Presence Of Intermediate-mass Black Hole Research Articles

Searching for Intermediate-mass Black Holes in Globular Clusters through Tidal Disruption Events

Intermediate-mass black holes (IMBHs) may be the link between stellar mass holes and the supermassive variety in the nuclei of galaxies, and globular clusters (GCs) may be one of the most promising environments for their formation. Here, we carry out a pilot study of the observability of tidal disruption events (TDEs) from 103 M ⊙ < M • < 105 M ⊙ IMBHs embedded in stellar cusps at the center of GCs. We model the long super-Eddington accretion phase and ensuing optical flare, and derive the disruption rate of main-sequence stars as a function of black hole mass and GC properties with the help of a 1D Fokker–Planck approach. The photospheric emission of the adiabatically expanding outflow dominates the observable radiation and peaks in the near-ultraviolet/optical bands, outshining the brightness of the (old) stellar population of GCs in Virgo for a period of months to years. A search for TDE events in a sample of nearly 4000 GCs observed at multiple epochs by the Next Generation Virgo Cluster Survey yields null results. Given our model predictions, this sample is too small to set stringent constraints on the present-day occupation fraction of GCs hosting IMBHs. Naturally, better simulations of the properties of the cluster central stellar distribution, TDE light curves, and rates, together with larger surveys of GCs are all needed to gain deeper insights into the presence of IMBHs in GCs.

Evidence of dark contents in the centre of NGC 6517

ABSTRACT Millisecond pulsars can serve as effective probes to investigate the presence of intermediate-mass black holes (IMBHs) within Galactic globular clusters (GCs). Based on the standard structure models for GCs, we conduct simulations to analyse the distributions of pulsar accelerations within the central region of NGC 6517. By comparing the measured accelerations of pulsars obtained from their period derivatives $\dot{P}$ to the simulated distribution profiles, we demonstrate that a central excess of dark mass is required to account for the measured accelerations. Our analysis, which relies on existing pulsar timing observations, is currently unable to differentiate between two possible scenarios: an IMBH precisely situated at the core of the cluster with mass ${\gtrsim} 9000^{+4000}_{-3000} \, {\rm M}_{\odot }$ or a central concentration of stellar-mass dark remnants with a comparable total mass. However, with additional acceleration measurements from a few more pulsars in the cluster, it will be possible to differentiate the source of the non-luminous matter.

Dynamical modelling of globular clusters: challenges for the robust determination of IMBH candidates

ABSTRACTThe presence or absence of intermediate-mass black holes (IMBHs) at the centre of Milky Way globular clusters (GCs) is still an open question. This is due to either observational restrictions or limitations in the dynamical modelling method; in this work, we explore the latter. Using a sample of high-end Monte Carlo simulations of GCs, with and without a central IMBH, we study the limitations of spherically symmetric Jeans models assuming constant velocity anisotropy and mass-to-light ratio. This dynamical method is one of the most widely used modelling approaches to identify a central IMBH in observations.With these models, we are able to robustly identify and recover the mass of the central IMBH in our simulation with a high-mass IMBH ($M_{\rm IMBH}/M_{\rm GC}\sim 4{{\ \rm per\ cent}}$). Simultaneously, we show that it is challenging to confirm the existence of a low-mass IMBH ($M_{\rm IMBH}/M_{\rm GC}\sim 0.3{{\ \rm per\ cent}}$), as both solutions with and without an IMBH are possible within our adopted error bars. For simulations without an IMBH, we do not find any certain false detection of an IMBH. However, we obtain upper limits that still allow for the presence of a central IMBH. We conclude that while our modelling approach is reliable for the high-mass IMBH and does not seem to lead towards a false detection of a central IMBH, it lacks the sensitivity to robustly identify a low-mass IMBH and to definitely rule out the presence of an IMBH when it is not there.

Wandering off the centre: a characterization of the random motion of intermediate-mass black holes in star clusters

