scholarly journals A strongly changing accretion morphology during the outburst decay of the neutron star X-ray binary 4U 1608−52

2020 ◽  
Vol 493 (1) ◽  
pp. 1318-1327 ◽  
Author(s):  
J van den Eijnden ◽  
N Degenaar ◽  
R M Ludlam ◽  
A S Parikh ◽  
J M Miller ◽  
...  
Keyword(s):  
Neutron Star ◽  
Dynamic Range ◽  
Signal To Noise Ratio ◽  
X Ray ◽  
Accretion Flow ◽  
Accretion Rates ◽  
Low Mass ◽  
Physical Changes ◽  
X Ray Binaries ◽  
Eddington Limit

ABSTRACT It is commonly assumed that the properties and geometry of the accretion flow in transient low-mass X-ray binaries (LMXBs) significantly change when the X-ray luminosity decays below ∼10−2 of the Eddington limit (LEdd). However, there are few observational cases where the evolution of the accretion flow is tracked in a single X-ray binary over a wide dynamic range. In this work, we use NuSTAR and NICER observations obtained during the 2018 accretion outburst of the neutron star LMXB 4U 1608−52, to study changes in the reflection spectrum. We find that the broad Fe–Kα line and Compton hump, clearly seen during the peak of the outburst when the X-ray luminosity is ∼1037 erg s−1 (∼0.05 LEdd), disappear during the decay of the outburst when the source luminosity drops to ∼4.5 × 1035 erg s−1 (∼0.002 LEdd). We show that this non-detection of the reflection features cannot be explained by the lower signal-to-noise ratio at lower flux, but is instead caused by physical changes in the accretion flow. Simulating synthetic NuSTAR observations on a grid of inner disc radius, disc ionization, and reflection fraction, we find that the disappearance of the reflection features can be explained by either increased disc ionization (log ξ ≳ 4.1) or a much decreased reflection fraction. A changing disc truncation alone, however, cannot account for the lack of reprocessed Fe–Kα emission. The required increase in ionization parameter could occur if the inner accretion flow evaporates from a thin disc into a geometrically thicker flow, such as the commonly assumed formation of a radiatively inefficient accretion flow at lower mass accretion rates.

scholarly journals Revealing the Physical Changes Taking Place in Z-Track Motion in Low Mass X-Ray Binaries

2004 ◽  
Vol 194 ◽  
pp. 206-206
Author(s):  
M. Bałucińska-Church ◽  
M. J. Church ◽  
G. Halai ◽  
A. Szostek
Keyword(s):  
Neutron Star ◽  
Emission Model ◽  
Spectral Evolution ◽  
X Ray ◽  
Astrophysical Problem ◽  
Low Mass ◽  
Physical Changes ◽  
First Time ◽  
X Ray Binaries

The explanation of the strong physical changes clearly taking place in the Z-track class of Low Mass X-ray Binaries has so far not been obtained, and this remains a significant astrophysical problem, without which we cannot claim to understand accretion in LMXB. We have for the first time applied the Birmingham emission model (2,3) to this problem to attempt to obtain a solution from the spectral evolution along the Z-track in the source GX 340+0 observed with Rossi- ХTE. In this model, X-ray emission consists of blackbody from the neutron star, plus Comptonized emission from an extended ADC.

scholarly journals The advection-dominated accretion flow for the anticorrelation between the X-ray photon index and the X-ray luminosity in neutron star low-mass X-ray binaries

2020 ◽  
Vol 496 (3) ◽  
pp. 2704-2714
Author(s):  
Erlin Qiao ◽  
B F Liu
Keyword(s):  
Neutron Star ◽  
Thermal Emission ◽  
X Ray ◽  
Radiative Efficiency ◽  
Accretion Flow ◽  
Self Similar Solution ◽  
Photon Index ◽  
Low Mass ◽  
Self Similar ◽  
X Ray Binaries

ABSTRACT Observationally, an anticorrelation between the X-ray photon index Γ (obtained by fitting the X-ray spectrum between 0.5 and 10 keV with a single power law) and the X-ray luminosity L0.5-10 keV, i.e. a softening of the X-ray spectrum with decreasing L0.5-10 keV, is found in neutron star low-mass X-ray binaries (NS-LMXBs) in the range of $L_{\rm 0.5\!-\!10\,keV}\sim 10^{34}\!-\!10^{36}\ \rm erg\ s^{-1}$. In this paper, we explain the observed anticorrelation between Γ and L0.5–10 keV within the framework of the self-similar solution of the advection-dominated accretion flow (ADAF) around a weakly magnetized NS. The ADAF model intrinsically predicts an anticorrelation between Γ and L0.5–10 keV. In the ADAF model, there is a key parameter, fth, which describes the fraction of the ADAF energy released at the surface of the NS as thermal emission to be scattered in the ADAF. We test the effect of fth on the anticorrelation between Γ and L0.5–10 keV. It is found that the value of fth can significantly affect the anticorrelation between Γ and L0.5–10 keV. Specifically, the anticorrelation between Γ and L0.5–10 keV becomes flatter with decreasing fth as taking fth = 0.1, 0.03, 0.01, 0.005, 0.003, and 0, respectively. By comparing with a sample of non-pulsating NS-LMXBs with well measured Γ and L0.5–10 keV, we find that indeed only a small value of 0.003 ≲ fth ≲ 0.1 is needed to match the observed anticorrelation between Γ and L0.5–10 keV. Finally, we argue that the small value of fth ≲ 0.1 derived in this paper further confirms our previous conclusion that the radiative efficiency of NSs with an ADAF accretion may not be as high as $\epsilon \sim {\dot{M} GM\over R_{*}}/{\dot{M} c^2}\sim 0.2$.

