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The systematic properties are largely unknown for the black hole X-ray binary Swift J151857.0–572147 newly discovered in the 2024 outburst. The nature of a black hole can be completely defined by specifying the mass and dimensionless spin parameter. Therefore, accurate measurement of the two fundamental parameters is important for understanding the nature of black holes. The joint spectral fitting of a reflection component with simultaneous observations from Insight-HXMT, NICER, and NuSTAR reveals for the first time a black hole dimensionless spin of ${0.84}_{-0.26}^{+0.17}$ and an inclination angle of ${21.1}_{-3.6}^{+4.5}$ degrees for this system. Monitoring of the soft state by NICER results in disk flux and temperature following ${F}_{\mathrm{disk}}\propto {T}_{\mathrm{in}}^{3.83\pm 0.17}$ . For the standard thin disk, ${L}_{\mathrm{disk}}\approx 4\pi {R}_{\mathrm{in}}^{2}\sigma {T}_{\mathrm{in}}^{4}$ , so the relationship between the flux and temperature of the disk we measured indicates that the inner radius of the disk is stable and the disk is in the innermost stable circular orbit. With an empirical relation built previously between the black hole outburst profile and the intrinsic power output, the source distance is estimated as 5.8 ± 2.5 kpc according to the outburst profile and peak flux observed by Insight-HXMT and NICER. Finally, a black hole mass of 3.67 ± 1.79–8.07 ± 4.20 M _⊙ can be inferred from a joint diagnostic of the aforementioned parameters measured for this system. This system is also consistent with most black hole X-ray binaries with high spin and a mass in the range of 5–20 M _⊙ .

We study the spectral and temporal properties of the black hole (BH) X-ray transient binary SLX 1746–331 during the 2023 outburst with NICER, NuSTAR, and Insight-HXMT observations. Through the joint fitting of the spectra from NICER, NuSTAR, and Insight-HXMT, the spin and inclination angles are measured for the first time as 0.85 ± 0.03 and 53° ± 0.°5, respectively. Accordingly, the mass of the compact star is updated from the previous lower limit of 3.3 ± 2.1 M _⊙ to 5.5 ± 3.6 M _⊙ , which is consistent with 5.2 ± 4.5 M _⊙ measured with an empirical mass–luminosity correlation of BH samples. With more NICER observations covering the later decay of the outburst, we confirm that the entire outburst was dominated by the disk emissions, and the thermal spectrum follows $F\propto {T}_{\mathrm{in}}^{3.974\pm 0.003}$ , until a luminosity of over 2 magnitudes lower than the maximum of the outburst.

We present the first polarimetric analysis of quasiperiodic oscillations (QPOs) in a black hole binary utilizing IXPE data. Our study focuses on Swift J1727.8–1613, which experienced a massive outburst that was observed by various telescopes across different wavelengths. The IXPE observation we studied was conducted during the hard-intermediate state. The polarization degree (PD) and polarization angle (PA) were measured at 4.28% ± 0.20% and 1.°9 ± 1.°4, respectively. Remarkably, significant QPO signals were detected during this observation, with a QPO frequency of approximately 1.34 Hz and a fractional rms amplitude of about 12.3%. Furthermore, we conducted a phase-resolved analysis of the QPO using the Hilbert–Huang transform technique. The photon index showed a strong modulation with respect to the QPO phase. In contrast, the PD and PA exhibit no modulations in relation to the QPO phase, which is inconsistent with the expectation of the Lense–Thirring precession of the inner flow. Further theoretical studies are needed to conform with the observational results.

Swift J1727.8–1613 is a black hole X-ray binary newly discovered in 2023. We perform spectral analysis with simultaneous Insight-HXMT, NICER, and NuSTAR observations when the source was approaching the hard intermediate state. Such a joint view reveals an additional hard component apart from the normally observed hard component with reflection in the spectrum, to be distinguished from the usual black hole X-ray binary systems. By including this extra component in the spectrum, we have measured a high spin of ${0.98}_{-0.07}^{+0.02}$ and an inclination of around ${40}_{-0.8}^{+1.2}$ °, which is consistent with NICER results reported before. However, we find that the additional spectral component cannot be exclusively determined due to the model degeneracy. Accordingly, a possible jet/corona configuration is adjusted to account for the spectral fitting with different model trials. The extra component may originate either from a relativistic jet or a jet base/corona underneath a slow jet.

IGR J17091–3624, in addition to GRS 1915 + 105, is the only black hole X-ray binary that displays “heartbeat”-like variability, characterized by structured flares with high amplitudes. In this study, we conduct a detailed phase-resolved analysis of the recently identified “heartbeat”-like Class X variability in IGR J17091–3624 during its 2022 outburst, utilizing data from NICER and NuSTAR observations. A shortage in the high-energy (>20 keV) X-ray flux is detected at peak phases of the soft-X-ray flare at a ∼15 σ confidence level from the phase-folded light curves. Furthermore, our phase-resolved spectral analysis reveals variations in the spectral shape, particularly showing significant synchronous variations in the disk temperature and flux with the count rate. These findings imply that the flare is primarily driven by instabilities within the accretion disk, consistent with previous studies of the well-known Class ρ variability in GRS 1915+105. However, we also observe a positive correlation between the disk temperature and flux over the flare cycle, which differs from the loop relation between the two parameters found in the Class ρ variability. This could suggest differences in the underlying physical processes between the two variability classes. Variations in the Comptonization component during flares are also observed: the electron temperature and covering fraction show anticorrelations with the disk flux, revealing potential interactions between the accretion disk and the corona during these flares.

