[Paper Review] X-ray and multiwavelength view of NGC 4278. A LINER-Seyfert connection?

George Younes, D. Porquet+4 more|arXiv (Cornell University)|Apr 28, 2010

Astrophysical Phenomena and Observations72 references16 citations

TL;DR

This study investigates NGC 4278, a low-luminosity active galactic nucleus (LLAGN) with a LINER-like spectrum, using multiwavelength X-ray, UV, optical, and radio data from XMM-Newton and Chandra. It finds that the source alternates between LINER-like and Seyfert-like states depending on X-ray flux: at low flux, emission is dominated by a jet or radiatively inefficient accretion flow (RIAF); at high flux, a thin accretion disk dominates, with short-term X-ray variability and a Seyfert-like SED.

ABSTRACT

(Abridged) Based on UV to X-ray and radio to UV flux ratios, some argue that low ionization emission line regions (LINERs) and low luminosity AGN (LLAGN) are a scaled-down version of their more luminous predecessors Seyfert galaxies. Others, based on the lack of X-ray short (hours) time-scale variability, the non-detection of an iron line at 6.4 keV, and the faint UV emission, suggest the truncation of the classical thin accretion disk in the inner regions of the AGN where a radiatively inefficient accretion flow (RIAF) structure forms. We investigate the LINER-Seyfert connection by studying the unabsorbed, AGN powered, LINER galaxy NGC 4278. We analyzed one XMM-Newton and seven Chandra X-ray observations of NGC 4278 spread over a three year period. We detected a flux increase by a factor of ~3 on a ~3 months time-scale and by a factor of 5 between the faintest and the brightest observation separated by ~3 years. During only the XMM-Newton observation, where the highest flux level is detected, we found a 10% flux increase on a ~1 hour time-scale. A combination of an absorbed power law (N(H)~10^20 cm^-2, Gamma~2.2) plus a thermal component (kT~0.6 keV) were able to fit the Chandra spectra. The XMM-Newton spectra, where the highest X-ray flux is detected, are well fitted with a single absorbed power-law. No Fe K(alpha) emission line is detected at 6.4 keV. We constructed SEDs based on simultaneous or quasi simultaneous observations and compared them to LINER, radio-loud, and radio-quiet quasar SEDs. We find that at a low X-ray flux the NGC 4278 SED resembles that of typical LINER sources where the radio to X-ray emission can be considered as originating from a jet and/or RIAF, whereas at a high X-ray flux, NGC 4278 SED is more like a low luminosity Seyfert SED. Consequently, NGC 4278 could exhibit both LINER and Seyfert nuclear activity depending on the strength of its X-ray emission.

Motivation & Objective

• To resolve the long-standing debate on whether LINERs are scaled-down Seyferts or fundamentally different via multiwavelength monitoring.
• To determine the accretion mode in NGC 4278, a low-Eddington-radiation LLAGN with a broad Hα line.
• To test whether X-ray variability and SED shape depend on flux state, indicating a switch between accretion modes.
• To assess the role of a thin disk vs. RIAF in powering the X-ray and UV emission in low-luminosity AGN.

Proposed method

• Analyzed 1 XMM-Newton and 7 Chandra X-ray observations spanning ~3 years to study X-ray variability on timescales from hours to years.
• Fitted X-ray spectra with absorbed power laws and thermal components to model the central engine, using photon index Γ and NH for absorption.
• Used simultaneous XMM-Newton UV and X-ray data from the optical monitor to construct SEDs for flux state comparison.
• Compared SEDs of NGC 4278 at high and low X-ray fluxes with those of LINERs, radio-quiet, and radio-loud quasars to identify emission origin.
• Measured optical fluxes from HST ACS and WFPC2 data to assess long-term optical variability.
• Set upper limits on Fe Kα line equivalent width (22 eV) and tested for short-timescale variability in hardness ratios.

Experimental results

Research questions

• RQ1 Does NGC 4278 exhibit X-ray variability on short (hours) and long (months, years) timescales, and what does this imply for its accretion state?
• RQ2 Is the X-ray spectrum best explained by a thin accretion disk or a radiatively inefficient accretion flow (RIAF) at different flux states?
• RQ3 How does the spectral energy distribution (SED) of NGC 4278 change with X-ray flux, and does it resemble LINERs or low-luminosity Seyferts?
• RQ4 Is the UV-to-X-ray flux ratio consistent with that of Seyfert galaxies during high X-ray states, supporting the 'scaled-down AGN' model?
• RQ5 Does the presence of a broad Hα line and point-like nucleus support a Seyfert-like central engine despite its LINER classification?

Key findings

• NGC 4278 showed a flux increase by a factor of ~3 over a few months and by a factor of 5 over ~3 years, indicating strong long-term variability.
• A 10% flux increase was detected on a few-hour timescale during the XMM-Newton observation, suggesting transient short-timescale variability.
• The XMM-Newton spectrum at highest flux was well-fit by an absorbed power law (Γ ≈ 2.1, NH ≈ 3.8×10²⁰ cm⁻²) with no need for a thermal component.
• Chandra spectra required an additional thermal component (kT ≈ 0.6 keV) at lower flux states, indicating a cooler plasma at low activity.
• No Fe Kα emission line at 6.4 keV was detected, with a 22 eV upper limit on equivalent width, disfavoring strong reflection or disk fluorescence.
• Simultaneous UV and X-ray data revealed a UV flux of ~1.2×10⁻¹⁵ erg cm⁻² s⁻¹ Å⁻¹, consistent with Seyfert-like UV/X-ray ratios during high X-ray states.