Spectral State Research Articles

Magnetic Flux Plays an Important Role during a Black Hole X-Ray Binary Outburst in Radiative Two-temperature General Relativistic Magnetohydrodynamic Simulations

Black hole (Bh) X-ray binaries cycle through different spectral states of accretion over the course of months to years. Although persistent changes in the Bh mass accretion rate are generally recognized as the most important component of state transitions, it is becoming increasingly evident that magnetic fields play a similarly important role. In this article, we present the first radiative two-temperature general relativistic magnetohydrodynamics simulations in which an accretion disk transitions from a quiescent state at an accretion rate of Ṁ∼10−10ṀEdd to a hard-intermediate state at an accretion rate of Ṁ∼10−2ṀEdd . This huge parameter space in mass accretion rate is bridged by artificially rescaling the gas density scale of the simulations. We present two jetted BH models with varying degrees of magnetic flux saturation. We demonstrate that in “standard and normal evolution” models, which are unsaturated with magnetic flux, the hot torus collapses into a thin and cold accretion disk when Ṁ≳5×10−3ṀEdd . On the other hand, in “magnetically arrested disk” models, which are fully saturated with vertical magnetic flux, the plasma remains mostly hot with substructures that condense into cold clumps of gas when Ṁ≳1×10−2ṀEdd . This suggests that the spectral signatures observed during state transitions are closely tied to the level of magnetic flux saturation.

SDSS J075217.84 + 193542.2: X-ray weighing of a secondary BH

Precise measurements of black hole (BH) masses are necessary to understand the coevolution of these sources and their host galaxies. Sometimes in the center of a galaxy, there is not one but two BHs. The BH duality of the quasar nucleus SDSS J075217.84 + 193542.2 (herein referred to as SDSS J0752) was recently proposed based on the observed strict periodicity of optical emission from the source. We tested this assumption using X-ray observations obtained using Swift/XRT (2008–2010). We fitted the SDSS J075217 spectrum using a Comptonization model and discovered soft X-ray variability in the 0.3–10-keV energy range. We pursued a scenario in which two supermassive BHs at the center of SDSS J0752 form a pair, and the less massive (secondary) BH periodically crosses/punctures the disk around the more massive (primary) BH. We associated these periodic crossings with tidal disruptions of the disk and, as a consequence, with an increase in X-rays observed as a flare in SDSS J0752. During such an X-ray flare event (2008–2010), we discovered a change in the source spectral states and the photon index saturation at the Γ ∼ 3 level with mass accretion rate Ṁ. For BH mass scaling, we used OJ 287, M101 ULX–1, and HLX–1 ESO 243–49 as reference sources and found that MSDSS = 9 × 107 solar masses, assuming that dSDSS = 500 Mpc. Thus, we obtained a lower limit to a BH mass due to the unknown inclination. In addition, we used the virial mass of the secondary BH based on Hα-line measurements, and we estimated the inclination of the binary at SDSS J0752, i = 80°, using a scaling technique.

A large population of neutron star low-mass X-ray binaries with long outburst recurrence time?

ABSTRACT Low-mass X-ray binaries (LMXBs) with neutron stars show quite different features that depend on the rate of mass transfer from the donor star. With a high transfer rate, the Z sources are in a persistent soft spectral state, and with a moderate transfer rate the transient Atoll sources have outburst cycles like the black hole X-ray binaries. The observations document very long outburst recurrence times for quite a number of sources. We follow with our computations the evolution of the accretion disc until the onset of the ionization instability. For sources with a low mass transfer rate, the accumulation of matter in the disc is essentially reduced due to the continuous evaporation of matter from the disc to the coronal flow. Different mass transfer rates result in nearly the same amount of matter accumulated for the outburst, which means that the outburst properties are similar for sources with short outburst cycles and sources with long outburst cycles, contrary to some expectations. Then, for systems with long recurrence time, less sources will be detected and the total population of LMXBs could be larger than it appears. This would relieve the apparent problem that the observed number of LMXBs as progenitors of millisecond pulsars (MSPs) is too small compared to the number of MSPs. Concerning the few quasi-persistent sources with year-long soft states, we argue that these states are not outbursts, but quasi-stationary hot states as in Z sources.

