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The formation process of intermediate-mass black holes (IMBHs), defined as those between 100 and 105 solar masses (M⊙), is debated. One potential origin is the growth of less-massive black holes merging with stars and compact objects within globular clusters (GCs). However, previous simulations have indicated that this process only produces IMBHs under 500 M⊙ before gravitational wave recoil ejects them from the GC. We performed star-by-star simulations of GC formation, finding that high-density star formation in a GCÕs parent giant molecular cloud can produce sufficient mergers of massive stars to overcome that mass threshold. We conclude that GCs can form with IMBHs more than 103 M⊙, which is sufficiently massive to be retained within the GC even with the expected gravitational wave recoil. [ABSTRACT FROM AUTHOR]

A dark star cluster (DSC) is a system in which the cluster potential is dominated by stellar remnants, such as black holes and neutron stars having larger masses than the long-lived low-mass stars. Due to mass segregation, these remnants are located in the central region of the cluster and form a dark core. We expect that at a few kpc from the Galactic Centre, the efficient evaporation of the lower-mass stars caused by the strong tidal force exposes the dark core, because the dynamical properties of the DSC are dominated by the remnants. Due to the invisibility of the remnants, finding a DSC by observation is challenging. In this project, we use N -body simulations to obtain models of DSCs and try to discern observables that signify a DSC. We consider four observables: the mass spectrum, the observational mass density profile, the observational velocity dispersion profile and the mass segregation. The models show that a DSC typically exhibits two distinct characteristics: for a given mass in stars and a given half-light radius, the expected velocity dispersion is underestimated when only visible stars are considered, and there is a lack of measurable mass segregation among the stars. These properties can be helpful for finding DSCs in observational data, such as the Gaia catalogue. [ABSTRACT FROM AUTHOR]

In the centers of dense star clusters, close encounters between stars and compact objects are likely to occur. We studied tidal disruption events of main-sequence (MS) stars by stellar-mass black holes (termed μTDEs), which can shed light on the processes occurring in these clusters, including being an avenue in the mass growth of stellar-mass BHs. Using the moving-mesh hydrodynamics code AREPO, we performed a suite of 58 hydrodynamics simulations of partial μTDEs of realistic, MESA-generated MS stars by varying the initial mass of the star (0.5 M⊙ and 1 M⊙), the age of the star (zero-age, middle-age and terminal-age), the mass of the black hole (10 M⊙ and 40 M⊙), and the impact parameter (yielding almost no mass loss to full disruption). We then examined the dependence of the masses, spins, and orbital parameters of the partially disrupted remnant on the initial encounter parameters. We find that the mass lost from a star decreases roughly exponentially with increasing approach distance and that a 1 M⊙ star loses less mass than a 0.5 M⊙ one. Moreover, a more evolved star is less susceptible to mass loss. Tidal torques at the closest approach spin up the remnant very close to break-up velocity when the impact parameter is low. The remnant star can be bound (eccentric) or unbound (hyperbolic) to the black hole; hyperbolic orbits occur when the star's central density concentration is relatively low and the black-hole-star mass ratio is high, which is the case for the disruption of a 0.5 M⊙ star. Finally, we provide best-fit analytical formulae for the aforementioned range of parameters that can be incorporated into cluster codes to model star-black-hole interaction more accurately. [ABSTRACT FROM AUTHOR]

ω Centauri is considered the most massive globular cluster of the Milky Way and likely the former nuclear star cluster of a Galaxy accreted by the Milky Way. It is speculated to contain an intermediate-mass black hole (IMBH) from several dynamical models. However, uncertainties regarding the location of the cluster centre or the retention of stellar remnants limit the robustness of the IMBH detections reported so far. In this paper, we derive and study the stellar kinematics from the highest-resolution spectroscopic data yet, using the Multi Unit Spectroscopic Explorer (MUSE) in the narrow field mode and wide field mode. Our exceptional data near the centre reveal for the first time that stars within the inner 20 arcsec (∼0.5 pc) counter-rotate relative to the bulk rotation of the cluster. Using this data set, we measure the rotation and line-of-sight velocity dispersion profile out to 120 arcsec with different centres proposed in the literature. We find that the velocity dispersion profiles using different centres match well with those previously published. Based on the counter–rotation, we determine a kinematic centre and look for any signs of an IMBH using the high-velocity stars close to the centre. We do not find any significant outliers >60 km s−1 within the central 20 arcsec, consistent with no IMBH being present at the centre of ω Centauri. A detailed analysis of Jeans' modelling of the putative IMBH will be presented in the next paper of the series. [ABSTRACT FROM AUTHOR]

We present the first results of the Dragon-II simulations, a suite of 19 N -body simulations of star clusters with up to 106 stars, with up to 33 per cent of them initially paired in binaries. In this work, we describe the main evolution of the clusters and their compact objects (COs). All Dragon-II clusters form in their centre a black hole (BH) subsystem with a density 10−100 times larger than the stellar density, with the cluster core containing |$50{-}80~{{\rm per\ cent}}$| of the whole BH population. In all models, the BH average mass steeply decreases as a consequence of BH burning, reaching values 〈 m BH〉 < 15 M⊙ within 10−30 relaxation times. Generally, our clusters retain only BHs lighter than 30 M⊙ over 30 relaxation times. Looser clusters retain a higher binary fraction, because in such environments binaries are less likely disrupted by dynamical encounters. We find that BH–main-sequence star binaries have properties similar to recently observed systems. Double CO binaries (DCOBs) ejected from the cluster exhibit larger mass ratios and heavier primary masses than ejected binaries hosting single CO binaries (SCOBs). Ejected SCOBs have BH masses m BH = 3−20 M⊙, definitely lower than those in DCOBs (m BH = 10−100 M⊙). [ABSTRACT FROM AUTHOR]

WDs and neutron stars are far-reaching and multi-faceted laboratories in the hunt for dark matter. We review detection prospects of wave-like, particulate, macroscopic and black hole dark matter that make use of several exceptional properties of compact stars, such as ultra-high densities, deep fermion degeneracies, low temperatures, nucleon superfluidity, strong magnetic fields, high rotational regularity, and significant gravitational wave emissivity. Foundational topics first made explicit in this document include the effect of the "propellor phase" on neutron star baryonic accretion, and the contribution of Auger and Cooper pair breaking effects to neutron star heating by dark matter capture. [ABSTRACT FROM AUTHOR]

