X-ray and multiwavelength view of NGC 4278

Abstract

Context. 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. Aims. We investigate the LINER-Seyfert connection by studying the unabsorbed LINER galaxy NGC 4278 that accretes at a low rate (L bol/Edd ∼ 7 × 10 −6 ) but exhibits a broad Hα line, and a point-like nucleus in radio, optical, UV and X-rays. Methods. 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. Results. 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 Chandra observations do not show short time-scale (minutes to hours) variability. A combination of an absorbed power law (N H ≈ 10 20 cm −2 , Γ = 2.2 +0.1 −0.2 ) 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 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 20 cm −2 . Neither a narrow nor a broad Fe Kα 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...

X-ray spectral variability of seven LINER nuclei withXMM-NewtonandChandradata

“…Despite this, the same spectral variation was found (Norm 2 ). Younes et al (2010) found for NGC 4278 a 31% (20%) variation at soft (hard) energies, where we found 26% (29%) variation for the same observations. Pian et al (2010) found long-term variability in two of their four type 1 LINERs using Swift data.…”

“…The nuclear variability of this source has been previously studied by Younes et al (2010). They concluded that long timescale (months) variability is detected with a flux increase of a factor of ∼3 on a timescale of a few months and a factor of 5 between the faintest and the brightest observation separated by about three years.…”

“…This percentage approaches zero in our own study, since none of our seven sources showed short timescale variability, according to the χ 2 r and the normalized excess variance, σ 2 NXS , (see Table 9). Younes et al (2010) found variability in ∼1.5 h for NGC 4278 on the XMM-Newton observation, where the flux increased by 10%. Using the same observation, we found a 3% flux increase in the same period, and a null probability of being a variable source.…”

X-ray spectral variability of LINERs selected from the Palomar sample

“…While the different Chandra observations did not show short-term variability, during the XMM-Newton observation Younes et al (2010) found a flux increase of a 10% in few hours. With the same dataset, HG13 obtained a 3% variation in the same time range, the difference being most probably due to the different apertures used for the analysis (10 vs. 25 ).…”

“…Pellegrini et al (2012) studied Chandra observations of NGC 4278 and found an X-ray luminosity decrease by a factor of ∼18 between 2005 and 2010. Younes et al (2010) detected a factor of ∼3 flux increase on a timescale of a few months and a variation of a factor of 5 between the faintest and brightest observations (separated by about three years). We used three of these observations (others were affected by pileup or did not meet the minimum count number), and found that our spectral fittings agreed well with theirs, although we found weaker variations in luminosities.…”