Chandra uncovers a hidden low-luminosity active galactic nucleus in the radio galaxy Hydra A (3C 218)

Rita M. Sambruna, George Chartas, Michael Eracleous, Richard F. Mushotzky, John A. Nousek

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We report the detection with Chandra of a low-luminosity active galactic nucleus (LLAGN) in the low-ionization nuclear emission-line region (LINER) hosted by Hydra A, a nearby (z = 0.0537) powerful FR I 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 ≳ 2 keV at the position of the compact radio core, embedded in diffuse thermal X-ray emission (kT ∼ 1 keV) at softer energies. The spectrum of the point source is well fitted by a heavily absorbed power law with intrinsic column density NintH ∼ 3 × 1022 cm-2 and photon index Γ ∼ 1.7. The intrinsic (absorption-corrected) luminosity is L2-10 keV ∼ 1.3 × 1042 ergs s-1. 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 I galaxies. These observations underscore the power of the X-rays and of Chandra in the quest for black holes.

Original languageEnglish (US)
Pages (from-to)L91-L94
JournalAstrophysical Journal
Issue number2 PART 2
StatePublished - Apr 1 2000

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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