Broad Band X-Ray Spectra of Two Low-luminosity Active Galactic Nuclei NGC 1566 and NGC 4941

TL;DR

This study presents the first broad band X-ray spectra of two low-luminosity AGNs, revealing details about their accretion disk reflection, torus geometry, and obscuration, with implications for AGN unification models.

Contribution

It provides the first broad band X-ray spectral analysis of NGC 1566 and NGC 4941, constraining torus geometry and reflection features in low-luminosity AGNs.

Findings

No significant broad iron-K extalpha ext{ } line detected.

Moderate reflection strength from the torus observed.

Obscured AGN fraction aligns with luminosity-dependent models.

Abstract

We report the first broad band X-ray spectra of the low luminosity active galactic nuclei (LLAGN) NGC 1566 (type 1.5) and NGC 4941 (type 2) observed with Suzaku and Swift/BAT covering the 0.5-195 keV band. Both targets have hard X-ray luminosities of ~ 10^{41-42} ergs/s in the 15-55 keV band. The spectra of the nucleus are well reproduced by a sum of partially or fully covered transmitted emission and its reflection from the accretion disk, reprocessed emission from the torus accompanied by a strong narrow iron-K\alpha line, and a scattered component (for NGC 4941). We do not significantly detect a broad iron-K\alpha line from the inner accretion disk in both targets, and obtain an upper limit on the corresponding solid angle of \Omega/2\pi < 0.3 in NGC 1566. The reflection strength from the torus is moderate, \Omega/2\pi =0.45^{+0.13}_{-0.10} in NGC 1566 and \Omega/2\pi =0.64^{+0.69}_{-0.27} in NGC 4941. Comparison of the equivalent width of the narrow iron-K\alpha line with a model prediction based on a simple torus geometry constrains its half-opening angle to be \theta_oa ~ 60-70 degree in NGC 4941. These results agree with the obscured AGN fraction obtained from hard X-ray and mid-infrared selected samples at similar luminosities. Our results support the implication that the averaged covering fraction of AGN tori is peaked at L 10^{42-43} ergs/s but decreases toward lower luminosities.