In this paper, we obtain a new class of stationary axisymmetric spacetimes by using the Gürses–Gürsey metric with an appropriate mass function in order to generate a rotating core of matter that may be smoothly matched to the exterior Kerr metric. The same stationary spacetimes may be obtained by applying a slightly modified version of the Newman–Janis algorithm to a nonrotating spherically symmetric seed metric. The starting spherically symmetric configuration represents a nonisotropic de Sitter-type fluid whose radial pressure p r satisfies an state equation of the form p r = − ρ , where the energy density ρ is chosen to be the Tolman-type-VII energy density [R. C. Tolman, Phys. Rev.55, 364 (1939)]. The resulting rotating metric is then smoothly matched to the exterior Kerr metric, and the main properties of the obtained geometries are investigated. All the solutions considered in this study are regular in the sense they are free of curvature singularities. Depending on the relative values of the total mass m and rotation parameter a, the resulting stationary spacetimes represent different kinds of rotating compact objects such as regular black holes, extremal regular black holes, and regular starlike configurations. [ABSTRACT FROM AUTHOR]

We present evidence for a 0.278(8) d ( =6.7 h) orbital period in the X-ray transient GRS 1716-249 (=N Oph 93), based on a superhump modulation detected during the 1995 mini-outburst plus ellipsoidal variability in quiescence. With a quiescent magnitude of r  = 23.19 ± 0.15 N Oph 93 is too faint to warrant a full dynamical study through dedicated time-resolved spectroscopy. Instead, we apply the FWHM– K 2 correlation to the disc H α emission line detected in Gran Telescopio Canarias spectra and obtain K 2 = 521 ± 52 km s−1. This leads to a mass function f (M) = 4.1 ± 1.2 M⊙, thus indicating the presence of a black hole in this historic X-ray transient. Furthermore, from the depth of the H α trough and the quiescent light curve we constrain the binary inclination to i  = 61 ± 15°, while the detection of superhumps sets an upper limit to the donor to compact star mass ratio q  = M 2/ M 1 ≲ 0.25. Our de-reddened (r − i) colour is consistent with a ≈ K6 main sequence star that fills its Roche lobe in a 0.278 d orbit. Using all this information we derive a compact object mass |$M_{1}=6.4^{+3.2}_{-2.0}$| M⊙ at 68 per cent confidence. We also constrain the distance to GRS 1716-249 to 6.9 ± 1.1 kpc, placing the binary ∼0.8 kpc above the Galactic Plane, in support of a large natal kick. [ABSTRACT FROM AUTHOR]

The observed radial accelerations of 462 early-type galaxies (ETGs) at their half-mass radii are discussed. They are compared to the baryonic masses of the same galaxies, which are derived from theoretical expectations for their stellar populations and cover a range from |$\approx 10^4 \, {\rm M}_{\odot }$| to |$\approx 10^{11} \, {\rm M}_{\odot }$|⁠. Both quantities are plotted against each other, and it is tested whether they lie (within errors) along theoretical radial acceleration relations (RARs). We choose the Newtonian RAR and two Milgromian, or MONDian RARs. At low radial accelerations (corresponding to low masses), the Newtonian RAR fails without non-baryonic dark matter, but the two MONDian ones may work, provided moderate out-of-equilibrium dynamics in some of the low-mass ETGs. However all three RARs fail at high accelerations (corresponding to high masses) if all ETGs have formed their stellar populations with the canonical stellar initial mass function (IMF). A much better agreement with the observations can however be accomplished, if the theory of the integrated galaxy-wide stellar initial mass functions (IGIMFs) is used instead. This is because the IGIMF-theory predicts the formation of an overabundance of stellar remnants during the lifetime of the massive ETGs. Thus their baryonic masses today are higher than they would be if the ETGs had formed with a canonical IMF. Also the masses of the stellar-mass black holes should be rather high, which would mean that most of them probably formed when the massive ETGs were not as metal-enriched as they are today. The IGIMF-approach confirms downsizing. [ABSTRACT FROM AUTHOR]

The origin of current angular momentum (AM) of the black hole (BH) in X-ray binary (XRB) is still unclear, which is related with the birth and/or the growth of the BH. Here, we collect the spin parameters a * measured in BH XRBs and find an apparent bimodal distribution centered at ∼ 0.17 and 0.83. We find a positive relation between the spin parameter and the orbital period/orbital separation through combining distinct XRB categories, including neutron star (NS) low-mass X-ray binaries (LMXBs), Roche lobe overflow (RLOF) BH XRBs, and wind-fed BH XRBs. It seems that the AM of the compact star and the binary orbit correlates by combining the different XRB systems. These positive relations imply that accretion process is a common mechanism for spinning up the compact star in these diverse XRB systems. We infer that the low and high spin BH XRBs may experience different evolution and accretion history, which corresponds to the bimodal distribution of the BH spin parameters. The low spin BHs (a * < 0.3) are similar to the NS LMXBs, the compact star of which is spun-up by the low-level accretion, and the high spin BHs (a * > 0.5) had experienced a short hypercritical accretion (⁠|$\gg \dot{M}_\mathrm{Edd}$|⁠) period, during which, the BH spin dramatically increased. [ABSTRACT FROM AUTHOR]

Tidal disruption events (TDEs) of stars operated by massive black holes (MBHs) will be detected in thousands by upcoming facilities such as the Vera Rubin Observatory. In this work, we assess the rates of standard total TDEs, destroying the entire star, and partial TDEs, in which a stellar remnant survives the interaction, by solving 1D Fokker–Planck equations. Our rate estimates are based on a novel definition of the loss cone whose size is commensurate with the largest radius at which partial disruptions can occur, as motivated by relativistic hydrodynamical simulations. Our novel approach unveils two important results. First, partial TDEs can be more abundant than total disruptions by a factor of a few to a few tens. Second, the rates of complete stellar disruptions can be overestimated by a factor of a few to a few tens if one neglects partial TDEs, as we find that many of the events classified as total disruptions in the standard framework are in fact partial TDEs. Accounting for partial TDEs is particularly relevant for galaxies harbouring a nuclear stellar cluster featuring many events coming from the empty loss cone. Based on these findings, we stress that partial disruptions should be considered when constraining the luminosity function of TDE flares; accounting for this may reconcile the theoretically estimated TDE rates with the observed ones. [ABSTRACT FROM AUTHOR]

