Bridging the gap between stellar-mass black holes and ultraluminous X-ray sources

TL;DR

This paper explores the connection between ultraluminous X-ray sources and stellar-mass black holes, proposing that ULXs are likely intermediate-mass black holes accreting at high rates, based on spectral and timing analysis.

Contribution

It reinterprets ULX properties within stellar-mass black hole accretion states, challenging the intermediate-mass black hole hypothesis with a new phenomenological model.

Findings

ULXs are consistent with black hole masses around 50-100 Msun.

ULXs accrete at approximately 20 times the Eddington rate.

XTE J1550-564 exemplifies the ultraluminous branch linking ULXs and stellar-mass black holes.

Abstract

The X-ray spectral and timing properties of ultraluminous X-ray sources (ULXs) have many similarities with the very high state of stellar-mass black holes (power-law dominated, at accretion rates greater than the Eddington rate). On the other hand, their cool disk components, large characteristic inner-disk radii and low characteristic timescales have been interpreted as evidence of black hole masses ~ 1000 Msun (intermediate-mass black holes). Here we re-examine the physical interpretation of the cool disk model, in the context of accretion states of stellar-mass black holes. In particular, XTE J1550-564 can be considered the missing link between ULXs and stellar-mass black holes, because it exhibits a high-accretion-rate, low-disk-temperature state (ultraluminous branch). On the ultraluminous branch, the accretion rate is positively correlated with the disk truncation radius and the bolometric disk luminosity, while it is anti-correlated with the peak temperature and the frequency of quasi-periodic-oscillations. Two prototypical ULXs (NGC1313 X-1 and X-2) also seem to move along that branch. We use a phenomenological model to show how the different range of spectral and timing parameters found in the two classes of accreting black holes depends on both their masses and accretion rates. We suggest that ULXs are consistent with black hole masses ~ 50-100 Msun, moderately inefficiently accreting at ~20 times Eddington.