X-ray views of neutron star low-mass X-ray binaries
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Abstract:
A neutron star low-mass X-ray binary is a binary stellar system with a neutron star and a low-mass companion star rotating around each other. In this system the neutron star accretes mass from the companion, and as this matter falls into the deep potential well of the neutron star, the gravitational potential energy is released primarily in the X-ray wavelengths. Such a source was first discovered in X-rays in 1962, and this discovery formally gave birth to the "X-ray astronomy". In the subsequent decades, our knowledge of these sources has increased enormously by the observations with several X-ray space missions. Here we give a brief overview of our current understanding of the X-ray observational aspects of these systems.Keywords:
X-ray binary
X-ray burster
Stellar mass
Star (game theory)
Low Mass
Cygnus X-3 is a strong X-ray source ( erg s-1) which is thought to consist of a compact object accreting matter from a helium star. We analytically find that the estimated ranges of mass-loss rate and orbital-period derivative for Cyg X-3 are consistent with two models: i) the system is detached and the mass loss from the system comes from the stellar wind of a massive helium star, of which only a fraction that allows for the observed X-ray luminosity is accreted, or ii) the system is semidetached and a Roche-lobe-overflowing low- or moderate-mass helium donor transfers mass to the compact object, followed by ejection of its excess over the Eddington rate from the system. These analytical results appear to be consistent with evolutionary calculations. By means of population synthesis we find that currently in the Galaxy there may exist ~1 X-ray binary with a black hole that accretes from a Wolf-Rayet star and ~1 X-ray binary in which a neutron star accretes matter from a Roche-lobe-overflowing helium star with mass . Cyg X-3 is probably one of these systems.
X-ray burster
Roche lobe
X-ray binary
Compact star
Black hole (networking)
Eddington luminosity
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X-ray binary
Low Mass
Intermediate polar
Accretion disc
Star (game theory)
X-ray burster
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We report on the optical spectroscopy of the eclipsing halo low-mass X-ray binary 2S 0921-630, which reveals the absorption-line radial velocity curve of the K0 III secondary star with a semiamplitude K2 = 92.89 ± 3.84 km s-1, a systemic velocity γ = 34.9 ± 3.3 km s-1, and an orbital period Porb of 9.0035 ± 0.0029 days (1 σ). Given the quality of the data, we find no evidence for the effects of X-ray irradiation. Using the previously determined rotational broadening of the mass donor and applying conservative limits on the orbital inclination, we constrain the compact object mass to be 2.0-4.3 M☉ (1 σ), ruling out a canonical neutron star at the 99% level. Since the nature of the compact object is unclear, this mass range implies that the compact object is either a low-mass black hole with a mass slightly higher than the maximum possible neutron star mass (2.9 M☉) or a massive neutron star. If the compact object is a black hole, it confirms the prediction of the existence of low-mass black holes, while if the object is a massive neutron star, its high mass severely constrains the equation of state of nuclear matter.
Compact star
X-ray binary
Black hole (networking)
Low Mass
Mass ratio
X-ray burster
Star (game theory)
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Centaurus X-4 (Cen X-4) is a relatively nearby neutron star low-mass X-ray binary that showed outbursts in 1969 and 1979, but has not shown a full outburst since. Due to its proximity and sustained period of quiescence, it is a prime target to study the coupling between accretion and jet ejection in quiescent neutron star low-mass X-ray binaries. Here, we present four MeerKAT radio observations at 1.3 GHz of Cen X-4, combined with NICER and Swift X-ray monitoring. During the first and most sensitive observation, Cen X-4 was in a fully quiescent X-ray state. The three later and shorter observations targeted a brief period of faint X-ray activity in January 2021, which has been referred to as a 'mis-fired' outburst. Cen X-4 is not detected in any of the four MeerKAT observations. We place these radio non-detections on the X-ray -- radio luminosity diagram, improving the constraints on the correlation between the two luminosities from earlier quiescent radio studies. We confirm that Cen X-4 is radio fainter than the transitional milli-second pulsar PSR J1023+0038 at the same X-ray luminosity. We discuss the radio behaviour of accreting neutron stars at low X-ray luminosity more generally and finally comment on future observing campaigns.
