Search

The primary formation channel for the stellar-mass Binary Black Holes which have been detected merging by the LIGO–Virgo–KAGRA (LVK) collaboration is yet to be discerned. One of the main reason is that the detection of an Electromagnetic counterpart to such Gravitational Wave (GW) events, which could signpost their formation site, has so far been elusive. Recently, 20 Active Galactic Nuclei flaring activities detected by the Zwicky Transient Facility have been investigated as potential counterparts of GW events by Graham et al. We present the results of a spatial correlation analysis that involves such events and uses the up-to-date posterior samples of 78 mergers, detected during the third observing run of the LVK collaboration. We apply a likelihood method which takes into account the exact position of the flares within the 3D sky map of the GW events. We place an upper limit of 0.155 at a 90 per cent credibility level on the fraction of the detected coalescences that are physically related to an observed flare, whose posterior probability distribution peaks at a null value. Finally, we show that the typically larger values of the masses of the GW-events, which host at least one flare in their localization volume, are also consistent with the no-connection hypothesis. This is because of a positive correlation between binary masses and the size of the localization uncertainties. [ABSTRACT FROM AUTHOR]

Pulsar timing arrays (PTAs) are ensembles of regularly observed millisecond pulsars timed to high precision. Each pulsar in an array could be affected by a suite of noise processes, most of which are astrophysically motivated. Analysing them carefully can be used to understand these physical processes. However, the primary purpose of these experiments is to detect signals that are common to all pulsars, in particular signals associated with a stochastic gravitational wave background. To detect this, it is paramount to appropriately characterize other signals that may otherwise impact array sensitivity or cause a spurious detection. Here, we describe the second data release and first detailed noise analysis of the pulsars in the MeerKAT Pulsar Timing Array, comprising high-cadence and high-precision observations of 83 millisecond pulsars over 4.5 yr. We use this analysis to search for a common signal in the data, finding a process with an amplitude of |$\log _{10}{A_{\mathrm{ CURN}}} = -14.25^{+0.21}_{-0.36}$| and spectral index |$\gamma _\mathrm{CURN} = 3.60^{+1.31}_{-0.89}$|⁠. Fixing the spectral index at the value predicted for a background produced by the inspiral of binary supermassive black holes, we measure the amplitude to be |$\log _{10}{A_{\mathrm{ CURN}}} = -14.28^{+0.21}_{-0.21}$| at a significance expressed as a Bayes factor of |$\ln (\mathcal {B}) = 4.46$|⁠. Under both assumptions, the amplitude that we recover is larger than those reported by other PTA experiments. We use the results of this analysis to forecast our sensitivity to a gravitational wave background possessing the spectral properties of the common signal we have measured. [ABSTRACT FROM AUTHOR]

Statistical analyses based on the spatial correlation between the sky maps of gravitational wave (GW) events and the positions of potential host environments are a powerful tool to infer the origin of the black hole binary mergers that have been detected by the LIGO, Virgo, and KAGRA instruments. In this paper, we tighten our previous constraints on the fraction of detected GW events that may have originated from active galactic nuclei (AGNs). We consider 159 mergers detected not later than 1st June, 2024, and the all-sky quasar catalogue Quaia. We increase by a factor of 5.3 and 114, the number of considered GW sources and AGN, respectively, also extending our analysis from redshift 0.3 to 1.5. This is possible thanks to the uniformity of the AGN catalogue and its high level of completeness, which we estimate as a function of redshift and luminosity. We find at a 95 per cent credibility level that un-obscured AGN with a bolometric luminosity higher than |$10^{44.5}\,{\rm erg\ s}^{-1}$| (⁠|$10^{45}\,{\rm erg\ s}^{-1}$|⁠) do not contribute to more than the 21 (11) per cent of the detected GW events. [ABSTRACT FROM AUTHOR]

A fraction of the ultraluminous X-ray (ULX) sources are known to be accreting neutron stars as they show coherent X-ray pulsations with pulse periods ranging from |${\sim} 1{\!-\!}30$|  s. While initially thought to host intermediate-mass black holes, ULXs have since been recognized as a diverse class of objects, including ULX pulsars. These pulsars require models specifically tailored to account for their unique accretion physics, distinct from those used for Galactic black hole binaries. The X-ray spectra of all Galactic accreting X-ray pulsars (including sources in the Magellanic Clouds) are dominated by a high-energy cut-off power law and some of the sources show a soft excess, some emission lines, cyclotron absorption features, etc. In this work, we undertake a comprehensive analysis of the broad-band X-ray spectra of five ULX pulsars using simultaneous XMM – Newton and NuSTAR observations and show that their X-ray spectra can be effectively described by spectral models, similar to those used for the local accretion-powered X-ray pulsars. A soft excess is detected in all the sources which is also consistent with the local X-ray pulsars that have low absorption column density. We have marginal detection or low upper limit on the presence of the iron K-alpha emission line from these sources, which is a key difference of the ULX pulsars with the local accreting X-ray pulsars. We discuss the implication of this on the nature of the binary companion and the accretion mechanism in the ULX pulsars. [ABSTRACT FROM AUTHOR]

Our current understanding is that an environment – mainly consisting of gas or stars – is required to bring massive black hole binaries (MBHBs) with total redshifted mass |$M_z\sim [10^{4},10^7]~{\rm M}_{\odot }$| to the LISA band from parsec separation. Even in the gravitational wave (GW) dominated final inspiral, realistic environments can non-negligibly speed up or slow down the binary evolution, or leave residual, measurable eccentricity in the LISA band. Despite this fact, most of the literature does not consider environmental effects or orbital eccentricity in modelling GWs from near-equal mass MBHBs. Considering either a circular MBHB embedded in a circumbinary disc or a vacuum eccentric binary, we explore if ignoring either secular gas effects (migration and accretion) or eccentric corrections to the GW waveform can mimic a failure of general relativity (GR). We use inspiral-only aligned-spin 3.5 post-Newtonian (PN) waveforms, a complete LISA response model, and Bayesian inference to perform a parameterized test of GR. For a 4-yr LISA observation of an MBHB with |$M_z=10^{5}~{\rm M}_{\odot }$|⁠ , primary-to-secondary mass ratio |$q=8$|⁠ , and component BHs' dimensionless spins |$\chi _{1,2}=0.9$| at redshift |$z=1$|⁠ , even a moderate gas–disc imprint (Eddington ratio |${\it f}_{\rm Edd}\sim 0.1$|⁠) or low initial eccentricity (⁠|$e_0\sim 10^{-2.5}$|⁠) causes a false violation of GR in several PN orders. However, correctly modelling either effect can mitigate systematics while avoiding significant biases in vacuum circular systems. The adoption of LISA makes it urgent to consider gas imprints and eccentricity in waveform models to ensure accurate inference for MBHBs. [ABSTRACT FROM AUTHOR]

There are indications that stellar-origin black holes (BHs) are efficiently paired up in binary black holes (BBHs) in active galactic nucleus (AGN) disc environments, which can undergo interactions with single BHs in the disc. Such binary–single interactions can potentially lead to an exceptionally high fraction of gravitational-wave mergers with measurable eccentricity in LIGO/Virgo/KAGRA. We here take the next important step in this line of studies by performing post-Newtonian N -body simulations between migrating BBHs and single BHs set in an AGN disc-like configuration, with a consistent inclusion of the central supermassive black hole (SMBH) in the equations of motion. With this set-up, we study how the fraction of eccentric mergers varies in terms of the initial size of the BBH semimajor axis relative to the Hill sphere, as well as how it depends on the angle between the BBH and the incoming single BH. We find that the fraction of eccentric mergers is still relatively large, even when the interactions are notably influenced by the gravitational field of the nearby SMBH. However, the fraction as a function of the BBH semimajor axis does not follow a smooth functional shape, but instead shows strongly varying features that originate from the underlying phase-space structure. The phase-space further reveals that many of the eccentric mergers are formed through prompt scatterings. Finally, we present the first analytical solution to how the presence of an SMBH in terms of its Hill sphere affects the probability for forming eccentric BBH mergers through chaotic three-body interactions. [ABSTRACT FROM AUTHOR]

