Constraining the X-ray reflection in low accretion-rate active galactic nuclei using XMM-Newton, NuSTAR, and Swift
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.
Additional Information© The Authors 2023. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This article is published in open access under the Subscribe-to-Open model. Subscribe to A&A to support open access publication. We thank the referee for the valuable comments that improved the manuscript. D.Y. acknowledges financial support from the Doctorate Fellowship program FIB-UV of the Universidad de Valparaíso and the Max Planck Society by a Max Planck partner group. LHG acknowledges funds by ANID – Millennium Science Initiative Program – ICN12_009 awarded to the Millennium Institute of Astrophysics (MAS). ELN acknowledges financial support from ANID Beca 21200718. CR acknowledges support from the Fondecyt Iniciacion grant 11190831 and ANID BASAL project FB210003. MB acknowledges support from the YCAA Prize Postdoctoral Fellowship. NOC acknowledges support from CONACyT. J.A.G. acknowledges support from NASA grant 80NSSC21K1567.
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