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Recently, citet{vitral2021does} detected a central concentration of dark objects in the core-collapsed globular cluster NGC 6397, which could be interpreted as a subcluster of stellar-mass black holes. However, it is well established theoretically that any significant number of black holes in the cluster would provide strong dynamical heating and is fundamentally inconsistent with this clusters core-collapsed profile. Claims of intermediate-mass black holes in core-collapsed clusters should similarly be treated with suspicion, for reasons that have been understood theoretically for many decades. Instead, the central dark population in NGC 6397 is exactly accounted for by a compact subsystem of white dwarfs, as we demonstrate here by inspection of a previously published model that provides a good fit to this cluster. These central subclusters of heavy white dwarfs are in fact a generic feature of core-collapsed clusters, while central black hole subclusters are present in all {em non/}-collapsed clusters.
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Intermediate-mass black holes (IMBHs) by definition have masses of $M_{rm IMBH} sim 10^{2-5}~M_odot$, a range with few observational constraints. Finding IMBHs in globular star clusters (GCs) would validate a formation channel for massive black-hole seeds in the early universe. Here, we simulate a 60-hour observation with the next-generation Very Large Array (ngVLA) of 728 GC candidates in the Virgo Cluster galaxy NGC,4472. Interpreting the radio detection thresholds as signatures of accretion onto IMBHs, we benchmark IMBH mass thresholds in three scenarios and find the following: (1) Radio analogs of ESO,243-49 HLX-1, a strong IMBH candidate with $M_{rm IMBH}^{rm HLX} sim 10^{4-5}~M_odot$ in a star cluster, are easy to access in all 728 GC candidates. (2) For the 30 GC candidates with extant X-ray detections, the empirical fundamental-plane relation involving black hole mass plus X-ray and radio luminosities suggests access to $M_{rm IMBH}^{rm FP} sim 10^{1.7-3.6}~M_odot$, with an uncertainty of 0.44 dex. (3) A fiducial Bondi accretion model was applied to all 728 GC candidates and to radio stacks of GC candidates. This model suggests access to IMBH masses, with a statistical uncertainty of 0.39 dex, of $M_{rm IMBH}^{rm B} sim 10^{4.9-5.1}~M_odot$ for individual GC candidates and $M_{rm IMBH}^{rm B,stack} sim 10^{4.5}~M_odot$ for radio stacks of about 100-200 GC candidates. The fiducial Bondi model offers initial guidance, but is subject to additional systematic uncertainties and should be superseded by hydrodynamical simulations of gas flows in GCs.
We compare the results of a large grid of N-body simulations with the surface brightness and velocity dispersion profiles of the globular clusters $omega$ Cen and NGC 6624. Our models include clusters with varying stellar-mass black hole retention fractions and varying masses of a central intermediate-mass black hole (IMBH). We find that an $sim 45,000$ M$_odot$ IMBH, whose presence has been suggested based on the measured velocity dispersion profile of $omega$ Cen, predicts the existence of about 20 fast-moving, $m>0.5$ M$_odot$ main-sequence stars with a (1D) velocity $v>60$ km/sec in the central 20 arcsec of $omega$ Cen. However no such star is present in the HST/ACS proper motion catalogue of Bellini et al. (2017), strongly ruling out the presence of a massive IMBH in the core of $omega$ Cen. Instead, we find that all available data can be fitted by a model that contains 4.6% of the mass of $omega$ Cen in a centrally concentrated cluster of stellar-mass black holes. We show that this mass fraction in stellar-mass BHs is compatible with the predictions of stellar evolution models of massive stars. We also compare our grid of $N$-body simulations with NGC 6624, a cluster recently claimed to harbor a 20,000 M$_odot$ black hole based on timing observations of millisecond pulsars. However, we find that models with $M_{IMBH}>1,000$ M$_odot$ IMBHs are incompatible with the observed velocity dispersion and surface brightness profile of NGC 6624,ruling out the presence of a massive IMBH in this cluster. Models without an IMBH provide again an excellent fit to NGC 6624.