Gravitational-wave observations by the laser interferometer gravitational-wave observatory (LIGO) and Virgo have provided us a new tool to explore the Universe on all scales from nuclear physics to the cosmos and have the massive potential to further impact fundamental physics, astrophysics, and cosmology for decades to come. In this paper we have studied the science capabilities of a network of LIGO detectors when they reach their best possible sensitivity, called A ♯ , given the infrastructure in which they exist and a new generation of observatories that are factor of 10 to 100 times more sensitive (depending on the frequency), in particular a pair of L-shaped cosmic explorer (CE) observatories (one 40 km and one 20 km arm length) in the US and the triangular Einstein telescope with 10 km arms in Europe. We use a set of science metrics derived from the top priorities of several funding agencies to characterize the science capabilities of different networks. The presence of one or two A ♯ observatories in a network containing two or one next generation observatories, respectively, will provide good localization capabilities for facilitating multimessenger astronomy (MMA) and precision measurement of the Hubble parameter. Two CE observatories are indispensable for achieving precise localization of binary neutron star events, facilitating detection of electromagnetic counterparts and transforming MMA. Their combined operation is even more important in the detection and localization of high-redshift sources, such as binary neutron stars, beyond the star-formation peak, and primordial black hole mergers, which may occur roughly 100 million years after the Big Bang. The addition of the Einstein Telescope to a network of two CE observatories is critical for accomplishing all the identified science metrics including the nuclear equation of state, cosmological parameters, the growth of black holes through cosmic history, but also make new discoveries such as the presence of dark matter within or around neutron stars and black holes, continuous gravitational waves from rotating neutron stars, transient signals from supernovae, and the production of stellar-mass black holes in the early Universe. For most metrics the triple network of next generation terrestrial observatories are a factor 100 better than what can be accomplished by a network of three A ♯ observatories. [ABSTRACT FROM AUTHOR]

The mid-infrared (MIR; |$\lambda \simeq 3\!\!-\!\!10\ \mu \mathrm{ m}$|⁠) bands offer a unique window into understanding accretion and jet formation in Galactic black hole X-ray binaries (BHXRBs). Although difficult to observe from the ground, the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) survey offers an excellent data set to study MIR variability when combined with contemporaneous X-ray data from Monitor of All-sky X-ray Image (MAXI) mission over an |$\approx\!\! 15$|  yr baseline. Using a new difference imaging pipeline for NEOWISE data, we present the first systematic MIR study of BHXRB outbursts. Analysing a sample of 16 sources, we show variability trends wherein (i) the MIR bands are dominated by jet emission during the hard states, constraining the electron power spectrum index (⁠|$p\approx 1\!\!-\!\!4$|⁠) in the optically thin regime and indicating emitting regions of few |$10\times$| gravitational radii when evolving towards a flat spectrum, (ii) the MIR luminosity (⁠|$L_{\mathrm{ IR}}$|⁠) scales as |$L_{\mathrm{ IR}} \propto L_\mathrm{ X}^{0.82\pm 0.12}$| with the |$2\!\!-\!\!10$|  keV X-ray luminosity (⁠|$L_\mathrm{ X}$|⁠) in the hard state, consistent with a jet, and (iii) thermal disc emission dominates the soft states as the jet switches off and suppresses (⁠|$\gtrsim\!\! 10\times$|⁠) the MIR emission into an inverted spectrum (spectral index |$\alpha \approx -1$|⁠). We highlight 'mini' outbursts detected in NEOWISE (including two previously unreported episodes of MAXI J1828−249 from September 2015 and September 2016) but missed in MAXI due to their faint fluxes' confusion, exhibiting MIR colours suggestive of thermal disc emission. We highlight that upcoming infrared surveys and the Rubin observatory will be powerful discovery engines for the distinctively large-amplitude and long-lived outbursts of BHXRBs, as an independent discovery route to X-ray monitors. [ABSTRACT FROM AUTHOR]

Binary population synthesis (BPS) is an essential tool for extracting information about massive binary evolution from gravitational-wave detections of compact object mergers. It has been successfully used to constrain the most likely permutations of evolution models among hundreds of alternatives, while initial condition models, in contrast, have not yet received the same level of attention. Here, we introduce bossa , a detailed initial sampling code including a set of 192 initial condition permutations for BPS, that captures both 'invariant' and 'varying' models, the latter accounting for a possible metallicity- and star formation rate-dependence of the initial mass function (IMF), as well as correlations between the initial primary mass, orbital period, mass ratio, and eccentricity of binaries. We include 24 metallicity-specific cosmic star formation history models and propose two alternate models for the mass-dependent binary fraction. We build a detailed pipeline for time-evolving BPS, such that each binary has well-defined initial conditions, and we are able to distinguish the contributions from populations of different ages. We discuss the meaning of the IMF for binaries and introduce a refined initial sampling procedure for component masses. We also discuss the treatment of higher order multiple systems when normalizing a binary sample. In particular, we argue for how a consistent interpretation of the IMF implies that this is not the distribution from which any set of component masses should be independently drawn, and show how the individual IMF of primaries and companions is expected to deviate from the full IMF. [ABSTRACT FROM AUTHOR]

Ninety gravitational-wave events have been detected by the LIGO–Virgo–KAGRA network and are released in the Gravitational-Wave Transient Catalog. Among these events, 83 cases are definitely binary black hole mergers, since the masses of all the objects involved significantly exceed the upper limit of neutron stars. The black holes in these merger events naturally form two interesting samples, a premerger sample that includes all the black holes before the mergers and a postmerger sample that consists of the black holes generated during the merging processes. The former represents black holes that once existed in the Universe, while the latter represents newly born black holes. Here we present a statistical analysis of these two samples. The nonparametric τ statistic method is adopted to correct for the observational selection effect. The Lynden-Bell C − method is further applied to derive the mass distribution and density function of black holes. It is found that the mass distribution can be expressed as a broken power-law function. More interestingly, the power-law index in the high-mass region is comparable for the two samples. The number density of black holes is found to depend on redshift as ρ (z) ∝ z −2.06— z −2.12 based on the two samples. The implications of these findings on the origin of black holes are discussed. [ABSTRACT FROM AUTHOR]

In the case of low-mass X-ray binaries, the companion star is often too faint for detection; therefore, there is no chance for dynamical studies to independently determine the mass of the compact object. In the absence of a mass estimate, one cannot make a distinction as to whether the binary hosts a black hole or neutron star. Therefore, the question arises whether this distinction can be made based on the X-ray data alone, even when there are no bursts or pulsations. These would automatically imply a neutron star, but they are not always present. Black hole systems are known to emit radiation with an unbroken power–law shape up to several hundred keV energies in their high/soft states. If the non-thermal Comptonization processes that are responsible for this are somehow related to the lack of a solid surface, and to the fact that more gravitational potential energy can be released for a black hole, then there would be a definite method to reliably distinguish between the two sources. This work intends to review the available observations and studies to compare how these two populations behave during their different spectral states. A conclusion can be made that high/soft-state spectra are really different for black holes and neutron stars, for the low/hard state; however, the same conclusion cannot be safely made. [ABSTRACT FROM AUTHOR]