Low-frequency quasiperiodic oscillations (LFQPOs) are commonly observed in X-ray light curves of black hole X-ray binaries (BHXRBs); however, their origin remains a topic of debate. In order to thoroughly investigate variations in spectral properties on the quasiperiodic oscillation (QPO) timescale, we utilized the Hilbert–Huang transform technique to conduct phase-resolved spectroscopy across a broad energy band for LFQPOs in the newly discovered BHXRB Swift J1727.8–1613. This is achieved through quasi-simultaneous observations from Neutron Star Interior Composition Explorer, Nuclear Spectroscopic Telescope Array, and Hard X-ray Modulation Telescope. Our analysis reveals that both the nonthermal and disk–blackbody components exhibit variations on the quasiperiodic oscillation (QPO) timescale, with the former dominating the QPO variability. For the spectral parameters, we observe modulation of the disk temperature, spectral indices, and reflection fraction with the QPO phase with high statistical significance (≳5 σ ). Notably, the variation in the disk temperature is found to precede the variations in the nonthermal and disk fluxes by ∼0.4–0.5 QPO cycles. We suggest that these findings offer further evidence that the type-C QPO variability is a result of geometric effects of the accretion flow.

After 49 yr of quiescence, 4U 1730–22 became active and had two outbursts in 2021 and 2022; the onset and tail of the outbursts were observed by NICER, which give us a peerless opportunity to study the state transition and its underlying mechanism. In this work, we take both the neutron star (NS) surface and accretion disk emission as the seed photons of the Comptonization and derive their spectral evolution in a bolometric luminosity range of 1%–15% L _Edd . In the high/soft state, the inferred inner disk radius and the NS radius are well consistent, which implies that the accretion disk is close to the NS surface. For the decay stage, we report a steep change of the accretion disk emission within 1 day, i.e., the soft-to-hard transition, which could be due to the propeller effect, and the corresponding NS surface magnetic field is 1.8–2.2 × 10 ^8 G. Moreover, the inner disk radius is truncated at the corotation radius, which is similar to the propeller effect detected from 4U 1608–52. The absence of the propeller effect in the hard-to-soft state transition implies that the transition between the magnetospheric accretion and the disk accretion is not the sole cause of the state transitions.

The black hole candidate system SLX 1746–331 was back to business in 2023, after a long silence of roughly 13 years. An outburst was observed thoroughly by Insight-HXMT and NICER. The outburst is characterized by spectral dominance of the soft state, where the joint Insight-HXMT and NICER spectral analysis shows the temperature dependence of the disk flux follows ${T}_{\mathrm{in}}^{3.98}$ , and thus suggests that the inner disk reaches its innermost stable circular orbit during almost the entire outburst. By assuming 0.3 L _Edd for the peak flux and an inclination angle of zero degrees, the lower limit of the compact object hosted in this system is estimated as 3.28 ± 2.14 M _⊙ . We also look into the relation between the disk temperature and disk flux for a sample of black hole systems, and by taking the disk temperature derived in the outburst of SLX 1746–331, such a relation results in a mass estimation of 5.2 ± 4.5 M _⊙ . Finally, the spin of the compact object is constrained to be larger than 0.8 with the spectral model KERRBB.

Staying in quiescence for 49 yr, 4U 1730–22 became active and had two outbursts in 2021 and 2022; 10 thermonuclear X-ray bursts were detected with Insight-HXMT. Among them, the faintest burst showed a double-peaked profile, placing the source as the seventh accreting neutron star (NS) exhibiting double-peaked type I X-ray bursts; the other bursts showed photospheric radius expansion (PRE). The properties of a double-peaked non-PRE burst indicate that it could be related to a stalled burning front. For the five bright PRE bursts, apart from the emission from the neutron star (NS) surface, we find the residuals both in the soft (10 keV) X-ray bands. Time-resolved spectroscopy reveals that the excess can be attributed to an enhanced preburst/persistent emission or the Comptonization of the burst emission by the corona. We find, the burst emission shows a rise until the photosphere touches down to the NS surface rather than the theoretical predicted constant Eddington luminosity. The shortage of the burst emission in the early rising phase is beyond the occlusion by the disk. We speculate that the findings above are due to that the obscured part (not only the lower part) of the NS surface is exposed to the line of sight due to the evaporation of the obscured material by the burst emission, or the burst emission is anisotropic ( ξ > 1) in the burst early phase. In addition, based on the fluxes of PRE bursts at their touchdown times, we derive a distance estimation as 9.0–12.4 kpc.