Despite recent observational efforts, unequivocal signs for the presence of intermediate-mass black holes (IMBHs) in globular clusters (GCs) have not been found yet. Especially when the presence of IMBHs is constrained through dynamical modeling of stellar kinematics, it is fundamental to account for the displacement that the IMBH might have with respect to the GC centre. In this paper we analyse the IMBH wandering around the stellar density centre using a set of realistic direct N-body simulations of star cluster evolution. Guided by the simulation results, we develop a basic yet accurate model that can be used to estimate the average IMBH radial displacement ($\left<r_\mathrm{bh}\right>$) in terms of structural quantities as the core radius ($r_\mathrm{c}$), mass ($M_\mathrm{c}$), and velocity dispersion ($\sigma_\mathrm{c}$), in addition to the average stellar mass ($m_\mathrm{c}$) and the IMBH mass ($M_\mathrm{bh}$). The model can be expressed by the equation $\left<r_\mathrm{bh}\right>/r_\mathrm{c}=A(m_\mathrm{c}/M_\mathrm{bh})^\alpha[\sigma_\mathrm{c}^2r_\mathrm{c}/(GM_\mathrm{c})]^\beta$, in which the free parameters $A,\alpha,\beta$ are calculated through comparison with the numerical results on the IMBH displacement. The model is then applied to Galactic GCs, finding that for an IMBH mass equal to 0.1% of the GC mass, the typical expected displacement of a putative IMBH is around $1''$ for most Galactic GCs, but IMBHs can wander to larger angular distances in some objects, including a prediction of a $2.5''$ displacement for NGC 5139 ($\omega$ Cen), and $>10''$ for NGC 5053, NGC 6366 and ARP2.

N-body modelling of globular clusters: masses, mass-to-light ratios and intermediate-mass black holes

We have determined the masses and mass-to-light ratios of 50 Galactic globular clusters by comparing their velocity dispersion and surface brightness profiles against a large grid of 900 N-body simulations of star clusters of varying initial concentration, size and central black hole mass fraction. Our models follow the evolution of the clusters under the combined effects of stellar evolution and two-body relaxation allowing us to take the effects of mass segregation and energy equipartition between stars self-consistently into account. For a subset of 16 well observed clusters we also derive their kinematic distances. We find an average mass-to-light ratio of Galactic globular clusters of $<M/L_V>=1.98 \pm 0.03$, which agrees very well with the expected M/L ratio if the initial mass function of the clusters was a standard Kroupa or Chabrier mass function. We do not find evidence for a decrease of the average mass-to-light ratio with metallicity. The surface brightness and velocity dispersion profiles of most globular clusters are incompatible with the presence of intermediate-mass black holes (IMBHs) with more than a few thousand $M_\odot$ in them. The only clear exception is $\omega$ Cen, where the velocity dispersion profile provides strong evidence for the presence of a $\sim$40,000 $M_\odot$ IMBH in the centre of the cluster.

Dynamical Evolution of Stellar Systems

Dynamical simulations have become a powerful tool to study the evolution of star clusters due to hardware and software progresses in recent years. Here, I review the state of the art of N-body and other simulation techniques and show what we have learned from these simulations about the dynamical evolution of star clusters. Special attention is given to the results on the lifetimes of star clusters as a function of their environment, the internal changes of the mass functions, the influence of primordial gas expulsion on the ratio of first to second generation stars in globular clusters, and the possible presence of intermediate-mass black holes in star clusters.

A spectral-timing model for ULXs in the supercritical regime

Ultraluminous X-ray sources (ULXs) with luminosities lying between ~3x10^{39} - 2x10^{40} erg/s represent a contentious sample of objects as their brightness, together with a lack of unambiguous mass estimates for the vast majority of the central objects, leads to a degenerate scenario where the accretor could be a stellar remnant (black hole or neutron star) or intermediate mass black hole (IMBH). Recent, high-quality observations imply that the presence of IMBHs in the majority of these objects is unlikely unless the accretion flow somehow deviates strongly from expectation based on objects with known masses. On the other-hand, physically motivated models for super-critical inflows can re-create the observed X-ray spectra and their evolution, although have been lacking a robust explanation for their variability properties. In this paper we include the effect of a partially inhomogeneous wind that imprints variability onto the X-ray emission via two distinct methods. The model is heavily dependent on both inclination to the line-of-sight and mass accretion rate, resulting in a series of qualitative and semi-quantitative predictions. We study the time-averaged spectra and variability of a sample of well-observed ULXs, finding that the source behaviours can be explained by our model in both individual cases as well as across the entire sample, specifically in the trend of hardness-variability power. We present the covariance spectra for these sources for the first time, which shed light on the correlated variability and issues associated with modelling broad ULX spectra.