scholarly journals A Machine Learning Approach For Classifying Low-mass X-ray Binaries Based On Their Compact Object Nature

2020 ◽  
Author(s):  
R Pattnaik ◽  
K Sharma ◽  
K Alabarta ◽  
D Altamirano ◽  
M Chakraborty ◽  
...  
Keyword(s):  
Machine Learning ◽  
Black Hole ◽  
Neutron Star ◽  
Binary Systems ◽  
Compact Object ◽  
X Ray ◽  
Average Accuracy ◽  
Machine Learning Approach ◽  
Low Mass ◽  
X Ray Binaries

Abstract Low Mass X-ray binaries (LMXBs) are binary systems where one of the components is either a black hole or a neutron star and the other is a less massive star. It is challenging to unambiguously determine whether a LMXB hosts a black hole or a neutron star. In the last few decades, multiple observational works have tried, with different levels of success, to address this problem. In this paper, we explore the use of machine learning to tackle this observational challenge. We train a random forest classifier to identify the type of compact object using the energy spectrum in the energy range 5-25 keV obtained from the Rossi X-ray Timing Explorer archive. We report an average accuracy of 87±13% in classifying the spectra of LMXB sources. We further use the trained model for predicting the classes for LMXB systems with unknown or ambiguous classification. With the ever-increasing volume of astronomical data in the X-ray domain from present and upcoming missions (e.g., SWIFT, XMM-Newton, XARM, ATHENA, NICER), such methods can be extremely useful for faster and robust classification of X-ray sources and can also be deployed as part of the data reduction pipeline.

scholarly journals Radiative efficiency of hot accretion flow and the radio/X-ray correlation in X-ray binaries

2014 ◽  
Vol 10 (S312) ◽  
pp. 139-140
Author(s):  
Fu-Guo Xie
Keyword(s):  
Black Hole ◽  
Active Galactic Nuclei ◽  
Accretion Rate ◽  
Galactic Nuclei ◽  
X Ray ◽  
Radiative Efficiency ◽  
Accretion Flow ◽  
Accretion Rates ◽  
Distinctive Property ◽  
X Ray Binaries

AbstractSignificant progresses have been made since the discovery of hot accretion flow, a theory successfully applied to the low-luminosity active galactic nuclei (LLAGNs) and black hole (BH) X-ray binaries (BHBs) in their hard states. Motivated by these updates, we re-investigate the radiative efficiency of hot accretion flow. We find that, the brightest regime of hot accretion flow shows a distinctive property, i.e. it has a constant efficiency independent of accretion rates, similar to the standard thin disk. For less bright regime, the efficiency has a steep positive correlation with the accretion rate, while for faint regime typical of advection-dominated accretion flow, the correlation is shadower. This result can naturally explain the observed two distinctive correlations between radio and X-ray luminosities in black hole X-ray binaries. The key difference in systems with distinctive correlations could be the viscous parameter, which determines the critical luminosity of different accretion modes.

scholarly journals Hardness Ratio in Evolving Low-Mass X-ray Binary Systems

1987 ◽  
Vol 125 ◽  
pp. 199-199
Author(s):  
J. Shaham ◽  
M. Tavani
Keyword(s):  
Neutron Star ◽  
Accretion Disk ◽  
Binary Systems ◽  
Spectral Component ◽  
Soft Component ◽  
Hard Component ◽  
X Ray ◽  
Low Mass ◽  
Additional Support ◽  
X Ray Binaries

Spectral observations of low-mass X-ray binaries (LMXBs) show that the soft component usually dominates over the hard one. These results provide additional support to an interpretation based on models of LMXBs in which the neutron star while, on the average, spinning up, is also experiencing a spinning down torque. Under these conditions, a fraction of the luminosity associated with the gravitational release of energy on the surface of the accreting neutron star may manifest itself as luminosity originating in the inner part of the accretion disk. It is probably possible to separate the two contributions; the stellar luminosity can be associated with the hard component of the spectrum and the disk luminosity, related to the exchange of energy due to the torque between the rapidly spinning neutron star and the accretion disk, can be associated with the soft spectral component.