Unipolar optical transitions in nanoclusters of ellipsoidal geometry

The quantum states of an ellipsoidal nanocluster of a heterophase system InAs/GaAs are studied using an exact analytical approach, in contrast to the generally accepted theoretical model based on the adiabatic approximation. It is shown that the spectrum of a nanoobject is formed from local groups, consisting of discrete levels, separated by terahertz frequency intervals. A double random degeneration of certain spectrum states is revealed. Allowed mid-infrared (IR) optical transitions between different spectral states are analyzed. The role of dimensional parameters and features of the shape of a nanoobject in the characteristics of unipolar transitions is assessed. The absorption spectrum for the transition in the lower part of the substructure spectrum is calculated.

Soft-state optical spectroscopy of the black hole MAXI J1305-704

The X-ray dipper MAXI J1305-704 is a dynamically confirmed black hole (BH) X-ray binary discovered a decade ago. While its only outburst has been studied in detail in X-rays, follow-up at other wavelengths has been scarce. We report here the results from an optical spectroscopy campaign across the outburst of MAXI J1305-704. We analysed two epochs of data obtained by the Magellan Clay Telescope during two consecutive nights, when the source was in a soft X-ray spectral state. We identified typical emission lines from outbursting low-mass X-ray binaries, such as the hydrogen Balmer series, He II 4686 Å and the Bowen blend. We focused our analysis on the prominent Hα line, which exhibits asymmetric emission and variable absorption components. We applied both traditional analytical methods and machine-learning techniques in order to explore the association of the absorption features with outflowing phenomena, and we conclude that they are best explained by broad absorption. This result is consistent with reports from other outbursting BHs, where optical outflows have predominantly been observed in the hard state. Further observations at different X-ray states are key to properly test whether this behaviour is universal and to determine the implications for the disc wind physics.

A dependable distance estimator to black hole low-mass X-ray binaries

ABSTRACT Black hole low-mass X-ray binaries (BH-LMXBs) are excellent observational laboratories for studying many open questions in accretion physics. However, determining the physical properties of BH-LMXBs necessitates knowing their distances. With the increased discovery rate of BH-LMXBs, many canonical methods cannot produce accurate distance estimates at the desired pace. In this study, we develop a versatile statistical framework to obtain robust distance estimates soon after discovery. Our framework builds on previous methods where the soft spectral state and the soft-to-hard spectral state transitions, typically present in an outbursting BH-LMXB, are used to place constraints on mass and distance. We further develop the traditional framework by incorporating general relativistic corrections, accounting for spectral/physical parameter uncertainties, and employing assumptions grounded in current theoretical and observational knowledge. We tested our framework by analysing a sample of 50 BH-LMXB sources using X-ray spectral data from the Swift/XRT, MAXI/GSC, and RXTE/PCA missions. By modelling their spectra, we applied our framework to 26 sources from the 50. Comparison of our estimated distances to previous distance estimates indicates that our findings are dependable and in agreement with the accurate estimates obtained through parallax and H i absorption methods. Investigating the accuracy of our constraints, we have found that estimates obtained using both the soft and transition spectral information have a median uncertainty (1σ) of 20 per cent, while estimates obtained using only the soft spectral state spectrum have a median uncertainty (1σ) of around 50 per cent. Furthermore, we have found no instrument-specific biases.

Spectral and type I X-ray burst studies of 4U 1702−429 using AstroSat observations

ABSTRACT 4U 1702−429, an atoll-type neutron star low-mass X-ray binary, was observed twice by the AstroSat/Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counters (LAXPC-20) on 2018 April 27 and 2019 August 8. Persistent emission spectra of the source were well fitted with the model combination - constant × tbabs (thcomp × diskbb+powerlaw). The parameters obtained from the spectral analysis revealed the source to be in a hard spectral state during the observations. Time-resolved spectral analyses were performed on the three type I X-ray bursts detected from the source. Burst analysis showed that the source underwent a photospheric radius expansion. Consequently, the radius of the neutron star and distance to the source (with isotropic and anisotropic burst emission) were obtained as 12.65$\substack{+0.90\\-0.86}$ km and 6.92$\substack{+0.16\\-0.09}$ and 8.43$\substack{+0.20\\-0.10}$ kpc, respectively.

