Soft State Research Articles

Evaluation of Several Explanations of the Strong X-Ray Polarization of the Black Hole X-Ray Binary 4U 1630-47

The Imaging X-ray Polarimetry Explorer observations of the X-ray binary 4U 1630–47 in the high soft state revealed high linear polarization degrees (PDs) rising from 6% at 2 keV to 10% at 8 keV. We discuss in this Letter three different mechanisms that impact the polarization of the observed X-rays: the reflection of gravitationally lensed emission by the accretion disk, reprocessing of the emission in outflowing plasma, and electron and ion anisotropies in the accretion disk atmosphere. We conducted detailed ray-tracing studies to evaluate the impact of the reflection of strongly gravitationally lensed emission on the PDs. Although the reflected emission can produce high PDs in the high-energy tail of the thermal emission component, we do not find models that describe the PDs and are consistent with independent estimates of the source distance. We discuss the energetics of another proposed mechanism: the emission or scattering of the X-rays in mildly relativistically moving plasma outflows. We argue that these models are disfavored as they require large mechanical luminosities on the order of, or even exceeding, the Eddington luminosity. We investigated the impact of electron and ion anisotropies but find that their impact on the observed PDs are likely negligible. We conclude with a discussion of all three effects and avenues for future research.

Characterizing the Broadband Reflection Spectrum of MAXI J1803-298 during Its 2021 Outburst with NuSTAR and NICER

MAXI J1803-298 is a transient black hole candidate discovered in 2021 May during an outburst that lasted several months. Multiple X-ray observations reveal recurring “dipping” intervals in several of its light curves, particularly during the hard/intermediate states, with a typical recurrence period of ∼7 hr. We report analysis of four NuSTAR observations of the source, supplemented with NICER data where available, over the duration of the outburst evolution covering the hard, intermediate, and the soft states. Reflection spectroscopy reveals the black hole to be rapidly spinning (a * = 0.990 ± 0.001) with a near edge-on viewing angle (i = 70° ± 1°). Additionally, we show that the light-curve dips are caused by photoelectric absorption from a moderately ionized absorber whose origin is not fully understood, although it is likely linked to material from the companion star impacting the outer edges of the accretion disk. We further detect absorption lines in some of the spectra, potentially associated with Fe xxv and Fe xxvi, indicative of disk winds with moderate to extreme velocities. During the intermediate state and just before transitioning into the soft state, the source showed a sudden flux increase, which we found to be dominated by soft disk photons and consistent with the filling of the inner accretion disk, at the onset of state transition. In the soft state, we show that models of disk self-irradiation provide a better fit and a preferred explanation to the broadband reflection spectrum, consistent with previous studies of other accreting sources.

An X-ray study of changing-look active galactic nuclei

A significant number of changing-look active galactic nuclei (CL AGNs) have been identified to date. In this work, we study what happens to the X-ray spectrum during CL events. We use the example of the nearby CL Seyfert named NGC 1566, which has been observed by Swift, NuSTAR, XMM-Newton and Suzaku . We applied the Comptonization model to describe the evolution of NGC 1566 X-ray spectra during outbursts and compared these results with the typical behavior of other AGNs to identify some differences and common properties that will ultimately help us better understand the physics of the CL phenomenon. We found that changes in the X-ray properties of NGC 1566 are characterized by a different combination of Sy1 (using 1H 0707--495 as a representative) and Sy2 properties (using NGC 7679 and Mrk 3 as their representatives). At high X-ray luminosities, NGC 1566 exhibits behavior typical of Sy1. At low luminosities, we see a transition of NGC 1566 from Sy1 behavior to an Sy2 pattern. We revealed the saturation of the spectral indices, alpha , for these four AGNs during outbursts ($ 0.9$, and $ mrk3 and we determined the masses of the black holes (BHs) in the centers of these AGNs; namely 10^7$ M$_ odot $, M$_ odot $, M$_ mrk3 odot $, and M$_ odot $, applying the scaling method. Our spectral analysis shows that the changing-look of NGC1566 from Sy1.2 to Sy1.9 in 2019 was accompanied by the transition of NGC 1566 to an accretion regime, which is typical for the intermediate and highly soft spectral states of other BHs. We also find that when going from Sy2 to Sy1, the spectrum of NGC 1566 shows an increase in the soft excess accompanied by a decrease in the Comptonized fraction ($0.1<f<0.5$), which is consistent with the typical behavior of BH sources during X-ray outburst decay. Our results strongly suggest that the broad variations in behavior observed among CL, Sy1, and Sy2 AGNs with different X-ray luminosities can be explained by changes in a single variable parameter (e.g., the ratio of the AGN's X-ray luminosity to its Eddington luminosity), without any need for incorporating additional differences in the Sy AGN parameters (e.g., inclination). Therefore, we find that the distinction between the Sy1, Sy2, and CL-AGN subclasses is effectively blurred.

