Constraining the X-ray reflection in low accretion-rate active galactic nuclei using XMM-Newton, NuSTAR, and Swift

Abstract

Context. An interesting feature of active galactic nuclei (AGN) accreting at low rates is the weakness of the reflection features in their X-ray spectra, which may result from the gradual disappearance of the torus with decreasing accretion rates. It has been suggested that low-luminosity AGN (LLAGN) would exhibit a different reflector configuration than high-luminosity AGN, covering a smaller fraction of the sky or simply having less material. Additionally, we note that the determination of the spectral index (Γ) and the cut-off energy of the primary power-law emission is affected by the inclusion of reflection models, showing their importance in studying accretion mechanisms. This is especially valid in the case of the LLAGN which has previously shown a high dispersion in the relation between Γ and the accretion rate. Aims. Our purpose is to constrain the geometry and column density of the reflector in a sample of LLAGN covering a broad X-ray range of energy by combining data from XMM-Newton+ NuSTAR + Swift. The spectral analysis also allows us to investigate the accretion mechanism in LLAGN. Methods. We used XMM-Newton+ NuSTAR + Swift observations of a hard X-ray flux-limited sample of 17 LLAGN from BASS/DR2 with accretion rates of λ Edd =L Bol /L Edd <10 −3 . We fit all spectra using the reflection model for torus (BORUS) and accretion disk (XILLVER) reflectors. Results. We found a tentative correlation between the torus column density and the accretion rate, with LLAGN showing a lower column density than the high-luminosity objects. We also confirm the relation between Γ and λ Edd , with a smaller scatter than previously reported, thanks to the inclusion of high-energy data and the reflection models. Our results are consistent with a break at λ Edd ∼ 10 −3 , which is suggestive of a different accretion mechanism compared with higher accretion AGN.

Coronal properties of low-accreting AGNs using Swift, XMM–Newton, and NuSTAR observations

“…This suggests that at low Eddington ratio, the average column density decreases. Diaz et al ( 2023 ) also found a similar relation between N tor H and λ Edd . The relation between N tor H and λ Edd also supports the Eddington ratio regulated unification and growth model (e.g.…”

“…Thus, we are unable to make a firm conclusion on the correlation/anticorrelation of λ Edd − . Recently, Diaz et al ( 2023 ) studied a sample of LLAGNs and found similar results. They argued that due to the small number of sources, they did not find a statistically significant correlation of -λ Edd .…”

The High-Energy X-ray Probe (HEX-P): the circum-nuclear environment of growing supermassive black holes

“…A root cause is the considerable challenge to select and classify true low-accretionpower AGN, especially at high line-of-sight column densities. NuSTAR has provided an unprecedented view into the hard X-ray properties of the circum-nuclear environment of lowluminosity AGN for the first time (Ursini et al, 2015;Annuar et al, 2017;Young et al, 2018;Younes et al, 2019;Annuar et al, 2020;Diaz et al, 2020;Baloković et al, 2021;Diaz et al, 2023). NuSTAR has also led to the discovery and classification of a few low-luminosity Compton-thick AGN (Annuar et al, 2017;Brightman et al, 2018;Da Silva et al, 2021), providing exciting evidence that suggests AGN can sustain a significant obscuration structure at low luminosities.…”

BASS. XLII. The Relation between the Covering Factor of Dusty Gas and the Eddington Ratio in Nearby Active Galactic Nuclei

“…It should be stressed that in stage 1, the covering factors could actually be lower than what was inferred here due to our AGN selection, which is missing AGN that accrete at very low Eddington ratios (log 4 Edd l < -). In fact, besides showing little absorption (see also She et al 2018), AGN accreting at very low λ Edd also typically display faint reflection features (e.g., Ptak et al 2004;Bianchi et al 2017;Diaz et al 2020Diaz et al , 2023Jana et al 2023;see Ho 2008 for a review of the subject).…”