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We report on Chandra X-ray observations of four candidate low-mass black hole(<10^6Msun) active galactic nuclei (AGNs) that have the estimated Eddingtonratios among the lowest (~10^(-2)) found for this class. The aims are tovalidate the nature of their AGNs and to confirm the low Eddington ratios thatare derived from the broad H_alpha line, and to explore this poorly studiedregime in the AGN parameter space. Among them, two objects with the lowestsignificance of the broad lines are also observed with Multi-Mirror Telescope,and the high-quality optical spectra taken confirm them as Seyfert 1 AGNs andas having small black hole masses. X-ray emission is detected from the nucleiof two of the galaxies, which is variable on timescales of 10^3s, whereas nosignificant (or only marginal at best) detection is found for the remainingtwo. The X-ray luminosities are on the order of 10^(41) ergs/s or even lower,on the order of 10^(40) ergs/s for non-detections, which are among the lowestregimes ever probed for Seyfert galaxies. The low X-ray luminosities, comparedto their black hole masses derived from H_alpha, confirm their low accretionrates assuming typical bolometric corrections. Our results hint at theexistence of a possibly large population of under-luminous low-mass black holesin the local universe. An off-nucleus ultra-luminous X-ray source (ULX) in oneof the dwarf galaxies is detected serendipitously, with a luminosity(6-9)x10^(39) ergs/s in 2-10 keV.

Chandra and MMT observations of low-mass black hole active galactic nuclei accreting at low rates in dwarf galaxies