The 2022 Outburst of IGR J17091–3624: Connecting the Exotic GRS 1915+105 to Standard Black Hole X-Ray Binaries

While the standard X-ray variability of black hole X-ray binaries (BHXBs) is stochastic and noisy, there are two known BHXBs that exhibit exotic “heartbeat”-like variability in their lightcurves: GRS 1915+105 and IGR J17091–3624. In 2022, IGR J17091–3624 went into outburst for the first time in the NICER/NuSTAR era. These exquisite data allow us to simultaneously track the exotic variability and the corresponding spectral features with unprecedented detail. We find that as in typical BHXBs, the outburst began in the hard state, then continued in the intermediate state, but then transitioned to an exotic soft state, where we identify two types of heartbeat-like variability (Class V and a new Class X). The flux energy spectra show a broad iron emission line due to relativistic reflection when there is no exotic variability, and absorption features from highly ionized iron when the source exhibits exotic variability. Whether absorption lines from highly ionized iron are detected in IGR J17091–3624 is not determined by the spectral state alone, but rather is determined by the presence of exotic variability; in a soft spectral state, absorption lines are only detected along with exotic variability. Our finding indicates that IGR J17091–3624 can be seen as a bridge between the most peculiar BHXB GRS 1915+105 and “normal” BHXBs, because it alternates between the conventional and exotic behaviors of BHXBs. We discuss the physical nature of the absorbing material and exotic variability in light of this new legacy data set.

Probing the accretion geometry of the atoll source 4U 1702−429 in different spectral states with NICER, NuSTAR, and AstroSat

ABSTRACT We perform a comprehensive spectral study of a carefully selected sample (total exposure ∼50.5 ks) of NICER observations of the atoll neutron star low-mass X-ray binary 4U 1702−429. Our sample encompasses nearly all classical spectral states found within the NICER data set. We require two thermal emission components, originating from the accretion disc and the boundary layer, to describe the soft state spectra in the energy band 0.3–10.0 keV. In contrast, in our model, only the disc component directly contributes to the intermediate/hard state. Additionally, we use a thermally Comptonized component (or a power law with pegged normalization) to represent the hard coronal emission in the soft and intermediate/hard state spectra. The boundary layer emerges as the principal source providing soft seed photons for Comptonization across all spectral states. In contrast to a previously held assertion regarding this source, our analyses reveal a decrease in the inner disc temperature coupled with the retreat of the inner disc from the neutron star surface as the source evolves from the soft to the intermediate/hard state. The reflection features are either absent or weak (∼3σ–4σ) in all these observations. Further investigation using broad-band NuSTAR (3.0–50.0 keV) and AstroSat spectra (1.3–25.0 keV) shows a slightly stronger iron emission line (∼5.8σ) in the NuSTAR spectra. However, this feature is not significantly detected in the AstroSat observation. The AstroSat data suggest a highly ionized disc, explaining the absence of reflection features. In the case of NuSTAR, a truncated disc is likely responsible for the weak reflection features.

Rms–Flux Slope in MAXI J1820+070: A Measure of Disk–Corona Coupling

The linear rms–flux relation has been well established in different spectral states of all accreting systems. In this work, we study the evolution of the frequency-dependent rms–flux relation of MAXI J1820+070 during the initial decaying phase of its 2018 outburst with Insight-HXMT over a broad energy range of 1–150 keV. As the flux decreases, we first observe a linear rms–flux relation at frequencies from 2 mHz to 10 Hz, while such a relation breaks at varying times for different energies, leading to a substantial reduction in the slope. Moreover, we find that the low-frequency variability exhibits the highest sensitivity to the break, which occurs prior to the hard-to-hard state transition time determined through time-averaged spectroscopy, and the time deviation increases with energy. The overall evolution of the rms–flux slope and intercept suggests the presence of a two-component Comptonization system. One component is radially extended, explaining the strong disk–corona coupling before the break, while the other component extends vertically, contributing to a reduction of disk–corona coupling after the break. A further vertical expansion of the latter component is required to accommodate the dynamic evolution observed in the rms–flux slope. In conclusion, we suggest that the rms–flux slope in the 1–150 keV band can be employed as an indicator of disk–corona coupling, and the hard-to-hard state transition in MAXI J1820+070 could be partially driven by changes in the coronal geometry.

