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Black hole binaries show equatorial disc winds at high luminosities, which apparently disappear during the spectral transition to the low/hard state. This is also where the radio jet appears, motivating speculation that both wind and jet are driven by different configurations of the same magnetic field. However, these systems must also have thermal winds, as the outer disc is clearly irradiated.We develop a predictive model of the absorption features from thermal winds, based on pioneering work of Begelman et al 1983. We couple this to a realistic model of the irradiating spectrum as a function of luminosity to predict the entire wind evolution during outbursts. We show that the column density of the thermal wind scales roughly with luminosity, and does not shut off at the spectral transition, though its visibility will be affected by the abrupt change in ionising spectrum. We re-analyse the data from H1743-322 which most constrains the difference in wind across the spectral transition and show that these are consistent with the thermal wind models.We include simple corrections for radiation pressure, which allows stronger winds to be launched from smaller radii. These winds become optically thick around Eddington, which may even explain the exceptional wind seen in one observation of GRO J1655-40. These data can instead be fit by magnetic wind models, but similar winds are not seen in this or other systems at similar luminosities. Hence we conclude that the majority (perhaps all) current data can be explained by thermal or thermal-radiative winds.
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GW190426_152155 was recently reported as one of the 39 candidate gravitational wave (GW) events in citet{2020arXiv201014527A}, which has an unusual source-frame chirp mass $sim 2.4M_{odot}$ and may be the first GW signal from a neutron star-black hole (NSBH) merger. Assuming an astrophysical origin, we reanalyze GW190426_152155 using several waveforms with different characteristics, and consider two different priors for the mass ratio of the binary (Uniform and LogUniform). We find that the results are influenced by the priors of mass ratio, and this candidate could also be from the merger of two low mass black holes (BH). In the case for a binary black hole (BBH) merger, the effective spin is likely negative and the effective precession spin is non-negligible. As for the NSBH merger, supposing the mass of the light object follow the distribution of current neutron stars (NSs) with a reasonably measured/constrained mass, the spin of the low mass BH is so small that is hard to generate bright electromagnetic emission. Finally, we estimate a merger rate of GW190426_152155-like systems to be $59^{+137}_{-51}~{rm Gpc}^{-3}~{rm yr}^{-1}$.
Possible models for the generation of electromagnetic (EM) radiation during the coalescence of neutron star-black hole binaries are considered. The mass of the remnant disk around the black hole during the coalescence of neutron stars and black holes is calculated by taking into account the equation of state for neutron stars and the rotation of the binary components before the coalescence. The parameters of binary systems before the coalescence (the mass ratio, the component rotation, the neutron star magnetic field) are calculated by the population synthesis method. The derived mass of the remnant disk around the black hole after the coalescence is used to estimate the kinetic energy of the relativistic jet launched by the Blandford-Znajek mechanism. A disk mass of more than $sim 0.05 M_odot$ required for the formation of short gamma-ray bursts is shown to be obtained in no more than 1-10% of the coalescences (depending on the equation of state). Less efficient common envelopes (a large parameter $alpha_{CE}$) lead to a noticeably larger percentage of events with astrophysically interesting EM energy release. For binaries with a large mass ratio, in which a magnetized neutron star is not subjected to tidal disruption before the coalescence, the possibility of the formation of an electrically charged rotating black hole (Wald charge) is considered and estimates of the maximum EM power released by such a black hole after the coalescence are made. The conversion of the emitted gravitational waves into electromagnetic ones in the relativistic lepton plasma generated in coalescing pulsar-black hole binaries at the pre-coalescence stage is discussed.
The Advanced LIGO and Advanced Virgo gravitational wave detectors have detected a population of binary black hole mergers in their first two observing runs. For each of these events we have been able to associate a potential sky location region represented as a probability distribution on the sky. Thus, at this point we may begin to ask the question of whether this distribution agrees with the isotropic model of the Universe, or if there is any evidence of anisotropy. We perform Bayesian model selection between an isotropic and a simple anisotropic model, taking into account the anisotropic selection function caused by the underlying antenna patterns and sensitivity of the interferometers over the sidereal day. We find an inconclusive Bayes factor of $1.3:1$, suggesting that the data from the first two observing runs is insufficient to pick a preferred model. However, the first detections were mostly poorly localised in the sky (before the Advanced Virgo joined the network), spanning large portions of the sky and hampering detection of potential anisotropy. It will be appropriate to repeat this analysis with events from the recent third LIGO observational run and a more sophisticated cosmological model.
A small cluster of massive stars residing in the Galactic center, collectively known as IRS13E, is of special interest due to its close proximity to Sgr A* and the possibility that an embedded intermediate-mass black hole (IMBH) binds its member stars. It has been suggested that colliding winds from two member stars, both classified as Wolf-Rayet type, are responsible for the observed X-ray, infrared and radio emission from IRS13E. We have conducted an in-depth study of the X-ray spatial, temporal and spectral properties of IRS13E, based on 5.6 Ms of ultra-deep Chandra observations obtained over 20 years. These X-ray observations show no significant evidence for source variability. We have also explored the kinematics of the cluster members, using Keck near-infrared imaging and spectroscopic data on a 14-yr baseline that considerably improve the accuracy of stars proper motions. The observations are interpreted using 3-dimensional hydrodynamical simulations of colliding winds tailored to match the physical conditions of IRS13E, leading us to conclude that the observed X-ray spectrum and morphology can be well explained by the colliding wind scenario, in the meantime offering no support for the presence of a putative IMBH. An IMBH more massive than a few $10^3{rm~M_odot}$ is also strongly disfavored by the stellar kinematics.
In large and complicated stellar systems like galaxies it is difficult to predict the number and characteristics of a black hole population. Such populations may be modelled as an aggregation of homogeneous (i.e. having uniform star formation history and the same initial chemical composition) stellar populations. Using realistic evolutionary models we predict the abundances and properties of black holes formed from binaries in these environments. We show that the black hole population will be dominated by single black holes originating from binary disruptions and stellar mergers. Furthermore, we discuss how black hole populations are influenced by such factors as initial parameters, metallicity, initial mass function, and natal kick models. As an example application of our results, we estimate that about 26 microlensing events to happen every year in the direction of the Galactic Bulge due to black holes in a survey like OGLE-IV. Our results may be used to perform in-depth studies related to realistic black hole populations, e.g. observational predictions for space survey missions like Gaia, or Einstein Probe. We prepared a publicly available database with the raw data from our simulations to be used for more in-depth studies.
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