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

TL;DR

This work investigates whether radiatively inefficient accretion flows (RIAFs) power the X-ray emission in two γ-ray–detected LLAGNs with jets, NGC 315 and NGC 4261, by combining NuSTAR and XMM-Newton data. Through timing and broad-band spectral analyses, the authors find no significant short-term variability and reveal distinct X-ray continua: NGC 315 exhibits a hard cutoff power-law with $E_{ m cut} = 18.45^{+8.00}_{-4.51}$ keV and a neutral Fe Kα line with high reflection $R = 0.61^{+0.18}_{-0.17}$, consistent with a truncated disk and bremsstrahlung cooling; NGC 4261 requires two power-law components (HPL and SPL) with Fe XXV at 6.71 keV and a relatively weak reflection $R = 0.18^{+0.15}_{-0.14}$, with the hard component likely arising from Comptonization in the RIAF and the soft component from the inner jet. The results support the interpretation that the X-ray emission in both LLAGNs is dominated by RIAF processes, though jet contributions are present in NGC 4261, and they demonstrate the diagnostic power of hard X-ray spectra for probing accretion physics in LLAGNs with jets. This work adds valuable hard X-ray samples for LLAGN studies and motivates future observations up to higher energies to constrain $E_{ m cut}$ and Fe-line features.

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 also adds samples to future research on LLAGNs. Methods. Weconducted an analysis of the X-ray data obtained from NuSTAR and XMM-Newton observations of NGC 315 and NGC 4261, encompassing 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 both NGC 315 and NGC 4261. The X-ray continuum emission of NGC 315 is suitable for cutoff power-law (PL) fitting, yielding a cutoff energy of Ecut = 18.45 keV, which is the lowest value found in LLAGNssofar. 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 are R = 0.61 for NGC 315 and R = 0.18 for NGC 4579. Neither NGC 315 nor NGC 4261 shows evidence of a Compton reflection bump. Conclusions. The X-ray spectral characteristics support the RIAF emission as 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 NGC315, while Comptonization within the RIAF is more likely responsible for the X-ray emission in NGC 4261.

Figures (6)

• Figure 1: XMM-Newton/pn and Nustar/FPMA background-corrected light curves with a time bin of 1 ks for NGC 315 (left panel) and NGC 4261 (right panel), corresponding to the energy ranges of 0.5--10 KeV and 3--30 keV, respectively. In each case, time zero corresponds to the start time of the observation by the respective satellite.
• Figure 2: Joint spectral fitting results for NGC 315. Left panel: Fitted spectrum comprising a PL, thermal emission from diffuse gas, and a Gaussian line. Right panel: Fitted spectrum comprising a cutoff PL, thermal emission from diffuse gas, and a Gaussian line.
• Figure 3: Contour plots of NGC 315 illustrating the relationships between the cutoff energy and photon spectral index (top panel) and between the cutoff energy and reflection fraction (bottom panel). Red, green, and blue lines represent 1$\sigma$, 2$\sigma$, and 3$\sigma$ contours, respectively.
• Figure 4: Contour plots of plasma temperature and optical depth obtained using the $\texttt{compTT}$ model for NGC 315 (top panel) and NGC 4261 (bottom panel). Red, green, and blue lines represent 1$\sigma$, 2$\sigma$, and 3$\sigma$ contours, respectively.
• Figure 5: Best-fit result of the NuSTAR+XMM-Newton spectrum for NGC 4261. The fitted spectrum includes a SPL, a HPL, thermal emission from diffuse gas, and a Gaussian line.