The cosmic expansion rate can be directly measured with gravitational-wave (GW) data of the compact binary mergers by jointly constraining the mass function of the population and the cosmological model via the so-called spectral sirens. Such a method relies on the features in the mass functions, which may originate from some individual subpopulations and hence become blurred/indistinct due to the superposition of different subpopulations. In this work, we propose a novel approach to constrain the cosmic expansion rate with subpopulations of GW events, named multispectral sirens. The advantage of the multispectral sirens compared to the traditional spectral sirens is demonstrated by the simulation with the mock data. The application of this approach to the GWTC-3 data yields H 0 = 73.3 − 25.6 + 29.9 Mpc − 1 km s − 1 (median and symmetric 68.3% credible interval), which is about 19% tighter than the result inferred with the traditional spectral sirens utilizing a powerlaw+peak mass function. The incorporation of the bright standard siren GW170817 with a uniform prior in [10, 200] (log-uniform prior in [20,140]) Mpc−1 km s−1 gives H 0 = 71.1 − 7.5 + 15.0 (70.3 − 7.1 + 12.9) Mpc − 1 km s − 1 (68.3% confidence level), corresponding to an improvement of ∼26% (23%) with respect to the measurement from sole GW170817. [ABSTRACT FROM AUTHOR]

Gravitational wave (GW) detection allows us to test general relativity in entirely new regimes. A prominent role takes the detection of quasi-normal modes (QNMs), which are emitted after the merger of a binary black hole (BBH) when the highly distorted remnant emits GWs to become a regular Kerr black hole (BH). The BH uniqueness theorems of Kerr black hole solutions in general relativity imply that the frequencies and damping times of QNMs are determined solely by the mass and spin of the remnant BH. Therefore, detecting QNMs offers a unique way to probe the nature of the remnant BH and to test general relativity. We study the detection of a merging BBH in the intermediate-mass range, where the inspiral–merger phase is detected by space-based laser interferometer detectors TianQin and LISA, while the ringdown is detected by the ground-based atom interferometer (AI) observatory AION. The analysis of the ringdown is done using the regular broadband mode of AI detectors as well as the resonant mode optimizing it to the frequencies of the QNMs predicted from the inspiral–merger phase. We find that the regular broadband mode allows constraining the parameters of the BBH with relative errors of the order 10 − 1 and below from the ringdown. Moreover, for a variety of systems considered, the frequencies and the damping times of the QNMs can be determined with relative errors below 0.1 and 0.2, respectively. We further find that using the resonant mode can improve the parameter estimation for the BBH from the ringdown by a factor of up to three. Utilizing the resonant mode significantly limits the detection of the frequency of the QNMs but improves the detection error of the damping times by around two orders of magnitude. [ABSTRACT FROM AUTHOR]

Black hole X-ray binary systems consist of a black hole accreting mass from its binary companion, forming an accretion disk. As a result, twin relativistic plasma ejections (jets) are launched towards opposite and perpendicular directions. Moreover, multiple broadband emission observations from X-ray binary systems range from radio to high-energy gamma rays. The emission mechanisms exhibit thermal origins from the disk, stellar companion, and non-thermal jet-related components (i.e., synchrotron emission, inverse comptonization of less energetic photons, etc.). In many attempts at fitting the emitted spectra, a static black hole is often assumed regarding the accretion disk modeling, ignoring the Kerr metric properties that significantly impact the geometry around the usually rotating black hole. In this work, we study the possible implications of the spin inclusion in predictions of the X-ray binary spectrum. We mainly focus on the most significant aspect inserted by the Kerr geometry, the innermost stable circular orbit radius dictating the disk's inner boundary. The outcome suggests a higher-peaked and hardened X-ray spectrum from the accretion disk and a substantial increase in the inverse Compton component of disk-originated photons. Jet-photon absorption is also heavily affected at higher energy regimes dominated by hadron-induced emission mechanisms. Nevertheless, a complete investigation requires the full examination of the spin contribution and the resulting relativistic effects beyond the disk truncation. [ABSTRACT FROM AUTHOR]

Around 50 years ago, the famous bet between Stephen Hawking and Kip Thorne on whether Cyg X-1 hosts a stellar-mass black hole became a well-known story in the history of black hole science. Today, Cyg X-1 is widely recognised as hosting a stellar-mass black hole with a mass of approximately 20 solar masses. With the advancement of X-ray telescopes, Cyg X-1 has become a prime laboratory for studies in stellar evolution, accretion physics, and high-energy plasma physics. In this review, we explore the latest results from X-ray observations of Cyg X-1, focusing on its implications for black hole spin, its role in stellar evolution, the geometry of the innermost accretion regions, and the plasma physics insights derived from its X-ray emissions. This review primarily focuses on Cyg X-1; however, the underlying physics applies to other black hole X-ray binaries and, to some extent, to AGNs. [ABSTRACT FROM AUTHOR]

Pulsar timing arrays (PTA) can detect continuous nanohertz gravitational waves (GW) emitted by individual supermassive black hole binaries. The data analysis procedure can be formulated within a time-domain, state-space framework, in which the radio timing observations are related to a temporal sequence of latent states, namely the intrinsic pulsar spin frequency. The achromatic wandering of the pulsar spin frequency is tracked using a Kalman filter concurrently with the pulse frequency modulation induced by a GW from a single source. The modulation is the sum of terms proportional to the GW strain at the Earth and at every pulsar in the array. Here, we generalize previous state-space formulations of the PTA problem to include the pulsar terms; that is, we copy the pulsar terms from traditional, non-state-space analyses over to the state-space framework. The performance of the generalized Kalman filter is tested using astrophysically representative software injections in Gaussian measurement noise. It is shown that including the pulsar terms corrects for previously identified biases in the parameter estimates (especially the sky position of the source) which also arise in traditional matched-filter analyses that exclude the pulsar terms. Additionally, including the pulsar terms decreases the minimum detectable strain by 14 per cent. Overall, the study verifies that the pulsar terms do not raise any special extra impediments for the state-space framework, beyond those studied in traditional analyses. The inspiral-driven evolution of the wave frequency at the Earth and at the retarded time at every pulsar in the array is also investigated. [ABSTRACT FROM AUTHOR]

Intermediate-mass black hole (IMBH) mergers with masses 104–106 M ⊙ are expected to produce gravitational waves detectable by the Laser Interferometer Space Antenna (LISA) with high signal-to-noise ratios from the present day to cosmic dawn. IMBH mergers are expected to take place within dwarf galaxies; however, the dynamics, timescales, and effect on their hosts are largely unexplored. In a previous study, we examined how IMBHs would pair and merge within nucleated dwarf galaxies. IMBHs in nucleated hosts evolve very efficiently, forming a binary system and coalescing within a few hundred million years. Although the fraction of dwarf galaxies (107 M ⊙ ≤ M ⋆ ≤ 1010 M ⊙) hosting nuclear star clusters is between 60% and 100%, this fraction drops to 20%–70% for lower-mass dwarfs (M ⋆ ≈ 107 M ⊙), with the largest drop in low-density environments. Here, we extend our previous study by performing direct N -body simulations to explore the dynamics and evolution of IMBHs within nonnucleated dwarf galaxies, under the assumption that IMBHs exist within these dwarfs. To our surprise, none of the IMBHs in our simulation suite merge within a Hubble time, despite many attaining high eccentricities e ∼ 0.7–0.95. We conclude that extremely low stellar density environments in the centers of nonnucleated dwarfs do not provide an ample supply of stars to interact with an IMBH binary, resulting in its stalling, in spite of triaxiality and high eccentricity, common means to drive a binary to coalescence. Our findings underline the importance of considering all detailed host properties to predict IMBH merger rates for LISA. [ABSTRACT FROM AUTHOR]

