The star ingesting luminosity of intermediate mass black holes in globular clusters

TL;DR

This paper investigates how intermediate mass black holes in globular clusters can produce observable X-ray emissions through star disruption events, using hydrodynamics simulations to understand their luminosity and activity cycles.

Contribution

It introduces a detailed model of star disruption and debris accretion around IMBHs in globular clusters, explaining their transient X-ray signals and duty cycle.

Findings

Transient X-ray signals can persist for tens of years at near Eddington luminosity.

IMBHs can remain active with high luminosity for hundreds of years after star disruption.

Quiescent globular clusters with IMBHs are more common than active ones, affecting observational strategies.

Abstract

The dynamics of stars in the inner regions of nearby globular clusters (GCs) such as G1 indicate the presence of central concentrated dark masses, and one would like to know whether these are indeed intermediate mass black holes (IMBHs). As the number of surrounding stars, and their motions, are roughly known, the capture rate can be estimated; the question then arises of whether the apparent quiescence of the nuclei of these GCs is compatible with the IMBH's presence. The role of debris from disrupted stars in activating quiescent nuclei of GCs is studied here employing three-dimensional hydrodynamics simulations. It is argued that when individual stars are disrupted, the bulk of the debris would be swallowed or expelled rapidly compared with the interval between successive disruptions. Only a small fraction of the tightly bound mass is likely to be swallowed, yielding radiation with high efficiency, the surplus being expelled in a radiation-driven wind. A transient (predominantly of soft X-ray emission) signal could persist steadily with L ~ L_Edd =10^{41}(M_h/10^3M_sun) erg/s for at most tens of years; thereafter the flare would rapidly fade. While the infall rate declines as t^{-5/3}, some material may be stored for a longer time in an accretion disk. The IMBH luminosity could then remain as high as L_X > 10^{39} erg/s for several hundreds of years after disruption. In a given object, this ultraluminous X-ray activity would have a duty cycle of order 10^{-4}. Quiescent GCs, those with IMBHs now starved of fuel, should greatly outnumber active ones; observational constraints would not then be stringent until we had observed enough candidates to constitute a proper ensemble average.(abridged)