Chandra uncovers a hidden Low-Luminosity AGN in the radio galaxy Hydra~A (3C~218)

Abstract

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${}_H^{int}\sim 3\times 10^{22}$ \nh and photon index $\Gamma \sim 1.7$. The intrinsic (absorption-corrected) luminosity is $L_{2-10keV}\sim 1.3\times 10^{42}$ \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.

figure 1 in color. accepted for publication in ApJ Letters

References in corpus (4)

• The Spectral Energy Distributions of Low-Luminosity Active Galactic Nuclei
• An X-ray Spectral Survey of Radio-Loud AGN With ASCA
• Chandra X-Ray Detection of the Radio Hotspots of 3C295
• ASCA Observation of the Low-Luminosity Seyfert 1.5 Galaxy NGC 5033

Cited by in corpus (16)

• Mechanical Feedback from Active Galactic Nuclei in Galaxies, Groups, and Clusters
• A High Resolution Study of the Hydra A Cluster with Chandra: Comparison of the Core Mass Distribution with Theoretical Predictions and Evidence for Feedback in the Cooling Flow
• The cluster-scale AGN outburst in Hydra A
• Measuring the kinetic power of AGN in the radio mode
• Radiative efficiency, variability and Bondi accretion onto massive black holes: from mechanical to quasar feedback in brightest cluster galaxies
• An X-ray survey of the 2Jy sample. I: is there an accretion mode dichotomy in radio-loud AGN?
• Thermal Emission as a Test for Hidden Nuclei in Nearby Radio Galaxies
• Radiation fields of disk, BLR and torus in quasars and blazars: implications for gamma-ray absorption
• A Chandra detection of the radio hotspot of 3C123
• Hydra A at Low Radio Frequencies
• Cold gas dynamics in Hydra-A: evidence for a rotating disk
• The HST view of the nuclear emission line region in low luminosity radio-galaxies
• Emission-Line Diagnostics of the Central Engines of Weak-Line Radio Galaxies
• The nature of the absorbing torus in compact Radio Galaxies
• Non-thermal X-ray Emission: An Alternative to Cluster Cooling Flows?
• Discovery of an Outstanding Disk in the cD Galaxy of the Hydra A Cluster