We implement a recent model proposed by Serrano–Liska (SL) (Alonso-Serrano and Liška, JHEP, 12:196, 2020) to study the ultra-compact star properties. The matter in the interior star is modeled by the quark model, deducted from QCD theory equipped with anisotropic pressure. Anisotropy is used to increase the compactness of the stars. We intend to see the signature of the "quantum gravity" effect through the SL model in ultra-compact stars. The SL model was motivated by quantum correction appearing in the black hole by adding a logarithmic term in gravity entropy used to derive the effective Einstein field equation. We expect this correction term in the SL model also affects ultra-compact star properties. The SL model with coupling constant c ~ in logarithmic term equipped with spherically symmetric metric yields correction terms O (c ~) that can be expressed by a function Ξ (r) . The Ξ (r) function vanishes at the star's exterior. We found that the mass-radius relation prediction by the SL model with anisotropic matter deviates from the one predicted by the standard Tolman–Oppenheimer–Volkoff (TOV) equation for c ~ ≥ 10 7 m 2 . We also have a sufficiently deep enough effective potential to produce a quasi-normal mode. We obtain the echo frequency of 15.2 kHz using maximum anisotropic pressure contribution and c ~ = 10 7 m 2 . Because the corresponding effective potential is almost indistinguishable from that of GR, this echo frequency value can be indistinguishable to one of GR, but not comparable to the result from GW170817 data analysis, i.e., 72 Hz. To circumvent this problem, we can decrease the value of echo frequency by increasing the magnitude of c ~ to orders of magnitude than c ~ = 10 7 m 2 . On the other hand, too large a strength from the logarithmic correction term is not physically favored because we learn from the black hole case that the logarithmic term is expected to be smaller than that of the Bekenstein term. Therefore, more precise gravitation echo measurements are crucial to understand this issue. [ABSTRACT FROM AUTHOR]

Discovery and characterization of black holes (BHs), neutron stars (NSs), and white dwarfs (WDs) with detached luminous companions (LCs) in wide orbits are exciting because they are important test beds for dark remnant (DR) formation physics as well as binary stellar evolution models. Recently, 187 candidates have been identified from Gaia's non-single star catalog as wide orbit (P orb/day > 45), detached binaries hosting DRs. We identify UV counterparts for 49 of these sources in the archival GALEX data. Modeling the observed spectral energy distribution (SED) spanning from the FUV-NUV to IR for these sources and stellar evolution models, we constrain the LC properties including mass, bolometric luminosity, and effective temperature for these 49 sources. Using the LC masses, and the astrometric mass function constrained by Gaia, we constrain the DR masses for these sources. We find that nine have masses clearly in the NS or BH mass range. Fifteen sources exhibit significant NUV excess and four show excess both in the FUV and NUV. The simplest explanation for these excess UV fluxes is that the DRs in these sources are WDs. Using SED modeling we constrain the effective temperature and bolometric luminosity for these 15 sources. Our estimated DR masses for all of these 15 sources are lower than the Chandrasekhar mass limit for WDs. Interestingly, five of these sources had been wrongly identified as NSs in the literature. [ABSTRACT FROM AUTHOR]

We present BIFROST , an extended version of the GPU-accelerated hierarchical fourth-order forward symplectic integrator code FROST. BIFROST (BI naries in FROST) can efficiently evolve collisional stellar systems with arbitrary binary fractions up to |$f_\mathrm{bin}=100~{{\ \rm per\ cent}}$| by using secular and regularized integration for binaries, triples, multiple systems, or small clusters around black holes within the fourth-order forward integrator framework. Post-Newtonian (PN) terms up to order PN3.5 are included in the equations of motion of compact subsystems with optional three-body and spin-dependent terms. PN1.0 terms for interactions with black holes are computed everywhere in the simulation domain. The code has several merger criteria (gravitational-wave inspirals, tidal disruption events, and stellar and compact object collisions) with the addition of relativistic recoil kicks for compact object mergers. We show that for systems with N particles the scaling of the code remains good up to N GPU ∼ 40 × N /106 GPUs and that the increasing binary fractions up to 100 per cent hardly increase the code running time (less than a factor ∼1.5). We also validate the numerical accuracy of BIFROST by presenting a number of star clusters simulations the most extreme ones including a core collapse and a merger of two intermediate mass black holes with a relativistic recoil kick. [ABSTRACT FROM AUTHOR]

Recent studies of nearby globular clusters have discovered excess dark mass in their cores, apparently in an extended distribution, and simulations indicate that this mass is composed mostly of white dwarfs (respectively stellar-mass black holes) in clusters that are core collapsed (respectively with a flatter core). We perform mass-anisotropy modelling of the closest globular cluster, M4, with intermediate slope for the inner stellar density. We use proper motion data from Gaia Early Data Release 3 (EDR3) and from observations by the Hubble Space Telescope. We extract the mass profile employing Bayesian Jeans modelling, and check our fits with realistic mock data. Our analyses return isotropic motions in the cluster core and tangential motions (β ≈ −0.4 ± 0.1) in the outskirts. We also robustly measure a dark central mass of roughly |$800\pm 300 \, \rm M_\odot$| , but it is not possible to distinguish between a point-like source, such as an intermediate-mass black hole (IMBH), and a dark population of stellar remnants of extent |${\approx} 0.016\, {\rm pc} \simeq 3300\, {\rm au}$|. However, when removing a high-velocity star from the cluster centre, the same mass excess is found, but more extended (⁠|${\sim} 0.034\, {\rm pc} \approx 7000\, {\rm au}$|). We use Monte Carlo N -body models of M4 to interpret the second outcome, and find that our excess mass is not sufficiently extended to be confidently associated with a dark population of remnants. Finally, we discuss the feasibility of these two scenarios (i.e. IMBH versus remnants), and propose new observations that could help to better grasp the complex dynamics in M4's core. [ABSTRACT FROM AUTHOR]

We study close encounters of a 1 M ⊙ middle-age main-sequence star (modeled using MESA) with massive black holes through hydrodynamic simulations, and explore in particular the dependence of the outcomes on the black hole mass. We consider here black holes in the intermediate-mass range, M BH = 100–104 M ⊙. Possible outcomes vary from a small tidal perturbation for weak encounters all the way to partial or full disruption for stronger encounters. We find that stronger encounters lead to increased mass loss at the first pericenter passage, in many cases ejecting the partially disrupted star on an unbound orbit. For encounters that initially produce a bound system, with only partial stripping of the star, the fraction of mass stripped from the star increases with each subsequent pericenter passage and a stellar remnant of finite mass is ultimately ejected in all cases. The critical penetration depth that separates bound and unbound remnants has a dependence on the black hole mass when M BH ≲ 103 M ⊙. We also find that the number of successive close passages before ejection decreases as we go from the stellar-mass black hole to the intermediate-mass black hole regime. For instance, after an initial encounter right at the classical tidal disruption limit, a 1 M ⊙ star undergoes 16 (5) pericenter passages before ejection from a 10 M ⊙ (100 M ⊙) black hole. Observations of periodic flares from these repeated close passages could in principle indicate signatures of a partial tidal disruption event. [ABSTRACT FROM AUTHOR]

