Occultation Mapping of the Central Engine in the Active Galaxy MCG −6-30-15

Abstract

The colossal power output of active galactic nuclei (AGNs) is believed to be fueled by the accretion of matter onto a supermassive black hole. This central accreting region of AGNs has hitherto been spatially unresolved, and its structure therefore unknown. Here we propose that a previously reported deep minimum in the X-ray intensity of the AGN MCG -6-30-15 was due to a unique X-ray occultation event and that it probes structure of the central engine on scales smaller than 1014 cm, or 1.4×10−7 arcsec. This resolution is more than a factor of ~3×106 greater than is possible with current X-ray optics. The data are consistent with a bright central source surrounded by a less intense ring, which we identify with the inner edge of an accretion disk. These may be the first direct measurements of the spatial structure and geometry of the accreting black hole system in an active galaxy. We estimate a mass lower limit for sub-Eddington accretion of 3.1×105 M☉. If the ring of X-ray emission is identified with the inner edge of an accretion disk, we get mass upper limits of 1.9×108 and 9.1×108 M☉ for a nonrotating and maximally rotating black hole, respectively. We point out that our occultation interpretation is controversial in the sense that X-ray variability in AGNs is normally attributed to intrinsic physical changes in the X-ray emission region, such as disk or coronal instabilities.