REJECTING PROPOSED DENSE MATTER EQUATIONS OF STATE WITH QUIESCENT LOW-MASS X-RAY BINARIES

ABSTRACT We investigate frequency correlations of low frequency (LF, <80 Hz) and kHz quasi-periodic oscillations (QPOs) using the complete RXTE data sets on six accreting millisecond X-ray pulsars (AMXPs) and compare them to those of non-pulsating neutron star (NS) low-mass X-ray binaries with known spin. For the AMXPs SAX J1808.4−3658 and XTE J1807−294, we find frequency-correlation power-law indices that, surprisingly, are significantly lower than in the non-pulsars, and consistent with the relativistic precession model (RPM) prediction of 2.0 appropriate to test-particle orbital and Lense–Thirring precession frequencies. As previously reported, power-law normalizations are significantly higher in these AMXPs than in the non-pulsating sources, leading to requirements on the NS specific moment of inertia in this model that cannot be satisfied with realistic equations of state. At least two other AMXPs show frequency correlations inconsistent with those of SAX J1808.4−3658 and XTE J1807−294, and possibly similar to those of the non-pulsating sources; for two AMXPs no conclusions could be drawn. We discuss these results in the context of a model that has had success in black hole (BH) systems involving a torus-like hot inner flow precessing due to (prograde) frame dragging, and a scenario in which additional (retrograde) magnetic and classical precession torques not present in BH systems are also considered. We show that a combination of these interpretations may accommodate our results.

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QUIESCENT THERMAL EMISSION OF NEUTRON STARS IN LMXBs

International Journal of Modern Physics D ◽

10.1142/s0218271810016786 ◽

2010 ◽

Vol 19 (06) ◽

pp. 805-810

Author(s):

ANABELA R. TURLIONE ◽

JOSE A. PONS ◽

DEBORAH N. AGUILERA

Keyword(s):

Thermal Conductivity ◽

Thermal Emission ◽

Accretion Rate ◽

Thermal Relaxation ◽

Dense Matter ◽

Light Curves ◽

Model Parameters ◽

X Ray ◽

Low Mass ◽

X Ray Binaries

Recent monitoring of the quiescent thermal emission from NSs in low mass X–ray binaries (LMXBs) after active periods (bursts) opened a new view to the physics of dense matter. Theoretical modeling of the thermal relaxation of the crust may be used to establish constraints on the thermal conductivity of matter, depending on the accretion rate. We present here cooling curves obtained from numerical simulations that fit the light curves for two sources (KS 1731-260, MXB 1659-29). We estimate the model parameters (accretion rate, thermal conductivity) that match the data and compare our results with previous constraints of neutron star crust properties in LMXBs.

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Iron K-Line Emission from X-Ray Binaries

International Astronomical Union Colloquium ◽

10.1017/s0252921100107389 ◽

1988 ◽

Vol 102 ◽

pp. 47-50

Author(s):

K. Masai ◽

S. Hayakawa ◽

F. Nagase

Keyword(s):

Accretion Disk ◽

Radiative Recombination ◽

Characteristic Temperature ◽

Line Emission ◽

Ionization Front ◽

Continuum Radiation ◽

X Ray ◽

Low Mass ◽

X Ray Binaries

AbstractEmission mechanisms of the iron Kα-lines in X-ray binaries are discussed in relation with the characteristic temperature Txof continuum radiation thereof. The 6.7 keV line is ascribed to radiative recombination followed by cascades in a corona of ∼ 100 eV formed above the accretion disk. This mechanism is attained for Tx≲ 10 keV as observed for low mass X-ray binaries. The 6.4 keV line observed for binary X-ray pulsars with Tx> 10 keV is likely due to fluorescence outside the He II ionization front.

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QPOs in compact binaries from small scale eruptions in an inner magnetized disk

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3410 ◽

2020 ◽

Author(s):

Nicolas Scepi ◽

Mitchell C Begelman ◽

Jason Dexter

Keyword(s):

Rapid Heating ◽

Small Scale ◽

Type B ◽

Periodic Oscillations ◽

X Ray ◽

Compact Binaries ◽

Heating And Cooling ◽

Low Mass ◽

X Ray Binaries ◽

Standard Disk

Abstract Dwarf novæ (DNe) and low mass X-ray binaries (LMXBs) are compact binaries showing variability on time scales from years to less than seconds. Here, we focus on explaining part of the rapid fluctuations in DNe, following the framework of recent studies on the monthly eruptions of DNe that use a hybrid disk composed of an outer standard disk and an inner magnetized disk. We show that the ionization instability, that is responsible for the monthly eruptions of DNe, is also able to operate in the inner magnetized disk. Given the low density and the fast accretion time scale of the inner magnetized disk, the ionization instability generates small, rapid heating and cooling fronts propagating back and forth in the inner disk. This leads to quasi-periodic oscillations (QPOs) with a period of the order of 1000 s. A strong prediction of our model is that these QPOs can only develop in quiescence or at the beginning/end of an outburst. We propose that these rapid fluctuations might explain a subclass of already observed QPOs in DNe as well as a, still to observe, subclass of QPOs in LMXBs. We also extrapolate to the possibility that the radiation pressure instability might be related to Type B QPOs in LMXBs.