X-Ray Spectra of Black Hole X-Ray Binaries with Returning Radiation

In the disk–corona model, the X-ray spectrum of a stellar-mass black hole in an X-ray binary is characterized by three components: a thermal component from a thin and cold accretion disk, a Comptonized component from a hot corona, and a reflection component produced by illumination of the cold disk by the hot corona. In this paper, we assume a lamppost corona, and we improve previous calculations of the X-ray spectrum of black hole X-ray binaries. The reflection spectrum is produced by the direct radiation from the corona as well as by the returning radiation of the thermal and reflection components and is calculated considering the actual spectrum illuminating the disk. If we turn the corona off, the reflection spectrum is completely generated by the returning radiation of the thermal component, as it may happen for some sources in soft spectral states. After choosing the radial density profile of the accretion disk, the ionization parameter is calculated self-consistently at any radial coordinate of the disk from the illuminating X-ray flux and the local electron density. We show the predictions of our model in different regimes, and we discuss its current limitations as well as the next steps to improve it.

A Bistate Intelligent Eutectogel Material with Water‐Regulated Stiffness for Diverse Designed Functionality

AbstractThe design of stiffness‐changeable synthetic materials that can mimic their biological counterparts in the production of adaptable structures and functions is highly important but remains challenging. Here, the development of a bioinspired intelligent material composed of a deep eutectic solvent (DES) and an ionically crosslinked polymer network that is capable of switching reversibly between hard plastic and soft elastic states in response to water is reported. The eutectogel stiffness can be dramatically switched and flexibly regulated by water, which can activate salt ionization and shield the ionic crosslink. This design principle is extensively applied to multiple polymers, producing a series of bistate materials that exhibit water‐regulated capabilities ranging from self‐healing, shape memory, remolding, antifreezing, anticompressing, and stretchability to conductivity. These bistate materials are used for diverse functions, such as performing mechanical tasks, sensing and monitoring human movements, lighting LEDs, and outputting signals. This work opens an avenue for the development of next‐generation stiffness‐changeable materials with diverse functionalities for various designed applications.

Unveiling the rebrightening mechanism of GRS 1915+105: Insights from a change in the quasi-periodic oscillations and from a wind analysis

We report a significant rebrightening event in the microquasar GRS 1915+105 that was observed in July 2021 with NICER and NuSTAR. This event was characterized by quasi-periodic oscillations (QPOs) in the soft state, but is typically free of these oscillations. It was also marked by an increase in the disk wind ionization degree. By employing the Hilbert-Huang transform (HHT), we decomposed the stable low-frequency QPO from the light curves using data from NICER and NuSTAR. Our spectral analysis shows a weak change in the Fe XXV absorption lines and a strong change in the Fe XXV absorption edge with QPO phase. Other spectral parameters, including the photon index and the seed photon temperature, correlate positively with the QPO phase, but the electron temperature is inversely correlated. Based on our findings we propose that the observed QPOs were caused by magnetic activity and not by precession. The magnetic field drove a failed disk wind of high-ionization low-velocity material. These results support the accretion ejection instability model and provide deeper insights into the dynamics of accretion-ejection processes that are magnetized by a black hole.

Quasi-in-situ observations of microstructural evolution during cyclic deformation of a Mg-Zn-Ca alloy

Quasi-in-situ electron backscatter diffraction (EBSD) was employed to investigate the microstructural evolution of a homogenized coarse-grained Mg-1.2Zn-0.1Ca alloy during cyclic deformation at a strain amplitude of 0.8 %. The random orientations of grains along with the soft state of the alloy resulted in symmetrical hysteresis loops and cyclic hardening. Besides the occurrence of twinning-detwinning, basal <a>/pyramidal II <c+a> and pyramidal I <a> slip systems were primarily activated. Grain boundaries with a higher misorientation angle of >35° served as preferential sites for strain localization.