Intermediate-mass black holes in globular clusters: observations and simulations

AbstractThe study of intermediate-mass black holes (IMBHs) is a young and promising field of research. If IMBHs exist, they could explain the rapid growth of supermassive black holes by acting as seeds in the early stage of galaxy formation. Formed by runaway collisions of massive stars in young and dense stellar clusters, intermediate-mass black holes could still be present in the centers of globular clusters, today. Our group investigated the presence of intermediate-mass black holes for a sample of 10 Galactic globular clusters. We measured the inner kinematic profiles with integral-field spectroscopy and determined masses or upper limits of central black holes in each cluster. In combination with literature data we further studied the positions of our results on known black-hole scaling relations (such asM•− σ) and found a similar but flatter correlation for IMBHs. Applying cluster evolution codes, the change in the slope could be explained with the stellar mass loss occurring in clusters in a tidal field over its life time. Furthermore, we present results from several numerical simulations on the topic of IMBHs and integral field units (IFUs). We ranN-body simulations of globular clusters containing IMBHs in a tidal field and studied their effects on mass-loss rates and remnant fractions and showed that an IMBH in the center prevents core collapse and ejects massive objects more rapidly. These simulations were further used to simulate IFU data cubes. For the specific case of NGC 6388 we simulated two different IFU techniques and found that velocity dispersion measurements from individual velocities are strongly biased towards lower values due to blends of neighboring stars and background light. In addition, we use the Astrophysical Multipurpose Software Environment (AMUSE) to combine gravitational physics, stellar evolution and hydrodynamics to simulate the accretion of stellar winds onto a black hole.

Searching for IMBHs in Galactic globular clusters through radial velocities of individual stars

AbstractI present an overview of our ongoing project aimed at building a new generation of velocity dispersion profiles ad rotation curves for a representative sample of Galactic globular clusters, from the the radial velocity of hundreds of individual stars distributed at different distances from the cluster center. The innermost portion of the profiles will be used to constrain the possible presence of intermediate-mass black holes. The adopted methodology consists of combining spectroscopic observations acquired with three different instruments at the ESO-VLT: the adaptive-optics assisted, integral field unit (IFU) spectrograph SINFONI for the innermost and highly crowded cluster cores, the multi-IFU spectrograph KMOS for the intermediate regions, and the multi-fiber instrument FLAMES/GIRAFFE-MEDUSA for the outskirts. The case of NGC 6388, representing the pilot project that motivated the entire program, is described in some details.

Tidal disruption of white dwarfs by intermediate mass black holes

Modeling ultra-close encounters between a white dwarf and a spinning, intermediate mass black hole requires a full general relativistic treatment of gravity. This paper summarizes results from such a study. Our results show that the disruption process and prompt accretion of the debris strongly depend on the magnitude and orientation of the black hole spin. On the other hand, the late-time accretion onto the black hole follows the same decay, ˙ M ∝ t −5/3 , estimated from Newtonian gravity disruption studies. The spectrum of the fallback material peaks in the soft X-rays and sustains Eddington luminosity for 1-3yrs after the disruption. The orientation of the black hole spin has also a profound effect on how the outflowing debris obscures the central region. The disruption produces a burst of gravitational radiation with characteristic frequencies of ∼3.2Hz and strain amplitudes of ∼10 −18 for galactic intermediate mass black holes. In a tidal disruption event, the spin of black hole (BH) will most likely not be aligned with the orbital angular momentum of the incoming star. As a consequence, the tidal debris when it falls back will form a tilted disk (1). For thin disks, the Bardeen-Petterson effect will help align the inner part of the disk with the BH spin (2), but for the thick disks expected from tidal disruptions, the alignment may not happen (3-5), potentially changing our conventional picture of these explosive events. Even more remarkable will be ultra-close encounters, where an incoming star and associated tidal debris will experience general relativistic effects. Examples of strong curvature effects altering the dynamics of the disruption are frame dragging and the location of the innermost stable orbit, responsible for changing the traditional S-shape debris observed in Newtonian gravity simulations to a shell-like appearance, engulfing the BH. There are very few tidal disruption numerical simulations that account for general relativistic effects (6- 9). This paper summarizes the results from a study published recently (10). The study was primarily aimed at investigating observational signatures associated with strong gravity that may shed light on the presence of intermediate mass BHs. It included estimates of electromagnetic transient radiation using a slim disk model to compute the spectrum during the fallback phase. We found that the sources shine at Eddington luminosity ∼10 41 erg s −1 for about 1-3yrs, and then fade approximately as ∝ t −5/3 . Furthermore, there is a 50/50 chance that the inner disk will be obscured by outflowing debris for a fully misaligned BH spin. The study also included estimates of the gravitational wave burst signal produced during the encounters.

