The polyhedral nature of LINERs: an XMM-Newton view of LINERs in radio galaxies

Aims:We investigate the origin of X-rays and the nature of accretion flow in 4 low-ionization nuclear emission-line regions (LINERs) hosted by radio galaxies, namely NGC 1692, PKS 0625-35, 3C 88, 3C 444, recently observed with XMM-Newton.
Methods: We combine the results from the time-averaged spectral analysis with model-independent information from X-ray temporal and spectral variability analyses, and with additional broadband information (specifically from the UV band, covered by the Optical Monitor aboard XMM-Newton, and from archival radio data).
Results: The values of the Eddington ratios L_bol/L_Edd of our sample span 2 orders of magnitude ranging between ~1×10^-5 and 1×10^-3. The 4 AGN are adequately fitted by the same continuum model that comprises at least one thermal component (kT ~ 0.65-1.45 keV) and a partially absorbed power law, whose relative contribution and photon index vary substantially from source to source. NGC 1692 and PKS 0625-35 have fairly steep power-law components (Γ ~ 2.5-2.9), perhaps indicative of synchrotron emission from the base of a jet. Conversely, the flat photon index derived for 3C 88 (Γ ~ 1.1) may be indicative of a heavily absorbed object. Finally, the time-averaged spectral properties of 3C 444 (Γ ~ 1.9 and an apparent line-like excess around 6.7 keV) are more in line with Seyfert-like objects. The temporal analysis reveals that PKS 0625-35 and 3C 88 are significantly variable in the soft (0.2-1 keV) energy band. PKS 0625-35 also shows suggestive evidence of spectral variability on timescales of months, with a spectral softening associated with the source brightening. NGC 1692 is only marginally variable in the soft band, whereas 3C 444 does not show significant variability on short timescales. The main findings from the broadband analysis can be summarized as follows: 1) 3C 444, PKS 0625-35, and NGC 1692 have α_OX values consistent with the α_OX - l_UV correlation found by Steffen et al. (2006, AJ, 131, 2826) for Seyfert-like objects. 2) No positive correlation is found between L_X and the inclination angle, suggesting that the X-ray emission is not beamed. 3) The values of the radio-loudness are inversely proportional to the Eddington ratio and locate our objects in between the “radio-loud” and “radio-quiet” branches in the R- l_UV plane proposed by Maoz (2007, MNRAS, 377, 1696).