Context. In the Milky Way the central massive black hole, SgrA, coexists with a compact nuclear star cluster that contains a sub-parsec concentration of fast-moving young stars called S-stars. Their location and age are not easily explained by current star formation models, and in several scenarios the presence of an intermediate-mass black hole (IMBH) has been invoked. Aims. We use GRAVITY astrometric and SINFONI, KECK, and GNIRS spectroscopic data of S2, the best known S-star, to investigate whether a second massive object could be present deep in the Galactic Centre (GC) in the form of an IMBH binary companion to SgrA. Methods. To solve the three-body problem, we used a post-Newtonian framework and consider two types of settings: (i) a hierarchical set-up where the star S2 orbits the SgrA-IMBH binary and (ii) a non-hierarchical set-up where the IMBH trajectory lies outside the S2 orbit. In both cases we explore the full 20-dimensional parameter space by employing a Bayesian dynamic nested sampling method. Results. For the hierarchical case we find the strongest constraints: IMBH masses >2000 M on orbits with smaller semi-major axes than S2 are largely excluded. For the non-hierarchical case, the chaotic nature of the problem becomes significant: the parameter space contains several pockets of valid IMBH solutions. However, a closer analysis of their impact on the resident stars reveals that IMBHs on semi-major axes larger than S2 tend to disrupt the S-star cluster in less than a million years. This makes the existence of an IMBH among the S-stars highly unlikely. Conclusions. The current S2 data do not formally require the presence of an IMBH. If an IMBH hides in the GC, it has to be either a low-mass IMBH inside the S2 orbit that moves on a short and significantly inclined trajectory or an IMBH with a semi-major axis >100. We provide the parameter maps of valid IMBH solutions in the GC and discuss the general structure of our results and how future observations can help to put even stronger constraints on the properties of IMBHs in the GC. [ABSTRACT FROM AUTHOR]

Strong dynamical interactions among stars and compact objects are expected in a variety of astrophysical settings, such as star clusters and the disks of active galactic nuclei. Via a suite of three-dimensional hydrodynamics simulations using the moving-mesh code arepo , we investigate the formation of transient phenomena and their properties in close encounters between an |$2\, {\rm M}_{\odot }$| or |$20\, {\rm M}_{\odot }$| equal-mass circular binary star and single |$20\, {\rm M}_{\odot }$| black hole (BH). Stars can be disrupted by the BH during dynamical interactions, naturally producing electromagnetic transient phenomena. Encounters with impact parameters smaller than the semimajor axis of the initial binary frequently lead to a variety of transients whose electromagnetic signatures are qualitatively different from those of ordinary disruption events involving just two bodies. These include the simultaneous or successive disruptions of both stars and one full disruption of one star accompanied by successive partial disruptions of the other star. On the contrary, when the impact parameter is larger than the semimajor axis of the initial binary, the binary is either simply tidally perturbed or dissociated into bound and unbound single stars ('micro-Hills' mechanism). The dissociation of |$20\, {\rm M}_{\odot }$| binaries can produce a runaway star and an active BH moving away from one another. Also, the binary dissociation can either produce an interacting binary with the BH, or a non-interacting, hard binary; both could be candidates of BH high- and low-mass X-ray binaries. Hence, our simulations especially confirm that strong encounters can lead to the formation of the (generally difficult to form) BH low-mass X-ray binaries. [ABSTRACT FROM AUTHOR]

The detection of the binary black hole merger GW190521, with primary black hole mass |$85^{+21}_{-14} {\rm M}_{\odot }$|⁠ , proved the existence of black holes in the theoretically predicted pair-instability gap (⁠|$\sim 60-120 \, {\rm M}_{\odot }$|⁠) of their mass spectrum. Some recent studies suggest that such massive black holes could be produced by the collision of an evolved star with a carbon–oxygen core and a main sequence star. Such a post-coalescence star could end its life avoiding the pair-instability regime and with a direct collapse of its very massive envelope. It is still not clear, however, how the collision shapes the structure of the newly produced star and how much mass is actually lost in the impact. We investigated this issue by means of hydrodynamical simulations with the smoothed particle hydrodynamics code StarSmasher , finding that a head-on collision can remove up to 12 per cent of the initial mass of the colliding stars. This is a non-negligible percentage of the initial mass and could affect the further evolution of the stellar remnant, particularly in terms of the final mass of a possibly forming black hole. We also found that the main sequence star can plunge down to the outer boundary of the core of the primary, changing the inner chemical composition of the remnant. The collision expels the outer layers of the primary, leaving a remnant with an helium-enriched envelope (reaching He fractions of about 0.4 at the surface). These more complex abundance profiles can be directly used in stellar evolution simulations of the collision product. [ABSTRACT FROM AUTHOR]

Motivated by the increasing interest in finding physically viable rotating sources, we present a new class of anisotropic rotating solutions. The energy–momentum tensor compatible with the metric is composed of anisotropic matter with a nonvanishing energy flow around the symmetry axis and vanishing viscosity. The new class of solutions can be used to find new possible sources for the Kerr metric, to obtain new regular black hole solutions and to study galaxies with a central rotating black hole and an halo of dark matter. As an example, we obtain a five-parameter class of solutions representing a two-way traversable wormhole smoothly matched to the Kerr one and satisfying all energy conditions outside the wormhole for a wide range of parameters, in particular for compact objects. Finally, with a simple modification of the aforementioned solution, we obtain a source for Kerr metric with a throat geometry, nonrepresenting a two-way traversable wormhole and satisfying all energy conditions. [ABSTRACT FROM AUTHOR]

Many recent works have shown that the angular resolution of ground-based detectors is too poor to characterize the anisotropies of the stochastic gravitational-wave background (SGWB). For this reason, we asked ourselves if a constellation of space-based instruments could be more suitable. We consider the Laser Interferometer Space Antenna (LISA), a constellation of multiple LISA-like clusters, and the Deci-hertz Interferometer Gravitational-wave Observatory (DECIGO). Specifically, we test whether these detector constellations can probe the anisotropies of the SGWB. For this scope, we considered the SGWB produced by two astrophysical sources: merging compact binaries, and a recently proposed scenario for massive black hole seed formation through multiple mergers of stellar remnants. We find that measuring the angular power spectrum of the SGWB anisotropies is almost unattainable. However, it turns out that it could be possible to probe the SGWB anisotropies through cross-correlation with the cosmic microwave background (CMB) fluctuations. In particular, we find that a constellation of two LISA-like detectors and CMB-S4 can marginally constrain the cross-correlation between the CMB lensing convergence and the SGWB produced by the black hole seed formation process. Moreover, we find that DECIGO can probe the cross-correlation between the CMB lensing and the SGWB from merging compact binaries. [ABSTRACT FROM AUTHOR]