HST's hunt for intermediate-mass black holes in star clusters

AbstractEstablishing or ruling out, either through solid mass measurements or upper limits, the presence of intermediate-mass black holes (IMBHs; with masses of 102 − 105 M⊙) at the centers of star clusters would profoundly impact our understanding of problems ranging from the formation and long-term dynamical evolution of stellar systems, to the nature of the seeds and the growth mechanisms of supermassive black holes. While there are sound theoretical arguments both for and against their presence in today's clusters, observational studies have so far not yielded truly conclusive IMBH detections nor upper limits. We argue that the most promising approach to solving this issue is provided by the combination of measurements of the proper motions of stars at the centers of Galactic globular clusters and dynamical models able to take full advantage of this type of data set. We present a program based on HST observations and recently developed tools for dynamical analysis designed to do just that.

The presence of intermediate-mass black holes in globular clusters and their connection with extreme horizontal branch stars

By means of a multimass isotropic and spherical model that includes the self-consistent treatment of a central intermediate-mass black hole (IMBH), the influence of this black hole on the morphological and physical properties of globular clusters is investigated. In agreement with recent numerical studies, it is found that a cluster (with mass M) hosting an IMBH (with mass M.) shows, outside the region of gravitational influence of the black hole, a core-like profile resembling a King profile with concentration c M. The mass range estimate 12-4.8 < log(M./M) < - 1.1 c - 0.69, depending on the morphological parameters, is deduced by considering a wide range of models. Applying this estimate to a set of 39 globular clusters, it is found that NGC 2808, NGC 6388, M80, M13, M62, M54 and Gl (in M31) probably host an IMBH. For them, the scaling laws M. ∼ 0.09(M/M ⊙ ) 0.7 M ⊙ and M. ∼ 50(σ obs /km s -1 ) 1.2 M ⊙ are identified from weighted least-squares fits. An important result of this 'collective' study is that a strong correlation exists between the presence of an extreme blue horizontal branch and the presence of an IMBH. In particular, the presence of a central IMBH in M13 and NGC 6388 could explain why these clusters possess extreme horizontal-branch stars, in contrast to their 'second parameter' counterparts M3 and 47 Tuc.

Ratios of star cluster core and half-mass radii: a cautionary note on intermediate-mass black holes in star clusters

There is currently much interest in the possible presence of intermediate-mass black holes in the cores of globular clusters. Based on theoretical arguments and simulation results it has previously been suggested that a large core radius -- or particularly a large ratio of the core radius to half-mass radius -- is a promising indicator for finding such a black hole in a star cluster. In this study N-body models of 100000 stars with and without primordial binaries are used to investigate the long-term structural evolution of star clusters. Importantly, the simulation data is analysed using the same processes by which structural parameters are extracted from observed star clusters. This gives a ratio of the core and half-mass (or half-light) radii that is directly comparable to the Galactic globular cluster sample. As a result, it is shown that the ratios observed for the bulk of this sample can be explained without the need for an intermediate-mass black hole. Furthermore, it is possible that clusters with large core to half-light radius ratios harbour a black-hole binary (comprised of stellar mass black holes) rather than a single massive black hole. This work does not rule out the existence of intermediate-mass black holes in the cores of at least some star clusters.

On the weakness of disc models in bright ULXs

It is sometimes suggested that phenomenological power law plus cool disc-blackbody models represent the simplest, most robust interpretation of the X-ray spectra of bright ultraluminous X-ray sources (ULXs); this has been taken as evidence for the presence of intermediate-mass black holes (BHs) (M similar to 10(3) M-circle dot) in those sources. Here, we assess this claim by comparing the cool disc-blackbody model with a range of other models. For example, we show that the same ULX spectra can be fitted equally well by subtracting a disc-blackbody component from a dominant power-law component, thus turning a soft excess into a soft deficit. Then, we propose a more complex physical model, based on a power-law component slightly modified at various energies by smeared emission and absorption lines from highly ionized, fast-moving gas. We use the XMM-Newton/EPIC (European Photon Imaging Camera) spectra of two ULXs in Holmberg II and NGC 4559 as examples. Our main conclusion is that the presence of a soft excess or a soft deficit depends on the energy range over which we choose to fit the 'true' power-law continuum; those small deviations from the power-law spectrum are well modelled by disc-blackbody components (either in emission or absorption) simply because they are a versatile fitting tool for most kinds of smooth, broad bumps. Hence, we argue that those components should not be taken as evidence for accretion disc emission, nor used to infer BH masses. Finally, we speculate that bright ULXs could be in a spectral state similar to (or an extension of) the steep power-law state of Galactic BH candidates, in which the disc is now completely comptonized and not directly detectable, and the power-law emission may be modified by the surrounding, fast-moving, ionized gas.