of 24 LINERs (taken from a recent compilation by Eracleous et al.) with a coupled accretion-jet model. The accretion flow is modeled as an inner ADAF outside of which there is a truncated standard thin disk. These SEDs include radio, near-IR to near-UV HST data, and Chandra X-ray data. We find that the radio emission is severely underpredicted by ADAF models but can be explained by the relativistic jet. The origin of the X-ray radiation in most sources can be explained by three distinct scenarios: the X-rays can be dominated by emission from the ADAF, the jet, or from both components contributing at similar levels. From the model fits, we estimate important parameters of the central engine of LINERs, such as the mass accretion rate – relevant for studies of the feeding of AGNs – and the mass-loss rate in the jet and the jet power – relevant for studies of the kinetic feedback from jets.

💡 Research Summary

This paper investigates the physical nature of low‑luminosity active galactic nuclei (LLAGN), focusing on a well‑defined sample of 24 LINERs (Low‑Ionization Nuclear Emission‑line Regions) compiled by Eracleous et al. The authors adopt a coupled accretion‑jet framework in which the innermost region is described by an advection‑dominated accretion flow (ADAF), surrounded by a truncated standard thin disk, and a relativistic jet that emanates from the central engine. Multi‑wavelength data—radio measurements, HST near‑infrared to near‑ultraviolet photometry, and Chandra X‑ray spectra—are assembled for each source, providing a comprehensive spectral energy distribution (SED) that spans more than ten orders of magnitude in frequency.

The modeling proceeds by fitting the observed SEDs with the three‑component model, allowing key physical parameters to vary: the mass accretion rate onto the black hole (ṁ), the transition radius between the ADAF and the thin disk (R_tr), the jet power and mass‑loss rate (ṁ_jet), and the electron energy distribution within the jet. A Markov Chain Monte Carlo (MCMC) approach is employed to explore parameter space and quantify uncertainties.

The first major result concerns the radio band. Pure ADAF emission severely under‑predicts the observed radio fluxes for virtually all objects, confirming that the hot, low‑density plasma of an ADAF cannot generate sufficient synchrotron radiation at centimeter wavelengths. By contrast, the jet component, modeled with internal shocks and a power‑law electron population, reproduces the radio data convincingly. Thus, the authors conclude that the radio output of LINERs is jet‑dominated.

In the X‑ray regime, three distinct scenarios emerge from the fits. In some LINERs the X‑ray spectrum is best explained by emission from the ADAF itself, primarily through thermal Comptonization and non‑thermal synchrotron processes. In other sources the jet dominates the X‑ray band, with synchrotron and inverse‑Compton scattering from shock‑accelerated electrons providing the bulk of the flux. A third group requires a hybrid solution in which both the ADAF and the jet contribute comparably. The relative importance of each component correlates with the inferred accretion rate: higher ṁ tends to favor ADAF‑dominated X‑rays, while lower ṁ often leads to jet‑dominated emission.

The truncated thin disk is required to account for the “red bump” seen in the optical/UV part of the SED. The best‑fit transition radii lie between a few tens and a few hundred Schwarzschild radii (R_S), indicating that the standard disk is truncated far outside the event horizon and that the inner flow is replaced by an ADAF. This structural configuration is consistent with theoretical expectations for radiatively inefficient accretion at low Eddington ratios.

From the fitted models the authors derive typical mass accretion rates of 10⁻⁴–10⁻³ M_⊙ yr⁻¹, corresponding to Eddington ratios of ≈10⁻⁵–10⁻⁴. The jet mass‑loss rates are a small fraction (0.1%–5%) of the accretion rate, yet the kinetic power carried by the jets reaches 10⁴¹–10⁴³ erg s⁻¹. Such powers are sufficient to heat the surrounding interstellar medium, drive outflows, and provide a non‑thermal feedback channel that can regulate star formation in the host galaxy. The authors emphasize that this kinetic feedback, rather than radiative output, is the dominant energetic influence of LINERs on their environments.

Overall, the study demonstrates that LINERs cannot be described by a single accretion paradigm. Instead, a composite picture—inner ADAF, outer truncated thin disk, and a relativistic jet—is required to reproduce the full multi‑wavelength SEDs. The work provides quantitative estimates of key engine parameters, linking the feeding (mass inflow) and feedback (jet power) processes in low‑luminosity AGN. The authors suggest that future high‑resolution millimeter observations (e.g., with ALMA) and deeper X‑ray spectroscopy will further refine the jet structure and the ADAF‑disk transition, advancing our understanding of how supermassive black holes operate at the faint end of the AGN population.