scholarly journals X-ray emission from magnetized neutron star atmospheres at low mass-accretion rates. I. Phase-averaged spectrum

2021 ◽  
Author(s):  
E. Sokolova-Lapa ◽  
M. Gornostaev ◽  
J. Wilms ◽  
R. Ballhausen ◽  
S. Falkner ◽  
...  
Keyword(s):  
Neutron Star ◽  
X Ray ◽  
Accretion Rates ◽  
Low Mass

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 The study of multipeaked type-I X-ray bursts in the neutron star low-mass X-ray binary 4U 1636 − 536 with RXTE

2020 ◽  
Vol 501 (1) ◽  
pp. 168-178
Author(s):  
Chen Li ◽  
Guobao Zhang ◽  
Mariano Méndez ◽  
Jiancheng Wang ◽  
Ming Lyu
Keyword(s):  
Neutron Star ◽  
Flux Ratio ◽  
Light Curves ◽  
Type I ◽  
X Ray ◽  
Soft State ◽  
Peak Flux ◽  
Low Mass ◽  
Two Peaks ◽  
Second Peak

ABSTRACT We have found and analysed 16 multipeaked type-I bursts from the neutron-star low-mass X-ray binary 4U 1636 − 53 with the Rossi X-ray Timing Explorer (RXTE). One of the bursts is a rare quadruple-peaked burst that was not previously reported. All 16 bursts show a multipeaked structure not only in the X-ray light curves but also in the bolometric light curves. Most of the multipeaked bursts appear in observations during the transition from the hard to the soft state in the colour–colour diagram. We find an anticorrelation between the second peak flux and the separation time between two peaks. We also find that in the double-peaked bursts the peak-flux ratio and the temperature of the thermal component in the pre-burst spectra are correlated. This indicates that the double-peaked structure in the light curve of the bursts may be affected by enhanced accretion rate in the disc, or increased temperature of the neutron star.

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 Daily multiwavelength Swift monitoring of the neutron star low-mass X-ray binary Cen X-4: evidence for accretion and reprocessing during quiescence

2013 ◽  
Vol 436 (3) ◽  
pp. 2465-2483 ◽  
Author(s):  
F. Bernardini ◽  
E. M. Cackett ◽  
E. F. Brown ◽  
C. D'Angelo ◽  
N. Degenaar ◽  
...  
Keyword(s):  
Neutron Star ◽  
X Ray ◽  
Low Mass

scholarly journals UV and X-ray observations of the neutron star LMXB EXO 0748–676 in its quiescent state

2020 ◽  
Vol 501 (1) ◽  
pp. 1453-1462
Author(s):  
A S Parikh ◽  
N Degenaar ◽  
J V Hernández Santisteban ◽  
R Wijnands ◽  
I Psaradaki ◽  
...  
Keyword(s):  
Neutron Star ◽  
Thermal Emission ◽  
Hubble Space Telescope ◽  
Dense Matter ◽  
Continuum Emission ◽  
Quiescent State ◽  
X Ray ◽  
Low Level ◽  
Low Mass ◽  
Different Origins

ABSTRACT The accretion behaviour in low-mass X-ray binaries (LMXBs) at low luminosities, especially at <1034 erg s−1, is not well known. This is an important regime to study to obtain a complete understanding of the accretion process in LMXBs, and to determine if systems that host neutron stars with accretion-heated crusts can be used probe the physics of dense matter (which requires their quiescent thermal emission to be uncontaminated by residual accretion). Here, we examine ultraviolet (UV) and X-ray data obtained when EXO 0748–676, a crust-cooling source, was in quiescence. Our Hubble Space Telescope spectroscopy observations do not detect the far-UV continuum emission, but do reveal one strong emission line, C iv. The line is relatively broad (≳3500 km s−1), which could indicate that it results from an outflow such as a pulsar wind. By studying several epochs of X-ray and near-UV data obtained with XMM–Newton, we find no clear indication that the emission in the two wavebands is connected. Moreover, the luminosity ratio of LX/LUV ≳ 100 is much higher than that observed from neutron star LMXBs that exhibit low-level accretion in quiescence. Taken together, this suggests that the UV and X-ray emission of EXO 0748–676 may have different origins, and that thermal emission from crust-cooling of the neutron star, rather than ongoing low-level accretion, may be dominating the observed quiescent X-ray flux evolution of this LMXB.

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 A VARIABLE NEAR-INFRARED COUNTERPART TO THE NEUTRON-STAR LOW-MASS X-RAY BINARY 4U 1705 – 440

2009 ◽  
Vol 692 (1) ◽  
pp. 73-80 ◽  
Author(s):  
Jeroen Homan ◽  
David L. Kaplan ◽  
Maureen van den Berg ◽  
Andrew J. Young
Keyword(s):  
Neutron Star ◽  
Near Infrared ◽  
X Ray ◽  
Low Mass

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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