The study of compact object populations has come a long way since the determination of the mass of the Hulse–Taylor pulsar, and we now count on more than 150 known Galactic neutron stars and black hole masses, as well as another 180 objects from binary mergers detected from gravitational-waves by the Ligo–Virgo–KAGRA Collaboration. With a growing understanding of the variety of systems that host these objects, their formation, evolution and frequency, we are now in a position to evaluate the statistical nature of these populations, their properties, parameter correlations and long-standing problems, such as the maximum mass of neutron stars and the black hole lower mass gap, to a reasonable level of statistical significance. Here, we give an overview of the evolution and current state of the field and point to some of its standing issues. We focus on Galactic black holes, and offer an updated catalog of 35 black hole masses and orbital parameters, as well as a standardized procedure for dealing with uncertainties. [ABSTRACT FROM AUTHOR]

After a star has exhausted all of its fuel by the process of nuclear fusion, it will collapse into a compact object. The three known stellar remnants of the universe are white dwarfs, neutron stars, and black holes. These three objects are characterized by their small size and extremely high densities. To model these compact objects, we must first understand their stellar structure. In this learning activity, we show the mathematical and numerical methods needed to model neutron stars both in the classical Newtonian and relativistic frameworks. [ABSTRACT FROM AUTHOR]

We present results from a suite of eight direct N -body simulations, performed with nbody6++ GPU , representing realistic models of rotating star clusters with up to 1.1 × 105 stars. Our models feature primordial (hard) binaries, a continuous mass spectrum, differential rotation, and tidal mass-loss induced by the overall gravitational field of the host galaxy. We explore the impact of rotation and stellar evolution on the star cluster dynamics. In all runs for rotating star clusters, we detect a previously predicted mechanism: an initial phase of violent relaxation followed by the so-called gravogyro catastrophe. We find that the gravogyro catastrophe reaches a finite amplitude, which depends in strength on the level of the bulk rotation, and then levels off. After this phase, the angular momentum is transferred from high-mass to low-mass particles in the cluster (both stars and compact objects). Simultaneously, the system becomes gravothermally unstable and collapses, thus undergoing the so-called gravothermal-gravogyro catastrophe. Comparing models with and without stellar evolution, we find an interesting difference. When stellar evolution is not considered, the whole process proceeds at a faster pace. The population of heavy objects tends to form a triaxial structure that rotates in the cluster centre. When stellar evolution is considered, we find that such a rotating bar is populated by stellar black holes and their progenitors. The triaxial structure becomes axisymmetric over time, but we also find that the models without stellar evolution suffer repeated gravogyro catastrophes as sufficient angular momentum and mass are removed by the tidal field. [ABSTRACT FROM AUTHOR]

Many astrophysical environments, from star clusters and globular clusters to the discs of active galactic nuclei, are characterized by frequent interactions between stars and the compact objects that they leave behind. Here, using a suite of 3D hydrodynamics simulations, we explore the outcome of close interactions between |$1\, \mathrm{M}_{\odot }$| stars and binary black holes (BBHs) in the gravitational wave regime, resulting in a tidal disruption event (TDE) or a pure scattering, focusing on the accretion rates, the back reaction on the BH binary orbital parameters, and the increase in the binary BH effective spin. We find that TDEs can make a significant impact on the binary orbit, which is often different from that of a pure scattering. Binaries experiencing a prograde (retrograde) TDE tend to be widened (hardened) by up to |$\simeq 20{{\ \rm per\ cent}}$|⁠. Initially circular binaries become more eccentric by |$\lesssim 10{{\ \rm per\ cent}}$| by a prograde or retrograde TDE, whereas the eccentricity of initially eccentric binaries increases (decreases) by a retrograde (prograde) TDE by |$\lesssim 5{{\ \rm per\ cent}}$|⁠. Overall, a single TDE can generally result in changes of the gravitational-wave-driven merger time-scale by order unity. The accretion rates of both black holes are very highly super-Eddington, showing modulations (preferentially for retrograde TDEs) on a time-scale of the orbital period, which can be a characteristic feature of BBH-driven TDEs. Prograde TDEs result in the effective spin parameter χ to vary by ≲0.02, while χ ≳ −0.005 for retrograde TDEs. [ABSTRACT FROM AUTHOR]

Most back hole and neutron star progenitors are found in triples or higher multiplicity systems. Here, we present a new triple stellar evolution code, |${\tt TSE}$|⁠ , which simultaneously takes into account the physics of the stars and their gravitational interaction. |${\tt TSE}$| is used to simulate the evolution of massive stellar triples in the galactic field from the zero-age main sequence until they form compact objects. To this end, we implement initial conditions that incorporate the observed high correlation between the orbital parameters of early-type stars. We show that the interaction with a tertiary companion can significantly impact the evolution of the inner binary. High eccentricities can be induced by the third-body dynamical effects, leading to a Roche lobe overflow or even to a stellar merger from initial binary separations 103– |$10^5\, \rm R_\odot$|⁠. In |$\sim 5\, {{\ \rm per\ cent}}$| of the systems, the tertiary companion itself fills its Roche lobe, while |$\sim 10\, {{\ \rm per\ cent}}$| of all systems become dynamically unstable. We find that between |$0.3{{\ \rm per\ cent}}$| and |$5{{\ \rm per\ cent}}$| of systems form a stable triple with an inner compact object binary, where the exact fraction depends on metallicity and the natal kick prescription. Most of these triples are binary black holes with black hole companions. We find no binary neutron star in any surviving triple, unless zero natal kicks are assumed. About half of all black hole binaries formed in our models are in triples, where in the majority, the tertiary black hole can perturb their long-term evolution. Our results show that triple interactions are key to a full understanding of massive star evolution and compact object binary formation. [ABSTRACT FROM AUTHOR]

A short review of spherically symmetric static regular black holes and spherically symmetric non-singular cosmological space-time is presented. Several models, including new ones, of regular black holes are considered. First, a large class of regular black holes having an inner de Sitter core with the related issue of a Cauchy horizon is investigated. Then, Black Bounce space-times, where the Cauchy horizon and therefore the related instabilities are absent, are discussed as valid alternatives to regular black holes with inner de Sitter cores. Friedman–Lemaître–Robertson–Walker space-times admitting regular bounce solutions are also discussed. In the general analysis concerning the presence or absence of singularities in the equations of motion, the role of a theorem credited to Osgood is stressed. [ABSTRACT FROM AUTHOR]