Gravitational waves (GWs) have revealed surprising properties of binary black hole (BBH) populations, but there is still mystery surrounding how these compact objects evolve. We apply Bayesian inference and an efficient method to calculate the BBH merger rates in the Shark host galaxies, to determine the combination of COMPAS parameters that outputs a population most like the GW sources from the LIGO, Virgo, and KAGRA (LVK) transient catalogue. For our COMPAS models, we calculate the likelihood with and without the dependence on the predicted number of BBH merger events. We find strong correlations between hyper-parameters governing the specific angular momentum (AM) of mass lost during mass transfer, the mass-loss rates of Wolf–Rayet stars via winds and the chemically homogeneous evolution (CHE) formation channel. We conclude that analysing the marginalized and unmarginalized likelihood is a good indicator of whether the population parameters distribution and number of observed events reflect the LVK data. In doing so, we see that the majority of the models preferred in terms of the population-level parameters of the BBHs greatly overpredict the number of events we should have observed to date. Looking at the smaller number of models that perform well with both likelihoods, we find that those with no CHE, AM loss occurring closer to the donor during the first mass-transfer event, and/or higher rates of mass-loss from Wolf–Rayet winds are generally preferred by current data. We find these conclusions to be robust to our choice of selection criteria. [ABSTRACT FROM AUTHOR]

Parsec-scale separation supermassive black hole binaries in the centre of gas-rich galaxy merger remnants could be surrounded by massive circumbinary discs (CBDs). Black hole mass and spin evolution during the gas-rich binary inspiral are crucial in determining the direction and power of relativistic jets that radio observations with LOFAR (Low-Frequency Array) and SKAO (Square Kilometer Array Observatory) will probe, and for predicting gravitational wave (GW) emission that the IPTA (International Pulsar Timing Array) and LISA (Laser Interferometer Space Antenna) will measure. We present 3D hydrodynamic simulations capturing gas-rich, self-gravitating CBDs around a |$2\times 10^6$| M |$_{\odot }$| supermassive black hole binary, that probe different mass ratios, eccentricities, and inclinations. We employ a subgrid Shakura–Sunyaev accretion disc to self-consistently model black hole mass and spin evolution together with super-Lagrangian refinement techniques to resolve gas flows, streams, and mini-discs within the cavity, which play a fundamental role in torquing and feeding the binary. We find that higher mass ratio and eccentric binaries result in larger cavities, while retrograde binaries result in smaller cavities. All of the simulated binaries are expected to shrink with net gravitational torques being negative. Unlike previous simulations, we do not find preferential accretion onto the secondary black hole. This implies smaller chirp masses at coalescence and hence a weaker GW background. Critically this means that spin alignment is faster than the binary inspiral time-scale even for low-mass ratios. When considering initially misaligned systems, the orientation of the mini-discs around each black hole can vary significantly. We discuss the implications of this behaviour for black hole spin alignment and highlight the need for broader parameter space studies of misaligned systems to understand the impact on black hole recoil velocities. [ABSTRACT FROM AUTHOR]

We perform a comprehensive timing and broadband spectral analysis using an AstroSat observation of the low-mass black hole X-ray binary H 1743–322 during its 2017 outburst. Additionally, we use two Swift/XRT observations, one of which is simultaneous with AstroSat and the other taken three days earlier, for timing analysis. The hardness–intensity diagram indicates that the 2017 outburst was a failed one, unlike the previous successful outburst in 2016. We detect type C quasi-periodic oscillation (QPO) in the simultaneous AstroSat and Swift/XRT observations at ∼0.4 Hz, whereas an upper harmonic is noticed at ∼0.9 Hz in the AstroSat data only. Although these features are found to be energy-independent, we notice a shift of ∼0.08 Hz in the QPO frequency over the interval of three days. We also investigate the nature of variability in the two consecutive failed outbursts in 2017 and 2018. We detect soft time lags of 23.2 ± 12.2 ms and 140 ± 80 ms at the type C QPO frequencies in 2017 AstroSat and 2018 XMM-Newton data, respectively. The lag–energy spectra from both the outbursts suggest that the soft lags may be associated with reflection features. The broadband spectral analysis indicates that the source was in the low/hard state during the AstroSat observation. Modeling of the disk and reflection continuum suggests the presence of an accretion disk that is significantly truncated by at least 27.4 r g from the innermost stable circular orbit when the source luminosity is ∼1.6% of the Eddington luminosity. [ABSTRACT FROM AUTHOR]

While there has been an increase in interest in the possibility of quasiperiodic oscillations (QPOs) in blazars, the search has hitherto been restricted to sources with well-sampled light curves. Objects with light curves that include gaps have been, to our knowledge, overlooked. Here, we study two such curves, which have the interesting feature of pertaining to relatively high-redshift blazars—FSRQs, PKS 2155-83, and PKS 2255-282—observed by the Fermi Large Area Telescope. Their redshifts border the "cosmic noon" era of galaxy formation and merging, and their light curves exhibit a distinctive pattern of repetitive high and low (gap dominant) states for 15.6 yr. To accommodate for the gaps in the curves, data are integrated over extended time intervals of 1 month and 2 months. The resulting curves were also examined using methods suitable for sparsely sampled data. This investigation of PKS 2155-83 and PKS 2255-282 suggests QPOs with periods of 4.69 ± 0.79 yr (3 σ) and 6.82 ± 2.25 yr (2.8 σ), respectively. The probability density functions of the blazars' fluxes, along with the correlation between their flux and spectral index, were also analyzed. Given the epochs in which the objects are observed, the plausibility of a binary black hole scenario as an origin of the apparent periodicity was examined. We estimated the prospective parameters of such a system using a simple geometric model. The total masses were estimated and found to be consistent, in principle, with independent (dynamical) measurements of the central black hole masses in the two host galaxies. [ABSTRACT FROM AUTHOR]

Understanding the connection between the properties of black holes (BHs) and their progenitors is interesting in many branches of astrophysics. Discovering BHs in detached orbits with luminous companions (LCs) promises to help establish this connection since the LC and BH progenitor are expected to have the same metallicity and formation time. We explore the possibility of detecting BH–LC binaries in detached orbits using photometric variations of the LC flux, induced by tidal ellipsoidal variation, relativistic beaming, and self-lensing. We create realistic present-day populations of detached BH–LC binaries in the Milky Way (MW) using binary population synthesis where we adopt observationally motivated initial stellar and binary properties, star formation history, and the present-day distribution of these sources in the MW based on detailed cosmological simulations. We test detectability of these sources via photometric variability by Gaia and TESS missions by incorporating their respective detailed detection biases as well as interstellar extinction. We find that Gaia is expected to resolve ∼300–1000 (∼700–1500) detached BH–LC binaries with signal-to-noise ratio (SNR) ≥ 10 (1) depending on the photometric precision and details of supernova physics. Similarly, the number of resolved BH–LC binaries with TESS is ∼50–200 (∼140–350). We find that 136 − 15 + 15 BH–LC binaries would be common between Gaia and TESS. Moreover, ∼60–70 (∼50–200) BH–LC binaries identifiable using photometry with SNR ≥ 10 may also be resolved using Gaia's radial velocity (astrometry). [ABSTRACT FROM AUTHOR]

Recent gravitational wave (GW) observations include possible detections of black hole—neutron star binary mergers. As with binary black hole mergers, numerical simulations help characterize the sources. For binary systems with neutron star components, the simulations help to predict the imprint of tidal deformations and disruptions on the GW signals. In a previous study, we investigated how the mass of the black hole has an impact on the disruption of the neutron star and, as a consequence, on the shape of the GWs emitted. We extend these results to study the effects of varying the compactness of the neutron star. We consider neutron star compactness in the 0.113–0.2 range for binaries with mass ratios of 3 and 5. As the compactness and the mass ratio increase, the binary system behaves during the late inspiral and merger more like a black hole binary. For the cases with the least compact neutron star, the GWs emitted, in terms of mismatches, are the most distinguishable from those by a binary black hole. The disruption of the star significantly suppresses the kicks on the final black hole. The disruption also affects, although not dramatically, the spin of the final black hole. Lastly, for neutron stars with low compactness, the quasi-normal ringing of the black hole after the merger does not show a clean quasi-normal ringing because of the late accretion of debris from the neutron star. [ABSTRACT FROM AUTHOR]

