Keywords

I estimate the frequencies of gravitational waves from jittering jets that explode core collapse supernovae (CCSNe) to crudely be 5–30 Hz, and with strains that might allow detection of Galactic CCSNe. The jittering jets explosion mechanism (JJEM) asserts that most CCSNe are exploded by jittering jets that the newly born neutron star (NS) launches within a few seconds. According to the JJEM, instabilities in the accreted gas lead to the formation of intermittent accretion disks that launch the jittering jets. Earlier studies that did not include jets calculated the gravitational frequencies that instabilities around the NS emit to have a peak in the crude frequency range of 100–2000 Hz. Based on a recent study, I take the source of the gravitational waves of jittering jets to be the turbulent bubbles (cocoons) that the jets inflate as they interact with the outer layers of the core of the star at thousands of kilometers from the NS. The lower frequencies and larger strains than those of gravitational waves from instabilities in CCSNe allow future, and maybe present, detectors to identify the gravitational wave signals of jittering jets. Detection of gravitational waves from local CCSNe might distinguish between the neutrino-driven explosion mechanism and the JJEM.

Under the assumption that jets explode all core collapse supernovae (CCSNe), I classify 14 CCSN remnants (CCSNRs) into five groups according to their morphology as shaped by jets, and attribute the classes to the specific angular momentum of the pre-collapse core. Point-symmetry (one CCSNR): According to the jittering jets explosion mechanism (JJEM) when the pre-collapse core rotates very slowly, the newly born neutron star (NS) launches tens of jet-pairs in all directions. The last several jet-pairs might leave an imprint of several pairs of "ears," i.e., a point-symmetric morphology. One pair of ears (eight CCSNRs): More rapidly rotating cores might force the last pair of jets to be long-lived and shape one pair of jet-inflated ears that dominates the morphology. S-shaped (one CCSNR): The accretion disk might precess, leading to an S-shaped morphology. Barrel-shaped (three CCSNRs): Even more rapidly rotating pre-collapse cores might result in a final energetic pair of jets that clear the region along the axis of the pre-collapse core rotation and form a barrel-shaped morphology. Elongated (one CCSNR): A very rapidly rotating pre-collapse core forces all jets to be along the same axis such that the jets are inefficient in expelling mass from the equatorial plane and the long-lasting accretion process turns the NS into a black hole. The two new results of this study are the classification of CCSNRs into five classes based on jet-shaped morphological features, and the attribution of the morphological classes mainly to the pre-collapse core rotation in the frame of the JJEM.

I build a toy model in the frame of the jittering jets explosion mechanism (JJEM) of core collapse supernovae that incorporates both the stochastically varying angular momentum component of the material that the newly born neutron star (NS) accretes and the constant angular momentum component, and show that the JJEM can account for the ≃2.5–5M_⊙ mass gap between NSs and black holes (BHs). The random component of the angular momentum results from pre-collapse core convection fluctuations that are amplified by post-collapse instabilities. The fixed angular momentum component results from pre-collapse core rotation. For slowly rotating pre-collapse cores the stochastic angular momentum fluctuations form intermittent accretion disks (or belts) around the NS with varying angular momentum axes in all directions. The intermittent accretion disk/belt launches jets in all directions that expel the core material in all directions early on, hence leaving an NS remnant. Rapidly rotating pre-collapse cores form an accretion disk with angular momentum axis that is about the same as the pre-collapse core rotation. The NS launches jets along this axis and hence the jets avoid the equatorial plane region. Inflowing core material continues to feed the central object from the equatorial plane increasing the NS mass to form a BH. The narrow transition from slow to rapid pre-collapse core rotation, i.e., from an efficient to inefficient jet feedback mechanism, accounts for the sparsely populated mass gap.

In this work, we present the probabilities of mergers of binary black holes (BBHs) and binary neutron stars (BNSs) as functions of stellar mass, metallicity, specific star formation rate (sSFR), and age for galaxies with redshift z ≤ 0.1. Using the binary-star evolution (BSE) code and some fitting formulae, we construct a phenomenological model of cosmic gravitational wave (GW) merger events. By using the Bayesian analysis method and the observations from Advanced LIGO and Virgo, we obtain the relevant parameters of the phenomenological model (such as the maximum black hole mass is ${93}_{-22}^{+73}\,{M}_{\odot }$). Combining the above model results with the galaxy catalog given by the EMERGE empirical galaxy model, we find the normalized probability of occurrence of a merger event varying with ${\mathrm{log}}_{10}(\mathrm{sSFR}/{\mathrm{yr}}^{-1})$ for galaxies with z ≤ 0.1 is different from that in previous studies, that is, two peaks exist in this work while there is only one peak (log₁₀(sSFR/yr ⁻¹) = −10) in the previous work. The sSFR value corresponding to the new peak is log₁₀(sSFR/yr ⁻¹) = −12 and in line with the value (${\mathrm{log}}_{10}(\mathrm{sSFR}/{\mathrm{yr}}^{-1})=-{12.65}_{-0.66}^{+0.44}$) of NGC 4493, the host galaxy of BNS merger event GW170817. The new peak is caused by today's quenched galaxies, which give a large contribution to the total SFR at high redshift in the EMERGE empirical galaxy model. Moreover, we find that the BNS mergers are most likely detected in galaxies with age ∼11 Gyr, which is greater than previous results (6−8 Gyr) and close to the age of NGC 4993, ${13.2}_{-0.9}^{+0.5}$ Gyr.