We perform non-hydrodynamical 2.5D simulations to study the dynamics of material above accretion disk based on the disk radiation pressure acting on dust. We assume a super-accreting underlying disk with the accretion rate of 10 times the Eddington rate with central black hole mass ranging from 10 7 up to 10 9 M ⊙ . Such high accretion rates are characteristic for extreme sources. We show that for high accretors the radiatively dust-driving mechanism based on the FRADO model always leads to a massive outflow from the disk surface, and the failed wind develops only at larger radii. The outflow rate strongly depends on the black hole mass, and an optically thick energy-driven solution can exceed the accretion rate for masses larger than 10 8 M ⊙ but momentum-driven outflow does not exceed the accretion rate even for super-Eddington accretion, therefore not violating the adopted stationarity of the disk. However, even in this case the outflow from the disk implies a strong mechanical feedback. [ABSTRACT FROM AUTHOR]

Recent analyses have shown that close encounters between stars and stellar black holes occur frequently in dense star clusters. Depending upon the distance at closest approach, these interactions can lead to dissipating encounters such as tidal captures and disruptions, or direct physical collisions, all of which may be accompanied by bright electromagnetic transients. In this study, we perform a wide range of hydrodynamic simulations of close encounters between black holes and main-sequence stars that collectively cover the parameter space of interest, and we identify and classify the various possible outcomes. In the case of nearly head-on collisions, the star is completely disrupted with roughly half of the stellar material becoming bound to the black hole. For more distant encounters near the classical tidal-disruption radius, the star is only partially disrupted on the first pericenter passage. Depending upon the interaction details, the partially disrupted stellar remnant may be tidally captured by the black hole or become unbound (in some cases, receiving a sufficiently large impulsive kick from asymmetric mass loss to be ejected from its host cluster). In the former case, the star will undergo additional pericenter passages before ultimately being disrupted fully. Based on the properties of the material bound to the black hole at the end of our simulations (in particular, the total bound mass and angular momentum), we comment upon the expected accretion process and associated electromagnetic signatures that are likely to result. [ABSTRACT FROM AUTHOR]

The article explores the discovery of a mysterious compact astronomical object, situated at the lower edge of the "mass gap" between neutron stars and black holes. It mentions that identified as a companion to the millisecond pulsar PSR J0514−4002E, the object's origin and classification as either the most massive neutron star or the least massive black hole pose intriguing astrophysical questions.

Abstract—The highest critical mass of neutron stars (NSs) was reviewed in the context of equation of state and observational results. It was predicted that the maximum NS mass () exists in the range – . However, recent observations of gravitational waves and other studied had suggested the higher mass limit of NSs, . The NS mass upto the value of is well understood, and with such a mass value it was meaningful to discuss the "mass gap" (m-gap) between the NS and black hole (BH) collapsars. The "m-gap" exist in between the highest mass of NS and the lowest mass of BH collapsars ( – ). In the mass distribution, the maximum population of NSs and BHs is located at and , respectively. However, recent observational results predicted filling the "m-gap" by the compact objects. In this paper, the concept of gravidynamics was reported to resolve the problem of peak likelihood value of gravitational mass at and the "m-gap" ( – ). This concept was based on a non-metric scalar-tensor model of gravitational interaction with localizable field energy. The gravidynamics model shows the total mass () of a compact relativistic object filled with matter of quark-gluon plasma of the radius /c km, consistent with the "m-gap". It was conceptualized that the total measurable gravitational mass of such an extremely dense object consists of both matter and field, which is described by scalar-tensor components. This model is also useful for predicting the collapsars within "m-gap". [ABSTRACT FROM AUTHOR]

AstroSat, launched in 2015, has observed all kinds of objects in the universe in X-rays and UV light. These objects range from isolated stars to stars in clusters, binary stars with compact companions like neutron stars and black holes, star-forming regions in the galaxies, and supermassive black holes in active nuclei of galaxies. In addition, it has carried out deep surveys of certain parts of the sky, reaching the farthest and the faintest objects in those regions. In this second part of the article, I highlight some of the important results obtained using AstroSat in the first five years of its operation. [ABSTRACT FROM AUTHOR]

It is found difficult to construct a complete general relativistic theory for an accretion disc around a relativistic compact star. There exists a trend to reproduce the effects of general relativistic attraction by using a pseudo-Newtonian approach. One such pseudo-Newtonian force for a spinning black hole sitting in quintessence background is constructed in this paper. Comparison with general relativity at different parametric values is picturised thoroughly. Next, an accretion model using this newly constructed pseudo-Newtonian force is built. The radial inward speed gradient is found to be expressed as a ratio of two functions. It can be realized easily that among these two functions, the denominator might vanish for a particular value of radial speed in the interval zero to speed of light. To make the flow physical, the numerator should vanish at the same radial distance. This will ultimately generate two speed profiles starting from the particular radial distance. This is the cause of generation for accretion and wind branches. It has been followed that the disc is truncated at a finite distance due to the fact that the specific angular momentum becomes larger than the Keplerian angular momentum beyond that. Effects of the spin of the central gravitating object are followed along with the intense effect of the quintessence parameters concerned. Accretion and wind density profiles are also studied to find the density variations near the central engine. Ratio of shear viscosity to entropy density is analyzed in the presence of quintessence for viscous accretion flows. [ABSTRACT FROM AUTHOR]

We present direct N -body simulations, carried out with nbody6+ + gpu , of young and compact low-metallicity (Z  = 0.0002) star clusters with 1.1 × 105 stars, a velocity dispersion of ∼15 |$\mathrm{km\, s^{-1}}$|⁠ , a half-mass radius R h = 0.6 pc, and a binary fraction of |$10{{\ \rm per\,cent}}$| including updated evolution models for stellar winds and (pulsation) pair-instability supernovae (PSNe). Within the first tens of megayears, each cluster hosts several black hole (BH) merger events which nearly cover the complete mass range of primary and secondary BH masses for current LIGO-Virgo-KAGRA gravitational wave detections. The importance of gravitational recoil is estimated statistically during post-processing analysis. We present possible formation paths of massive BHs above the assumed lower PSN mass-gap limit (⁠|$45\, {\rm M}_\odot$|⁠) into the intermediate-mass black hole (IMBH) regime (⁠|$\gt 100\, {\rm M}_\odot$|⁠) which include collisions of stars, BHs, and the direct collapse of stellar merger remnants with low core masses. The stellar evolution updates result in the early formation of heavier stellar BHs compared to the previous model. The resulting higher collision rates with massive stars support the rapid formation of massive BHs. For models assuming a high accretion efficiency for star–BH mergers, we present a first-generation formation scenario for GW190521-like events: a merger of two BHs which reached the PSN mass-gap merging with massive stars. This event is independent of gravitational recoil and therefore conceivable in dense stellar systems with low escape velocities. One simulated cluster even forms an IMBH binary (153, 173 M⊙) which is expected to merge within a Hubble time. [ABSTRACT FROM AUTHOR]