We apply the Lagrangian method to study the post-Newtonian evolution of a compact binary system with environmental effects, including a dark matter spike, and obtain the resulting gravitational wave emission. This formalism allows one to incorporate post-Newtonian effects up to any desired known order, as well as any other environmental effect around the binary, as long as their dissipation power or force formulae are known. In particular, in this work, we employ this method to study a black hole–black hole binary system of mass ratio 10 5 by including post-Newtonian effects of order 1PN and 2.5PN, as well as the effect of relativistic dynamical friction. We obtain the modified orbits and the corresponding modified gravitational waveform. Finally, we contrast these modifications against the LISA sensitivity curve in frequency space and show that this observatory can detect the associated signals. [ABSTRACT FROM AUTHOR]

Context. The observed spectra from black hole (BH) X-ray binaries (XRBs) typically consist of two primary components. A multitemperature blackbody originating from the accretion disk in the soft X-ray, and a power law-like component in the hard X-ray, due to the Comptonization of soft photons by the hot corona. The illumination of the disk by the corona gives rise to another key component known as reflection. A fraction of the incident hard X-ray radiation is naturally absorbed and re-emitted as a blackbody at lower energies and referred to as the "reprocessed blackbody". Aims. For densities relevant to XRBs and typical ionization values, the reprocessed blackbody may become significant in the soft X-ray region (approximately 0.1–1.0 keV) and should be noticeable in the observed spectra as a consequence of reflection. The absence of any blackbody component in the low/hard state of a BH XRB may not be consistent with the reflection of highly irradiating flux, observed as a power law from an appropriately dense disk of XRB. Methods. We focus on the low/hard state of the BH XRB MAXI J1820+070. In contrast to previous works, we simultaneously fit the shape and flux of the reflection spectra. This allowed us to estimate the correct density and ionization of the slab as well as the corresponding reprocessed blackbody. Results. Our fitting of the representative observation of the BH XRB low/hard state suggests that the disk may, in principle, extend very close to the BH, even though the reprocessed thermal emission (due to disk illumination) remains cold (and thus low) enough to be consistent with the data in contrast to the results of a previous study. The inner reflection component is highly ionized and its fit is primarily driven by its contribution to the continuum, rather than by the shape of the relativistic iron line. Conclusions. The reprocessed blackbody cannot help determine whether the disk extends close to the BH or not in the hard state. For this specific observation, the flux in inner reflection component turns out to be quite low with respect to the outer reflection or power law. The outflowing slab corona covering the inner region of the disk could be the plausible geometry of the source, with the underlying disk approaching near to the BH. [ABSTRACT FROM AUTHOR]

The variability data for the BL Lacertae object TXS 1902+556 in the optical and |$\gamma$| -ray wavebands were obtained from the 0.76-m Katzman Automatic Imaging Telescope and the Fermi Large Area Telescope (Fermi -LAT), covering periods of 14.4 and 14.7 yr, respectively. The variability properties were systematically analysed, with particular emphasis on the first comprehensive investigation of radiation variation in the optical waveband. Four well-established techniques were employed for this purpose: the Lomb–Scargle periodogram, REDFIT program, Jurkevich method, and discrete correlation function (DCF) approach. The optical waveband exhibits quasi-periodic oscillations (QPO) with a time-scale of |$P_{\rm O}=276.8\pm 6.1$|  d at a significance level |$3.87\sigma$|⁠ , while the |$\gamma$| -ray waveband does not exhibit any significant periodicity. However, it should be noted that the QPO time-scale is consistent with the Sun-gaps in the optical light curve within 2 |$\sigma$| uncertainties. The optical QPO behaviour is most likely attributed to the helical motion of the jet driven by the orbital motion in a supermassive black hole binary system. Moreover, we have provided an explanation for the absence of QPO in the |$\gamma$| -ray light curves. Furthermore, utilizing the DCF method, a weak correlation between the variability in the optical and |$\gamma$| -ray wavebands was observed, suggesting that the emission of TXS 1902+556 may be generated through a combination of synchrotron self-Compton (SSC) and external Compton (EC) processes, or a leptonic–hadronic hybrid process. [ABSTRACT FROM AUTHOR]

The gravitational wave signature of a binary black hole (BBH) merger is dependent on its component mass and spin. If such black holes originate from rapidly rotating progenitors, the large angular momentum reserve in the star could drive a collapsar-like supernova explosion, hence substantially impacting these characteristics of the black holes in the binary. To examine the effect of stellar rotation on the resulting black hole mass and spin, we conduct a one-dimensional general relativistic study of the end phase of the stellar collapse. We find that the resulting black hole mass at times differs significantly from the previously assumed values. We quantify the dependence of the black hole spin magnitude on the hydrodynamics of the accretion flow, providing analytical relations for calculating the mass and spin based on the progenitor's pre-collapse properties. Depending on the nature of the accretion flow, our findings have implications for the black hole upper mass gap resulting from pair-instability supernovae, the maximum mass of a maximally rotating stellar black hole, and the maximum effective spin of a BBH formed in a tidally locked helium star–black hole binary. [ABSTRACT FROM AUTHOR]

Continuous nanohertz gravitational waves from individual supermassive black hole binaries may be detectable with pulsar timing arrays. A novel search strategy is developed, wherein intrinsic achromatic spin wandering is tracked simultaneously with the modulation induced by a single gravitational wave source in the pulse times of arrival. A two-step inference procedure is applied within a state-space framework, such that the modulation is tracked with a Kalman filter, which then provides a likelihood for nested sampling. The procedure estimates the static parameters in the problem, such as the sky position of the source, without fitting for ensemble-averaged statistics such as the power spectral density of the timing noise, and therefore complements traditional parameter estimation methods. It also returns the Bayes factor relating a model with a single gravitational wave source to one without, complementing traditional detection methods. It is shown via astrophysically representative software injections in Gaussian measurement noise that the procedure distinguishes a gravitational wave from pure noise down to a characteristic wave strain of |$h_0 \approx 2 \times 10^{-15}$|⁠. Full posterior distributions of model parameters are recovered and tested for accuracy. There is a bias of |$\approx 0.3$| rad in the marginalized one-dimensional posterior for the orbital inclination |$\iota$|⁠ , introduced by dropping the so-called pulsar terms. Smaller biases |$\lesssim 10~{{\ \rm per\ cent}}$| are also observed in other static parameters. [ABSTRACT FROM AUTHOR]

The gravitational wave (GW) signal from binary black hole (BBH) mergers is a promising probe of Population III (Pop III) stars. To fully unleash the power of the GW probe, one important step is to understand the relative importance and features of different BBH evolution channels. We model two channels, isolated binary stellar evolution (IBSE) and nuclear star cluster-dynamical hardening (NSC-DH), in one theoretical framework based on the semi-analytical code a-sloth , under various assumptions on Pop III initial mass function (IMF), initial binary statistics and high- z nuclear star clusters (NSCs). The NSC-DH channel contributes |$\sim 8\!-\!95{{\ \rm per\ cent}}$| of Pop III BBH mergers across cosmic history, with higher contributions achieved by initially wider binary stars, more top-heavy IMFs, and more abundant high- z NSCs. The dimensionless stochastic GW background (SGWB) produced by Pop III BBH mergers has peak values |$\Omega ^{\rm peak}_{\rm GW}\sim 10^{-11}\!-\!8\times 10^{-11}$| around observer-frame frequencies |$\nu \sim 10\!-\!100\ \rm Hz$|⁠. The Pop III contribution can be a non-negligible (⁠|$\sim 2\!-\!32{{\ \rm per\ cent}}$|⁠) component in the total SGWB at |$\nu \lesssim 10\ \rm Hz$|⁠. The estimated detection rates of Pop III BBH mergers by the Einstein Telescope are |$\sim 6\!-\!230$| and |$\sim 30\!-\!1230\ \rm yr^{-1}$| for the NSC-DH and IBSE channels, respectively. Pop III BBH mergers in NSCs are more massive than those from IBSE, so they dominate the Pop III SGWB below 20 Hz in most cases. Besides, the detection rate of Pop III BBH mergers involving at least one intermediate-mass BH above |$100\ \rm M_\odot$| by the Einstein Telescope is |$\sim 0.5\!-\!200\ \rm yr^{-1}$| in NSCs but remains below |$0.1\ \rm yr^{-1}$| for IBSE. [ABSTRACT FROM AUTHOR]