We used a dataset of archival Hubble Space Telescope images obtained through the F555W, F814W and F656N filters, to perform a complete search for objects showing $Halpha$ emission in the globular cluster NGC 6397. As photometric diagnostic, we used the $(V-Halpha)_0$ color excess in the $(V-Halpha)_0$-$(V-I)_0$ color-color diagram. In the analysed field of view, we identified 53 $Halpha$ emitters. In particular, we confirmed the optical counterpart to 20 X-ray sources (7 cataclysmic variables, 2 millisecond pulsars and 11 active binaries) and identified 33 previously unknown sources, thus significantly enlarging the population of known active binaries in this cluster. We report the main characteristics for each class of objects. Photometric estimates of the equivalent width of the $Halpha$ emission line, were derived from the $(V-Halpha)_0$-excess and, for the first time, compared to the spectroscopic measurements obtained from the analysis of MUSE spectra. The very good agreement between the spectroscopic and photometric measures fully confirmed the reliability of the proposed approach to measure the $Halpha$ emission. The search demonstrated the efficiency of this novel approach to pinpoint and measure $Halpha$-emitters, thus offering a powerful tool to conduct complete census of objects whose formation and evolution can be strongly affected by dynamical interactions in star clusters.
We present results of a study of the central regions of NGC 6397 using Hubble Space Telescopes Advanced Camera for Surveys, focusing on a group of 24 faint blue stars that form a sequence parallel to, but brighter than, the more populated sequence of carbon-oxygen white dwarfs (CO WDs). Using F625W, F435W, and F658N filters with the Wide Field Channel we show that these stars, 18 of which are newly discovered, have magnitudes and colors consistent with those of helium-core white dwarfs (He WDs) with masses ~ 0.2-0.3 Msun. Their H-alpha--R625 colors indicate that they have strong H-alpha absorption lines, which distinguishes them from cataclysmic variables in the cluster. The radial distribution of the He WDs is significantly more concentrated to the cluster center than that of either the CO WDs or the turnoff stars and most closely resembles that of the clusters blue stragglers. Binary companions are required to explain the implied dynamical masses. We show that the companions cannot be main-sequence stars and are most likely heavy CO WDs. The number and photometric masses of the observed He WDs can be understood if ~1-5% of the main-sequence stars within the half-mass radius of the cluster have white dwarf companions with orbital periods in the range ~1-20 days at the time they reach the turnoff. In contrast to the CO WD sequence, the He WD sequence comes to an end at R625 ~ 24.5, well above the magnitude limit of the observations. We explore the significance of this finding in the context of thick vs. thin hydrogen envelope models and compare our results to existing theoretical predictions. In addition, we find strong evidence that the vast majority of the CO WDs in NGC 6397 down to Teff ~ 10,000 K are of the DA class. Finally, we use the CO WD sequence to measure a distance to the cluster of 2.34 +- 0.13 kpc.
Neutron stars can be destroyed by black holes at their center accreting material and eventually swallowing the entire star. Here we note that the accretion model adopted in the literature, based on Bondi accretion or variations thereof, is inadequate for small black holes -- black holes whose Schwarzschild radius is comparable to, or smaller than, the neutrons de Broglie wavelength. In this case, quantum mechanical aspects of the accretion process cannot be neglected, and give rise to a completely different accretion rate. We show that for the case of black holes seeded by the collapse of bosonic dark matter, this is the case for electroweak-scale dark matter particles. In the case of fermionic dark matter, typically the black holes that would form at the center of a neutron star are more massive, unless the dark matter particle mass is very large, larger than about 10$^{10}$ GeV. We calculate the lifetime of neutron stars harboring a small black hole, and find that black holes lighter than $sim 10^{11}$ kg quickly evaporate, leaving no trace. More massive black holes destroy neutron stars via quantum accretion on time-scales much shorter than the age of observed neutron stars.
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