The X-ray Properties of Million Solar Mass Black Holes

TL;DR

This study investigates the X-ray properties of low-mass (~1 million solar masses) black holes and finds no significant change in accretion structure across a range of Eddington ratios, with implications for cosmic reionization.

Contribution

It provides new Chandra X-ray observations of low-mass black holes and compares their properties across different Eddington ratios, suggesting possible underestimation of accretion rates and implications for accretion models.

Findings

No statistical difference in X-ray to UV luminosity ratios between low- and high-Eddington ratio low-mass AGN.

Presence of an X-ray weak tail in the aox distribution of low-L/Ledd AGN.

Potential underestimation of Eddington ratios by current methods.

Abstract

We present new Chandra X-ray observations of seven low-mass black holes (~1e6 Msun) accreting at low Eddington ratios between -2.0<log L/Ledd<-1.5. We compare the X-ray properties of these seven low-mass active galactic nuclei (AGN) to a total of 73 other low-mass AGN in the literature with published Chandra observations (with Eddington ratios extending from -2.0<log L/Ledd<-0.1). We do not find any statistical differences between low- and high-Eddington ratio low-mass AGN in the distributions of their X-ray to ultraviolet luminosity ratios (aox), or in their X-ray spectral shapes. Furthermore, the aox distribution of low-L/Ledd AGN displays an X-ray weak tail that is also observed within high-L/Ledd objects. Our results indicate that between -2<log L/Ledd<-0.1, there is no systematic change in the structure of the accretion flow for active galaxies hosting 1e6 Msun black holes. We examine the accuracy of current bolometric luminosity estimates for our low-L/Ledd 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 aox. Alternatively, if current Eddington ratios are in fact 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 aox distribution for low-mass black holes may have favorable consequences for the epoch of cosmic reionization being driven by AGN.