GW170817 is the only gravitational-wave event for which a confirmed Îł -ray counterpart, GRB 170817A, has been detected. Here, we present a method to search for another type of Îł -ray signal, a Îł -ray burst precursor, associated with a compact binary merger. If emitted shortly before the coalescence, a high-energy electromagnetic (EM) flash travels through a highly dynamical and relativistic environment, created by the two compact objects orbiting each other. Thus, the EM signal arriving at an Earth observer could present a somewhat predictable time-dependent modulation. We describe a targeted search method for light curves exhibiting such a modulation, parameterized by the observer-frame component masses and binary merger time, using Fermi-GBM data. The sensitivity of the method is assessed based on simulated signals added to GBM data. The method is then applied to a selection of potentially interesting compact binary mergers detected during the second (O2) and third (O3) observing runs of Advanced LIGO and Advanced Virgo. We find no significant modulated Îł -ray precursor signal associated with any of the considered events. [ABSTRACT FROM AUTHOR]

AstroSat, India's first UV and X-ray astronomy satellite, has completed over six years of observations while in orbit around the Earth. It has carried out detailed studies of all kinds of objects in the Universe ranging from single stars, stars in clusters, binary stars with compact companions like neutron stars and black holes, star formation in galaxies, supermassive black holes in active nuclei of galaxies, etc. Here, I give a short description of its UV and X-ray telescopes and detectors. Some of the most important results obtained using AstroSat will be described in Part II. [ABSTRACT FROM AUTHOR]

Hawking predicted that the formation of a black hole by gravitational collapse causes quantum particle creation and the spectrum of the particles is almost thermal. This phenomenon is called Hawking radiation. Recently, it has been predicted that particle creation may drastically change from Hawking radiation to a strong double burst if the gravitational collapse suddenly stops just before the formation of the event horizon. In contrast with Hawking radiation, the burst may be so strong that it can be of observational interest even for collapsing objects with astrophysical mass scales. However, the burst phenomenon has been predicted through studies of idealized models in which a spherical hollow shell begins to collapse, but stops shrinking and eventually settles down to a static "star". Therefore, one might guess that it could be particular to the hollow shell model. In this paper, we study particle creation due to the gravitational collapse of a spherical object whose interior is filled with matter. In this model, we obtain similar results to those in the case of the hollow shell model. This implies that the double burst is a robust property of particle creation by the sudden braking of gravitational collapse. [ABSTRACT FROM AUTHOR]

We propose quantum simulations of 1 + 1D radial sections of different black hole spacetimes (Schwarzschild, Reissner–Nordstrøm, Kerr and Kerr–Newman), by means of a dc-SQUID array embedded on an open transmission line. This was achieved by reproducing the spatiotemporal dependence of 1 + 1D sections of the spacetime metric with the propagation speed of the electromagnetic field in the simulator, which can be modulated by an external magnetic flux. We show that the generation of event horizons—and therefore Hawking radiation—in the simulator could be achieved for non-rotating black holes, although we discuss limitations related to fluctuations of the quantum phase. In the case of rotating black holes, it seems that the simulation of ergospheres is beyond reach. [ABSTRACT FROM AUTHOR]

The general relativity vacuum black holes (BHs) can be scalarised in models where a scalar field non-minimally couples to the Gauss-Bonnet (GB) invariant. Such GB scalarisation comes in two flavours, depending on the GB sign that triggers the phenomenon. Hereafter these two cases are termed GB+scalarisation. For vacuum BHs, only GB+ scalarisation is possible in the static case, while GB-scalarisation is spin induced. But for electrovacuum BHs, GB- is also charged induced. We discuss the GB- scalarisation of Reissner-Nordström and Kerr-Newman BHs, discussing zero modes and constructing fully non-linear solutions. Some comparisons with GB+ scalarisation are given. To assess the generality of the observed features, we also briefly consider the GB+ scalarisation of stringy dilatonic BHs and coloured BHs which provide qualitative differences with respect to the electrovacuum case, namely on the distribution and existence of regions triggering G- scalarisation. [ABSTRACT FROM AUTHOR]

We assess the feasibility of detecting the compact object remnants from Population III (Pop III) stars in nearby dense star clusters, where they become luminous again as X-ray binaries (XRBs) and tidal disruption events (TDEs) via strong tidal encounters. Analytically modelling the formation of Pop III stars, coupled with a top-heavy initial mass function predicted by numerical simulations, we derive the number of (active) Pop III XRBs and TDEs in the present-day Milky Way (MW) nuclear star cluster as |${\sim} 0.06\!-\!0.3 $| and ≲4 × 10−6, rendering any detection unlikely. The detection probability, however, can be significantly boosted when surveying all massive star clusters from the MW and neighbouring galaxy clusters. Specifically, we predict ∼1.5–6.5 and ∼40–2800 active Pop III XRBs in the MW and the Virgo Cluster, respectively. Our Pop III XRBs are dominated (⁠|${\sim} 99{{\ \rm per\ cent}}$|⁠) by black holes with a typical mass and luminosity of |${\sim} 45\, \rm M_{\odot }$| and |${\sim} 10^{36}\, \rm erg\, s^{-1}$|⁠. Deep surveys of nearby (⁠|${\lesssim} 30\!-\!300\, \rm Mpc$|⁠) galaxy clusters for such Pop III XRBs are well within reach of next-generation X-ray telescopes, such as Athena and Lynx. [ABSTRACT FROM AUTHOR]

The main aim of our study is to explore some relativistic configurations of compact object solution in the background of f R gravity, by adopting the Krori-Barua spacetime. In this regard, we establish the field equations for spherically symmetric spacetime along with charged anisotropic matter source by assuming the specific form of the metric potentials, i.e., ν r = B r 2 + C and λ r = A r 2 . Further, to calculate the constant values, we consider the Bardeen model as an exterior spacetime at the surface boundary. To ensure the viability of the f R gravity model, the physical characteristics including energy density, pressure components, energy bonds, equilibrium condition, Herrera cracking concept, mass-radius relation, and adiabatic index are analyzed in detail. It is observed that all the outcomes by graphical exploration and tabular figures show that the Bardeen black hole model describes the physically realistic stellar structures. [ABSTRACT FROM AUTHOR]