Partial spectral flow in the D1D5 CFT

The two-dimensional \U0001d4a9 = 4 superconformal algebra has a free field realization with four bosons and four fermions. There is an automorphism of the algebra called spectral flow. Under spectral flow, the four fermions are transformed together. In this paper, we study partial spectral flow where only two of the four fermions are transformed. Partial spectral flow is applied to the D1D5 CFT where a marginal deformation moves the CFT away from the free point. The partial spectral flow is broken by the deformation. We show that this effect can be studied due to a transformation of the deformation which is well-defined under partial spectral flow. As a result in the spectrum, we demonstrate how to compute the second-order energy lift of a D1D5P state through its partial spectral flowed state. We find that D1D5P states related by partial spectral flow do not have the same lift in general.

X-Ray Polarized View of the Accretion Geometry in the X-Ray Binary Circinus X-1

Cir X-1 is a neutron star X-ray binary characterized by strong variations in flux during its eccentric ∼16.6 day orbit. There are also strong variations in the spectral state, and it has historically shown both atoll and Z state properties. We observed the source with the Imaging X-ray Polarimetry Explorer during two orbital segments, 6 days apart, for a total of 263 ks. We find an X-ray polarization degree in these segments of 1.6% ± 0.3% and 1.4% ± 0.3% at polarization angles of 37° ± 5° and −12° ± 7°, respectively. Thus, we observed a rotation of the polarization angle by 49° ± 8° along the orbit. Because variations of accretion flow, and then of the hardness ratio, are expected during the orbit, we also studied the polarization binned in hardness ratio and found the polarization angle differing by 67° ± 11° between the lowest and highest values of the hardness ratio. We discuss possible interpretations of this result that could indicate a possible misalignment between the symmetry axes of the accretion disk and the Comptonizing region caused by the misalignment of the neutron star’s angular momentum with respect to the orbital one.

Three-dimensional GRMHD Simulations of Neutron Star Jets

Neutron stars and black holes in X-ray binaries are observed to host strong collimated jets in the hard spectral state. Numerical simulations can act as a valuable tool in understanding the mechanisms behind jet formation and its properties. Although there have been significant efforts in understanding black hole jets from general relativistic magnetohydrodynamic (GRMHD) simulations in recent years, neutron star jets still remain poorly explored. We present the results from three-dimensional GRMHD simulations of accreting neutron stars with oblique magnetospheres for the very first time. The jets in our simulations are produced due to the anchored magnetic field of the rotating star in analogy with the Blandford–Znajek process. We find that for accreting stars, the star–disk magnetic field interaction plays a significant role, and as a result, the jet power becomes directly proportional to , where Φjet is the open flux in the jet. The jet power decreases with increasing stellar magnetic inclination, and finally, for an orthogonal magnetosphere, it reduces by a factor of ≃2.95 compared to the aligned case. We also find that in the strong propeller regime, with a highly oblique magnetosphere, the disk-induced collimation of the open stellar flux preserves parts of the striped wind, resulting in a striped jet.

High Hard X-Ray Polarization in Cygnus X-1 Confined to the Intermediate Hard State: Evidence for a Variable Jet Component

Cygnus X-1, the well-known accreting black hole system, exhibits several observational features hinting at an intricate interplay between the accretion disk, its atmosphere known as the corona, and the putative relativistic jet. It has been extensively studied using all available observational methods, including using the newly available technique of sensitive X-ray polarimetry. X-ray polarization characteristics are distinct for coronal and jet emissions. The low X-ray polarization measured below ∼100 keV is understood as arising from the corona. In contrast, the high polarization measurements reported above ∼400 keV required a separate jet-dominated spectral component, which spectroscopy does not demonstrate conclusively. Here we report precise polarization measurements in the 100–380 keV region made during three different subclasses of spectral states of the source using the CZTI instrument onboard AstroSat. A high polarization (23% ± 4%) is found mainly in the Intermediate Hard State of the source, and the energy-resolved measurements smoothly connect the coronal and the jet regimes. When high polarization is observed, the simultaneous spectral data hints at a separate power-law component above 100 keV. We examine the possible sources of this energy-dependent high polarization in Cygnus X-1.