The First Detection of X-Ray Polarization in a Newly Discovered Galactic Transient Swift J151857.0-572147

We study the spectropolarimetric properties of a newly discovered black hole (BH) X-ray binary Swift J151857.0-572147 jointly using Imaging X-ray Polarimetry Explorer (IXPE) and NuSTAR observations during 2024 March. The analysis of IXPE data reports the first detection of X-ray with a polarization degree (PD) of 1.34 ± 0.27 and a polarization angle (PA) of −13.°69 ± 5.°85 using a model-independent approach, while the model-dependent analysis gives a PD of 1.18 ± 0.23 and a PA of −14.°01 ± 5.°80. The joint spectral analysis of the broadband data and NuSTAR analysis in isolation constrain the mass of the central BH between ∼9.2 ± 1.6 and 10.1 ± 1.7M ⊙ and a moderate spin parameter of ∼0.6 ± 0.1–0.7 ± 0.2 with a disk inclination of ∼35° ± 7°–46° ± 15°. The power-law photon index and cutoff energy are 2.19 ± 0.03–2.47 ± 0.06 and ∼36 ± 4–78 ± 10 keV, suggesting a transition to the soft spectral state. Additionally, a relatively lower corona size of 6 ± 1–9 ± 2r S , a low mass outflow rate ( <3%ṀEdd ), and the best-fitted halo accretion is less compared to the disk accretion rate further confirm the same state. The low PD detected in the soft state can be due to repeated scattering inside the dense corona, and the dominant emission from the disk agrees with the low spin and low disk inclination. The hydrogen column density obtained from the fit is relatively high at ∼4–5 × 1022 cm−2.

Estimating the Spin of MAXI J1348–630 from Intermediate and Soft States Using Insight-HXMT Data

We present a broadband spectral analysis of the soft-intermediate and soft states using Insight-HXMT observations of the black hole binary MAXI J1348–630 during its outburst in 2019. We employ a combination of reflection and continuum fitting methods to measure the spin of the black hole and determine a spin of a = 0.79 ± 0.13, which is consistent with most of the previous results. Additionally, we investigate some sources of systematic uncertainty that could bias the measurement. The valid spectral hardening factor can partially account for the spin evolution observed during the outburst. Other potential factors, such as assumptions about the corona’s structure and their interaction with reflected photons, can also affect the accuracy of spin measurements. Furthermore, our analysis reveals that MAXI J1348–630 exhibits a significantly high disk density (∼1022 cm−3), with a moderate iron abundance of approximately 2.5 times solar.

A novel photocurable polymeric solid–solid phase change material for intelligent temperature regulators

The advancement of solid–solid phase change materials (SSPCMs) with crystalline segments embedded onto crosslinked polymer backbones offers a promising solution to the leakage problems associated with traditional phase change materials. Despite its potential, this technology faces challenges, including energy-intensive synthesis processes of SSPCMs and difficulties in custom designing materials for confined spaces. Herein, we introduce a novel strategy for the rapid preparation of polyurethane acrylate-based SSPCMs via UV-initiated polymerization. By leveraging crystalline segments from polyethylene glycol, the resulting SSPCM exhibits an impressive enthalpy of 110.4 J⋅g−1, enabling seamless transitions from rigid to soft states during solid–solid phase changes. When applied in portable electronic devices, this SSPCM can be synthesized in-situ to meet various size requirements, demonstrating exceptional temperature regulation performance based on latent thermal energy storage mechanisms. Furthermore, by integrating thin layers of carbon nanotubes (CNTs) onto the rough surfaces of SSPCM films and employing a face-to-face configuration to convert temperature-sensitive mechanical changes into electric resistance variations, we have engineered high-temperature alarms for SSPCM-based temperature regulators. The advantages of this preparation strategy, combined with the thermal regulation capabilities and innovative temperature alarm design, underscore the significant potential for intelligent and adaptive applications of SSPCMs in advanced technological landscapes.

