Black Hole Mass Limits for Optically Dark X-ray Bright Sources in Elliptical Galaxies

TL;DR

This paper introduces a method to set strict upper limits on black hole masses in optically dark X-ray sources within elliptical galaxies by analyzing the absence of optical counterparts, refining previous mass estimates.

Contribution

The study presents a novel optical counterpart search technique using HST data to constrain black hole masses in X-ray sources, improving upon prior dynamical friction limits.

Findings

Six sources have black hole masses less than 5000 solar masses.

An ultra-luminous X-ray source in NGC 4486 has a mass below 1244 solar masses.

The method effectively constrains black hole masses in optically dark X-ray sources.

Abstract

Estimation of the black hole mass in bright X-ray sources of nearby galaxies is crucial to the understanding of these systems and their formation. However, the present allowed black hole mass range spans five order of magnitude (10Msun < M < 10^5 Msun) with the upper limit obtained from dynamical friction arguments. We show that the absence of a detectable optical counterpart for some of these sources, can provide a much more stringent upper limit. The argument is based only on the assumption that the outer regions of their accretion disks is a standard one. Moreover, such optically dark X-ray sources cannot be foreground stars or background active galactic nuclei, and hence must be accreting systems residing within their host galaxies. As a demonstration we search for candidates among the point-like X-ray sources detected with Chandra in thirteen nearby elliptical galaxies. We use a novel technique to search for faint optical counterparts in the HST images whereby we subtract the bright galaxy light based on isophotal modeling of the surface brightness. We show that for six sources with no detectable optical emission at the 3$-$sigma level, their black hole masses M_{BH} < 5000Msun. In particular, an ultra-luminous X-ray source (ULX) in NGC 4486 has M_{BH} < 1244 Msun. We discuss the potential of this method to provide stringent constraints on the black hole masses, and the implications on the physical nature of these sources.