X-ray polarization changes with the state transition in Cygnus X–1

ABSTRACT We studied the polarization properties of Cygnus X–1 in both hard and soft spectral states with imaging X-ray polarimetry explorer (IXPE) observations. The polarization degree is $\sim 4~{{\ \rm per\ cent}}$, and $\sim 2.6-2.8~{{\ \rm per\ cent}}$ in the hard and soft states, respectively. The polarization angle is observed along the jet axis and remains the same in both states. Energy-dependent analysis revealed an increasing polarization with the energy. The observed polarization rules out the lamp-post corona and prefers a conical corona if the coronal geometry remains the same in the hard and soft states. The disc is not detected in the hard state but is found in the soft state in the IXPE spectra. The spectral analysis revealed that the disc emission contributes $\sim 20~{{\ \rm per\ cent}}$ of the total emission in the soft state. The scattering of the seed photons inside the corona is likely to reduce the polarization. This could explain the different polarization in different spectral states. Additionally, if the disc polarization is perpendicular to that of the corona, it could also reduce the polarization in the soft state.

Pair Plasma Cascade in Rotating Black Hole Magnetospheres with a Split Monopole Flux Model

An electron-positron cascade in the magnetospheres of Kerr black holes (BHs) is a fundamental ingredient to fueling the relativistic γ-ray jets seen at the polar regions of galactic supermassive BHs (SMBHs). This leptonic cascade occurs in the spark gap region of a BH magnetosphere where the unscreened electric field parallel to the magnetic field is present; hence, it is affected by the magnetic field structure. A previous study explored the case of a thin accretion disk, representative of active galactic nuclei. Here we explore the case of a quasi-spherical gas distribution, as is expected to be present around the SMBH Sgr A* in the center of our Milky Way galaxy, for example. The properties and efficiency of the leptonic cascade are studied. The findings of our study and the implications for SMBH systems in various spectral and accretion states are discussed. The relationships and scalings derived from varying the mass of the BH and background photon spectra are further used to analyze the leptonic cascade process to power jets seen in astronomical observations. In particular, one finds the efficiency of the cascade in a quasi-spherical gas distribution peaks at the jet axis. Observationally, this should lead to a more prominent jet core, in contrast to the thin disk accretion case, where it peaks around the jet–disk interface. One also finds the spectrum of the background photons plays a key role. The cascade efficiency is maximum for a spectral index of 2, while harder and softer spectra lead to a less efficient cascade.

Accretion Flow Properties of EXO 1846-031 during Its Multi-peaked Outburst after Long Quiescence

We study the recent outburst of the black hole candidate EXO 1846-031, which went into an outburst in 2019 after almost 34 yr in quiescence. We use archival data from the Swift/XRT, MAXI/GSC, NICER/XTI, and NuSTAR/FPM satellites/instruments to study the evolution of the spectral and temporal properties of the source during the outburst. The low-energy (2–10 keV) X-ray flux of the outburst shows multiple peaks, making it a multipeak outburst. Evolving type-C quasi-periodic oscillations are observed in the NICER data in the hard, hard-intermediate, and soft-intermediate states. We use the physical two-component advective flow (TCAF) model to analyze the combined spectra of multiple satellite instruments. According to the TCAF model, the accreting matter is divided into Keplerian and sub-Keplerian parts, and the variation in the observed spectra in different spectral states arises out of the variable contributions of these two types of accreting matter in the total accretion rate. Studying the evolution of the accretion rates and other properties of the accretion flow obtained from the spectral analysis, we show that the multiple peaks in the outburst flux arise out of the variable supply of accreting matter from the pile-up radius. We determine the probable mass of the black hole to be from the spectral analysis with the TCAF model. We also estimate the viscous timescale of the source in this outburst to be ∼8 days from the peak difference of the Keplerian and sub-Keplerian mass-accretion rates.