NICER and Swift/XRT monitoring of the 2023 outburst of Swift J1727.7–1613

Aims. The X-ray transient Swift J1727.7–1613 was first detected on August 24 2023 by Swift/BAT and INTEGRAL. We investigated data from the Neutron star Interior Composition Explorer (NICER) and the Neil GehrelsSwift Observatory taken between August and October 2023. Methods. We studied diagnostic diagrams, energy spectra, and short term variability. The observations cover the initial rise of the outburst in the hard state and the transition to the soft state. We focused on the evolution of quasi-periodic oscillations (QPOs) using power-density spectra and on the evolution of the spectral parameters. Results. The overall evolution of Swift J1727.7–1613 is consistent with this source being a low-mass black hole X-ray binary. Based on the Lense-Thirring precession interpretation of type-C QPOs, we obtained outer radii for the hot inner flow and found that the overall evolution of these radii agrees well with the evolution of the inner disc radii obtained from fits to the energy spectra. This result holds on all timescales tested in this study and supports the Lense-Thirring precession interpretation of type-C QPOs.

Variable Ionized Disk Winds in MAXI J1803−298 Revealed by NICER

We present the results from the NICER observation data of MAXI J1803−298 across the entire 2021 outburst. In the intermediate and soft state, we detect significant absorption lines at ∼7.0 and ∼6.7 keV, arising from X-ray disk winds outflowing with a velocity of hundreds of km s−1 along our line of sight. The fitting results from the photoionized model suggest that the winds are driven by thermal pressure and the mass-loss rate is low. We find a clear transition for iron from predominantly H-like to predominantly He-like during the intermediate-to-soft state transition. Our results indicate this transition for iron is caused by the evolution of the illuminating spectrum and the slow change of the geometric properties of the disk winds together. The coexistence of disk winds and quasiperiodic oscillation features in the intermediate state is also reported. Our study makes MAXI J1803−298 the first source in which a transition from optical winds to X-ray winds is detected, offering new insights into the evolution of disk winds across an entire outburst and long-term coupling of accretion disks and mass outflows around accreting black holes.

The Peculiar Disk Evolution of 4U 1630-472 Observed by Insight-HXMT During its 2022 and 2023 Outbursts

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∝Teff3.92±0.13 and F∝Teff4.91±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.

Wavelet analysis of low-frequency quasi-periodic oscillations in MAXI J1803−298 observed with Insight-HXMT and NICER

ABSTRACT With data observed by the Hard X-ray Modulation Telescope (Insight-HXMT) and the Neutron star Interior Composition Explorer (NICER), we study low-frequency quasi-periodic oscillations (QPOs) of the black hole candidate MAXI J1803−298 during the 2021 outburst. Based on the hardness–intensity diagram and the difference in the QPOs properties, type-C and type-B QPOs are found in the low-hard state and soft-intermediate state, respectively. After searching for and classifying QPOs in the Fourier domains, we extract the QPO component and study it with wavelet analysis. The QPO and no-QPO time intervals are separated by the confidence level, so that the S-factor, which is defined as the ratio of the QPO time interval to the total length of the good time interval, is calculated. We found S-factors decrease with QPOs frequency for type-C QPOs but stay stable around zero for type-B QPOs. The relation of S-factor of type-C QPOs and photon energy and the correlation of S-factor and counts are also studied. Different correlations between the S-factor and counts for different energy bands indicate different origins of QPOs in high- and low-energy bands, which may be due to a dual-corona scenario.

On soft factors and transmutation operators

The well known soft theorems state the specific factorizations of tree level gravitational (GR) amplitudes at leading, sub-leading and sub-sub-leading orders, with universal soft factors. For Yang-Mills (YM) amplitudes, similar factorizations and universal soft factors are found at leading and sub-leading orders. Then it is natural to ask if the similar factorizations and soft factors exist at higher orders. In this note, by using transformation operators proposed by Cheung, Shen and Wen, we reconstruct the known soft factors of YM and GR amplitudes, and prove the nonexistence of higher order soft factor of YM or GR amplitude which satisfies the universality.