scholarly journals Mass transfer on a nuclear timescale in models of supergiant and ultra-luminous X-ray binaries

2019 ◽  
Vol 628 ◽  
pp. A19 ◽  
Author(s):  
M. Quast ◽  
N. Langer ◽  
T. M. Tauris
Keyword(s):  
Mass Transfer ◽  
Black Hole ◽  
Neutron Star ◽  
Roche Lobe ◽  
High Mass ◽  
X Rays ◽  
X Ray ◽  
Mass Ratios ◽  
X Ray Binaries ◽  
Eddington Limit

Context. The origin and number of the Galactic supergiant X-ray binaries is currently not well understood. They consist of an evolved massive star and a neutron star or black-hole companion. X-rays are thought to be generated from the accretion of wind material donated by the supergiant, while mass transfer due to Roche-lobe overflow is mostly disregarded because the high mass ratios of these systems are thought to render this process unstable. Aims. We investigate how the proximity of supergiant donor stars to the Eddington limit, and their advanced evolutionary stage, may influence the evolution of massive and ultra-luminous X-ray binaries with supergiant donor stars (SGXBs and ULXs). Methods. We constructed models of massive stars with different internal hydrogen and helium gradients (H/He gradients) and different hydrogen-rich envelope masses, and exposed them to slow mass-loss to probe the response of the stellar radius. In addition, we computed the corresponding Roche-lobe overflow mass-transfer evolution with our detailed binary stellar evolution code, approximating the compact objects as point masses. Results. We find that a H/He gradient in the layers beneath the surface, as it is likely present in the well-studied donor stars of observed SGBXs, can enable mass transfer in SGXBs on a nuclear timescale with a black-hole or a neutron star accretor, even for mass ratios in excess of 20. In our binary evolution models, the donor stars rapidly decrease their thermal equilibrium radius and can therefore cope with the inevitably strong orbital contraction imposed by the high mass ratio. We find that the orbital period derivatives of our models agree well with empirical values. We argue that the SGXB phase may be preceded by a common-envelope evolution. The envelope inflation near the Eddington limit means that this mechanism more likely occurs at high metallicity. Conclusion. Our results open a new perspective for understanding that SGBXs are numerous in our Galaxy and are almost completely absent in the Small Magellanic Cloud. Our results may also offer a way to find more ULX systems, to detect mass transfer on nuclear timescales in ULX systems even with neutron star accretors, and shed new light on the origin of the strong B-field in these neutron stars.

scholarly journals The Beat-Frequency Interpretation of Kilohertz Quasi-periodic Oscillations in Neutron Star Low-Mass X-Ray Binaries

1998 ◽  
Vol 501 (1) ◽  
pp. L95-L99 ◽  
Author(s):  
Dimitrios Psaltis ◽  
Mariano Méndez ◽  
Rudy Wijnands ◽  
Jeroen Homan ◽  
Peter G. Jonker ◽  
...  
Keyword(s):  
Neutron Star ◽  
Beat Frequency ◽  
Periodic Oscillations ◽  
X Ray ◽  
Frequency Interpretation ◽  
Low Mass ◽  
X Ray Binaries

scholarly journals Neutron star QPOs from oscillating, precessing hot, thick flow

2019 ◽  
Vol 491 (3) ◽  
pp. 3245-3250
Author(s):  
P Chris Fragile
Keyword(s):  
Neutron Star ◽  
Low Frequency ◽  
Outer Radius ◽  
Horizontal Branch ◽  
X Ray ◽  
Best Fitting ◽  
Low Mass ◽  
The Moment ◽  
X Ray Binaries ◽  
Simultaneous Oscillation

ABSTRACT Across black hole (BH) and neutron star (NS) low-mass X-ray binaries (LMXBs), there appears to be some correlation between certain high- and low-frequency quasi-periodic oscillations (QPOs). In a previous paper, we showed that for BH LMXBs, this could be explained by the simultaneous oscillation and precession of a hot, thick, torus-like corona. In the current work, we extend this idea to NS LMXBs by associating the horizontal branch oscillations (HBOs) with precession and the upper-kiloHertz (ukHz) QPO with vertical epicyclic motion. For the Atoll source 4U 1608-52, the model can match many distinct, simultaneous observations of the HBO and ukHz QPO by varying the inner and outer radius of the torus, while maintaining fixed values for the mass (MNS) and spin (a*) of the NS. The best-fitting values are MNS = 1.38 ± 0.03 M⊙ and a* = 0.325 ± 0.005. By combining these constraints with the measured spin frequency, we are able to obtain an estimate for the moment of inertia of INS = 1.40 ± 0.02 × 1045 g cm2, which places constraints on the equation of state. The model is unable to fit the lower-kHz QPO, but evidence suggests that QPO may be associated with the boundary layer between the accretion flow and the NS surface, which is not treated in this work.

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