![]() In this sense, one can think of the KBHsSH indeed as a combined system of a BS with a horizon at its center, and therefore it shares traits of both objects.īSs are very peculiar stars that first appeared in the literature in the late 1960s (Kaup 1968 Ruffini & Bonazzola 1969), and they are the realization of a complex scalar field bound by its self-gravity. In their domain of existence, they connect Kerr BHs (that is, with no hair) with pure solitonic solutions, also known as boson stars (BS), which are regular everywhere and feature no horizons. These are known as scalarized Kerr black holes (KBHsSH) and they are the object of study of this paper. Remarkably, by dropping the assumption that the matter fields must be stationary and axisymmetric, Herdeiro and Radu found solutions in the context of GR where BHs have hair (Herdeiro & Radu 2014b, 2015), by minimally coupling to gravity a complex scalar field that depends on time and on the axial coordinate while its energy-momentum tensor still possesses the respective isometries see Herdeiro et al. 2018 Doneva & Yazadjiev 2018 Silva et al. 2020a), and Gauss–Bonnet theories (Kanti et al. 1992 Kleihaus & Kunz 1998, 2001 Kleihaus et al. Still, in four dimensions, hairy BHs have been described in different theories of gravity, such as Einstein–Yang–Mills (Bizon 1990 Künzle & Masood-ul-Alam 1990 Volkov & Galtsov 1990 Breitenlohner et al. Notwithstanding their significance, there are many ways with which to circumvent them and discover different solutions. Half a century ago, a series of theorems laid the ground for the Kerr hypothesis (Israel 1967 Carter 1971 Robinson 1975) according to these no-hair theorems, the only stationary, axisymmetric, asymptotically flat, regular outside of the horizon solution to four-dimensional GR when the matter fields feature the same isometries as the spacetime is the Kerr BH. Despite all of this success, the question of whether general relativity (GR) is the most accurate classical theory of gravity, or whether astrophysically relevant BHs are uniquely described by the Kerr solution, is not yet fully answered. While electromagnetic observations leave room for BH mimickers as viable alternatives, the waveforms obtained from gravitational wave detectors strongly support the existence of BHs. Moreover, hundreds of X-ray binaries, many of which have a BH candidate at the center, have been detected thus far, thanks to missions like RXTE and Suzaku, and two decades-long observations of stars moving at the center of our Milky Way indicate that a massive BH is dwelling there (Ghez et al. 2019), for the compact object powering M87. 2020), and more recently, the first imaging of a BH contender has been realized (Akiyama et al. Since 2016, there have been over 40 confident BH–BH merging events detected (Abbott et al. The last few years have been very exciting for black hole (BH) physics as new ways of observing them became reality. Nevertheless, by constraining the parameters through different observations, the luminosity profile could in turn be used to constrain the Noether charge and characterize the spacetime, should KBHsSH exist. Furthermore, Q cannot be extracted asymptotically from the metric functions. Because of the existence of a conserved scalar charge, Q, these solutions are nonunique in the ( M, J) parameter space. We compare the results in batches with the same spin parameter j but different normalized charges, and the profiles are richly diverse. All of the solutions for which the stable circular orbital velocity (and angular momentum) curve is continuous are used for building thin and optically thick disks around them, from which we extract the radiant energy fluxes, luminosities, and efficiencies. Some of these traits are incompatible with the thin-disk approach thus, we identify and map out various regions in parameter space. In this paper, we first investigate the equatorial circular orbit structure of Kerr black holes with scalar hair (KBHsSH) and highlight their most prominent features, which are quite distinct from the exterior region of ordinary bald Kerr black holes, i.e., peculiarities that arise from the combined bound system of a hole with an off-center, self-gravitating distribution of scalar matter.
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