THE X-RAY PROPERTIES OF MILLION SOLAR MASS BLACK HOLES

Abstract

We present new Chandra X-ray observations of seven low-mass black holes (MBH approximate to 10(6) M-circle dot) accreting at low-bolometric Eddington ratios between -2.0 less than or similar to log Lbol/LEdd less than or similar to -1.5. We compare the X-ray properties of these seven low-mass active galactic nuclei (AGNs) to a total of 73 other low-mass AGNs in the literature with published Chandra observations (with Eddington ratios extending from -2.0 less than or similar to log L-bol/L-Edd less than or similar to -0.1). We do not find any statistical differences between the low and high Eddington ratio low-mass AGNs in the distributions of their X-ray to ultraviolet luminosity ratios (alpha(ox)), or in their X-ray spectral shapes. Furthermore, the alpha(ox) distribution of low-L-bol/L-Edd AGNs displays an X-ray weak tail that is also observed within high-L-bol/L-Edd objects. Our results indicate that between -2 less than or similar to log L-bol/L-Edd less than or similar to -0.1, there is no systematic change in the structure of the accretion flow for active galaxies hosting 10(6) M-circle dot black holes. We examine the accuracy of current bolometric luminosity estimates for our low-L-bol/L-Edd objects with new Chandra observations, and it is plausible that their Eddington ratios could be underestimated by up to an order of magnitude. If so, then in analogy with weak emission line quasars, we suggest that accretion from a geometrically thick, radiatively inefficient “slim disk” could explain their diverse properties in alpha(ox). Alternatively, if current Eddington ratios are correct (or overestimated), then the X-ray weak tail would imply that there is diversity in disk/corona couplings among individual low-mass objects. Finally, we conclude by noting that the alpha(ox) distribution for low-mass black holes may have favorable consequences for the epoch of cosmic reionization being driven by AGN.