We study the spectral properties of the black hole X-ray transient binary 4U 1630–472 during its 2022 and 2023 outbursts with Insight-HXMT observations. We find that the outbursts are in peculiar soft states. The effect of the hardening factor on the disk temperature is taken into account by the kerrbb model, and the flux and temperature of the disk are found to follow F ∝ T eff 3.92 ± 0.13 and F ∝ T eff 4.91 ± 1.00 for the two outbursts, respectively. The flux–temperature relation is roughly consistent with a standard disk. By fitting with a p -free model, the p -value, the exponent of the radial dependence of the disk temperature, is found to have an anticorrelation with disk temperature. Combining a joint diagnostic analysis with a diagram of the relation between the nonthermal fraction and luminosity, we find a possible scenario that the disk evolves from 2022 to 2023 toward a slim one with a decreasing radiation efficiency, where such an evolution may depend on the fraction of the nonthermal emission in the high soft state. [ABSTRACT FROM AUTHOR]

Periodograms are widely employed for identifying periodicity in time series data, yet they often struggle to accurately quantify the statistical significance of detected periodic signals when the data complexity precludes reliable simulations. We develop a data-driven approach to address this challenge by introducing a null-signal template (NST). The NST is created by carefully randomizing the period of each cycle in the periodogram template, rendering it non-periodic. It has the same frequentist properties as a periodic signal template, and we show with simulations that the distribution of false positives is the same as with the original periodic template, regardless of the underlying data. Thus, performing a periodicity search with the NST acts as an effective simulation of the null (no-signal) hypothesis, without having to simulate the noise properties of the data. We apply the NST method to the supermassive black hole binaries (SMBHB) search in the Palomar Transient Factory (PTF), where Charisi et al. had previously proposed 33 high signal-to-noise candidates utilizing simulations to quantify their significance. Our approach reveals that these simulations do not capture the complexity of the real data. There are no statistically significant periodic signal detections above the non-periodic background. To improve the search sensitivity, we introduce a Gaussian quadrature based algorithm for the Bayes Factor with correlated noise as a test statistic. We show with simulations that this improves sensitivity to true signals by more than an order of magnitude. However, the Bayes Factor approach also results in no statistically significant detections in the PTF data. [ABSTRACT FROM AUTHOR]

Modified theories of gravity predict deviations from general relativity (GR) in the propagation of gravitational waves (GWs) across cosmological distances. A key prediction is that the GW luminosity distance will vary with redshift, differing from the electromagnetic (EM) luminosity distance due to varying effective Planck mass. We introduce a model-independent, data-driven approach to explore these deviations using multimessenger observations of dark standard sirens [binary black holes (BBH)]. By combining GW luminosity distance measurements from dark sirens with baryon acoustic oscillation measurements, BBH redshifts inferred from cross-correlation with spectroscopic or photometric galaxy surveys, and sound horizon measurements from the cosmic microwave background, we can make a data-driven test of GR (jointly with the Hubble constant) as a function of redshift. Using the multimessenger technique with the spectroscopic DESI galaxy survey, we achieve precise measurements of deviations in the effective Planck mass variation with redshift. For the Cosmic Explorer and Einstein Telescope (CEET), the best precision is approximately 3.6 per cent, and for LIGO–Virgo–KAGRA (LVK), it is 7.4 per cent at a redshift of |$\rm {z = 0.425}$|⁠. Additionally, we can measure the Hubble constant with a precision of about 1.1 per cent from CEET and 7 per cent from LVK over 5 yr of observation with a 75 per cent duty cycle. We also explore the potential of cross-correlation with photometric galaxy surveys from the Rubin Observatory, extending measurements up to a redshift of |$\rm {z \sim 2.5}$|⁠. This approach can reveal potential deviations from models affecting GW propagation using numerous dark standard sirens in synergy with DESI and the Rubin Observatory. [ABSTRACT FROM AUTHOR]

We report a detailed spectro-temporal analysis of the black hole low mass X-ray binary 4U 1543−475 during its 2021 outburst using the data from the Large Area X-ray Proportional Counter and the Soft X-ray Telescope instruments on board AstroSat. We studied the energy and frequency dependency of the source variability to probe the origin of the disc/coronal fluctuations. Following the state transition (from soft to intermediate state), the emergence of a band-limited noise component is observed along with the power law noise when the disk is recovering from a sudden decrease in the inner disk radius. A possible correlation between the low-frequency root mean square (RMS) variability amplitude and the covering fraction of the non-thermal component is detected. During the final AstroSat observation, a flip-flop phenomenon is reported, where rapid variation in RMS occurs in concurrence with sudden flux transition. An indication of the evolution of inner disk temperature along with a significant change in thermal flux was observed during the flip-flop phase, arguing for a disk instability-driven origin for this phenomenon. Our results suggest that the long-term variability evolution is primarily affected by the coronal changes, whereas the disk behavior governs the short-term variability evolution. [ABSTRACT FROM AUTHOR]

Through decades of observations and studies, the characteristics of black hole X-ray binaries (XRBs) have been gradually revealed. However, the structure of the accretion disk remains uncertain. In the power density spectrum of the BHXRB (Black Hole X-ray Binaries), there is a correlation between the break frequency and the quasi-periodic oscillation (W-K relation), and it can constrain the current model of accretion disk. The W-K relation in black hole XRBs is studied based on the observations of five black hole XRBs by Insight -HXMT (Hard X-ray Modulation Telescope). The result shows that the W-K relation is valid in all the three telescopes with different energy bands. Moreover, a correlation between the break frequency and the inner radius of the accretion disk is found for MAXI J1535-571, which is self-consistent with the model of a truncated accretion disk. If the observed power density spectra result from the propagation of fluctuation in mass accretion rate, it can be further inferred that the inner radius of the accretion disk is close to the innermost stable circular orbit, and the black hole might be a high-spin system. [ABSTRACT FROM AUTHOR]

Relativistic discrete ejecta launched by black hole X-ray binaries (BH XRBs) can be observed to propagate up to parsec-scales from the central object. Observing the final deceleration phase of these jets is crucial to estimate their physical parameters and to reconstruct their full trajectory, with implications for the jet powering mechanism, composition, and formation. In this paper, we present the results of the modelling of the motion of the ejecta from three BH XRBs: MAXI J1820 |$+$| 070, MAXI J1535–571, and XTE J1752–223, for which high-resolution radio and X-ray observations of jets propagating up to |$\sim$| 15 arcsec (⁠|$\sim$| 0.6 pc at 3 kpc) from the core have been published in the recent years. For each jet, we modelled its entire motion with a dynamical blast-wave model, inferring robust values for the jet Lorentz factor, inclination angle and ejection time. Under several assumptions associated to the ejection duration, the jet opening angle and the available accretion power, we are able to derive stringent constraints on the maximum jet kinetic energy for each source (between |$10^{43}$| and |$10^{44}$| erg, including also H1743–322), as well as placing interesting upper limits on the density of the ISM through which the jets are propagating (from |$n_{\rm ISM} \lesssim 0.4$| cm |$^{-3}$| down to |$n_{\rm ISM} \lesssim 10^{-4}$| cm |$^{-3}$|⁠). Overall, our results highlight the potential of applying models derived from gamma-ray bursts to the physics of jets from BH XRBs and support the emerging picture of these sources as preferentially embedded in low-density environments. [ABSTRACT FROM AUTHOR]

