A Chandra Survey of Milky Way Globular Clusters. III. Searching for X-ray Signature of Intermediate-mass Black Holes

Abstract

Globular clusters (GCs) are thought to harbor the long-sought population of intermediate-mass black holes (IMBHs). We present a systematic search for a putative IMBH in 81 Milky Way GCs, based on archival Chandra X-ray observations. We find in only six GCs a significant X-ray source positionally coincident with the cluster center, which have 0.5-8 keV luminosities between $\sim 1\times 10^{30}\mathrm{erg}{\mathrm{s}}^{-1}$ to $\sim 4\times 10^{33}\mathrm{erg}{\mathrm{s}}^{-1}$. However, the spectral and temporal properties of these six sources can also be explained in terms of binary stars. The remaining 75 GCs do not have a detectable central source, most with $3\sigma$ upper limits ranging between $10^{29-32}\mathrm{erg}{\mathrm{s}}^{-1}$ over 0.5-8 keV, which are significantly lower than predicted for canonical Bondi accretion. To help understand the feeble X-ray signature, we perform hydrodynamic simulations of stellar wind accretion onto a $1000{\mathrm{M}}_{\odot }$ IMBH from the most-bound orbiting star, for stellar wind properties consistent with either a main-sequence (MS) star or an asymptotic giant branch (AGB) star. We find that the synthetic X-ray luminosity for the MS case ($\sim 10^{19}\mathrm{erg}{\mathrm{s}}^{-1}$) is far below the current X-ray limits. The predicted X-ray luminosity for the AGB case ($\sim 10^{34}\mathrm{erg}{\mathrm{s}}^{-1}$), on the other hand, is compatible with the detected central X-ray sources, in particular the ones in Terzan 5 and NGC 6652. However, the probability of having an AGB star as the most-bound star around the putative IMBH is very low. Our study strongly suggests that it is very challenging to detect the accretion-induced X-ray emission from IMBHs, even if they were prevalent in present-day GCs.

21 pages, 10 figures, accepted by MNRAS

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