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Register a new userThe Chandra view of the 3C/FRI sample of low luminosity radio-galaxies
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We present results from Chandra observations of the 3C/FRI sample of low luminosity radio-galaxies. We detected a power-law nuclear component in 12 objects out of the 18 with available data. In 4 galaxies we detected nuclear X-ray absorption at a level of about N_H= (0.2-6)e22 cm-2. X-ray absorbed sources are associated with the presence of highly inclined dusty disks (or dust filaments projected onto the nuclei) seen in the HST images. This suggests the existence of a flattened X-ray absorber, but of much lower optical depth than in classical obscuring tori. We thus have an un-obstructed view toward most FR~I nuclei while absorption plays only a marginal role in the remaining objects. Three pieces of evidence support an interpretation for a jet origin for the X-ray cores: i) the presence of strong correlations between the nuclear luminosities in the radio, optical and X-ray bands, extending over 4 orders of magnitude and with a much smaller dispersion (about 0.3 dex) when compared to similar trends found for other classes of AGNs, pointing to a common origin for the emission in the three bands; ii) the close similarity of the broad-band spectral indices with the sub-class of BL Lac objects sharing the same range of extended radio-luminosity, in accord with the FRI/BL Lacs unified model; iii) the presence of a common luminosity evolution of spectral indices in both FRI and BL Lacs. The low luminosities of the X-ray nuclei, regardless of their origin, strengthens the interpretation of low efficiency accretion in low luminosity radio-galaxies.
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We present spectral results, from Chandra and XMM-Newton observations, of a sample of 22 low-redshift (z < 0.1) radio galaxies, and consider whether the core emission originates from the base of a relativistic jet, an accretion flow, or contains contributions from both. We find correlations between the unabsorbed X-ray, radio, and optical fluxes and luminosities of FRI-type radio-galaxy cores, implying a common origin in the form of a jet. On the other hand, we find that the X-ray spectra of FRII-type radio-galaxy cores is dominated by absorbed emission, with $N_{rm H} > 10^{23}$ atoms cm$^{-2}$, that is likely to originate in an accretion flow. We discuss several models which may account for the different nuclear properties of FRI- and FRII-type cores, and also demonstrate that both heavily obscured, accretion-related, and unobscured, jet-related components may be present in all radio-galaxy nuclei. Any absorbed, accretion-related, components in FRI-type galaxies have low radiative efficiencies.
We report the detection with Chandra of a Low-Luminosity AGN (LLAGN) in the Low Ionization Emission Line Region (LINER) hosted by Hydra A, a nearby (z=0.0537) powerful FRI radio galaxy with complex radio and optical morphology. In a 20 ks ACIS-S exposure during the calibration phase of the instrument, a point source is detected at energies $grtsim$ 2 keV at the position of the compact radio core, embedded in diffuse thermal X-ray emission ($kT sim 1$ keV) at softer energies. The spectrum of the point source is well fitted by a heavily absorbed power law with intrinsic column density N$_H^{int} sim 3 times 10^{22}$ h and photon index $Gamma sim 1.7$. The intrinsic (absorption-corrected) luminosity is $L_{2-10 keV} sim 1.3 times 10^{42}$ lum. These results provide strong evidence that an obscured AGN is present in the nuclear region of Hydra~A. We infer that the optical/UV emission of the AGN is mostly hidden by the heavy intrinsic reddening. In order to balance the photon budget of the nebula, we must either postulate that the ionizing spectrum includes a UV bump or invoke and additional power source (shocks in the cooling flow or interaction with the radio jets). Using an indirect estimate of the black hole mass and the X-ray luminosity, we infer that the accretion rate is low, suggesting that the accretion flow is advection dominated. Finally, our results support current unification schemes for radio-loud sources, in particular the presence of the putative molecular torus in FR~Is. These observations underscore the power of the X-rays and of chandra in the quest for black holes.
We study the properties of the emission line regions in two samples of low luminosity radio-galaxies (LLRG), focusing on the compact emission line region (CELR) revealed to be a characteristic feature of these objects by HST narrow-band imaging. We find a strong correlation between line and optical continuum nuclear emission, suggesting that the optical cores (most likely of non thermal origin) can be directly associated to the source of ionizing photons, i.e. that we are seeing a jet-ionized narrow line region. A photon budget argument indicates that the optical nuclear sources produce a sufficient photon flux provided that the covering factor of the circum-nuclear gas is rather large, on average ~ 0.3. Analysis of HST images and spectra suggests that the CELR may take the form of a pc-scale, high filling factor, structure, possibly an optically thin torus. Estimates of the CELR mass lead to values as small as 10 - 1000 solar masses and photon counting sets a limit to the BLR mass of 0.01 solar masses. When considered together with the low accretion rate and the tenuous torus structure, a general paucity of gas in the innermost regions of LLRG emerges as the main characterizing difference from more powerful AGN.
(ABRIDGED) We present here the results from new Very Long Baseline Array observations at 1.6 and 5 GHz of 19 galaxies of a complete sample of 21 UGC FRI radio galaxies. New Chandra data of two sources, viz., UGC00408 and UGC08433, are combined with the Chandra archival data of 13 sources. The 5 GHz observations of ten core-jet sources are polarization-sensitive, while the 1.6 GHz observations constitute second epoch total intensity observations of nine core-only sources. Polarized emission is detected in the jets of seven sources at 5 GHz, but the cores are essentially unpolarized, except in M87. Polarization is detected at the jet edges in several sources, and the inferred magnetic field is primarily aligned with the jet direction. This could be indicative of magnetic field shearing due to jet-medium interaction, or the presence of helical magnetic fields. The jet peak intensity $I_ u$ falls with distance $d$ from the core, following the relation, $I_ upropto d^a$, where $a$ is typically -1.5. Assuming that adiabatic expansion losses are primarily responsible for the jet intensity dimming, two limiting cases are considered: [1] the jet has a constant speed on parsec-scales and is expanding gradually such that the jet radius $rpropto d^0.4$; this expansion is however unobservable in the laterally unresolved jets at 5 GHz, and [2] the jet is cylindrical and is accelerating on parsec-scales. Accelerating parsec-scale jets are consistent with the phenomenon of magnetic driving in Poynting flux dominated jets. Chandra observations of 15 UGC FRIs detect X-ray jets in nine of them. The high frequency of occurrence of X-ray jets in this complete sample suggests that they are a signature of a ubiquitous process in FRI jets.
We examine the optical properties of the nuclei of low luminosity radio-galaxies using snapshot HST images of the B2 sample. In agreement with the results obtained from the analysis of the brighter 3C/FRI sample, we find a correlation between fluxes (and luminosities) of the optical and radio cores. This provides further support for the interpretation that the optical nuclear emission in FRI is dominated by synchrotron emission and that accretion in these sources takes place in a low efficiency radiative regime. In the framework of the FRI/BL Lacs unified scheme, we find that the luminosity difference between FRI and BL Lac nuclei can be reproduced with a common beaming factor in both the radio and the optical band, independent of the extended radio luminosity, thus supporting such a scenario. The corresponding bulk Lorentz factor is significantly smaller than is expected from observational and theoretical considerations in BL Lacs: this can be interpreted as due to a velocity structure in the jet, with a fast spine surrounded by a slower layer.
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