In astronomy, the study of compact stellar remnants — white dwarfs, neutron stars, black holes — has attracted much attention for addressing fundamental principles of physics under extreme conditions in the core of compact objects. In a recent argument, Maurya et al. [Eur. Phys. J. C 77, 45 (2017)] have proposed an exact solution depending on a specific spacetime geometry. Here, we construct equilibrium configurations of compact stars for the same spacetime that make it interesting for modeling high density physical astronomical objects. All calculations are carried out within the framework of the five-dimensional Einstein–Gauss–Bonnet gravity. Our main interest is to explore the dependence of the physical properties of these compact stars depending on the Gauss–Bonnet coupling constant. The interior solutions have been matched to an exterior Boulware–Deser solution for 5 D spacetime. Our finding ensures that all energy conditions hold, and the speed of sound remains causal, everywhere inside the star. Moreover, we study the dynamical stability of stellar structure by taking into account the modified field equations using the theory of adiabatic radial oscillations developed by Chandrasekhar. Based on the observational data for radii and masses coming from different astronomical sources, we show that our model is compatible and physically relevant. [ABSTRACT FROM AUTHOR]

Using a lattice equation of state combined with the D-dimensional Tolman–Oppenheimer–Volkoff equation and the Friedmann equations, we investigate the possibility of the formation of compact objects as well as the time evolution of the scale factor and the density profile of a self-gravitating material cluster. The numerical results show that in a (2 + 1)-dimensional space–time, the mass is independent of the central pressure. Hence, the formation of only compact objects with a finite constant mass similar to the white dwarf is possible. However, in a (3 + 1)-dimensional space–time, self-gravity leads to the formation of compact objects with a large gap of mass and the corresponding phase diagram has the same structure as the one for Neutron Star. The results also show that beyond certain critical central pressure, the star is unstable against gravitational collapse, and it may end in a black hole. Analysis of space–times of higher dimensions shows that gravity has the stronger effect in 3 + 1 dimensions. Numerical solutions of the Friedmann equations show that the effect of the curvature of space–time increases with the increasing temperature, but decreases with the increasing dimensionality beyond  D = 3. [ABSTRACT FROM AUTHOR]

A new compact stars nonsingular model is presented with the generalized Bardeen–Hayward mass function. Generalized Bardeen–Hayward described the regular black hole, however, due to its regularity or nonsingular nature we were inspired to construct an anisotropic compact stars model. Along with the ansatz mass function, we coupled it with a linear equation of state (EoS) to find the solutions of field equations. Indeed, the new solutions are physically viable in all physical ground. The energy conditions and Tolman–Oppenheimer–Volkoff (TOV)-equation are well satisfied signifying that the mass distribution is physically possible and at equilibrium. Also, the static stability criterion, the causality condition and Abreu's stability condition hold good and therefore configurations are physically static stable. The same condition is further supported by the condition that the adiabatic index, which is greater than the Newtonian limit, i.e. Γ r (r) > 4 / 3. It is also noticed that the bag constant B g is proportional to the surface density in our model. [ABSTRACT FROM AUTHOR]

We model the effects of collisions and close encounters on the stellar populations observed in the Milky Way nuclear stellar cluster (NSC). Our analysis is based on N -body simulations in which the NSC forms by accretion of massive stellar clusters around a supermassive black hole. We attach stellar populations to our N -body particles and follow the evolution of their stars, and the rate of collisions and close encounters. The most common encounters are collisions between pairs of main-sequence (MS) stars, which lead to mergers: destructive collisions between MS stars and compact objects are rare. We find that the effects of collisions on the stellar populations are small for three reasons. First, our models possess a core that limits the maximum stellar density. Secondly, the velocity dispersion in the NSC is similar to the surface escape velocities of the stars, which minimizes the collision rate. Finally, whilst collisions between MS stars destroy bright giants by accelerating their evolution, they also create them by accelerating the evolution of lower mass stars. These two effects approximately cancel out. We also investigate whether the G2 cloud could be a fuzzball: a compact stellar core that has accreted a tenuous envelope in a close encounter with a red giant. We conclude that fuzzballs with cores below |$2\, {\rm M_{\odot }}$| have thermal times-scales too short to reproduce G2. A fuzzball with a black hole core could reproduce the surface properties of G2 but the production rate of such objects in our model is low. [ABSTRACT FROM AUTHOR]

Accretion disks around supermassive black holes in active galactic nuclei produce continuum radiation at ultraviolet and optical wavelengths. Physical processes in the accretion flow lead to stochastic variability of this emission on a wide range of time scales. We measured the optical continuum variability observed in 67 active galactic nuclei and the characteristic time scale at which the variability power spectrum flattens. We found a correlation between this time scale and the black hole mass extending over the entire mass range of supermassive black holes. This time scale is consistent with the expected thermal time scale at the ultraviolet-emitting radius in standard accretion disk theory. Accreting white dwarfs lie close to this correlation, suggesting a common process for all accretion disks. [ABSTRACT FROM AUTHOR]

The study of Planet Nine has received a lot of attention of late. Recently it has been proposed that Planet Nine could be a Primordial Black Hole (PBH) lurking in our solar system. Earlier it was suggested by the present authors that Planet Nine could be degenerate object constituted mostly of dark matter (DM) particles, having their own distinct characteristic properties which were elaborated upon. Here we do a comparative study on whether the Planet Nine is a PBH or a DM Object, which would be relevant for future observations. [ABSTRACT FROM AUTHOR]

We present spherically symmetric simulations of the thermonuclear explosion of a white dwarf admixed with an extended component of fermionic dark matter, using the deflagration model with the deflagration–detonation transition. In all the dark matter admixed models we have considered, the dark matter is left behind after the explosion as a compact dark star. The presence of dark matter lengthens the deflagration phase to produce a similar amount of iron-group elements and more thermoneutrinos. Dark matter admixed models also give dimmer but slowly declining light curves, consistent with some observed peculiar supernovae. Our results suggest a formation path for dark compact objects that mimic sub-solar-mass black holes as dark gravitational sources. [ABSTRACT FROM AUTHOR]

In dense star clusters, such as globular and open clusters, dynamical interactions between stars and black holes (BHs) can be extremely frequent, leading to various astrophysical transients. Close encounters between a star and a stellar mass BH make it possible for the star to be tidally disrupted by the BH. Due to the relative low mass of the BH and the small cross-section of the tidal disruption event (TDE) for cases with high penetration, disruptions caused by close encounters are usually partial disruptions. The existence of the remnant stellar core and its non-negligible mass compared to the stellar mass BH alters the accretion process significantly. We study this problem with SPH simulations using the code Phantom , with the inclusion of radiation pressure, which is important for small mass BHs. Additionally, we develop a new, more general method of computing the fallback rate which does not rely on any approximation. Our study shows that the powerlaw slope of the fallback rate has a strong dependence on the mass of the BH in the stellar mass BH regime. Furthermore, in this regime, self-gravity of the fallback stream and local instabilities become more significant, and cause the disrupted material to collapse into small clumps before returning to the BH. This results in an abrupt increase of the fallback rate, which can significantly deviate from a powerlaw. Our results will help in the identification of TDEs by stellar mass BHs in dense clusters. [ABSTRACT FROM AUTHOR]