LSST will catalogue the light curves of up to 100 million quasars. Among these there can be |$\sim$| 100 ultra-compact massive black hole (MBH) binaries, whose gravitational waves (GWs) can be detected 5–15 yr later by LISA. Here, we assume such a LISA detection occurred, and assess whether or not its electromagnetic (EM) counterpart can be identified as a periodic quasar in archival LSST data. We use the binary's properties derived from the LISA waveform, including the evolution of its orbital frequency, its total mass, distance, and sky localization, to predict the redshift, magnitude, and historical periodicity of the quasar expected in the LSST data. We then use Monte Carlo simulations to compute the false alarm probability (FAP), i.e. the number of quasars in the LSST catalogue matching these properties by chance, based on the (extrapolated) quasar luminosity function, the cadence of LSST, and intrinsic 'damped random walk' quasar variability. We analyse four fiducial LISA binaries, with masses and redshifts of |$(M_{\rm bin}/{\rm M_{\odot }},z) = (3\times 10^5,0.3)$|⁠ , |$(3\times 10^6,0.3)$|⁠ , |$(10^7,0.3)$|⁠ , and |$(10^7,1)$|⁠. While noise and aliasing due to LSST's cadence produces false periodicities by chance, we find that the frequency chirp of the LISA source during the LSST observations washes out these noise peaks and allows the genuine source to stand out in appropriately scaled Lomb–Scargle periodograms. We find that all four fiducial binaries can be uniquely identified, with |${\rm FAP}\lt 10^{-5}$|⁠ , a week or more before merger. This should enable follow-up EM observations targeting individual EM counterparts during their inspiral stage. [ABSTRACT FROM AUTHOR]

Context. It is generally accepted that all massive galaxies host supermassive black holes (BHs) in their center and that mergers of two galaxies lead to the formation of BH binaries. The most interesting among them comprise the mergers in their final state, that is to say with parsec (3.2 light years) or sub-parsec orbital separations. It is possible to detect these systems with binary self-lensing. Aims. Here we report the potential detection of a central supermassive BH binary in the active galaxy (AGN) NGC 1566 based on a microlensing outburst. The light curve of the outburst – based on observations with the All Sky Automated Survey for SuperNovae – lasted from the beginning of 2017 until the beginning of 2020. The steep symmetric light curve as well as its shape look very different with respect to normal random variations in AGN. Results. However, the observations could be easily reproduced with a best-fit standard microlensing light curve. Based on the light curve, we derived a characteristic timescale of 155 days. During the outburst, the continuum as well as the broad line intensities varied; however, the narrow emission lines did not. This is an indication that the lensing object orbits the AGN nucleus between the broad line region (BLR) and the narrow line region (NLR), that is, at a distance on the order of 250 light days. The light curve can be reproduced by a lens with a BH mass of 5 × 105 M⊙. This implies a mass ratio to the central AGN on the order of 1–10. [ABSTRACT FROM AUTHOR]

Surveys in the Milky Way and Large Magellanic Cloud have revealed that the majority of massive stars will interact with companions during their lives. However, knowledge of the binary properties of massive stars at low metallicity, and therefore in conditions approaching those of the Early Universe, remain sparse. We present the Binarity at LOw Metallicity (BLOeM) campaign, an ESO large programme designed to obtain 25 epochs of spectroscopy for 929 massive stars in the Small Magellanic Cloud, allowing us to probe multiplicity in the lowest-metallicity conditions to date (Z = 0.2 Z⊙). BLOeM will provide (i) the binary fraction, (ii) the orbital configurations of systems with periods of P ≲ 3 yr, (iii) dormant black-hole binary candidates (OB+BH), and (iv) a legacy database of physical parameters of massive stars at low metallicity. Main sequence (OB-type) and evolved (OBAF-type) massive stars are observed with the LR02 setup of the GIRAFFE instrument of the Very Large Telescope (3960–4570 Å resolving power R = 6200; typical signal-to-noise ratio(S/N) ≈70–100). This paper utilises the first nine epochs obtained over a three-month time period. We describe the survey and data reduction, perform a spectral classification of the stacked spectra, and construct a Hertzsprung-Russell diagram of the sample via spectral-type and photometric calibrations. Our detailed classification reveals that the sample covers spectral types from O4 to F5, spanning the effective temperature and luminosity ranges 6.5 ≲ Teff/kK ≲ 45 and 3.7 < log L/L⊙ < 6.1 and initial masses of 8 ≲ Mini ≲ 80 M⊙. The sample comprises 159 O-type stars, 331 early B-type (B0–3) dwarfs and giants (luminosity classes V–III), 303 early B-type supergiants (II–I), and 136 late-type BAF supergiants. At least 82 stars are OBe stars: 20 O-type and 62 B-type (13% and 11% of the respective samples). In addition, the sample includes 4 high-mass X-ray binaries, 3 stars resembling luminous blue variables, 2 bloated stripped-star candidates, 2 candidate magnetic stars, and 74 eclipsing binaries. [ABSTRACT FROM AUTHOR]

Context. Merging compact objects such as binary black holes provide a promising probe for the physics of dark matter (DM). The gravitational waves emitted during inspiral potentially allow one to detect DM spikes around black holes. This is because the dynamical friction force experienced by the inspiralling black hole alters the orbital period and thus the gravitational wave signal. Aims. The dynamical friction arising from DM can potentially differ from the collisionless case when DM is subject to self-interactions. This paper aims to understand how self-interactions impact dynamical friction. Methods. To study the dynamical friction force, we use idealised N-body simulations, where we include self-interacting dark matter. Results. We find that the dynamical friction force for inspiralling black holes would be typically enhanced by DM self-interactions compared to a collisionless medium (ignoring differences in the DM density). At lower velocities below the sound speed, we find that the dynamical friction force can be reduced by the presence of self-interactions. Conclusions. DM self-interactions have a significant effect on the dynamical friction for black hole mergers. Assuming the Chandrasekhar formula may underpredict the deceleration due to dynamical friction. [ABSTRACT FROM AUTHOR]

Binary black holes (BBHs) that are born from the evolution of Population III (Pop. III) stars are one of the main high-redshift targets for next-generation ground-based gravitational-wave (GW) detectors. Their predicted initial mass function and lack of metals make them the ideal progenitors of black holes above the upper edge of the pair-instability mass gap, that is, with a mass higher than ~134 (241) M⊙ for stars that become (or do not become) chemically homogeneous during their evolution. We investigate the effects of cluster dynamics on the mass function of BBHs that are born from Pop. III stars by considering the main uncertainties on the mass function of Pop. III stars, the orbital properties of the binary systems, the star cluster mass, and the disruption time. In our dynamical models, at least ~5% and up to 100% BBH mergers in Pop. III star clusters have a primary mass m1 above the upper edge of the pair-instability mass gap. In contrast, only ≲3% isolated BBH mergers have a primary mass above the gap, unless their progenitors evolved as chemically homogeneous stars. The lack of systems with a primary and/or secondary mass inside the gap defines a zone of avoidance with sharp boundaries in the plane of the primary mass-mass ratio. Finally, we estimate the merger rate density of BBHs. In the most optimistic case, we find a maximum of ℛ ~ 200 Gpc-3 yr-1 at ɀ ~ 15 for BBHs that formed via dynamical capture. For comparison, the merger rate density of isolated Pop. III BBHs is ℛ ≤ 10 Gpc−3 yr−1 for the same model of Pop. III star formation history. [ABSTRACT FROM AUTHOR]