Detached and wide-orbit black hole-star binaries (BHSBs) can generate three types of periodic photometric signals: Ellipsoidal Variation, Doppler beaming and Self-Lensing (SL), providing a proxy to discover these black holes. We estimate the relative amplitude of the three signals for such systems and the detectability for black holes of a photometric telescope like Kepler in several steps. We estimate the searchable star number by assuming every star has a black hole companion, and apply the occurrence of BHSBs in field stars to estimate the detectable black hole signals. We consider three types of Initial Mass Function (IMF) model with different high end exponential slopes. "When spot and white noise are both considered, there is about one detectable signal for SL and less than one event is expected for beaming and Ellipsoidal Variation signal in Kepler Input Catalog stars with the standard IMF model.” to “Due to contamination by stellar spots and white noise, one may expect one detectable signal for SL and less than one detectable signal for both beaming and Ellipsoidal Variation in Kepler Input Catalog stars with the standard IMF model." On the other hand, if we assume that only white noise affects the detection efficiency of the BHSBs, we expect about 10 Ellipsoidal Variation signals and 17 beaming signals to be detectable while the number of SL signals remains unchanged.

Motivated by the determination of black hole masses with gravitational-wave observations, we calculate the evolution of massive stars through presupernova stages and obtain the mass distribution of black holes. In the first part, we calculate the evolution of He stars with masses of 30–120 M_⊙. We study in detail how convective carbon shell burning controls pair-instability pulsations before and during oxygen burning and determine their final fates. In the second part, we calculate the evolution of H-rich stars with initial masses of 13–80 M_⊙ until Fe core collapse and obtain the possible black hole mass range by applying the criterion of the compactness parameters. From these models, we predict the mass distribution of black holes for stars that undergo Fe core collapse and pair-instability pulsation. The predicted masses for black holes range from 4.2 to 46 M_⊙, which are consistent with the gravitational-wave observations.

Mass distribution of black holes in low-mass X-ray binaries previously suggested the existence of a ∼2–5 M_⊙ mass gap between the most massive neutron stars and the least massive black holes, while some recent evidence appears to support that this mass gap is being populated. Whether there is a mass gap or not can potentially shed light on the physics of supernova explosions that form neutron stars and black holes, although significant mass accretion of neutron stars including binary mergers may lead to the formation of mass-gap objects. In this review, I collect the compact objects that are probable black holes with masses being in the gap. Most of them are in binaries, their mass measurements are obviously subject to some uncertainties. Current observations are still unable to confidently infer an absence or presence of the mass gap. Ongoing and future surveys are expected to build the mass spectrum of black holes which can be used to constrain the process of their formation especially in binaries. I describe the theoretical predictions for the formation of black holes in various types of binaries, and present some prospects of searching for black holes via electromagnetic and gravitational wave observations.

We report spectral and timing analysis of the black hole transient MAXI J1631–479 during the hard intermediate state of its 2019 outburst from the Insight-Hard X-ray Modulation Telescope (Insight-HXMT) observations. We find that the energy dependence of the type-C quasi-periodic oscillation (QPO) frequency evolves with time: during the initial rise of a small flare (∼MJD 58526.0-58527.1), the QPO frequency increases with increasing energy from ∼1 to ∼100 keV, and then the frequency remains constant after MJD 58527.1. We discover a possible new phenomenon of Fe line's QPO frequency jump that has never been observed for other black hole transients: during the small flare, the QPO frequency around the Fe line energy is higher than any other energy band, with the frequency difference Δf = 0.25 ± 0.08 Hz between 5.5–7.5 keV and other energy bands. The spectral analysis shows that the evolution of QPOs is related to the equivalent width of the narrow Fe line, and its equivalent width increases during this small flare. We propose that the QPO frequency difference results from the differential precession of a vertically extended jet, and the higher QPO frequency of Fe line could be caused by the layered jet when the jet scale increases. At the same time, the evolution of QPOs is related to the accretion rate, while the energy dependence of QPOs supports the existence of deceleration in the vertically distributed jet.

The acceleration of LMXB 4U 1820-30 derived from its orbital-period derivative ${\dot{P}}_{{\rm{b}}}$ was supposed to be the evidence for an Intermediate-mass black hole (IMBH) in the Galactic globular cluster (GC) NGC 6624. However, we find that the anomalous ${\dot{P}}_{{\rm{b}}}$ is mainly due to the gravitational wave emission, rather than the acceleration in cluster potential. Using the standard structure models of GCs, we simulate acceleration distributions for pulsars in the central region of the cluster. By fitting the acceleration of J1823-3021A with the simulated distribution profiles (maximum values), it is suggested that an IMBH with mass $M\gtrsim {950}_{-350}^{+550}\,{M}_{\odot }$ may reside in the cluster center. We further show that the second period derivative $\ddot{P}$ of J1823-3021A is probably due to the gravitational perturbation of a nearby star.

Recent observations of AdLIGO and Virgo have shown that the spin measurements in binary black hole (BH) systems are typically small, which is consistent with the predictions by the classical isolated binary evolution channel. In this standard formation channel, the progenitor of the first-born BH is assumed to have efficient angular momentum transport. The BH spins in high-mass X-ray binaries (HMXBs), however, have consistently been found to be extremely high. In order to explain the high BH spins, the inefficient angular momentum transport inside the BH progenitor is required. This requirement, however, is incompatible with the current understanding of conventional efficient angular momentum transport mechanism. We find that this tension can be highly alleviated as long as the hypercritical accretion is allowed. We show that, for a case study of Cygnus X-1, the hypercritical accretion cannot only be a good solution for the inconsistent assumption upon the angular momentum transport within massive stars, but match its other properties reported recently.