DEEP CHANDRA OBSERVATIONS OF THE COMPACT STARBURST GALAXY HENIZE 2-10: X-RAYS FROM THE MASSIVE BLACK HOLE

Abstract

We present follow-up X-ray observations of the candidate massive black hole (BH) in the nucleus of the low-mass, compact starburst galaxy Henize 2-10. Using new high-resolution observations from the Chandra X-ray Observatory totaling 200. ks in duration, as well as archival Chandra observations from 2001, we demonstrate the presence of a previously unidentified X-ray point source that is spatially coincident with the known nuclear radio source in Henize 2-10 (i. e., the massive BH). We show that the hard X-ray emission previously identified in the 2001 observation is dominated by a source that is distinct from the nucleus, with the properties expected for a highmass X-ray binary. The X-ray luminosity of the nuclear source suggests the massive BH is radiating significantly below its Eddington limit (similar to 10-6 L-Edd), and the soft spectrum resembles other weakly accreting massive BHs including Sagittarius A*. Analysis of the X-ray light curve of the nucleus reveals the tentative detection of a similar to 9 hr periodicity, although additional observations are required to confirm this result. Our study highlights the need for sensitive high-resolution X-ray observations to probe low-level accretion, which is the dominant mode of BH activity throughout the universe.