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Drifts of the marginally stable burning frequency in the X-ray binaries 4U 1608–52 and Aql X–1

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab159 ◽

2021 ◽

Vol 502 (2) ◽

pp. 1856-1863

Author(s):

G C Mancuso ◽

D Altamirano ◽

M Méndez ◽

M Lyu ◽

J A Combi

Keyword(s):

Frequency Drift ◽

Soft Or ◽

Significance Level ◽

X Ray ◽

Intermediate States ◽

Source State ◽

Nuclear Burning ◽

Low Mass ◽

X Ray Binaries ◽

Stable Burning

ABSTRACT We detect millihertz quasi-periodic oscillations (mHz QPOs) using the Rossi X-ray Time Explorer (RXTE) from the atoll neutron-star (NS) low-mass X-ray binaries 4U 1608–52 and Aql X–1. From the analysis of all RXTE observations of 4U 1608–52 and Aql X–1, we find mHz QPOs with a significance level >3σ in 49 and 47 observations, respectively. The QPO frequency is constrained between ∼4.2 and 13.4 mHz. These types of mHz QPOs have been interpreted as being the result of marginally stable nuclear burning of He on the NS surface. We also report the discovery of a downward frequency drift in three observations of 4U 1608–52, making it the third source that shows this behaviour. We only find strong evidence of frequency drift in one occasion in Aql X–1, probably because the observations were too short to measure a significant drift. Finally, the mHz QPOs are mainly detected when both sources are in the soft or intermediate states; the cases that show frequency drift only occur when the sources are in intermediate states. Our results are consistent with the phenomenology observed for the NS systems 4U 1636–53 and EXO 0748–676, suggesting that all four sources can reach the conditions for marginally stable burning of He on the NS surface. These conditions depend on the source state in the same manner in all four systems.

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Search for intermittent X-ray pulsations from neutron stars in low-mass X-ray binaries

Publications of the Astronomical Society of Australia ◽

10.1017/pasa.2021.6 ◽

2021 ◽

Vol 38 ◽

Author(s):

Yunus Emre Bahar ◽

Manoneeta Chakraborty ◽

Ersin Göğüş

Keyword(s):

Neutron Stars ◽

Orbital Period ◽

Oscillation Frequency ◽

Orbital Motion ◽

X Rays ◽

X Ray ◽

Second Stage ◽

Corrected Data ◽

Low Mass ◽

X Ray Binaries

Abstract We present the results of our extensive binary orbital motion corrected pulsation search for 13 low-mass X-ray binaries. These selected sources exhibit burst oscillations in X-rays with frequencies ranging from 45 to 1 122 Hz and have a binary orbital period varying from 2.1 to 18.9 h. We first determined episodes that contain weak pulsations around the burst oscillation frequency by searching all archival Rossi X-ray Timing Explorer data of these sources. Then, we applied Doppler corrections to these pulsation episodes to discard the smearing effect of the binary orbital motion and searched for recovered pulsations at the second stage. Here we report 75 pulsation episodes that contain weak but coherent pulsations around the burst oscillation frequency. Furthermore, we report eight new episodes that show relatively strong pulsations in the binary orbital motion corrected data.

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4U 1608–52 as a quasi-persistent X-ray source

Publications of the Astronomical Society of Japan ◽

10.1093/pasj/psaa099 ◽

2020 ◽

Vol 72 (6) ◽

Author(s):

Vojtěch Šimon

Keyword(s):

Accretion Disk ◽

Time Segment ◽

X Ray ◽

Band Emission ◽

Heating And Cooling ◽

Soft State ◽

Cyclic Variations ◽

Low Mass ◽

X Ray Binaries ◽

Disk Region

Abstract 4U 1608–52 is a soft X-ray transient. The analysis presented here of a particular part of its X-ray activity uses observations of RXTE/ASM and Swift/BAT. We show a time segment (MJD 54262–MJD 55090) (828 d) in which 4U 1608–52 behaved as a quasi-persistent X-ray source with a series of bumps, with a complicated relation between the evolution of fluxes in the soft (1.5–12 keV) and the hard (15–50 keV) X-ray regions. We ascribe these bumps to a series of propagations of heating and cooling fronts over the inner disk region without any transitions to the true quiescence. 4U 1608–52 oscillated around the boundary between the dominance of the Comptonized component and the dominance of the multicolor accretion disk in its luminosity. Only some of the bumps in this series were accompanied by a transition from the hard to the soft state; if it occurred, it displayed a strong hysteresis effect. The hard-band emission with the dominant Comptonized component was present for most of this active state and showed a cycle of about 40 d. We argue that the cyclic variations of flux come from the inner disk region, not, e.g., from a jet. We also discuss the observed behavior of 4U 1608–52 in the context of other quasi-persistent low-mass X-ray binaries.

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

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3899 ◽

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.

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