TY - JOUR
T1 - Chandra uncovers a hidden low-luminosity active galactic nucleus in the radio galaxy Hydra A (3C 218)
AU - Sambruna, Rita M.
AU - Chartas, George
AU - Eracleous, Michael
AU - Mushotzky, Richard F.
AU - Nousek, John A.
N1 - Funding Information:
R. M. S. acknowledges support from NASA contract NAS-38252. We are grateful to Gordon Garmire and the ACIS team for making these observations possible. We thank Joe Pesce for help with Figure 2, Niel Brandt for the ASMOOTH routine, and Pat Broos and Scott Koch for assistance with data retrieving and for the TARA software. Finally, we are grateful to the referee, Yuichi Terashima, for his prompt and thoughtful comments and suggestions.
PY - 2000/4/1
Y1 - 2000/4/1
N2 - 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.
AB - 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.
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U2 - 10.1086/312569
DO - 10.1086/312569
M3 - Article
AN - SCOPUS:0034164212
SN - 0004-637X
VL - 532
SP - L91-L94
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 2 PART 2
ER -