The emission mechanism responsible for the bulk of energy from radio to X–rays in low ionization emission line regions ( LINERs ) and Low Luminosity Active Galactic Nuclei ( LLAGN ) has been long debated .
Based on UV to X–ray and radio to UV flux ratios , some argue that LINERs/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 compared to typical AGNs , 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 LINER galaxy NGC 4278 that accretes at a low rate ( L _ { bol / Edd } \sim 7 \times 10 ^ { -6 } ) but exhibits a broad H \alpha line , and a point-like nucleus in radio , optical , UV and X-rays .
We analyzed one XMM-Newton and seven Chandra X-ray observations of NGC 4278 spread over a three year period , allowing the study of the X–ray variability at different time-scales ( hours , months , years ) .
We also examined the radio to X-ray spectral energy distribution to constrain the accretion mode in the nucleus of NGC 4278 .
Long time-scale ( months ) variability is observed where the flux increased by a factor of \sim 3 on a time-scale of a few months and by a factor of 5 between the faintest and the brightest observation separated by \sim 3 years .
During the XMM-Newton observation , where the highest flux level is detected , we found a 10 \% flux increase on a short time-scale of a few hours , while the light curves for the different Chandra observations do not show short time-scale ( minutes to hours ) variability .
A combination of an absorbed power law ( N _ { H } \approx 10 ^ { 20 } cm ^ { -2 } , \Gamma = 2.2 ^ { +0.1 } _ { -0.2 } ) plus a thermal component ( kT \approx 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 an absorbed power–law with no need for a thermal component as the emission from the power–law component is dominant .
The power–law photon index is \sim 2.1 and the hydrogen column density is of the order of 10 ^ { 20 } cm ^ { -2 } .
Neither a narrow nor a broad Fe K { \alpha } emission line at 6.4 keV are detected with a 22 eV and 118 eV upper limits derived on their equivalent widths .
We derive optical fluxes from archival HST ACS observations and detected optical variability on time–scales of years .
For the first time for this source , thanks to the optical/UV monitor on board XMM-Newton , we obtained simultaneous UV and X-ray flux measurements .
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 .