Stellar-wind variability in Cygnus X-1 from high-resolution excess variance spectroscopy with Chandra

Context. Stellar winds of massive stars are known to be driven by line absorption of UV photons, a mechanism that is prone to instabilities, causing the wind to be clumpy. The clumpy structure hampers wind mass-loss estimates, limiting our understanding of massive star evolution. The wind structure also impacts accretion in high-mass X-ray binary (HMXB) systems. Aims. We aim to analyse the wavelength-dependent variability of X-ray absorption in the wind to study its structure. Such an approach is possible in HMXBs, where the compact object serves as an X-ray backlight. We probe different parts of the wind by analysing data taken at superior and inferior conjunctions. Methods. We applied excess variance spectroscopy to study the wavelength-dependent soft (2–14 Å) X-ray variability of the HMXB Cygnus X-1 in the hard spectral state. Excess variance spectroscopy quantifies the variability of an object above the statistical noise as a function of wavelength, which allows us to study the variability of individual spectral lines. This technique was applied to high-resolution gratings spectra provided by Chandra, accounting for various systematic effects. The frequency dependence is investigated by changing the time binning. Results. The strong orbital phase dependence we observe in the excess variance is consistent with column-density variations predicted by a simple model for a clumpy wind. We identify spikes of increased variability with spectral features found by previous spectroscopic analyses of the same data set, most notably from silicon in over-dense clumps in the wind. In the silicon line region, the variability power is redistributed towards lower frequencies, hinting at increased line variability in large clumps. In prospect of the microcalorimetry missions that are scheduled to launch within the next decade, excess variance spectra present a promising approach to constraining the wind structure, especially if accompanied by models that consider changing ionisation.

Correlated spectral and recurrence variations of Cygnus X-1

ABSTRACT We present results of recurrence analysis of the black hole X-ray binary Cygnus X-1 using combined observations from the Rossi X-ray Timing Explorer All-sky Monitor and the Japanese Monitor of All-sky X-ray Image aboard the International Space Station. From the time-dependent windowed recurrence plot (RP), we compute 10 recurrence quantities that describe the dynamical behaviour of the source and compare them to the spectral state at each point in time. We identify epochs of state changes corresponding to transitions into highly deterministic or highly stochastic dynamical regimes and their correlation to specific spectral states. We compare k-Nearest Neighbors and Random Forest models for various sizes of the time-dependent RP. The spectral state in Cygnus X-1 can be predicted with greater than 95 per cent accuracy for both types of models explored across a range of RP sizes based solely on the recurrence properties. The primary features from the RP that distinguish between spectral states are the determinism, Shannon entropy, and average line length, all of which are systematically higher in the hard state compared to the soft state. Our results suggest that the hard and soft states of Cygnus X-1 exhibit distinct dynamical variability and the time domain alone can be used for spectral state classification.

Comprehensive X-ray and multiwavelength study of ULXs in NGC 1566

ABSTRACT This paper presents a comprehensive X-ray and multiwavelength study of ultraluminous X-ray sources (ULXs) in NGC 1566 using archival Chandra, Swift/XRT, JWST, and HST observations. The main results are, first, from the hardness ratio diagram, where spectral state transitions from hard to soft as seen in typical Galactic high-mass X-ray binaries for ULX-3 were observed. Secondly, a new transient ULX candidate (ULX-4) was identified, reaching a peak luminosity of ∼1040 erg s−1. Thirdly, the optical and NIR (near-infrared) counterparts of the ULXs were searched from the precise astrometric calculations. For ULX-1 and ULX-2, evidence was found that the observed NIR emission is due to the circumbinary disc/dust disrupted by X-rays. Lastly, the optical observations suggest that the possible donor star of ULX-3 is a B-type supergiant. In the case of ULX-4, the multiwavelength properties are not clear since many sources are detected within the astrometric error radius.