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X-ray binary
X-ray burster
Stellar mass
Accretion disc
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We consider transient behavior in low-mass X-ray binaries (LMXBs). We show that if this results from a disk instability, the secondary star must be significantly evolved when mass transfer starts, particularly if the primary is a neutron star. For P ≲ 2 days, most neutron star systems will be persistent X-ray sources, whereas the slower orbital evolution of black hole systems means that most of them are likely to be transient. Both types of transient system must have extreme mass ratios (<0.1). For orbital periods P ≳ 2 days, most LMXBs will be transient regardless of whether the primary is a neutron star or a black hole.
Black hole (networking)
Transient (computer programming)
X-ray binary
Low Mass
X-ray transient
X-ray burster
Stellar mass
Primary (astronomy)
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To explain the presence of a relatively massive black hole in 2A0620−00 we propose a new evolutionary scenario for the formation of short-period, low-mass X-ray binaries. The progenitor in this scenario is a triple star in which a massive close binary is accompanied at large distance by a late dwarf. After the evolution of the close binary into an ordinary X-ray binary, the compact object is engulfed by its expanding massive companion, and spirals in to settle at its centre. The resulting Thorne–|$\rm{\dot Zytkow}$| supergiant gradually expands until it attains the size of the late-dwarf orbit. Then a second spiral-in phase ensues, leading to the formation of a low-mass close binary. Depending on the initial parameters of the massive binary, this close low-mass binary may contain either a back hole or a neutron star.
X-ray binary
Roche lobe
Low Mass
Orbital period
Compact star
X-ray burster
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Abstract A magnetized neutron star may appear as a radio pulsar or an X-ray source. The latter is connected with a binary system where accretion from a normal star onto the neutron star produces X-ray emission. At the end of the evolution of a normal non-massive star, accretion stops and the neutron star becomes a recycled radio pulsar. Further evolution may lead to an additional transition from a radio pulsar to a low mass X-ray binary (LMXB). The formation of a single recycled pulsar is considered and a new mechanism of “enhanced evaporation” in globular clusters is analyzed.
X-ray pulsar
X-ray binary
Low Mass
X-ray burster
Star (game theory)
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Following the new outburst of the Galactic neutron star low-mass X-ray binary and 2.1 Hz X-ray pulsar GRO J1744-28 (ATels #10073, #10079), we performed target of opportunity observations of this source with the Australia Telescope Compact Array (ATCA).
X-ray burster
X-ray binary
Low Mass
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Based on the model of the accretion-induced magnetic field decay of a neutron star (NS), millisecond pulsars (MSPs) will obtain their minimum magnetic field when the NS magnetosphere radius shrinks to the stellar surface during the binary accretion phase. We find that this minimum magnetic field is related to the accretion rate |$\dot{M}$| as |$B_{\rm min}\sim 2.0\times 10^7{\, \rm G}\, (\dot{M}/\dot{M}_{\rm min})^{1/2}$|, where |$\dot{M}_{\rm min}=4.6\times 10^{15}\rm \, g\,s^{-1}$| is the average minimum accretion rate required for MSP formation and is constrained by the long-term accretion time, which corresponds to the companion lifetime, being less than the Hubble time. The value of Bmin is consistent with that of observed radio MSPs and accreting MSPs in low-mass X-ray binaries, which can be found the illustrated case of the minimum and present field strength of SAX J1808.4–3658. The prediction of the minimum magnetic field of MSPs would be the lowest field strength of NSs in the Universe, which could constrain the evolution mechanism of the magnetic field of accreting NSs.
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Millisecond
X-ray pulsar
X-ray burster
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We explore the population of candidate pre-low-mass X-ray binaries in which a neutron star accretes mass from the wind of a low-mass companion (mass < 2Msun) in the framework of a binary population synthesis study. The simulated accretion-luminosity distribution shows a primary peak close to 1e31 erg/s and a secondary peak near 1e28 erg/s. The relative contribution of the two peaks depends primarily on the magnitude of the kick velocity imparted to the neutron star at birth. The secondary peak is negligible for average kick velocities larger than 200 km/s, but becomes dominant for average kick velocities smaller than 50 km/s. Regardless of the relative contributions of the two peaks, our calculations suggest that pre-low-mass X-ray binaries may provide a non-negligible contribution to the population of discrete low-luminosity X-ray sources in the Galaxy.
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X-ray binary
X-ray burster
Primary (astronomy)
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