Keywords

The equilibrium configuration of a solid strange star in the final inspiral phase with another compact object is generally discussed, and the starquake-related issue is revisited, for a special purpose to understand the precursor emission of binary compact star merger events (e.g., that of GRB211211A). As the binary system inspirals inward due to gravitational wave radiation, the ellipticity of the solid strangeon star increases due to the growing tidal field of its compact companion. Elastic energy is hence accumulated during the inspiral stage which might trigger a starquake before the merger when the energy exceeds a critical value. The energy released during such starquakes is calculated and compared to the precursor observation of GRB211211A. The result shows that the energy might be insufficient for binary strangeon-star case unless the entire solid strangeon star shatters, and hence favors a black hole-strangeon star scenario for GRB211211A. The timescale of the precursor as well as the frequency of the observed quasi-periodic-oscillation have also been discussed in the starquake model.

Previous simulations studying winds only focus on the line force due to photons from central active galactic nuclei. What properties of the winds will be when including the re-radiation force due to the scattered and reprocessed photons (i.e., the re-radiation effect)? We perform simulations to study the large-scale dynamics of accretion disk winds driven by radiation line force and re-radiation force. For the fiducial run, we find that the re-radiation force drives stronger outflows during the early stages. When the flows get into the steadiness, the UV radiation due to spectral lines dominates total radiation and the re-radiation effect could be negligible. The opening angle of winds narrows as the initial gas density increases. The larger the gas density is, the stronger the re-radiation effect will be. For M_BH = 10⁶M_⊙, ε = 0.3, the outflows do become much stronger with the re-radiation effect and the winds still cannot escape from gravitational potential. We find that the detection probability of ultra-fast outflows and the properties of the winds are both consistent with the observations.

The Swift/XRT detected the X-ray afterglow of long burst GRB 220117A, which began to rebrighten 300 s after triggering and followed a single power-law decay segment after thousands of seconds of the orbital observation gap. This segment is different from the shallow decay segment (plateau) and flare, and may belong to a giant X-ray bump. We investigated this segment by the fall-back accretion model and found that the model can interpret this segment with reasonable parameter values. Within this physical model scenario, the fall-back accretion rate reaches a peak value ∼1.70 × 10⁻⁵M_⊙ s⁻¹ around 300 s in the central engine frame, which is compatible with the late mass supply rate of some low-metallicity massive progenitor stars. The initial black hole (BH) spin is ${a}_{0}={0.64}_{-0.26}^{+0.24}$ and implies that this re-brightening signature requires a larger BH spin. The total accretion mass during the fall-back process is M_acc = (3.09 ± 0.02) × 10⁻²M_⊙. The jet energy from the fall-back accretion is (9.77 ± 0.65) × 10⁵² erg, with a ratio of 0.066 to the isotropic-equivalent radiation energies of the GRB prompt phase in the 1–10⁴ keV band. The fall-back radius r_p corresponding to the peak time of fall-back t_p is (3.16 ± 0.05) × 10¹⁰ cm, which is consistent with the typical radius of Wolf–Rayet stars. In summary, our results provide additional support for the origin of the long burst from the core collapse of Wolf–Rayet stars, and its late central engine activity is likely due to the fall-back accretion process.

We present the timing analysis of the nonlinear variability in two black hole low mass X-ray binaries MAXI J1820+070 and MAXI J1535-571 by using the bicoherence, a measure of phase coupling at different Fourier frequencies. We found different patterns, e.g., "cross" and "hypotenuse," for LFQPOs in different outburst states. When they can be clearly distinguished, bicoherence patterns are similar over a wide energy range of 1–100 keV. It is intriguing that in some type-C QPOs we found the patterns that are normally observed in type-B QPOs. On the contrary, the "hypotenuse" pattern, a characteristic of type-C QPOs, was detected in a type-B QPO. This suggests that different types of QPOs may originate from similar underlying mechanisms. In addition, we speculate that the nonlinear variability may be a promising approach to disentangle distinct QPO models which assume different interactions between the broadband noise and QPO components.

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.

The Galactic black hole candidate MAXI J0637-430 was first discovered by MAXI/GSC on 2019 November 2. We study the spectral properties of MAXI J0637-430 by using the archived NuSTAR data and Swift/XRT data. After fitting the eight spectra by using a disk component and a powerlaw component model with absorption, we select the spectra with relatively strong reflection components for detailed X-ray reflection spectroscopy. Using the most state-of-art reflection model, relxillCp, the spectral fitting measures a black hole spin a_* > 0.72 and the inclination angle of the accretion disk i = ${46.1}_{-5.3}^{+4.0}$ degrees, at a 90% confidence level. In addition, the fitting results show an extreme supersolar iron abundance. Combined with the fitting results of reflection model reflionx_hd, we consider that this unphysical iron abundance may be caused by a very high-density accretion disk (n_e > 2.34 × 10²¹ cm⁻³) or a strong Fe Kα emission line. The soft excess is found in the soft state spectral fitting results, which may be an extra free–free heating effect caused by high density of the accretion disk. Finally, we discuss the robustness of black hole spin obtained by X-ray reflection spectroscopy. The result of relatively high spin is self-consistent with broadened Fe Kα line. Iron abundance and disk density have no effect on the spin results.