We report two epochs of simultaneous near-infrared (IR) and X-ray observations of the low-mass X-ray binary black hole candidate Swift J1753.5–0127 with a subsecond time resolution during its long 2005–2016 outburst. Data were collected strictly simultaneously with VLT/ISAAC (KS band, 2.2 μm) and RXTE (2–15 keV) or XMM-Newton (0.7–10 keV). A clear correlation between the X-ray and the IR variable emission is found during both epochs but with very different properties. In the first epoch, the near-IR variability leads the X-ray by ∼130 ms, which is the opposite of what is usually observed in similar systems. The correlation is more complex in the second epoch, with both anti-correlation and correlations at negative and positive lags. Frequency-resolved Fourier analysis allows us to identify two main components in the complex structure of the phase lags: the first component, characterised by a near-IR lag of a few seconds at low frequencies, is consistent with a combination of disc reprocessing and a magnetised hot flow; the second component is identified at high frequencies by a near-IR lag of ≈0.7 s. Given the similarities of this second component with the well-known constant optical/near-IR jet lag observed in other black hole transients, we tentatively interpret this feature as a signature of a longer-than-usual jet lag. We discuss the possible implications of measuring such a long jet lag in a radio-quiet black hole transient. [ABSTRACT FROM AUTHOR]

Massive binary evolution models are needed to predict massive star populations in star-forming galaxies, the supernova diversity, and the number and properties of gravitational wave sources. Such models are often computed using so-called rapid binary evolution codes, which approximate the evolution of the binary components based on detailed single star models. However, about one-third of the interacting massive binary stars undergo mass transfer during core hydrogen-burning (Case A mass transfer), whose outcome is difficult to derive from single star models. For this work, we used a large grid of detailed binary evolution models for primaries in the initial mass range 10–40 M⊙ with a Large and Small Magellanic Cloud composition, to derive analytic fits for the key quantities needed in rapid binary evolution codes, that is, the duration of core hydrogen-burning, and the resulting donor star mass. We find that systems with shorter orbital periods produce up to 50% lighter stripped donors and have a lifetime up to 30% larger than wider systems. Both quantities strongly depend on the initial binary orbital period, but the initial mass ratio and the mass-transfer efficiency of the binary have little impact on the outcome. Our results are easily parameterisable and can be used to capture the effects of Case A mass transfer more accurately in rapid binary evolution codes. [ABSTRACT FROM AUTHOR]

Black-hole (BH) high-mass X-ray binary (HMXB) systems are likely to be the progenitors of BH-BH mergers detected in gravitational waves by LIGO/Virgo/KAGRA (LVK). Yet merging BHs reach higher masses (∼100 M⊙) than BHs in HMXBs (∼20 M⊙) and typically exhibit lower spins (aBH ≲ 0.25 with a larger values tail) than what is often claimed for BHs in HMXBs (aBH ≳ 0.9). This could suggest that these two classes of systems belong to different populations, but here we show that this may not necessarily be the case. The difference in masses is easily explained as the known HMXB-BHs are in galaxies with relatively high metallicity, so their progenitor stars are subject to strong mass loss from winds, leading to relatively low-mass BH at core collapse. Conversely, LVK is also able to detect BHs from low-metallicity galaxies that are known to naturally produce more massive stellar-origin BHs. However, the difference in spin is more difficult to explain. Models with efficient angular momentum transport in stellar interiors produce slowly spinning progenitors for both LVK and HMXB BHs. Known HMXBs have orbital periods that are too long for efficient tidal spin-up and are also unlikely to have undergone significant accretion spin-up. Instead, we show that the derived value of the BH spin depends strongly on how the HMXB accretion disc emission is modelled. We argue that since Cyg X-1 is never observed to be in a soft spectral state, the appropriate spectral models must take into account the Comptonisation of the disc photosphere. We show that such models are consistent with low spin values, namely: aBH ∼ 0.1. This was recently confirmed by other teams for both Cyg X-1 and LMC X-1 and here we show this is also the case for M33 X-7. We conclude that all known HMXB BHs can exhibit a low spin, in accordance with the results of stellar evolution models. Hence, the observations presented in this work are consistent with LVK BHs and HMXB BHs belonging to the same population. [ABSTRACT FROM AUTHOR]

The origin of Galactic cosmic rays (CRs) is unknown even though they have traditionally been connected to supernovae based on energetic arguments. In the past decades, Galactic black holes in X-ray binaries (BHXBs) have been proposed as candidate sources of CRs, which revises the CR paradigm. BHXBs launch two relativistic jets during their outbursts, but recent observations suggested that these jets may be launched even during quiescence. A0620−00 is a well-studied object that shows indications of jet emission. We study the simultaneous radio-to-X-ray spectrum of this source that was detected while the source was in quiescence to better constrain the jet dynamics. Because most BHXBs spend their lifetimes in quiescence (qBHXBs), we used the jet dynamics of A0620−00 to study a population of 105 such sources distributed throughout the Galactic disc, and a further 104 sources that are located in the boxy bulge around the Galactic centre. While the contribution to the CR spectrum is suppressed, we find that the cumulative intrinsic emission of qBHXBs from both the boxy bulge and from the Galactic disc adds to the diffuse emission that various facilities detected from radio to TeV γ rays. We examined the contribution of qBHXBs to the Galactic diffuse emission and investigated the possibility of SKA, INTEGRAL, and CTAO to detect individual sources in the future. Finally, we compare the predicted neutrino flux to the recently presented Galactic diffuse neutrino emission by IceCube. [ABSTRACT FROM AUTHOR]

We present the results of a search for continuous gravitational wave signals (CGWs) in the second data release (DR2) of the European Pulsar Timing Array (EPTA) Collaboration. The most significant candidate event from this search has a gravitational wave frequency of 4–5 nHz. Such a signal could be generated by a supermassive black hole binary (SMBHB) in the local Universe. We present the results of a follow-up analysis of this candidate using both Bayesian and frequentist methods. The Bayesian analysis gives a Bayes factor of 4 in favour of the presence of the CGW over a common uncorrelated noise process. In contrast, the frequentist analysis estimates the p-value of the candidate to be < 1%, also assuming the presence of common uncorrelated red noise. However, comparing a model that includes both a CGW and a gravitational wave background (GWB) to a GWB only, the Bayes factor in favour of the CGW model is only 0.7. Therefore, we cannot conclusively determine the origin of the observed feature, nor can we rule it out as a CGW source. We present results of simulations that demonstrate that data containing a weak gravitational wave background can be misinterpreted as data including a CGW and vice versa, providing two plausible explanations for the EPTA DR2 data. Further investigations combining data from all PTA collaborations will be needed to reveal the true origin of this feature. [ABSTRACT FROM AUTHOR]

Theoretical predictions suggest that very massive stars have the potential to form through multiple collisions and eventually evolve into intermediate-mass black holes (IMBHs) within Population III star clusters embedded in mini dark matter haloes. In this study, we investigate the long-term evolution of Population III star clusters, including models with a primordial binary fraction of |$f_{\rm b}=0$| and 1, using the N -body simulation code petar. We comprehensively examine the phenomenon of hierarchical triple black holes in the clusters, specifically focusing on their merging inner binary black holes (BBHs), with post-Newtonian correction, by using the tsunami code. Our findings suggest a high likelihood of the inner BBHs containing IMBHs with masses on the order of |$\mathcal {O}(100)\,{\rm M}_{\odot }$|⁠ , and as a result, their merger rate could be up to |$0.1{\rm Gpc}^{-3}{\rm yr}^{-3}$|⁠. The orbital eccentricities of some merging inner BBHs oscillate over time periodically, known as the Kozai–Lidov oscillation, due to dynamical perturbations. Detectable merging inner BBHs for mHz GW detectors LISA/TianQin/Taiji concentrate within |$z\lt 3$|⁠. More distant sources would be detectable for CE/ET/LIGO/KAGRA/DECIGO, which are sensitive from |$\mathcal {O}(0.1)$| Hz to |$\mathcal {O}(100)$| Hz. Furthermore, compared with merging isolated BBHs, merging inner BBHs affected by dynamical perturbations from tertiary BHs tend to have higher eccentricities, with a significant fraction of sources with eccentricities closing to 1 at mHz bands. GW observations would help constrain the formation channels of merging BBHs, whether through isolated evolution or dynamical interaction, by examining eccentricities. [ABSTRACT FROM AUTHOR]