Hard X-ray view of two γ-ray-detected low-luminosity active galactic nuclei: NGC 315 and NGC 4261

School of Physics, Beijing Institute of Technology, Beijing 100081, China

^⋆ Corresponding author: j.zhang@bit.edu.cn

Received: 19 July 2025
Accepted: 3 October 2025

Abstract

Aims. The accretion disk of low-luminosity active galactic nuclei (LLAGNs) is a radiatively inefficient accretion flow (RIAF). Our goal is to find evidence of RIAF radiation from LLAGNs with jets and analyze their radiation properties, which will provide samples that can be used in future research on LLAGNs.

Methods. We conducted an analysis of the X-ray data obtained from NuSTAR and XMM-Newton observations of NGC 315 and NGC 4261 that encompasses both timing and spectral investigations. The joint X-ray spectra of the two LLAGNs were fitted using various functional forms and radiative models in XSPEC.

Results. No significant variability on timescales of days is observed for either NGC 315 or NGC 4261. The X-ray continuum emission of NGC 315 is suitable for cutoff power-law (PL) fitting. This fitting yields a cutoff energy (E_cut) of 18.45^+8.00_−4.51 keV, which is the lowest value found in LLAGNs so far. In contrast, the X-ray continuum of NGC 4261 is composed of two PL components, with no signs of a cutoff energy. A prominent neutral Fe Kα line is observed in NGC 315, while an ionized Fe XXV line is seen in NGC 4261. The derived reflection fractions (R) are 0.61^+0.18_−0.17 for NGC 315 and 0.18^+0.15_−0.14 for NGC 4261. Neither NGC 315 nor NGC 4261 shows evidence of a Compton reflection bump.

Conclusions. The X-ray spectral characteristics indicate RIAF emission is the dominant origin of the X-rays in both sources, although an additional soft PL component originating from the inner jet is observed in NGC 4261. The higher reflection fraction compared to other LLAGNs, along with the detection of a neutral Fe Kα line, suggests the existence of a truncated accretion disk with a relatively small radius in NGC 315. Bremsstrahlung radiation appears to be the dominant cooling mechanism for the plasma in NGC 315, while Comptonization within the RIAF is more likely responsible for the X-ray emission in NGC 4261.