The power law and reflection emission have been observed in the X-ray spectra of both black hole X-ray binaries (BHXRBs) and active galactic nuclei (AGNs), indicating a common physical origin of the X-ray emission from these two types of sources. The relevant parameters describing the shape of both components and the potential correlation between these parameters can provide important clues on the geometric and physical properties of the disk and the corona in these sources. In this work, we present a positive correlation between the photon index Γ and the reflection strength R for the low-mass BHXRBs in the hard state by modeling NuSTAR data, which is qualitatively consistent with the previous studies. We compare our results with the predictions from different theoretical disk-corona models. We show that the R − Γ correlation found in this work seems to favor the moving corona model proposed by Beloborodov. Our results indicate that the coronal geometry varies significantly among BHXRBs. We further compare our results with that of AGNs. We find that the reflection strength R is smaller than unity in the hard state of BHXRBs, while it can be as large as ∼5 in AGNs, which implies that the variations of the disk-coronal geometry of AGNs are more vigorous than that of the BHXRBs in the hard state.

The observations of varying broad iron lines during the state transition of the black hole X-ray binaries have been accumulating. In this work, the relation between the normalized intensity and the width of iron lines is investigated, in order to understand better the variation of iron lines and possibly its connection to state transition. Considering the uncertainties due to ionization and illuminating X-rays, only the effects of geometry and gravity are taken into account. Three scenarios were studied, i.e., the continuous disk model, the innermost annulus model, and the cloud model. As shown by our calculations, at given iron width, the line flux of the cloud model is smaller than that of the continuous disk model; while for the innermost annulus model, the width is almost unrelated with the flux. The range of the line strength depends on both the BH spin and the inclination of the disk. We then apply to the observation of MAXI J1631-479 by Nuclear Spectroscopic Telescope Array during its decay from the soft state to the intermediate state. We estimated the relative line strength and width according to the spectral fitting results in Xu et al., and then compared with our theoretical width–flux relation. It was found that the cloud model was more favored. We further modeled the iron line profiles, and found that the cloud model can explain both the line profile and its variation with reasonable parameters.

The fast blue optical transients (FBOTs) are a new population of extragalactic transients of unclear physical origin. A variety of mechanisms has been proposed including failed supernova explosion, shock interaction with a dense medium, young magnetar, accretion onto a compact object and stellar tidal disruption event, but none is conclusive. Here we report the discovery of a possible X-ray quasi-periodicity signal with a period of ∼250 s (at a significance level of 99.76%) in the brightest FBOT AT2018cow through the analysis of XMM-Newton/PN data. The signal is independently detected at the same frequency in the average power density spectrum from data taken from the Swift telescope, with observations covering from 6 to 37 days after the optical discovery, though the significance level is lower (94.26%). This suggests that the quasi-periodic oscillation (QPO) frequency may be stable over at least 1.1 × 10⁴ cycles. Assuming the ∼250 s QPO to be a scaled-down analog of that typically seen in stellar mass black holes, a black hole mass of ∼10³–10⁵ solar masses could be inferred. The overall X-ray luminosity evolution could be modeled with a stellar tidal disruption by a black hole of ∼10⁴ solar masses, providing a viable mechanism to produce AT2018cow. Our findings suggest that other bright FBOTs may also harbor intermediate-mass black holes.

Absorption lines with high blueshifted velocities are frequently found in the ultraviolet (UV) and X-ray spectra of luminous active galactic nuclei (AGNs). This implies that high-velocity winds/outflows are common in AGNs. In order to study the formation of high-velocity winds, especially ultrafast outflows (UFOs), we perform two-dimensional magnetohydrodynamic (MHD) simulations. Initially, a magnetic field is set to be weaker than the gas pressure at the disk surface. In our simulations, line force operates on the region like filaments because the X-ray radiation from corona is shielded by dense gas in the inner region at some angle. The location of filaments changes with time and then the line-driven winds are exposed to X-rays and become highly ionized. The line force at the UV bands does not directly drive the highly ionized winds. In the sense of time average, the properties of high-velocity winds meet the formation condition of UFOs. Compared with line force, the function of magnetic field is negligible in directly driving winds. In the MHD model, the region around the rotational axis becomes magnetic-pressure dominated, which prevents gases from spreading to higher latitudes and then enhances the gas column density at middle and low latitudes (20°–70°). Higher column density is helpful to shield X-ray photons, which causes the line force to be more effective in the MHD model than in the hydrodynamic model. Higher-velocity winds with a broader opening angle are produced in the MHD model.