IXPE View of BH XRBs during the First 2.5 Years of the Mission

Accreting stellar-mass black holes represent unique laboratories for studying matter and radiation under the influence of extreme gravity. They are highly variable sources going through different accretion states, showing various components in their X-ray spectra from the thermal emission of the accretion disc dominating in the soft state to the up-scattered Comptonisation component from an X-ray corona in the hard state. X-ray polarisation measurements are particularly sensitive to the geometry of the X-ray scatterings and can thus constrain the orientation and relative positions of the innermost components of these systems. The IXPE mission has observed about a dozen stellar-mass black holes with masses up to 20 solar masses in X-ray binaries with different orientations and in various accretion states. The low-inclination sources in soft states have shown a low fraction of polarisation. On the other hand, several sources in soft and hard states have revealed X-ray polarisation higher than expected, which poses significant challenges for theoretical interpretation, with 4U 1630–47 being one of the most puzzling sources. IXPE has measured the spin of three black holes via the measurement of their polarisation properties in the soft emission state. In each of the three cases, the new results agree with the constraints from the spectral observations. The polarisation observations of the black hole X-ray transient Swift J1727.8–1613 across its entire outburst has revealed that the soft-state polarisation is much weaker than the hard-state polarisation. Remarkably, the observations furthermore show that the polarisation of the bright hard state and that of the 100 times less luminous dim hard state are identical within the accuracy of the measurement. For sources with a radio jet, the electric field polarisation tends to align with the radio jet, indicating the equatorial geometry of the X-ray corona, e.g., in the case of Cyg X–1. In the unique case of Cyg X–3, where the polarisation is perpendicular to the radio jet, the IXPE observations reveal the presence and geometry of obscuring material hiding this object from our direct view. The polarisation measurements acquired by the IXPE mission during its first 2.5 years have provided unprecedented insights into the geometry and physical processes of accreting stellar-mass black holes, challenging existing theoretical models and offering new avenues for understanding these extreme systems.

Polarization Study of Swift J151857.0–572147 with IXPE Observation

We present an analysis of the Imaging X-ray Polarimetry Explorer observation from a newly discovered transient source: Swift J151857.0–572147. The obtained polarization degree (PD) and angle are 0.3% ± 0.3% and −24° ± 26° respectively in 2–8 keV within 68% confidence level errors, and polarization results are below MDP99 in all energy bins, with the upper limit on PD of 0.8% in the 2–8 keV energy range. No quasi-periodic oscillations (QPOs) are detected in this observation. The polarization and QPO analyses support the hypothesis that the source was in the high soft state, and the results are consistent with predictions for a thin accretion disk model.

Insight-HXMT, NICER, and NuSTAR Views to the Newly Discovered Black Hole X-Ray Binary Swift J151857.0–572147

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 Fdisk∝Tin3.83±0.17 . For the standard thin disk, Ldisk≈4πRin2σTin4 , 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 ± 4.20M ⊙ 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 ⊙.

Conductive hydrogels‐based self‐sensing soft robot state perception and trajectory tracking

AbstractSoft robots face significant challenges in proprioceptive sensing and precise control due to their highly deformable and compliant nature. This paper addresses these challenges by developing a conductive hydrogel sensor and integrating it into a soft robot for bending angle measurement and motion control. A quantitative mapping between the hydrogel resistance and the robot's bending gesture is formulated. Furthermore, a nonlinear differentiator is proposed to estimate the angular velocity for closed‐loop control, eliminating the reliance on conventional sensors. Meanwhile, a controller is designed to track both structural and nonstructural trajectories. The proposed approach integrates advanced soft sensing materials and intelligent control algorithms, significantly improving the proprioception and motion accuracy of soft robots. This work bridges the gap between novel material design and practical control applications, opening up new possibilities for soft robots to perform delicate tasks in various fields. The experimental results demonstrate the effectiveness of the proposed sensing and control approach in achieving precise and robust motion control of the soft robot.

Accretion Properties of Soft X-Ray Transient XTE J1856+053 during Its 2023 Outburst

Soft X-ray transients (SXTs) are a subclass of the low-mass X-ray binaries that occasionally show a sudden rise in their soft X-ray luminosity; otherwise, they remain in an extremely faint state. We investigate the accretion properties of the SXT XTE J1856+053 during its 2023 outburst obtained by NICER and NuSTAR data in July. We present detailed results on the timing and spectral analysis of the X-ray emission during the outburst. The power spectral density shows no quasi-periodic oscillation features. The source’s spectrum on July 19 can be well fitted with a multicolor blackbody component, a power-law component, and a reflection component with a broadened iron emission line. NICER spectra can be well fitted by considering a combination of a blackbody and a power law. The source exhibits a transition within just 5 days from a soft state to an intermediate state during the outburst decline phase. The inner accretion disk has a low inclination angle (∼18°). The spectral analysis also suggests a high-spin (a > 0.9) black hole as the central accreting object.