2010/04/23 by George Younes, G. Younes, D. Porquet +5
Physics and Astronomy · #Astrophysical Phenomena and Observations #Astrophysics and Cosmic Phenomena #Galaxies: Formation, Evolution, Phenomena #astro-ph.GA
paper · pdf · doi:10.1051/0004-6361/201014371
14 pages, 15 figures, accepted for publication in Astronomy and Astrophysics
openalex publication_date 2010/04/23 · arxiv created 2010/04/28 · arxiv updated 2015/05/18 · openalex created_date 2020/11/23 · openalex updated_date 2026/08/01
<i>Context. <i/>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.<i>Aims. <i/>We investigate the LINER-Seyfert connection by studying the unabsorbed LINER galaxy NGC 4278 that accretes at a low rate (<i>L<i/><sub>bol/Edd<sub/> ~ 7 × 10<sup>-6<sup/>) but exhibits a broad H<i>α<i/> line, and a point-like nucleus in radio, optical, UV and X-rays.<i>Methods. <i/>We analyzed one XMM-Newton and seven <i>Chandra<i/> 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.<i>Results. <i/>Long time-scale (months) variability is observed where the flux increased by a factor of ~3 on a time-scale of a few months and by a factor of 5 between the faintest and the brightest observation separated by ~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 <i>Chandra<i/> observations do not show short time-scale (minutes to hours) variability. A combination of an absorbed power law (<i>N<i/><sub>H<sub/> <i>≈<i/> 10<sup>20<sup/> cm<sup>-2<sup/>, <i>Γ<i/> = 2.2) plus a thermal component (<i>kT<i/> <i>≈<i/> 0.6 keV) were able to fit the <i>Chandra<i/> spectra. The <i>XMM-Newton<i/> 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 2.1 and the hydrogen column density is of the order of 10<sup>20<sup/> cm<sup>-2<sup/>. Neither a narrow nor a broad Fe K 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 <i>HST<i/> 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 <i>XMM-Newton<i/>, 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.